HK1241679A1 - Pulse sequences for low field magnetic resonance - Google Patents

Description

Pulse sequence for low field magnetic resonance

Background

Magnetic Resonance Imaging (MRI) provides an important imaging modality for many applications and is widely used in clinical and research environments to produce images of the interior of the human body. As a general rule, MRI is based on detecting Magnetic Resonance (MR) signals, which are electromagnetic waves emitted by atoms in response to state changes caused by an applied electromagnetic field. For example, Nuclear Magnetic Resonance (NMR) techniques include: MR signals emitted from nuclei of excited atoms are detected upon realignment or relaxation of nuclear spins of atoms in an object being imaged (e.g., atoms in human tissue). The detected MR signals may be processed to generate images, which in the context of medical applications, allow for investigation of internal structures and/or biological processes within the body for diagnostic, therapeutic and/or research purposes.

MRI offers an attractive imaging modality for biological imaging due to its ability to produce non-invasive images with relatively high resolution and contrast without the safety concerns of other modalities (e.g., without the need to expose the imaged subject to ionizing radiation such as x-rays or introduce radioactive substances into the body). In addition, MRI can capture information about structures and/or biological processes that other modalities are less suitable for acquiring or are unable to acquire. For example, MRI is particularly suited to provide contrast between soft tissues. However, conventional MRI techniques suffer from a number of drawbacks, which may include, for a given imaging application: the relatively high cost of the equipment, limited availability, and/or difficulty in gaining access to the clinical MRI scanner, the length of the image acquisition process, etc.

The trend in clinical MRI is to increase the field strength of the MRI scanner to improve one or more of scan time, image resolution and image contrast, which in turn increases the cost of MRI imaging. Most installed MRI scanners operate using at least 1.5 tesla (T) or 3 tesla, which refers to the main magnetic field B of the scanner₀The field strength of (a). The rough cost of clinical MRI scanners is estimated to be on the order of one million dollars per tesla, even without taking into account the substantial operating, service, and maintenance costs involved in operating such MRI scanners.

In addition, conventional high-field MRI systems typically require large superconducting magnets and associated electronics to generate a strong, uniform static magnetic field (B) in which an object (e.g., a patient) is imaged_o). The size of such systems is considerable, with typical MRI equipment including multiple rooms for the magnet, electronics, thermal management system, and console areas. The size and expense of MRI systems often limits their use in facilities such as hospitals and academic research centers that have sufficient space and resources to purchase and maintain them. The high cost and large space requirements of high-field MRI systems result in limited availability of MRI scanners. Thus, the following clinical situations often exist: MRI scanning would be beneficial but is impractical or infeasible due to the limitations described above and as discussed in further detail below.

Disclosure of Invention

The inventors have recognized that performing low-field magnetic resonance imaging can be facilitated by operating in a low-field environment using pulse sequences developed by the inventors.

Some embodiments provide a low-field Magnetic Resonance Imaging (MRI) system, comprising: a plurality of magnetic components including a magnetic field configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation; and at least one controller configured to operate one or more of the plurality of magnetic components according to at least one low-field zero-echo time (LF-ZTE) pulse sequence.

Some embodiments provide a method for operating a low-field magnetic resonance imaging system, the system comprising a plurality of magnetic components including a magnetic coil configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation. The method comprises the following steps: generating a low-field main magnetic field B using at least one first magnetic component₀(ii) a And controlling, using at least one controller, one or more of the plurality of magnetic components according to at least one low-field zero-echo time (LF-ZTE) pulse sequence.

Some embodiments provide at least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by a low-field MRI system comprising a plurality of magnetic components including a magnetic component configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated: generating a low-field main magnetic field B using at least one first magnetic component₀(ii) a And according to at least one low fieldA zero echo time (LF-ZTE) pulse sequence to operate one or more of the plurality of magnetic components.

Some embodiments provide a low-field Magnetic Resonance Imaging (MRI) system, comprising: a plurality of magnetic components including a magnetic field configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation; and at least one controller configured to operate one or more of the plurality of magnetic components according to at least one Low Field Refocusing (LFR) pulse sequence, wherein an RF excitation pulse in the at least one LFR pulse sequence is associated with a decrease in B₀The flip angle of the effect of the non-uniformity on the net transverse magnetization is related.

Some embodiments provide a method for operating a low-field magnetic resonance imaging system, the system comprising a plurality of magnetic components including a magnetic coil configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation. The method comprises the following steps: operating at least one first magnetic component to generate a low-field main magnetic field B₀(ii) a And controlling one or more of the plurality of magnetic components using at least one controller according to at least one Low Field Refocusing (LFR) pulse sequence, wherein RF excitation pulses in the at least one LFR pulse sequence are reduced by B₀The flip angle of the effect of the non-uniformity on the net transverse magnetization is related.

Some embodiments provide at least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by a low-field MRI system comprising a plurality of magnetic components including a magnetic component configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated: operating at least one firstMagnetic means to generate a low field main magnetic field B₀(ii) a And operating one or more of the plurality of magnetic components according to at least one Low Field Refocusing (LFR) pulse sequence, wherein RF excitation pulses in the at least one LFR pulse sequence are reduced by B₀The flip angle of the effect of the non-uniformity on the net transverse magnetization is related.

Some embodiments provide a low-field Magnetic Resonance Imaging (MRI) system, comprising: a plurality of magnetic components configured to generate a magnetic field comprising a low-field main magnetic field B₀A plurality of magnetic components comprising a plurality of magnetic fields configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation; and at least one controller configured to operate one or more of the plurality of magnetic components according to a pulse sequence designed to compensate for non-uniformity of one or more of the plurality of magnetic fields at least in part by causing the one or more of the plurality of magnetic components to apply a series of RF pulses having at least one parameter that varies during a respective series of pulse repetition periods of the pulse sequence.

Some embodiments provide a method for operating a low-field magnetic resonance imaging system, the system comprising a plurality of magnetic components configured to generate a magnetic field comprising a low-field main magnetic field B₀A plurality of magnetic components comprising a plurality of magnetic fields configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation. The method comprises the following steps: operating at least one first magnetic component to generate a low-field main magnetic field B₀(ii) a And controlling one or more of the plurality of magnetic components using at least one controller according to a pulse sequence designed to compensate for non-uniformity of one or more of the plurality of magnetic fields at least in part by causing the plurality of magnetic components to apply a series of RF pulses, the series of RF pulses including a plurality of pulses having a first pulse width and a second pulse width, the second pulse width being different from the first pulse widthThe RF pulses have at least one parameter that varies during a respective series of pulse repetition periods of the pulse sequence.

Some embodiments provide at least one non-transitory computer-readable storage medium storing processor-executable instructions, when configured to generate a magnetic field comprising a low-field main magnetic field B₀The instructions, when executed by a low-field MRI system of a plurality of magnetic components of a plurality of magnetic fields, the instructions enabling the low-field MRI system to perform operations including generating a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated, the plurality of magnetic components generating at least one magnetic field: operating at least one first magnetic component to generate a low-field main magnetic field B₀(ii) a And operating one or more of the plurality of magnetic components according to a pulse sequence designed to compensate for inhomogeneity of one or more of the plurality of magnetic fields at least in part by causing the plurality of magnetic components to apply a series of RF pulses having at least one parameter that varies during a respective series of pulse repetition periods of the pulse sequence.

Drawings

Various aspects and embodiments of the disclosed technology will be described with reference to the following drawings. It should be understood that the drawings are not necessarily drawn to scale. Items appearing in multiple figures are denoted by the same reference numeral in all of the figures in which they appear.

Fig. 1 is a block diagram of exemplary components of a low-field MRI system, according to some embodiments of the technology described herein.

Fig. 2A is a diagram illustrating one pulse repetition period of a low-field zero-echo time pulse sequence in accordance with some embodiments of the technology described herein.

Fig. 2B is a diagram illustrating two consecutive pulse repetition periods of an LF-ZTE pulse sequence, according to some embodiments of the techniques described herein.

Fig. 2C is a diagram illustrating an LF-ZTE pulse sequence including one or more contrast preparation portions, according to some embodiments of the techniques described herein.

Fig. 2D is a diagram illustrating a portion of an LF-ZTE pulse sequence including a T1 contrast preparation portion, according to some embodiments of the techniques described herein.

Figure 2E is a diagram illustrating a portion of an Electron Paramagnetic Resonance (EPR) pulse sequence including an LF-ZTE pulse sequence according to some embodiments of the techniques described herein.

Fig. 2F is a diagram illustrating a portion of an LF-ZTE pulse sequence including a navigation pulse sequence, according to some embodiments of the techniques described herein.

Fig. 2G is a diagram illustrating a portion of an LF-ZTE pulse sequence including a water/fat separation contrast preparation sequence, according to some embodiments of the techniques described herein.

Fig. 3 is a flow chart of an illustrative process for performing MR imaging in a low-field MR system using a low-field zero-echo time pulse sequence.

Fig. 4 is a graph illustrating one pulse repetition period of a low field-equilibrium steady-state free precession (LF-bSSFP) sequence, according to some embodiments of the technology described herein.

Fig. 5 is a graph illustrating the effect of magnetic field non-uniformity on transverse magnetization at different flip angles according to some embodiments of the technology described herein.

Fig. 6 is a flow diagram of an illustrative process for performing MR imaging in a low-field MR system using an LF-bSSFP sequence in accordance with some embodiments of the technology described herein.

Fig. 7 is a schematic diagram of a low-field Radio Frequency (RF) coil, in accordance with some embodiments of the technology described herein.

Fig. 8 illustrates inputting and correspondingly outputting current to and from a low-field RF coil in the time domain without applying pre-emphasis (pre-emphasis), in accordance with some embodiments of the techniques described herein.

Fig. 9 illustrates how a current input to a low-field LF coil is attenuated by a low-field RF coil in the frequency domain and how pre-emphasis may be used to counteract this attenuation, in accordance with some embodiments of the techniques described herein.

Fig. 10 illustrates pre-emphasis waveforms in the frequency domain in accordance with some embodiments of the techniques described herein.

Fig. 11 illustrates inputting and correspondingly outputting current to and from a low-field RF coil in the time domain with pre-emphasis applied, in accordance with some embodiments of the techniques described herein.

Fig. 12A and 12B illustrate a biplane magnet configuration according to some embodiments.

Fig. 13 illustrates an exemplary stationary biplane low-field MRI system for use in conjunction with one or more other modalities.

Fig. 14A and 14B illustrate an exemplary tilted biplane low-field MRI system for use in conjunction with one or more other modalities.

Fig. 15A and 15B illustrate a mobile low-field MRI system according to some embodiments.

Detailed Description

The MRI scanner market is dominated by high field systems for the most part and is dedicated to medical or clinical MRI applications. As used herein, "high field" generally refers to MRI systems currently used in clinical settings, and more particularly to MRI systems operating with a main magnetic field at or above 1.5T (i.e., the B0 field), although clinical systems operating between 0.5T and 1.5T are also generally considered "high field". In contrast, "low field" generally refers to an MRI system operating at a B0 field that is less than or equal to about 0.2T.

The attractiveness of high-field MRI systems includes improved resolution and/or reduced scan times relative to lower-field systems, thereby facilitating the push for higher and higher field strengths for clinical and medical MRI applications. However, as discussed above, increasing the field strength of MRI systems increases the cost and complexity of MRI scanners, thus limiting their usability and preventing their use as a versatile and/or generally available imaging solution.

Low-field MR has been explored in limited environmental conditions for non-imaging research purposes and narrow and specific contrast-enhanced imaging applications, but has traditionally been considered unsuitable for producing clinically useful images. For example, resolution, contrast, and/or image acquisition time are not generally considered suitable for clinical purposes, including but not limited to tissue differentiation, blood flow or perfusion imaging, Diffusion Weighted (DW) or Diffusion Tensor (DT) imaging, functional mri (fmri), and the like. At least some of the difficulties in obtaining clinically useful images using low-field MRI include the fact that: in general, pulse sequences designed for high-field MRI are not suitable for low-field environments, for reasons discussed in further detail below.

Briefly, MRI involves placing an object to be imaged (e.g., all or a portion of a patient) in a static uniform magnetic field B₀To net magnetize (usually represented by a net magnetization vector) the atoms of the object at B₀The field is aligned in the direction (align). One or more transmit coils are then used to generate a pulsed magnetic field B₁The pulse magnetic field B₁Having a magnetic field B₀The frequency related to the precession rate of the atomic spins of the atoms such that the net magnetization of the atoms is transverse to B₀A component is generated in the direction of the field. In B₁After the field is switched off, the transverse component of the net magnetization vector precesses, with its amplitude decaying with time, until the net magnetization and B₀The direction of the field is realigned. The process produces one or more connections that may be passed through the MRI systemThe MR signals are detected by the induced voltage in the winding.

Furthermore, MRI involves the use of gradient coils to generate the main magnetic field B₀Gradients are induced in the signal so that MR signals emanating from a particular spatial location within the subject can be identified (i.e., gradient coils are used to spatially encode the detected MR signals). MR images are formed in part by pulsing one or more transmit coils and/or gradient coils in a particular sequence (referred to as a "pulse sequence") and sensing the MR signals induced by the pulse sequence using one or more receive coils. The detected MR signals may then be processed (e.g., "reconstructed") to form an image. The pulse sequence generally describes the order and timing at which the transmit/receive coils and gradient coils operate to prepare the magnetization of the subject and acquire the resulting MR data. For example, the pulse sequence may indicate the order of the transmit pulses, the gradient pulses, and the acquisition time at which the receive coils acquire MR data.

Although many pulse sequences for high-field MRI have been developed, pulse sequences defined for high-field MRI are not suitable for application in low-field environments. Significant differences in operating parameters, particularly significant reduction in signal-to-noise ratio (SNR), between high-field MRI and low-field MRI require different approaches to design pulse sequences suitable for low-field MRI. The inventors have developed pulse sequences specifically designed for low-field MRI that address various shortcomings of low-field environments and utilize other techniques to reduce acquisition time and improve the quality of low-field MRI. The significant differences in operating parameters of the low-field MRI pulse sequence developed by the present inventors from the conventional high-field MRI pulse sequence are shown in tables 1 and 2 below. In addition, the inventors have developed low-field MRI for different contrast types, such as T₁Weighted sum T₂Weighted imaging, diffusion weighted imaging, arterial spin labeling (perfusion imaging), Overhauser imaging, etc., where each contrast type has a specific set of considerations in a low-field environment.

Signal-to-noise ratio of MR signals and main magnetic field B₀Is related to the strength of and is driving the clinical system in a high field environmentOne of the main factors of operation. Therefore, the MR signal strength in the low field is relatively small, making the design of the pulse sequence critical. As discussed in further detail below, the inventors developed pulse sequences that increase the SNR and/or reduce the MR data acquisition time to facilitate improved low-field MRI (e.g., by improving resolution, making the acquisition time satisfactory, etc.).

As discussed above, the small SNR of low-field MRI is a significant challenge to performing low-field MRI. A technique for addressing low SNR is to repeat the MR data acquisition multiple times for a particular spatial encoding (e.g., by repeating a pulse sequence with the same or similar operating parameters) and average the obtained MR signals. However, although averaging improves SNR, repeated acquisition increases the total acquisition time. To address this problem, the inventors have developed a number of "fast averaging" pulse sequences that employ averaging to increase the signal-to-noise ratio of the acquired MR signals, but allow such averaging to be performed quickly, thereby reducing the total amount of time to acquire an image. Such a fast averaging pulse sequence results in improved MR imaging in low SNR (e.g., low field) environments. The term "averaging" is used herein to describe any type of scheme for combining signals, including absolute averaging (e.g., average), weighted averaging, or any other technique that may be used to increase SNR by combining MR data from multiple acquisitions.

The inventors have realized that a suitable class of fast averaging pulse sequences comprises zero echo time pulse sequences. The inventors have developed a pulse sequence, referred to herein as a low-field zero-echo time (LF-ZTE) pulse sequence, that is specifically designed for use and/or optimal performance in a low-field environment. The LF-ZTE pulse sequence may include RF pulses that induce relatively small flip angles (e.g., flip angles between fifteen and fifty degrees), which allow multiple acquisitions to be averaged more quickly with correspondingly shorter relaxation times, and thus with less time between successive acquisitions. In turn, a faster single acquisition allows multiple acquisitions to be quickly averaged. In addition, as described in more detail below, the LF-ZTE pulse sequence allows one or more receive coils to operate and receive MR signals with longer periods within the pulse sequence to increase the amount of signal obtained, thereby increasing the SNR obtained. Therefore, to obtain the desired SNR, fewer repetitions of the MR signal to be averaged are required. Thus, in some embodiments, a low-field MRI system may include one or more components (e.g., one or more transmit coils, one or more receive coils, one or more gradient coils, etc.) configured to operate according to one or more LF-ZTE pulse sequences, as discussed in further detail below.

Another type of fast averaging pulse sequence that the inventors have developed and specifically designed for use and/or optimal performance in low-field environments is the low-field refocusing (LFR) pulse sequence. The refocusing pulse sequence is characterized by: the pulse sequence has a portion configured to refocus the magnetization to a known state. For example, the LFR pulse sequence may include at least one RF pulse that induces a large flip angle of the net magnetization vector (e.g., a flip angle greater than 30 degrees, more preferably about 70 degrees or greater) and a refocused phase that drives the net magnetization vector toward the same large flip angle after the acquired relaxation period occurs. The refocusing phase may comprise applying gradient fields having a strength and polarity such that the sum of the field strengths of each gradient field is substantially zero (or intended to be close to zero) for the duration of the pulse repetition period. For example, the gradient field applied during the refocusing phase may be equal and opposite to the gradient field applied during the encoding phase of the pulse repetition period. Such sequences are called "balanced".

Between successive MR data acquisitions, the LFR pulse sequence need not wait for the net magnetization and B₀The field realigns (e.g., continuous acquisition can be obtained without waiting for the transverse magnetization vector to decrease to 0). In this way, successive acquisitions may be performed more quickly, which in turn allows for a fast averaging of multiple acquisitions, such that such averaging is performed. Some examplesEmbodiments include balanced pulse sequences developed by the present inventors for low-field environments, referred to herein as low-field balanced steady-state free precession (LF-bSSFP) pulse sequences, some examples of which are described in more detail below. Thus, in some embodiments, a low-field MRI system may include one or more components (e.g., control components configured to drive one or more transmit coils, one or more receive coils, one or more gradient coils, etc.) configured to operate according to one or more LFR (e.g., LF-bsfp) pulse sequences, as discussed in further detail below.

Typically, when applying a pulse sequence, there is a time delay between the time when the one or more transmit coils stop transmitting RF excitation pulses and the time when the one or more receive coils are able to accurately detect MR signals from the subject. This time delay is significantly attributed to the so-called "ringing" of the transmit coil, whereby the coil absorbs energy from the transmitted RF pulses and subsequently "rings" due to coil coupling (e.g., the absorbed energy dissipates at the resonant frequency of the coil). Until the coil ringing has sufficiently decayed (this period may be referred to as a "ring down" period), the receive coil (which may be the same coil as the receive coil in some embodiments) cannot be used to detect the MR signal.

The inventors have recognized that the RF pulses transmitted by the transmit coil can be designed to reduce coil ringing by shortening the ring-down period, thereby increasing the acquisition time in the pulse sequence used (e.g., LF-ZTE pulse sequence), which in turn increases the SNR of the MR signal. Thus, in some embodiments, the low-field MRI system may be configured to operate using RF pulses designed to reduce the length of the ring down period. For example, the RF pulses may be shaped to cancel attenuation of the RF pulses induced by the transmit coil by pre-emphasizing the RF pulses in proportion to an inverse function of a transfer function of the transmit coil. This is described in more detail below with reference to fig. 7 to 11.

Additionally or alternatively, the ring down period may be shortened by introducing a damping circuit in series or parallel with the transmit coil, the damping circuit being designed to dampen the energy absorbed by the transmit coil from the transmitted RF pulse. The damping circuit may be turned on for a period of time after the transmit coil completes transmitting to perform damping, and then may be turned off before the transmit coil begins transmitting again. The damping circuit may be designed in various ways. In some embodiments, for example, the damping circuit may include an n-channel metal oxide semiconductor field effect transistor (nMOS FET) having a source terminal connected to a ground terminal, a drain terminal connected to a signal from the transmit coil after the tuner, and a gate terminal connected to a fast digital input/output line. In some cases, the damping circuit may also include a low value resistor in series with the drain and the signal line. Such a damping circuit may be used to shorten the ring down by dumping its energy quickly into the nMOS FET and/or resistor.

Conventional high-field MRI systems use RF pulses to generate oscillating B₁A field in which the carrier frequency of each RF pulse is designed to be constant over its duration. The present inventors have recognized that oscillating B can be generated by using frequency modulated RF pulses₁The field to obtain an improved low-field MRI system in which the carrier frequency of each RF pulse varies with time over its duration. Examples of frequency modulated RF pulses include chirped pulses and adiabatic RF pulses. The carrier frequency of the adiabatic pulses may vary (e.g., in response to modulation) according to a quadratic or geometric function. Generation of B using frequency modulated RF pulses₁MRI system of fields versus main magnetic field B using RF pulses with a constant carrier frequency₀And B₁The non-uniformity in the field is less sensitive. However, frequency modulated RF pulses are not used in conventional high-field clinical MRI systems because they are longer in duration and have higher power than constant frequency pulses, such that the use of frequency modulated pulses will result in impermissible heating of the subject's tissue (i.e., typically resulting in a Specific Absorption Rate (SAR) that is more than prescribed to be allowed).

The inventors have realised that frequency modulated low field pulses may be usedLow-field MRI because at low fields, the power level of such pulses can be reduced to remain below acceptable or required SAR limits. Thus, in some embodiments, a low-field MRI system may be configured to generate oscillating B using frequency-modulated RF pulses₁Field, which can reduce the main magnetic field B of a low-field MRI system₀Field sum B₁Sensitivity to inhomogeneities in the field. In this way, since B is paired₀The insensitivity of field inhomogeneities increases, so the quality of the images obtained by low-field MRI systems can be improved.

The inventors have further recognized that LF-ZTE sequences may be suitable for the environment of Overhauser enhanced mri (omri). According to some embodiments, a low-field MRI system may be configured to operate using an LF-ZTE pulse sequence having one or more contrast preparation portions. For example, in some embodiments, a low-field MRI system may use an LF-ZTE pulse sequence including one or more Electron Paramagnetic Resonance (EPR) pulses to generate OMRI images, which provides a mechanism for imaging free radicals to provide, for example, brain trauma detection. As another example, in some embodiments, a low-field MRI system may use an LF-ZTE pulse sequence that may include one or more portions for preparing water/fat contrast imaging for a subject. In other embodiments, a low-field MRI system may use an LF-ZTE pulse sequence that includes one or more T1 contrast preparation portions, one or more T2 contrast preparation portions, one or more arterial spin labeling contrast preparation portions, and/or one or more diffusion weighted contrast preparation portions.

As discussed above, a benefit of low-field MRI is that it facilitates the deployment of relatively low-cost MRI systems that can be installed and maintained in virtually any location and/or can be designed to be portable/mobile to increase the availability of the system from a cost and accessibility standpoint. Accordingly, such low-field MRI systems may operate in less regular environments with respect to noise and/or may operate in varying environments for portable/mobile systems. The present inventors have recognized the advantages of an "environment informed" or adaptive pulse sequence that is configured to dynamically change based on the environment in which a given low-field MRI system operates. For example, one or more parameters of the pulse sequence may be dynamically varied based on one or more measurements obtained from the environment (e.g., measurements of one or more field sensors), measurements of the MRI system (e.g., measurements of the generated magnetic field, temperature measurements, etc.), and/or measurements of the scanned object (e.g., patient motion, etc.). It should be understood that any of the low-field pulse sequences described herein may be configured to: the context notification is performed by allowing one or more parameters of the pulse sequence to vary based on one or more measurements of the context and/or system.

According to some embodiments, a low-field MRI system (e.g., a portable low-field MRI system) may be used in "noisy" environments (e.g., in environments with electromagnetic interference that interferes, for example, with the operation of the low-field MRI system at least partially), and pulse sequences may be selected and/or adjusted based on the nature of the noise in the environment (e.g., parameters of the pulse sequences may be modified). As another example, a low-field MRI system may be employed to image an object that moves during image acquisition, and the pulse sequences may be selected and/or adjusted to reduce the effect of motion of the object during acquisition (e.g., by using pulse sequences with acquisition periods that are as short as possible). As another example, one or more components of the low-field MRI system may move relative to the subject during acquisition, and the pulse sequence may be selected and/or adjusted to reduce the effects of the movement of the one or more MRI system components.

It should be appreciated that the embodiments described herein may be implemented in any of numerous ways. Examples of specific implementations are provided below for illustrative purposes only. It should be understood that the provided embodiments and features/capabilities may be used alone, together, or in any combination of two or more, as the aspects of the technology described herein are not limited in this respect.

FIG. 1 is a block diagram of exemplary components of an MRI system 100. In the illustrative example of fig. 1, the MRI system 100 includes a computing device 104, a controller 106, a pulse sequence memory 108, a power management system 110, and a magnetic component 120. It should be appreciated that the system 100 is illustrative and that the low-field MRI system may have one or more other components of any suitable type in addition to or in place of the components shown in fig. 1.

As shown in FIG. 1, the magnetic member 120 includes B₀A magnet 122, shim coils 124, RF transmit and receive coils 126, and gradient coils 128. B is₀The magnet 122 may be used to generate, at least in part, a main magnetic field B₀。B₀The magnet 122 may be any suitable type of magnet capable of generating a main magnetic field (e.g., low field strength of about 0.2T or less), and may include one or more B s₀Coils, calibration coils, etc. Shim coils 124 may be used to help one or more magnetic fields improve the B generated by the magnet 122₀Uniformity of the field. The gradient coils 128 may be arranged to provide gradient fields and may for example be arranged to generate gradients in the magnetic field in three substantially orthogonal directions (X, Y, Z) to locate the position where the MR signals are induced.

The RF transmit and receive coil 126 may include one or more transmit coils that may be used to generate RF pulses to induce the magnetic field B₁. The one or more transmit/receive coils may be configured to: any suitable type of RF pulse is generated that is configured to excite an MR response in the subject and to detect the resulting MR signals that are emitted. The RF transmit and receive coils 126 may include one or more transmit coils and one or more receive coils. The configuration of the transmit/receive coils varies with implementation and may include a single coil for both transmit and receive, separate coils for transmit and receive, multiple coils for transmit and/or receive, or any combination for implementing a single channel or parallel MRI system. Thus, the transmit/receive magnetic components are commonly referred to as Tx/Rx coils to refer generally to for MVarious configurations of the transmit and receive components of the RI system. Each magnetic component 120 may be configured in any suitable manner. For example, in some embodiments, one or more of the magnetic components 120 may be any of the components described in U.S. patent application No. 14/845652 entitled "Low field magnetic Resonance Imaging Methods and Apparatus," filed on 9, 4, 2015 ("the' 652 application"), the entire contents of which are incorporated herein by reference.

The one or more transmit coils may be configured to generate any suitable type of RF pulse. For example, one or more transmit coils may be configured to generate one or more RF pulses each having a constant carrier frequency over its duration. As another example, one or more transmit coils may be configured to: one or more frequency modulated RF pulses (e.g., chirped RF pulses, adiabatic RF pulses, etc.) are generated such that the carrier frequency of the frequency modulated pulses varies over the course of their duration. As yet another example, one or more transmit coils may be configured to generate one or more electron paramagnetic resonance pulses. As yet another example, one or more transmit coils may be used to generate RF pulses designed to reduce coil ringing.

The power management system 110 includes electronics that provide operating power to one or more components of the low-field MRI system 100. For example, as discussed in more detail below, the power management system 110 may include one or more power supplies, gradient power amplifiers, transmit coil amplifiers, and/or any other suitable power electronics necessary to provide suitable operating power to energize and operate components of the low-field MRI system 100.

As shown in FIG. 1, the power management system 110 includes a power supply 112, one or more amplifiers 114, a transmit/receive switch 116, and a thermal management component 118. The power supply 112 includes electronics that provide operating power to the magnetic components 120 of the low-field MRI system 100. For example, the power supply 112 may include one or more B₀Coil (e.g. B)₀Magnet 122) provides operating power to generateElectronics for the main magnetic field of the low-field MRI system. In some embodiments, power supply 112 is a single-pole Continuous Wave (CW) power supply, however, any suitable power supply may be used. The transmit/receive switch 116 may be used to select whether the RF transmit coil is being operated or the RF receive coil is being operated.

The one or more amplifiers 114 may include: one or more RF receive (Rx) preamplifiers that amplify MR signals detected by one or more RF receive coils (e.g., coil 124); one or more RF transmit (Tx) amplifiers configured to provide power to one or more RF transmit coils (e.g., coil 126); one or more gradient power amplifiers configured to provide power to one or more gradient coils (e.g., gradient coil 128); and a shim amplifier configured to provide power to one or more shim coils (e.g., shim coil 124).

The thermal management component 118 provides cooling for components of the low-field MRI system 100 and may be configured to provide cooling away from one or more components of the low-field MRI system 100 by facilitating the transfer of thermal energy generated by those components. Thermal management component 118 may include, but is not limited to, components that perform water-based or air-based cooling, which may be integrated with or disposed proximate to a heat-generating MRI component, including, but not limited to B₀Coils, gradient coils, shim coils, and/or transmit/receive coils. The thermal management component 118 may include any suitable heat transfer medium (including, but not limited to, air and water) to transfer heat from the components of the low-field MRI system 100.

As shown in fig. 1, the low-field MRI system 100 includes a controller 106 (sometimes referred to as a console in an MRI environment) configured to send instructions to the power management system 110 and receive information from the power management system 110. The controller 106 may be configured to implement one or more pulse sequences for determining instructions to send to the power management system 110 to operate the magnetic components 120 in a desired sequence, for example, by operating one or more transmit coils and/or gradient coils in a particular sequence defined by the pulse sequences. The pulse sequence generally describes the order and timing at which the transmit/receive coils and gradient coils operate to prepare the magnetization of the subject and acquire the resulting MR data. For example, the pulse sequence may indicate the order of the transmit pulses, the gradient pulses, and the acquisition time at which the receive coils acquire MR data, as discussed in further detail below.

The controller 106 may be configured to control the power management system 110 to operate the magnetic component 120 according to an LF-ZTE pulse sequence, a low field balanced steady state free precession (LF-bSSFP) pulse sequence, a low field gradient echo pulse sequence, a low field spin echo pulse sequence, a low field inversion recovery pulse sequence, arterial spin labeling, Diffusion Weighted Imaging (DWI), and/or any other suitable pulse sequence. Pulse sequences for low-field MRI can be applied to different contrast types, such as T1-weighted and T2-weighted imaging, diffusion-weighted imaging, arterial spin labeling (perfusion imaging), Overhauser imaging, etc., where each contrast type has a particular set of considerations in a low-field environment. The controller 106 may be implemented as hardware, software, or any suitable combination of hardware and software, as the aspects of the present disclosure provided herein are not limited in this respect.

In some embodiments, the controller 106 may be configured to implement the pulse sequences by obtaining information about the pulse sequences from a pulse sequence repository 108, the pulse sequence repository 108 storing information for each of the one or more pulse sequences. The information for a particular pulse sequence stored by the pulse sequence repository 108 may be any suitable information that enables the controller 106 to implement a particular pulse sequence. For example, the information of the pulse sequences stored in the pulse sequence repository 108 may include: one or more parameters for operating the magnetic component 120 according to the pulse sequence (e.g., parameters for operating the RF transmit and receive coils 126, parameters for operating the gradient coils 128, etc.), one or more parameters for operating the power management system 110 according to the pulse sequence, one or more programs including instructions that, when executed by the controller 106, cause the controller 106 to control the system 100 to operate according to the pulse sequence, and/or any other suitable information. The information stored in the pulse sequence repository 108 may be stored on one or more non-transitory storage media.

As shown in fig. 1, the controller 106 also interacts with the computing device 104, and the computing device 104 is programmed to process the received MR data. For example, the computing device 104 may process the received MR data to generate one or more MR images using any suitable image reconstruction process or processes. The controller 106 may provide information regarding the one or more pulse sequences to the computing device 104 for processing of data by the computing device. For example, the controller 106 may provide information regarding one or more pulse sequences to the computing device 104, and the computing device may perform image reconstruction processing based at least in part on the provided information.

The computing device 104 may be any electronic device and typically includes one or more processors configured (e.g., programmed) to process the acquired MR data and generate one or more images of the imaged subject. In some implementations, the computing device 104 may be a stationary electronic device, such as a desktop computer, a server, a rack-mounted computer, or any other suitable stationary electronic device that may be configured to process MR data and generate one or more images of an imaged subject. Alternatively, the computing device 104 may be a portable device, such as a smartphone, personal digital assistant, laptop, tablet computer, or any other portable device that may be configured to process MR data and generate one or more images of an imaged subject.

It should be understood that the controller 106 may be a single integrated controller or may include separate controllers for performing the functions of the system 100. In some implementations, computing device 104 may include multiple computing devices of any suitable type, as aspects are not limited in this respect. The user 102 may interact with a computing device 104 (e.g., a workstation) to control aspects of the low-field MR system 100 (e.g., program the system 100 to operate according to a particular pulse sequence, adjust one or more parameters of the system 100, etc.) and/or view images obtained by the low-field MR system 100. According to some embodiments, the computing device 104 and the controller 106 form a single controller, while in other embodiments, the computing device 104 and the controller 106 each include one or more controllers. It should be understood that the functions performed by the computing device 104 and the controller 106 may be distributed in any manner over any combination of one or more controllers, as the aspects are not limited to use with any particular implementation or architecture. The controller 106 and computing device 104 typically include one or more processors capable of executing instructions contained in computer code, such as software programs, firmware instructions, etc., to perform one or more functions related to the operation of the system 100.

As described above, the present inventors have recognized that it may be advantageous to operate a low-field MRI system according to an LF-ZTE pulse sequence, such as the low-field MRI system 100 described above. Aspects of LF-ZTE pulse sequences according to some embodiments are described in more detail below with reference to fig. 2A-2G and fig. 3.

Fig. 2A is a graph having a duration T illustrating a LF-ZTE pulse sequence according to some embodiments of the techniques described herein_RA graph of one pulse repetition period 200. Initially, gradient fields G of respective operating strengths 204a, 204b and 204c are generated at the gradient coils_x、G_yAnd G_zWhile applying the duration T_FOf the RF pulse 202. Gradient field G_x、G_yAnd G_zApplied in substantially orthogonal directions. Next, for a duration Δ that allows the system to switch from transmit mode to receive mode_T/RAfter a delay 206, the receive coil is operated to be at duration T_ObtainingMR data is acquired during the acquisition interval 208. At a duration T_GDuring the subsequent interval 210 (e.g., toward or at the trailing end of interval 210), the strength of the one or more gradient fields changes to one or more other values. As shown in fig. 2A, during interval 210M, G_xAnd G_yIs changed, but during interval 210, field G_zIs constant in intensity. In some embodiments, the duration of the LF-ZTE pulse repetition may be 1 millisecond to 25 milliseconds.

Although in the embodiment shown in fig. 2A, the gradient field strengths 204a, 204b, and 204c are shown as being constant over the pulse repetition period 200 (except for the end of the interval 210 where the strength is shown as becoming another constant value), in other embodiments, the gradient field G is shown as being constant_x、G_yAnd G_zMay vary during the pulse repetition period. For example, the strength of one or more gradient fields may be modulated within the pulse repetition period to compensate for the presence of the time-varying eddy current field. As another example, the strength of one or more gradient fields may be modulated within the pulse repetition period to improve spatial encoding efficiency. As yet another example, reducing the strength of one or more gradient fields during transmission of RF pulses enables the use of lower bandwidth pulses to excite MR signals on the same region (e.g., slice) of the target. Thus, in some embodiments, the strength of one or more gradient fields may be reduced during the transmission of the RF pulse 202.

As can be seen from fig. 2A, the gradient coils are operated without being switched on and off for the entire duration of the pulse repetition period 200, as is the case with other sequences. Gradually changing the strength of the gradient fields generated by the gradient coils may place less burden on various components of the MRI system (e.g., a low-field MRI system), such as power amplifiers, for example, not having to drive fast and large current changes. It can also be observed that the pulse repetition time T_RIs the duration T of the RF pulse_FDuration delta of transmit/receive delay 206_T/RDuration T of acquisition period 208_ObtainingAnd duration T of the gradient switching interval_GAnd (4) summing. That is to say that the first and second electrodes,

T_R＝T_F+Δ_T/R+T_obtaining+T_G。

The RF pulse 202 may induce a flip angle of any suitable degree. For example, the RF pulse 202 may induce a flip angle of between 15 degrees and 50 degrees, and in some cases may induce a flip angle of 90 degrees or less. In some embodiments, RF pulse 202 may induce a small flip angle to minimize the time required for relaxation before another RF pulse can be applied to the next pulse repetition period. For example, RF pulses may be used to induce a flip angle of less than 60 degrees. As another example, RF pulses may be used to induce a flip angle of less than 40 degrees. As yet another example, RF pulses may be used to induce a flip angle of less than 20 degrees. As yet another example, RF pulses may be used to induce flip angles of less than 15 degrees. As discussed above, it may be advantageous to use a low flip angle in low SNR environments, as this allows for efficient averaging of multiple acquisitions: a lower flip angle results in a faster relaxation time and thus a faster averaging of multiple acquisitions.

The RF pulse 202 may be any suitable type of RF pulse. For example, the RF pulse 202 may be a pulse having a constant carrier frequency over its duration. As another example, the RF pulse 202 may have a varying carrier frequency over its duration. As yet another example, the RF pulse 202 may be designed to reduce the duration length of the delay interval 206, as discussed above, the delay interval 206 is significantly caused by coil ringing. For example, the pulse 202 may be shaped such that the pulse 202 suppresses or attenuates coil ringing. As one non-limiting example, the pulse 202 may be pre-emphasized in the frequency domain or in the time domain based on the transfer function of the transmit coil and/or any other suitable model of how the transmit coil attenuates the frequency and phase of the input signal. The pre-emphasis RF pulses are described in more detail below with reference to fig. 7-11. It should be appreciated that these examples are illustrative, and that RF pulse 202 may be any suitable type of RF pulse, as aspects of the techniques described herein are not limited in this respect.

FIG. 2B shows an illustrative LF-ZTE sequence including the pulse repetition period 200 shown in FIG. 2A and subsequent pulse repetitionTwo periods of period 220. As shown in fig. 2B, the gradient switching interval 210 ends and the repetition period 220 begins when the gradient field strengths have changed to their respective next values. In the present example, at the end of the gradient switching interval 210, the field Gx achieves a strength 224a (different from its previous strength 204a), the field Gy achieves a strength 224b (different from its previous strength 204b), and the field Gz maintains the same strength (204 c). Three gradient field coils G when applied at respective fields 224a, 224b and 204c_x、G_yAnd G_zTime, generating duration T_FAnd another RF pulse 222. The RF pulse 222 may be any suitable type of pulse, examples of which are provided herein, and the RF pulse 222 may be the same type of pulse as the RF pulse 202, or may be a different type of pulse than the RF pulse 202. Next, for a duration Δ that allows the system to switch from transmit mode to receive mode_T/RAfter a delay 226, the receive coil is operated to be at duration T_ACQMR data is acquired during the acquisition interval 228. At a duration T_GDuring the subsequent interval 230, the strength of the one or more gradient fields changes to one or more other values.

As can be appreciated from fig. 2A and 2B, each cycle of the LF-ZTE pulse sequence includes: transmitting an RF pulse; a delay period until one or more receive coils can acquire data; and acquiring MR signals while the gradient fields are set to a particular combination of intensity values (which may be time-varying in some embodiments). The LF-ZTE sequence may comprise a plurality of such periods, one for each particular combination of gradient field strengths, to obtain data from which an image of the object may be reconstructed. The acquisition of data for a particular combination of gradient field strengths corresponds to the trajectory of the measurement object image in the 3D fourier transform. Thus, the number of repetition periods in the LF-ZTE sequence depends on the number of 3D fourier "measurements" to be obtained for generating the image of the object.

In some embodiments, the LF-ZTE pulse sequence may contain one or more contrast preparation sequences. For example, as shown in fig. 2C, the LF-ZTE sequence may include a contrast preparation pulse sequence 240, followed by one or more LF-ZTE pulse repetition periods 242 (e.g., one or more LF-ZTE pulse repetition periods shown in fig. 2A), followed by another contrast preparation pulse sequence 244, followed by one or more LF-ZTE pulse repetition periods 246 (e.g., one or more LF-ZTE pulse repetition periods shown in fig. 2A), and so on. Each contrast pulse preparation sequence may comprise one or more RF pulses and/or one or more gradient field pulses. An example of a contrast preparation sequence is provided below, although it should be understood that the contrast preparation pulse sequence (e.g., sequences 240 and 244) may be of any suitable type such that any suitable type of contrast preparation may be used as part of the LF-ZTE sequence to acquire a corresponding contrast weighted image.

For example, T1 contrast preparation may be used with LF-ZTE sequences by interleaving (interleave) one or more T1 contrast preparation sequences with one or more pulse repetition periods of the LF-ZTE sequences (e.g., one or more pulse repetition periods 200 described with reference to fig. 2A). As shown in fig. 2D, applying a T1 contrast preparation sequence may include: applying an RF pulse associated with a flip angle of 180 degrees such that the RF pulse causes a rotation of the net magnetization of the imaged atoms by 180 degrees, and waiting for a duration T before applying an LF-ZTE pulse repetition period 254 (e.g., pulse repetition period 200 described with reference to FIG. 2A)_DelayDelay interval 252. As another example, arterial spin label contrast preparation may be used with LF-ZTE sequences by interleaving one or more arterial spin label contrast preparation sequences with the LF-ZTE sequences such that each arterial spin label preparation sequence includes RF pulses associated with a 180 degree flip angle, and the transmit pulses and acquisition periods are timed to detect MR signals as a function of blood flow and/or perfusion.

As another example, the LF-ZTE pulse sequence may be modified to allow acquisition of data for generating Overhauser-enhanced MR images. To this end, the LF-ZTE pulse sequence may be interleaved with one or more EPR pulse sequences. As shown in figure 2E of the drawings,applying the EPR pulse sequence may comprise: applying an EPR pulse 260 and waiting for a duration T before applying an LF-ZTE pulse repetition period 264 (e.g., pulse repetition period 200 described with reference to FIG. 2A)_DelayDelay interval 262.

As yet another example, the LF-ZTE pulse sequence may be modified to allow acquisition of data that may be used to compensate for movement of the subject during imaging. To this end, the LF-ZTE pulse sequence may be interleaved with one or more "navigator" pulse sequences that may be used to collect data that may be compared over time to identify and correct for motion of the imaged object during the image generation process. As shown in fig. 2F, applying the sequence of navigation pulses may include: applying a low-flipping RF pulse 270, and waiting for a duration T before applying an LF-ZTE pulse repetition period using a particular set of gradient field values_DelayDelay interval 272. The sequence of MR signals obtained after applying the same low flip angle pulse followed by the LF-ZTE pulse repetition period using the same set of gradient field values may be used to detect and/or track motion of the object and/or may be used to compensate for such motion during image reconstruction.

As yet another example, a LF-ZTE pulse sequence may be used to map the main magnetic field B₀The pulse sequences of the fields are interleaved. For example, the LF-ZTE pulse sequence may be used to map B₀The acquired sequence of inhomogeneities in the field is interleaved. Is designed to measure B₀Any of a variety of acquisitions of field off-resonance intensity may be used to map B₀Inhomogeneities in the field, such as multiple echo time gradient echo acquisitions. This acquired sequence may be referred to as B₀A non-resonant field mapping sequence. The resulting B can then be subsequently used during image reconstruction₀Mapping of inhomogeneities in the field to compensate for B₀Any artifacts (artifacts) caused by the non-uniformity, resulting in an improved MR image.

As yet another example, the water/fat separation contrast may be prepared for use with the LF-ZTE pulse sequences by interleaving one or more water/fat separation contrast pulse sequences with one or more pulse repetition periods of the LF-ZTE sequences (e.g., the one or more pulse repetition periods 200 described with reference to fig. 2A). Applying a water/fat separation contrast preparation sequence may comprise: a series of RF pulses associated with different flip angles and polarities are applied before one or more LF-ZTE pulse repetition periods are applied. For example, as shown in fig. 2G, applying a water/fat separation contrast preparation sequence includes: an RF pulse 280 associated with a 90 degree flip angle is applied, four RF pulses 282, 284, 286 and 288, each associated with a 180 degree flip angle, are sequentially applied, and an RF pulse 290 associated with a 90 degree flip angle and having an opposite polarity to the RF pulse 280 is applied. After application of these RF pulses, one or more pulse repetition periods of the LF-ZTE sequence may be performed-as shown in fig. 2G, a pulse repetition period 292 is applied after the RF pulse 290. Different types of water/fat separation contrast preparation can be achieved by different delays between the RF pulses 280 and 290 and different intensities of the RF pulses 280 to 290.

It should be appreciated that the LF-ZTE pulse sequences described above are merely exemplary, and that the pulse sequences may be modified in different ways, including adding additional preparation components to facilitate MR data acquisition according to any number of different protocols and/or contrast types, as the LF-ZTE pulse sequences are not limited to the examples described herein. It should be further understood that each pulse repetition period of the LF-ZTE sequence may be repeated multiple times (e.g., between 2 and 10 repetitions) with the same or similar parameters (e.g., the same or similar RF pulses, the same or similar gradient fields, etc.), such that signals acquired across the repeated acquisition periods may be averaged. The number of repetitions of the averaged MR signal may be selected depending on the required resolution and/or image acquisition time.

Fig. 3 is a flow diagram of an illustrative process 300 for performing low-field MR imaging using a low-field zero-echo time pulse sequence with contrast preparation. Process 300 may be performed by any suitable low-field MRI system and may be performed, for example, by using low-field MRI system 100 described with reference to fig. 1.

The process 300 begins at act 302, where a contrast preparation pulse sequence is applied to an object being imaged at act 302. The contrast preparation pulse sequence may comprise one or more RF pulses and/or one or more gradient field pulses. When the contrast preparation pulse sequence comprises a plurality of pulses, the pulses may be applied according to any suitable timing scheme (e.g., simultaneously, at least partially overlapping, sequentially, etc.). Any of a number of types of contrast preparation pulse sequences may be applied, including, but not limited to, the examples of contrast preparation pulse sequences described above, such as the T1 contrast preparation pulse sequence, arterial spin labeling contrast preparation pulse sequence, EPR pulse sequence, navigation pulse sequence, and water/fat contrast preparation pulse sequence.

Next, process 300 proceeds to act 304, where, at act 304, a gradient field G is generated_x、G_yAnd G_zIs set to a desired intensity (e.g., intensities 204a, 204b, and 204c as shown in fig. 2A). Once the gradient fields are set to the desired strength, and while the gradient fields are at the desired strength, RF pulses are transmitted at act 306. Any suitable type of RF pulse may be transmitted at act 306, examples of which are provided herein. After the RF pulses are transmitted at act 306, the gradient fields remain set to their respective strengths, and at act 308, the low-field MRI system performing process 300 switches from the transmit mode to the receive mode. The switching may occur over a period of time of any suitable duration, and for example over a period of time sufficiently long to attenuate the coil ringing sufficiently for the receive coil to acquire MR signals.

After the system has switched to the receive mode at act 308, the receive coils are used to acquire MR signals at act 310. During act 310, the gradient coils continue to operate such that acquisition occurs when the gradient fields have the strengths they set at act 304. The MR signals obtained at act 310 may be stored for later use in generating an MR image of the subject.

After the MR signals have been acquired at act 310, the process 300 proceeds to decision block 312 where a determination is made at 312 as to whether another MR signal should be acquired for another combination of gradient field values. This determination may be made in any suitable manner. As described above for the gradient field G_x、G_yAnd G_zTo acquire a trajectory of the MR signal corresponding to the image of the measurement object in the 3D fourier transform. Thus, in some embodiments, it may be determined whether another MR signal should be acquired for another combination of gradient field values based on whether at least one or more points of the 3D fourier transform should be measured. Thus, the number of points (and thus iterations of acts 304-310 of process 300) may depend on the desired MR image resolution, where a higher resolution typically requires more iterations.

When it is determined at decision block 312 that another MR signal is to be acquired for another combination of gradient field strength values, process 300 returns via the YES branch to act 304 where the gradient field strength values are set to another set of values at act 304. The gradient fields may be set to a combination of intensities according to a trajectory in a 3D fourier transform of the image for which measurements are desired, which in turn may depend at least in part on the mode in which 3D fourier space (sometimes referred to as "k-space") is explored. Any suitable pattern may be used to sample k-space (i.e., the order of points in which MR signals are acquired in k-space), as the aspects of the techniques described herein are not limited in this respect. After the gradient field strength values are set at act 304, acts 306 through 310 and decision block 312 are repeated.

On the other hand, when it is determined at decision block 312 that another MR signal is not to be acquired, process 300 proceeds via the no branch to act 314, where an MR image of the subject is generated at act 314 using the acquired MR signals (e.g., using the one or more MR signals obtained at act 310 of process 300). This may be done in any suitable way, and may for example be done by (optionally) pre-processing the acquired signals, applying a fourier transform (e.g. a 3D fourier transform) to the pre-processed signals to obtain an initial image, and (optionally) processing the initial image to obtain a final image. Preprocessing the acquired signals may include: demodulating the acquired signal, down-sampling the acquired signal (e.g., after demodulating the acquired signal), correcting for motion of the object, and/or correcting for any other type of artifact. Processing the initial image may include: correct for grid effects, correct for RF non-uniformities, and perform any other suitable image processing.

It should be understood that process 300 is illustrative and that variations of process 300 exist. For example, although in the embodiment of fig. 3, the contrast preparation pulse sequence is applied only initially, in other embodiments, the contrast preparation pulse sequence may be applied multiple times. For example, in some embodiments, when it is determined at decision block 312 that another MR signal is to be acquired for another combination of gradient field strength values, process 300 returns via the yes branch to act 302 (instead of act 304 as shown in fig. 3), and a contrast preparation pulse sequence may be applied at act 302.

As noted above, the present inventors have recognized that low-field refocusing (LFR) pulse sequences are another type of pulse sequence that is particularly suited for low-field MRI environments due, at least in part, to the speed with which the pulse sequence may be performed. One non-limiting example of an LFR pulse sequence designed by the present inventors includes a low field-equilibrium steady-state free precession (LF-bSSFP) pulse sequence. The inventors have also recognized, albeit by reason of pair B₀With strict constraints on field uniformity and/or specific absorption rate, balanced steady-state free-precession pulse sequences may not be suitable for high-field MRI, but the LF-bSSFP pulse sequences described in further detail below provide an attractive solution, in part, due to the generally superior uniformity that can be achieved at low field strengths.

As mentioned above, the LF-bSSFP pulse sequence is only one example of a more general class of LFR pulse sequences that includes other low-field refocusing pulse sequences. For example, a general class of LFR pulse sequences includes pulse sequences obtained by modifying a low-field pulse sequence (e.g., a low-field gradient echo pulse sequence, a low-field spin echo pulse sequence, etc.) by introducing a refocusing phase toward (e.g., at) an end of one or more (e.g., all) of the repetition periods of the low-field pulse sequence. The introduction of a refocusing phase into the repetition period serves to invert or undo the magnetic dephasing that was generated by the application of the gradient field prior to the application of the refocusing phase. For example, a refocusing phase may be introduced into the pulse repetition period such that the sum of the field strengths of each gradient field across the duration of the pulse repetition period is zero. This is explained in more detail below with reference to the LF-bSSFP pulse sequence. Thus, the LFR pulse sequence may be used as a framework to support other pulse sequences (e.g., spin echo, gradient echo, echo plane, etc.) to facilitate implementation of such sequences in low-field environments.

FIG. 4 is a block diagram illustrating an exemplary LF-bSSFP sequence having a duration T in accordance with some embodiments of the technology described herein_RAlthough in other embodiments any suitable flip angle (e.g., a flip angle in the range of 30 to 150 degrees) may be used, as aspects of the techniques described herein are not limited in this respect the flip angle shown above the pulse 402 ± α indicates that in some embodiments the polarity of the RF pulse may be flipped for each pulse repetition period (i.e., the RF pulse associated with flip angle α is applied during one pulse repetition period, the RF pulse associated with flip angle- α is applied during a subsequent pulse repetition period, the RF pulse associated with flip angle α is applied during a subsequent pulse repetition period, etc.).

As shown in fig. 4, no gradient field is applied during the application of the RF pulse 402. However, in other embodiments, the gradient field may be applied during the application of the RF pulse 402 such that the RF pulse 402 may be designed to include a range of one or more frequencies for exciting a desired portion (e.g., a desired slice or slab) of the object being imaged.

After application of the RF pulse 402, during a so-called "pre-phasing" phase of the LF-bSSFP sequence, a gradient field G is applied with respective operating strengths 404a, 404b and 404c_x、G_yAnd G_z. Next, after having a duration of T_ObtainingDuring an acquisition phase 407, the receive coil acquires MR signals while two of the gradient fields are switched off and the polarity of one of the gradient fields is reversed. For example, as shown in FIG. 4, a gradient field G_yAnd G_zAre switched off (their strength is set to 0) and the gradient field G_xIs reversed in polarity, the gradient field G_xIs set to 406 c. After the acquisition phase 407, during the so-called "refocusing" phase of the LF-bSSFP sequence, a gradient field G is applied with respective operating strengths 408a, 408b and 408c_x、G_yAnd G_zSuch that the dephasing due to the application of the gradient field is reversed or undone during the predetermined phase and the acquisition phase. The strength and polarity of the gradient fields are selected such that the sum of the field strengths of each field spans the duration T of the pulse repetition period 400_RIs zero (which is why this sequence is called "balanced"). Thus, in some embodiments, the strength and polarity of the gradient field are selected such that the following conditions are met:

and

conventionally, the flip angle α associated with the RF pulses is selected to maximize the net transverse magnetization when applying the b-SFFP sequence, hi particular, flip angle α may be set to T according to the following equation₁And T₂Function of relaxation time:

at low field, T₁And T₂The relaxation times are approximately equal so that using the above formula would produce an RF pulse associated with a flip angle of 90 degrees. However, the present inventors have realized that in low-field environments, RF pulses associated with a flip angle of 90 degrees may not be the best choice. Based on this insight, the inventors have realized that B is assumed to be₀In the case of uniformity of (B), rather than selecting the flip angle to maximize the net transverse magnetization, the flip angle can be selected to reduce B₀The effect of the non-uniformity on the net transverse magnetization. FIG. 5 shows the net transverse magnetization (M) for each of a plurality of flip angles_T) With longitudinal magnetization (M)₀) And the ratio of (A) to the main magnetic field B₀The deviation (in degrees) of uniformity of (c). This relationship is illustrated by curves 502, 504 and 506 for flip angles of 90 degrees, 70 degrees and 45 degrees, respectively. As can be appreciated from FIG. 5, flip angles less than 90 degrees (e.g., 70 degrees) span a wider range of B₀The non-uniformity provides a higher net magnetization (average).

Thus, in some embodiments, a low-field MRI system may be configured to use an LF-bSSFP sequence such that RF excitation pulses in the LF-bSSFP sequence and B reduction are combined₀The flip angle of the effect of the non-uniformity on the net transverse magnetization is related. For example, in some embodiments, a flip angle of less than 90 degrees may be used. As another example, a flip angle in the range of 60 degrees to 80 degrees may be used. As yet another example, a flip angle in the range of 65 degrees to 75 degrees may be used, and as yet another example, a flip angle of about 70 degrees is used. It should be understood that the LF-bSSFP pulse sequence described above is only forAre merely exemplary and may be modified in various ways, e.g., to include various preparation elements (e.g., preparation pulses or pulse sequences) for various contrast types, including, but not limited to, OMRI enhanced imaging, T1 and T2 weighted imaging, DW imaging, arterial spin labeling, etc., as the aspects are not limited in this respect.

As can be appreciated from the embodiment shown in fig. 4, the flip angle induced by the RF pulse may vary across the pulse repetition period of the pulse sequence. Varying the flip angle across the pulse repetition period and averaging the MR signals so obtained may eliminate or reduce B₁The sequence of flip angles may be a sequence of alternating flip angles (e.g., +/- α as shown in FIG. 4) or any other suitable flip angle (e.g., a sequence of any suitable flip angles in the range of 30 to 150 degrees.) for example, in some embodiments, the sequence of flip angles used may be generated from a signal model (e.g., a parametric signal model) and the signal model may then be used to perform image reconstruction from the obtained MR signals.

The inventors have also realized that the pulse sequence may be designed to identify and/or compensate the main magnetization B₀Non-uniformities in the field to reduce or eliminate the effects of the non-uniformities on the resulting image. The inventors have developed a system that can be used to identify and/or compensate for B in low-field as well as high-field environments₀A non-uniform pulse sequence. In the presence of main magnetic field non-uniformity, images can be generated compared to when conventional pulse sequences are usedSuch a pulse sequence may be used to generate a higher quality image than would be possible. Generate more uniform B₀Conventional approaches to fields rely on additional and expensive hardware components (e.g., additional magnetic components). On the other hand, as described herein, the use of pulse sequences to compensate for main magnetic field inhomogeneity provides a lower cost solution because such pulse sequences can be used to generate medically relevant MR images despite the relatively high level of main magnetic field inhomogeneity.

Thus, in some embodiments, one or more parameters of the RF pulses across the pulse repetition period of a pulse sequence may be varied to identify and/or compensate for B₀Non-uniformities in the field to reduce or eliminate the effects of the non-uniformities on the generated image. For example, the frequency and/or phase of the RF pulses across the pulse repetition period may be varied to compensate for B₀Non-uniformity. One or more parameters of the RF pulse may be changed in any type of MR pulse sequence including high-field and low-field MR pulse sequences, for example, low-field zero-echo time (LF-ZTE) pulse sequences and low-field refocusing (LFR) pulse sequences such as low-field balanced steady-state free precession (LF-bSSFP) pulse sequences.

In some embodiments, the frequency of the RF pulses used in the pulse sequence may vary across a small range of frequencies. For example, the center frequency of the RF pulses in a series of RF pulses corresponding to a respective series of pulse repetition periods of the pulse sequence may vary within a range of 10Hz to 25Hz, 10Hz to 100Hz, 100Hz to 200Hz, or 10Hz to 200Hz, such that the maximum difference between the center frequencies of any two RF pulses in the series falls within the range. The center frequency of the RF pulses in the series of RF pulses may vary linearly during the corresponding series of pulse repetition periods. For example, in some embodiments, the center frequency of an RF pulse in a series of RF pulses may be changed according to a step scan across a range of frequencies (e.g., from the lowest frequency to the highest frequency in the range, or from the highest frequency to the lowest frequency in the range) using a fixed step size, such that the center frequency of the RF pulse may be changed by a fixed amount corresponding to the step size between each pair of consecutive pulses in the series of RF pulses.

As another example, in an LF-bSSFP sequence, the center frequency of the RF pulses in the pulse sequence may vary within a frequency range determined based on the duration of a single pulse repetition period of the pulse sequence. For example, the center frequency of the RF pulses in the pulse sequence may be ± 1/T of the center frequency selected for a particular flip angle_RInternal variation of wherein T_RIs the duration of the pulse repetition period of the pulse train. As a specific, non-limiting example, in a pulse sequence where the duration of the pulse repetition period is in the range of 3 to 50 milliseconds, the frequency of the RF pulses may vary by a few tens of hertz (e.g., the frequency may vary from the center frequency by ± 10Hz, which may be selected for a particular flip angle) or by a few hundred hertz (e.g., the frequency may vary from the center frequency by ± 100Hz, which may be selected for a particular flip angle). Changing the frequency in this way allows for a change in frequency due to B₀The variations due to the non-uniformity compensate the acquired MR signals. Furthermore, varying the RF pulse frequency in this manner allows B to be generated₀A map of the inhomogeneities, which can then be used to recover (unwarp) an image in which the inhomogeneities cause distortions in the encoded gradient field. Restoration of the image may be performed in any suitable manner, and may be performed, for example, by using a B-based₀The mapping of the inhomogeneities and the gradient field values are performed in such a way that distortion correction values are calculated which apply distortion correction to the image on a pixel-by-pixel basis.

In some embodiments, the frequency of the RF pulses used in the pulse sequence may vary over a wider range of frequencies. For example, the center frequency of the RF pulses in a series of RF pulses corresponding to a respective series of pulse repetition periods of the pulse sequence may vary within a range of 200Hz to 1kHz, 500Hz to 10kHz, or 10kHz to 100kHz, such that the maximum difference between the center frequencies of any two RF pulses in the series falls within the range. As described above, the center frequency may vary linearly, and in some embodiments the center is varied linearly by scanning the range in fixed stepsFrequency. The bandwidth of the RF pulses at a single frequency is too low to cover the entire B₀In the case of ranges, varying the RF pulse frequency in this manner can be used to cover the entire B in multiple acquisitions₀Range (when B is₀When there is non-uniformity in the field). Varying the RF pulse frequency over a wider range also enables compensation of (even larger) non-uniformities in the received MR signal and generation of B, as is the case with varying the RF pulse frequency over a small range₀A non-uniformity map, wherein the map can be used to recover an image.

In some embodiments, B may be generated from a set of images obtained using a pulse sequence (or multiple pulse sequences) in which the frequency of the RF pulses varies across the pulse repetition period of the sequence₀And (4) mapping the nonuniformity. This set of images can be used to estimate B voxel by voxel₀And (4) mapping the nonuniformity. For example, in some embodiments, the amplitude and phase across a particular voxel of the set of images may be used to estimate B at the particular voxel₀The strength of the field. However, B₀The non-uniformity map may be estimated in any other suitable manner from data obtained using pulse sequences with varying RF pulse frequencies, as the aspects of the techniques described herein are not limited in this respect.

In some embodiments, the phase of the RF transmit pulses may vary across the pulse repetition period of the pulse sequence. Changing the phase of the RF pulses increases the signal-to-noise ratio (SNR) of the received MR signal because sensing signals that are inconsistent with the changing transmit RF phase are cancelled when the MR signals obtained by using pulses with changing phases are averaged. Furthermore, in some embodiments, frequency shifting may be achieved using changing the phase of the RF pulse to simulate a frequency change.

It will be appreciated from the above that various characteristics of the RF pulses may vary over the duration of the pulse repetition sequence. These characteristics include, but are not limited to, the flip angle induced by the RF pulse, the frequency of the pulse, and the phase of the RF pulse. One or more of these characteristics may be varied simultaneously.This variation provides a number of benefits, including: compensation B₀And B₁Non-uniformity in the field; mapping B₀Non-uniformity in the field; and removing artifacts (e.g., recovery) in the generated image using the generated map; and increasing the signal-to-noise ratio of the obtained MR signals.

The inventors have further realized that varying the characteristics of the RF pulses within the duration of the pulse repetition sequence may help to correct the acquired MR signals for frequency drifts of the main magnetic field that may occur during operation of the MR system (e.g. due to heating of the MR system during operation). Thus, in some embodiments, the frequency drift of the main magnetic field may be measured during the pulse sequence, and the center frequency of the RF pulses in the pulse sequence may be adjusted based on the measured frequency drift. In this way, the pulse sequence can be adapted to the frequency drift of the main magnetic field. The frequency drift of the main magnetic field may be measured by using a temperature probe, a voltage sensor and/or in any other suitable manner.

Fig. 6 is a flow diagram of an illustrative process 600 for performing MR imaging in a low-field MR system using an LF-bSSFP sequence in accordance with some embodiments of the technology described herein. Process 600 may be performed by any suitable low-field MRI system and may be performed, for example, by using low-field MRI system 100 described with reference to fig. 1.

Process 600 begins at act 602, where an RF pulse is transmitted at act 602. Examples of RF pulses that may be used are provided herein. In some embodiments, the RF pulses may be selected to reduce B₀The flip angle α of the effect of the non-uniformity on the net transverse magnetization correlates in other embodiments, assuming uniform B₀In the case of a field, the RF pulse may be associated with a flip angle selected to maximize the net transverse magnetization.

Next, process 600 proceeds to act 604 ("predetermined phase" phase), where gradient fields of a first combination having respective intensities (e.g., intensities 404a, 404b, and 404c) are applied to encode MR signals at act 604.Next, the process 600 proceeds to act 606 (the "acquisition" phase), where the receive coils acquire MR signals while two of the three gradient fields (e.g., the phase encoding field and the frequency encoding field) are turned off and the polarity of one of the magnetic fields (e.g., the slice selection field) is reversed, act 606. It should be noted that during the acquisition phase one of the gradient fields remains switched on. Next, process 600 proceeds to act 608 ("refocusing" phase), at act 608, applying gradient fields using the selected strength and polarity such that the average strength of each of the magnetic fields spans the duration T of the pulse repetition period of the pulse sequence_RIs 0.

Next, the process 600 proceeds to decision block 610, at decision block 610 a determination is made whether another MR signal should be acquired for another combination of gradient field values. This determination may be made in any suitable manner. As described above for the gradient field G_x、G_yAnd G_zTo acquire points in the 3D fourier transform of the image of the MR signal corresponding to the object of measurement. Thus, in some embodiments, it may be determined whether another MR signal should be acquired for another combination of gradient field values based on whether at least one or more points of the 3D fourier transform should be measured. Thus, the number of points (and thus iterations of acts 602-608 of process 600) may depend on the desired MR image resolution, where a higher resolution typically requires more iterations.

When it is determined at decision block 610 that another MR signal is to be acquired for another combination of gradient field strength values, process 600 returns via the yes branch to act 602, at act 602 another RF pulse is transmitted. As discussed above, in some embodiments, the transmitted RF pulse may be associated with a flip angle (-a) that has an opposite sign to the flip angle (a) associated with the RF pulse transmitted during the immediately preceding pulse period. Acts 604 to 608 are then repeated, another acquisition of MR signals being repeatedly performed during acts 604 to 608, and one or more of the gradient field intensity values being set to one or more different values.

On the other hand, when it is determined at decision block 610 that another MR signal is not to be acquired, process 600 proceeds via the no branch to act 612, where an MR image of the subject is generated at act 612 using the acquired MR signals (e.g., using one or more of the MR signals obtained at act 606 of process 600). This may be done in any suitable manner, and may be done, for example, in any manner described with reference to act 314 of process 300.

It should be understood that process 600 is illustrative and that variations of process 600 exist. For example, although the LF-bSSFP sequence shown in fig. 6 does not include a contrast preparation pulse sequence, in some embodiments, the LF-bSSFP sequence may be interleaved with one or more contrast preparation pulse sequences, e.g., to provide a support framework for implementing such pulse sequences in a low-field environment.

As described above, conventional pulse sequences developed for high-field MRI are generally unsuitable for application in low-field environments due, at least in part, to the significant differences in operating parameters of high-field MRI and low-field MRI. Some of these differences are shown in tables 1 and 2 below. Table 1 shows a side-by-side comparison of operating parameters for a conventional high-field ZTE pulse sequence and a low-field ZTE (LF-ZTE) pulse sequence developed by the present inventors. Table 2 shows a side-by-side comparison of operating parameters for a conventional high-field bSSFP pulse sequence and a low-field bSSFP (LF-bSSFP) pulse sequence developed by the present inventors.

TABLE 1

TABLE 2

As described above, in some embodiments, the RF pulses may be pre-emphasized to counteract attenuation induced to the RF pulses by the RF transmit coil (which may also be a receive coil), which in turn serves to reduce the coil ringing effects described above. As described in more detail below, the RF pulses may be pre-emphasized based on the transfer function of the RF transmit coil (e.g., by modulating the RF pulses using the inverse of the coil transfer function). The following description is made with reference to fig. 7 to 11. Pre-emphasis may be applied to any incoming RF signal for the purpose of synthesizing the desired or wider bandwidth output from the transmit RF coil.

Fig. 7, 8 and 9 illustrate how an RF transmit coil (e.g., an RF coil in a low-field MRI system) modifies the input current based on the transfer function of the coil. Fig. 7 is a schematic diagram of an illustrative low-field RF transmit coil. Fig. 8 shows the input current for a 60 microsecond pulse having a center frequency of 868KHz (top view), and the corresponding output current measured across L1 (bottom view) in the coil circuit shown in fig. 7. As shown in fig. 8, the output current measured across L1 is significantly different from the input current to the RF coil. In particular, the output current measured across L1 is a delayed-limited (band-limited) version of the input current. Fig. 9 also shows in the frequency domain the attenuation induced by the RF coil of fig. 7 on the input RF pulse. Specifically, fig. 9 shows a frequency spectrum of an input current (solid line), a transfer function of an RF coil (broken line), and a frequency spectrum of an output current (dot-dash line). It can be seen that the RF coil passes the center frequency of the input current, but attenuates its higher and lower frequency sidebands. These sidebands provide the fast rising and falling edges of the pulse modulation of the input waveform. The attenuation of these sidebands will introduce a delay or hysteresis in the time domain. The duration of the output signal is therefore longer than the duration of the input signal, and the resulting RF pulse has a longer transit time than if the input current had not been attenuated by the transmit coil.

In some embodiments, the attenuation induced by the RF coil to the input current (e.g., of sidebands or other frequency components of the waveform before or after its center frequency) may be cancelled by pre-emphasis of the input current by a suitable pre-emphasis function. For example, the input current may be pre-emphasized using an inverse function of the RF coil transfer function as a pre-emphasis function. The pre-emphasis may be performed in the time domain (e.g., using convolution), in the frequency domain (e.g., using a discrete fourier transform), or in any other suitable manner.

As an example, fig. 9 shows how the attenuation of the input current (shown in the upper diagram of fig. 8) induced by the RF coil can be counteracted by pre-emphasis. Fig. 9 shows the frequency spectrum (dashed line) of an input waveform pre-emphasized using the inverse function of the coil transfer function (in both phase and frequency). Fig. 10 shows an example of such a pre-emphasis function in the frequency domain. The upper graph of fig. 10 shows how the amplitude of the pre-emphasis function depends on the frequency, and the lower graph of fig. 10 shows how the phase of the pre-emphasis function is a function of the frequency. As can be seen in fig. 9, the side lobes of the input signal are emphasized such that subsequent attenuation thereof by the coil transfer function substantially matches the input current to the output current, as shown in the upper (input current) and lower (output current) plots of fig. 11. Essentially, pre-emphasis using the pre-emphasis function shown in fig. 10 increases the amplitude of the sidebands in the frequency domain so that they are delivered at the desired amplitude after the RF coil circuit.

The inventors have developed a number of system configurations on which low-field MRI can be performed using the pulse sequence techniques described herein. Fig. 12A and 12B illustrate biplane magnetic configurations that may be used in low-field MRI systems in conjunction with the pulse sequence techniques described herein. FIG. 12A schematically illustrates a B configured to produce, at least in part, a signal suitable for low-field MRI₀Biplane magnetic of a part of the fieldAnd (3) a body. The biplane magnet 1200 includes two outer coils 1210a and 1210b and two inner coils 1212a and 1212 b. When a suitable current is applied to the coils, a magnetic field is generated in the direction shown by the arrow to produce a field of view B between the coils₀Field, when properly designed and constructed B₀Time of field, B₀The field may be suitable for low-field MRI. The term "coil" is used herein to refer to any conductor or combination of conductors of any geometry having at least one "turn" that conducts an electrical current to generate a magnetic field to form an electromagnet.

It should be appreciated that the biplane geometry shown in FIG. 12A is generally not suitable for high-field MRI because it is difficult to obtain sufficiently uniform B at high field strengths₀A field. High-field MRI systems typically use solenoid geometry (and superconducting wires) to achieve a sufficiently uniform high field strength for high-field MRI. Biplane B shown in FIG. 12A₀The magnet provides a generally open geometry, facilitating its use in many situations where high-field MRI systems cannot be used. For example, the generally open geometry provides improved access to the patient to facilitate combining low-field MRI with one or more other modalities, techniques, and/or surgical procedures, including those that are difficult or impossible to perform using conventional high-field closed-bore configurations. Furthermore, the open geometry may be used with patients who suffer from claustrophobia and who may refuse to be imaged with conventional high-field solenoid coil geometries. Furthermore, the biplane design may be advantageous for use with larger patients due to its open design and, in some cases, the generally larger field of view possible under low field strength and uniformity requirements. In addition, the generally open design facilitates access to the patient being imaged and may improve the ability to position the patient within the field of view, for example, an unconscious, sedated or anesthetized patient.

The present inventors have further recognized that the open geometry allows for patient contact, facilitating during other clinical procedures, such as during surgical or other procedures where some measure of patient contact is desired or requiredMRI is used. In general, combining MRI with other modalities and/or clinical procedures using conventional MRI is not possible due to the closed configuration and/or the high field strengths involved. The biplane geometry in fig. 12A is merely exemplary, and other configurations may be used. For example, according to some embodiments, a "one-sided" geometry is used, where B₀The magnet consists essentially of a single side, as opposed to a pair of opposing sides in the bi-planar geometry shown.

FIG. 12B shows the fabrication of B using a lamination technique₀A hybrid biplane magnet, or a portion thereof, and/or one or more other magnetic components manufactured for low-field MRI. For example, in the exemplary biplane magnet 1200' shown in fig. 12B, laminates 1220a and 1220B replace the inner coils 1212a and 1212B to create a hybrid magnet. Laminates 1220a and 1220B may include one or more B's fabricated thereon₀Any number of laminated layers of coils, gradient coils, correction coils, and/or shim coils, etc., or portions thereof, to facilitate generation of a magnetic field for low-field MRI. Suitable hybrid magnets using lamination techniques are described in the' 652 application. In other embodiments, lamination techniques may be used to achieve B as a whole₀A magnet (e.g., instead of coils 1210a and 1210 b).

The exemplary laminates 1220a and 1220b may additionally or alternatively have one or more gradient coils or portions thereof fabricated thereon to encode the spatial location of received MR signals as a function of frequency or phase. According to some embodiments, the laminate comprises at least one electrically conductive layer patterned to form one or more gradient coils or portions of one or more gradient coils, the electrically conductive layer being capable of generating or contributing to provide a spatially encoded magnetic field adapted to provide a detected MR signal when operated in a low-field MRI system. For example, laminate 1220a and/or laminate 1220b may include: a first gradient coil configured to selectively change B in a first (X) direction₀The field to perform frequency encoding in that direction; a second gradient coil configured toArranged to selectively vary B in a second (Y) direction substantially orthogonal to the first direction₀Fields to perform phase encoding; and/or a third gradient coil configured to selectively change B in a third (Z) direction substantially orthogonal to the first and second directions₀A field to enable slice selection for volumetric imaging applications.

The example laminates 1220a and 1220B may additionally or alternatively include additional magnetic components, such as one or more correction coils or shim coils arranged to generate a magnetic field to support the system, for example, to increase strength and/or improve B₀Uniformity of the field counteracts deleterious field effects, such as those produced by operation of gradient coils, loading effects of the imaged subject, other devices nearby or used in conjunction therewith, or otherwise supports the magnetism of a low-field MRI system. The biplane magnets shown in fig. 12A and 12B may be fabricated using conventional coils, lamination techniques, or a combination of both, and may be used to provide magnetic components for low-field MRI systems, as discussed in further detail below.

Fig. 13 illustrates a system 1300 showing a patient 1385 seated within the fields of view of biplane magnets 1315A and 1315B, the biplane magnets 1315A and 1315B including magnetic components configured to perform low-field MRI with an external cover or housing that may further include other components such as internal shielding, electrical connections, power and control electronics, etc., and may generally be low-field magnetic components (e.g., B₀Magnets, gradient coils, transmit/receive coils, etc.) provide a measure of environmental protection.

Fig. 14A and 14B illustrate the system 1400 having a tilted configuration, wherein the magnetic components 1410A and 1410B are disposed within a frame that includes a seating portion 1435, the seating portion 1435 being adjustably oriented at an angle to accommodate a patient placed between the magnetic components in a tilted position. The angled portion of the system may be adjustable to facilitate a desired positioning of the patient between the magnetic components such that a desired portion of the patient is within the field of view of the magnet. Additionally or alternatively, the magnetic component may be adjustable within the housing 1415 to provide additional flexibility in positioning the magnetic component relative to the patient. The magnetic components 1410A and 1410B may be connected via one or more suitable cables to power electronics that may be mounted on a gantry or housed using another suitable movable structure to facilitate portability of the MRI system. These example systems are generally open and may have the advantages described above.

Fig. 15A-15B illustrate a portable or mobile low-field MRI system 1500 suitable for performing the techniques described herein, according to some embodiments. The system 1500 may include magnetic and electrical components, and possibly other components (e.g., thermal management devices, consoles, etc.), arranged together on a single, generally movable and deformable structure. The system 1500 may be designed to have at least two configurations; a configuration suitable for movement and storage and a configuration suitable for operation. Fig. 15A shows system 1500 for moving and/or storing when stationary, and fig. 15B shows system 1500 for operating when deformed. System 1500 includes a portion 1590A that can slide into portion 1590B and retract from portion 1590B when deforming the system from its mobile configuration to its operating configuration, as indicated by the arrows shown in fig. 15B. Portion 1590A may house power electronics, a console (which may include an interface device such as a touch panel display), and a thermal management device. Portion 1590A may also include other components for operating system 1500 as desired. The movable system includes casters or wheels 1572 that enable the system to be rolled to a desired position and brakes 1574 that fix the system when the desired position is reached (see fig. 15B).

Portion 1590B includes the magnetic components of the low-field MRI system 1500. When deformed into a configuration suitable for operating the system to perform MRI (as shown in fig. 15B), the support surfaces of portions 1590A and 1590B provide a surface on which a patient may lie. A slidable bed or surface 1584 may be provided to facilitate sliding the patient into a position such that the portion of the patient to be imaged is within the field of view of the low-field MRI magnetic component. System 1500 provides a portable compact configuration of a low-field MRI system to facilitate use of the device in situations where it is not conventionally available.

Fig. 15A-15B illustrate an example of a convertible low-field MRI system utilizing a bi-planar magnet forming an imaging region between housings 1586A and 1586B. The housings 1586A and 1586B house the magnetic components for the convertible system 1500. According to some embodiments, the magnetic component may be produced, manufactured and arranged using only lamination techniques, only conventional techniques, or using a combination of both lamination and conventional techniques (e.g., using hybrid techniques). The convertible low-field MRI system 1500 enables the system to be brought in front of a patient to facilitate operation in a variety of situations.

Having thus described several aspects and embodiments of the technology set forth in the disclosure, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described herein. For example, various other means and/or structures for performing the function and/or obtaining the result and/or one or more of the advantages described herein will be readily apparent to those of ordinary skill in the art, and each such variation and/or modification is considered to be within the scope of the embodiments described herein. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, embodiments of the invention may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, devices, and/or methods described herein, if such features, systems, articles, materials, devices, and/or methods are not mutually inconsistent, is included within the scope of the present disclosure.

The above-described embodiments may be implemented in any of a number of ways. One or more aspects and embodiments of the present disclosure relating to the performance of a process or method may utilize program instructions that are executable by an apparatus (e.g., a computer, processor, or other device) to perform or control the performance of a process or method. In this regard, the various inventive concepts may be embodied as a non-transitory computer readable storage medium (or multiple non-transitory computer readable storage media) (e.g., a computer memory, one or more floppy discs, optical discs, magnetic tapes, flash memories, circuit configurations in field programmable gate arrays or other semiconductor devices, or other tangible computer storage medium) encoded with one or more programs that, when executed on one or more computers or other processors, perform methods that implement one or more of the various embodiments described above. The computer-readable medium or media may be portable such that the one or more programs stored thereon can be loaded onto one or more different computers or other processors to implement various ones of the above-described aspects. In some implementations, the computer-readable medium may be a non-transitory medium.

The terms "program" or "software" are used herein in a generic sense to refer to any type of computer code or set of computer-executable instructions that can be employed to program a computer or other processor to implement various aspects, as discussed above. In addition, it should be understood that according to one aspect, one or more computer programs that when executed perform methods of the present disclosure need not reside on a single computer or processor, but may be distributed in a modular fashion amongst a number of different computers or processors to implement various aspects of the present disclosure.

Computer-executable instructions may be in many forms, such as program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Typically, the functionality of the program modules may be combined or distributed as desired in various embodiments.

In addition, the data structures may be stored in any suitable form on a computer readable medium. For simplicity of illustration, the data structure may be shown with fields that are related by location in the data structure. Likewise, such relationships can be achieved by allocating memory for fields in a computer-readable medium having locations that convey relationships between the fields. However, any suitable mechanism (including by using pointers, tags, or other mechanisms that establish relationships between data elements) may be used to establish relationships between information in fields of a data structure.

When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be implemented in any of a variety of forms, such as a rack-mounted computer, a desktop computer, a laptop computer, or a tablet computer, as non-limiting examples. Additionally, a computer may be embedded in a device not normally considered a computer but with suitable processing capabilities, including a Personal Digital Assistant (PDA), a smart phone, or any other suitable portable or fixed electronic device.

In addition, a computer may have one or more input and output devices. These devices may be used, among other things, to present a user interface. Examples of output devices that may be used to provide a user interface include: a printer or display screen for visually presenting output, and a speaker or other sound generating device for audibly presenting output. Examples of input devices that may be used for the user interface include keyboards, and pointing devices, such as mice, touch pads, and digitizing tablets. As another example, a computer may receive input information through speech recognition or in other audible format.

Such computers may be interconnected IN any suitable form by one or more networks, including local or wide area networks (e.g., enterprise networks) and Intelligent Networks (INs) or the internet. Such networks may be based on any suitable technology and may operate according to any suitable protocol and may include wireless networks, wired networks, or fiber optic networks.

Further, as described, some aspects may be embodied as one or more methods. The actions performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed which perform acts in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood to control dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.

The indefinite articles "a" and "an" as used herein in the specification and in the claims are to be understood as meaning "at least one" unless clearly indicated to the contrary.

The phrase "and/or" as used herein in the specification and claims should be understood to mean "either or both" of the elements so combined, i.e., elements that are present in combination in some cases and separately in other cases. Multiple elements listed with "and/or" should be interpreted in the same manner, i.e., "one or more" of the elements so combined. In addition to the elements specifically identified by the "and/or" clause, other elements may optionally be present, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, when used in conjunction with open-ended language such as "including," references to "a and/or B" may refer in one embodiment to a only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than a); in yet another embodiment refers to a and B (optionally including other elements), and the like.

As used herein in the specification and claims, the phrase "at least one" in reference to a list of one or more elements should be understood to mean at least one element selected from any one or more elements in the list of elements, but not necessarily including at least one of each and all elements specifically listed in the list of elements, and not excluding any combination of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified in the list of elements to which the phrase "at least one" refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of a and B" (or, equivalently, "at least one of a or B" or, equivalently "at least one of a and/or B") may refer in one embodiment to: at least one, optionally including more than one, a, B is absent (and optionally including elements other than B); in another embodiment, refers to: at least one, optionally including more than one, B, a is absent (and optionally including elements other than a); in yet another embodiment, refers to: at least one, optionally including more than one, a, and at least one, optionally including more than one, B (and optionally including other elements), and the like.

Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having," "containing," "involving," and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," "consisting of … … and the like are to be construed as open-ended, i.e., to mean including but not limited to. The transitional phrase "consisting of and" consisting essentially of shall be the transitional phrases closed or semi-closed, respectively.

Claims (63)

1. A low-field Magnetic Resonance Imaging (MRI) system, comprising:

a plurality of magnetic components including a magnetic field configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation; and

at least one controller configured to operate one or more of the plurality of magnetic components according to at least one low-field zero-echo time (LF-ZTE) pulse sequence.

2. The low-field MRI system of claim 1, further comprising:

at least one non-transitory computer-readable medium communicatively coupled to the at least one controller and storing at least one parameter for the at least one LF-ZTE pulse sequence,

wherein the at least one controller is configured to operate one or more of the plurality of magnetic components at least in part by:

accessing the at least one parameter stored on the at least one non-transitory computer-readable storage medium; and

operating one or more of the plurality of magnetic components according to the at least one parameter.

3. The low-field MRI system of claim 1 or any other preceding claim, wherein the plurality of magnetic components comprises components selected from at least one magnet, at least one shim coil, at least one RF transmit coil, at least one RF receive coil, and at least one gradient coil.

4. The low-field MRI system of claim 1 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having an intensity equal to or less than about 0.2T and greater than or equal to about 0.1T₀A field.

5. The low-field MRI system of claim 1 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 0.1T and greater than or equal to about 50mT₀A field.

6. According to claim 1 or anyThe low-field MRI system of the other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 50mT and greater than or equal to about 20mT₀A field.

7. The low-field MRI system of claim 1 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 20mT and greater than or equal to about 10mT₀A field.

8. The low-field MRI system of claim 1 or any other preceding claim, wherein the at least one controller is configured to operate one or more of the plurality of magnetic components according to the LF-ZTE pulse sequence at least in part by performing a set of actions comprising:

(A) applying a plurality of gradient fields;

(B) while applying the plurality of gradient fields:

applying at least one RF pulse; and

acquiring magnetic resonance signals after a threshold amount of time has elapsed after applying the at least one RF pulse; and

(C) adjusting the strength of at least one of the plurality of gradient fields to at least one other value.

9. The low-field MRI system of claim 8 or any other preceding claim, wherein applying the at least one RF pulse comprises: generating at least one frequency modulated RF pulse, and transmitting the at least one frequency modulated RF pulse.

10. The low-field MRI system of claim 8 or any other preceding claim, wherein applying the at least one RF pulse comprises: at least one RF pulse is generated having a duration and amplitude that deflects the net magnetization vector of the imaged object by an angle of less than 30 degrees.

11. The low-field MRI system of claim 8 or any other preceding claim, wherein applying the at least one RF pulse comprises: pre-emphasis the at least one RF pulse based at least on a transfer function of a transmit coil of the low-field MRI system to obtain at least one pre-emphasized RF pulse, and transmit the at least one pre-emphasized RF pulse using the transmit coil.

12. The low-field MRI system of claim 8 or any other preceding claim, wherein the adjusting of the strength of the at least one of the plurality of gradient fields is performed without turning off the at least one of the plurality of gradient fields.

13. The low-field MRI system of claim 8 or any other preceding claim, wherein the set of acts (a), (B), and (C) are performed within 1ms to 25 ms.

14. The low-field MRI system of claim 8 or any other preceding claim, wherein the set of actions further comprises: (D) applying at least one contrast preparation portion prior to performing act (a).

15. The low-field MRI system of claim 14, wherein applying the at least one contrast preparation portion comprises: at least one Electron Paramagnetic Resonance (EPR) pulse is applied.

16. The low-field MRI system of claim 14, wherein applying the at least one contrast preparation portion comprises: applying a contrast preparation fraction selected from the group consisting of at least one water/fat contrast separation fraction, at least one T1 contrast preparation fraction, at least one T2 contrast preparation fraction, at least one arterial spin labeling contrast preparation fraction, and at least one diffusion weighted contrast preparation fraction.

17. A method for operating a low-field magnetic resonance imaging system, the system comprising a plurality of magnetic components including a magnetic field configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated, the method comprising:

using the at least one first magnetic component to generate the low-field main magnetic field B₀(ii) a And

controlling one or more of the plurality of magnetic components according to at least one low-field zero-echo time (LF-ZTE) pulse sequence.

18. At least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by a low-field MRI system comprising a plurality of magnetic components including a magnetic component configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated:

generating the low-field main magnetic field B using the at least one first magnetic component₀(ii) a And

operating one or more of the plurality of magnetic components according to at least one low-field zero-echo time (LF-ZTE) pulse sequence.

19. A low-field Magnetic Resonance Imaging (MRI) system, comprising:

a plurality of magnetic components including a magnetic field configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation; and

at least one controller configured to operate one or more of the plurality of magnetic components according to at least one Low Field Refocusing (LFR) pulse sequence,

wherein the RF excitation pulse in the at least one LFR pulse sequence is associated with a decrease B₀The flip angle of the effect of the non-uniformity on the net transverse magnetization is related.

20. The low-field MRI system of claim 19, wherein the at least one controller is configured to operate one or more of the plurality of magnetic components according to at least one low-field balanced steady-state free precession (LF-bSSFP) pulse sequence.

21. The low-field MRI system of claim 20, further comprising:

at least one non-transitory computer-readable medium communicatively coupled to the at least one controller and storing at least one parameter for the at least one LF-bSSFP pulse sequence,

wherein the at least one controller is configured to operate one or more of the plurality of magnetic components at least in part by:

accessing the at least one parameter stored on the at least one non-transitory computer-readable storage medium; and

operating one or more of the plurality of magnetic components according to the at least one parameter.

22. The low-field MRI system of claim 19 or any other preceding claim, wherein the plurality of magnetic components comprises components selected from at least one magnet, at least one shim coil, at least one RF transmit coil, at least one RF receive coil, and at least one gradient coil.

23. According to claim 19 or any otherThe low-field MRI system of the preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 0.2T and greater than or equal to about 0.1T₀A field.

24. The low-field MRI system of claim 19 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 0.1T and greater than or equal to about 50mT₀A field.

25. The low-field MRI system of claim 19 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 50mT and greater than or equal to about 20mT₀A field.

26. The low-field MRI system of claim 19 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 20mT and greater than or equal to about 10mT₀A field.

27. The low-field MRI system of claim 19 or any other preceding claim, wherein the controller is configured to operate one or more of the plurality of magnetic components in accordance with the LF-bSSFP pulse sequence at least in part by performing a set of actions comprising:

(A) applying an RF pulse;

(B) applying a plurality of gradient fields after applying the RF pulses;

(C) after applying the plurality of gradient fields, acquiring magnetic resonance signals while applying one of the plurality of gradient fields;

(D) after acquiring the magnetic resonance signals, refocusing magnetic moments of the imaged subject at least in part by applying gradient fields having strengths and/or polarities selected such that a sum of field strengths of each gradient field is zero for a duration of a pulse repetition period of the LF-bSSFP pulse sequence.

28. The low-field MRI system of claim 27 or any other preceding claim, wherein applying the at least one RF pulse comprises: generating at least one frequency modulated RF pulse, and transmitting the at least one frequency modulated RF pulse.

29. The low-field MRI system of claim 27 or any other preceding claim, wherein B is reduced₀The flip angle of the effect of non-uniformity on the net transverse magnetization is in the range of 60 degrees to 80 degrees.

30. The low-field MRI system of claim 27 or any other preceding claim, wherein B is reduced₀The flip angle of the effect of non-uniformity on the net transverse magnetization is in the range of 65 degrees to 75 degrees.

31. The low-field MRI system of claim 27 or any other preceding claim, wherein B is reduced₀The flip angle of the effect of non-uniformity on the net transverse magnetization is less than 90 degrees.

32. The low-field MRI system of claim 27 or any other preceding claim, wherein applying the at least one RF pulse comprises: pre-emphasis the at least one RF pulse based at least on a transfer function of a transmit coil of the low-field MRI system to obtain at least one pre-emphasized RF pulse, and transmit the at least one pre-emphasized RF pulse using the transmit coil.

33. The low-field MRI system of claim 27 or any other preceding claim, wherein the set of actions further comprises: (E) applying at least one contrast preparation portion prior to performing act (a).

34. The low-field MRI system of claim 27 or any other preceding claim, wherein applying the at least one contrast preparation portion comprises: applying a contrast preparation fraction selected from the group consisting of at least one water/fat contrast separation fraction, at least one T1 contrast preparation fraction, at least one T2 contrast preparation fraction, at least one arterial spin labeling contrast preparation fraction, and at least one diffusion weighted contrast preparation fraction.

35. A method for operating a low-field magnetic resonance imaging system, the system comprising a plurality of magnetic components including a magnetic field configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated, the method comprising:

operating the at least one first magnetic component to generate the low-field main magnetic field B₀(ii) a And

controlling one or more of the plurality of magnetic components according to at least one Low Field Refocusing (LFR) pulse sequence,

wherein the RF excitation pulse in the at least one LFR pulse sequence is associated with a decrease B₀The flip angle of the effect of the non-uniformity on the net transverse magnetization is related.

36. At least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by a low-field MRI system comprising a plurality of magnetic components including a magnetic component configured to generate a low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated:

operating the at least one first magnetic component to generate the low-field main magnetic field B₀(ii) a And

operating one or more of the plurality of magnetic components according to at least one Low Field Refocusing (LFR) pulse sequence,

wherein the RF excitation pulse in the at least one LFR pulse sequence is associated with a decrease B₀The flip angle of the effect of the non-uniformity on the net transverse magnetization is related.

37. A low-field Magnetic Resonance Imaging (MRI) system, comprising:

a plurality of magnetic components configured to generate a magnetic field comprising a low-field main magnetic field B₀The plurality of magnetic components comprising a plurality of magnetic fields configured to generate the low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when in operation; and

at least one controller configured to operate one or more of the plurality of magnetics components according to a pulse sequence designed to compensate for non-uniformity of one or more of the plurality of magnetic fields at least in part by causing one or more of the plurality of magnetics components to apply a series of RF pulses having at least one parameter that varies during a respective series of pulse repetition periods of the pulse sequence.

38. The low-field MRI system of claim 37, wherein the pulse sequence is designed to compensate for non-uniformities in the main magnetic field.

39. The low-field MRI system of claim 37 or any other preceding claim, wherein the plurality of magnetic fields comprises B₁A magnetic field, and the pulse sequence is designed to compensate for the B₁Inhomogeneity in the magnetic field.

40. The low-field MRI system of claim 39 or any other preceding claim, wherein a flip angle induced by an RF pulse in the series of RF pulses varies during the respective series of pulse repetition periods.

41. The low-field MRI system of claim 37 or any other preceding claim, wherein a phase of an RF pulse of the series of RF pulses varies during the respective series of pulse repetition periods.

42. The low-field MRI system of claim 37 or any other preceding claim, wherein a center frequency of RF pulses in the series of RF pulses varies during the respective series of pulse repetition periods.

43. The low-field MRI system of claim 42 or any other preceding claim, wherein a center frequency of RF pulses in the series of RF pulses varies linearly during the respective series of pulse repetition periods.

44. The low-field MRI system of claim 42 or any other preceding claim, wherein a center frequency of an RF pulse in the series of RF pulses varies by a fixed amount between each pair of consecutive RF pulses in the series of RF pulses.

45. The low-field MRI system of claim 42 or any other preceding claim, wherein a maximum difference between center frequencies of RF pulses in the series of RF pulses is less than 25 Hz.

46. The low-field MRI system of claim 42 or any other preceding claim, wherein a maximum difference between center frequencies of RF pulses in the series of RF pulses is between 10Hz and 200 Hz.

47. The low-field MRI system of claim 42 or any other preceding claim, wherein a maximum difference between center frequencies of RF pulses in the series of RF pulses is between 200Hz and 1000 Hz.

48. The low-field MRI system of claim 42 or any other preceding claim, wherein a maximum difference between center frequencies of RF pulses in the series of RF pulses is between 500Hz and 10 kHz.

49. The low-field MRI system of claim 42 or any other preceding claim, wherein a maximum difference between center frequencies of RF pulses in the series of RF pulses is between 10kHz and 100 kHz.

50. The low-field MRI system of claim 42 or any other preceding claim, wherein a center frequency of RF pulses in the series of RF pulses varies over a frequency range that depends on a duration of a single pulse repetition period of the pulse sequence.

51. The low-field MRI system of claim 37 or any other preceding claim, wherein the at least one controller is further configured to generate an image of the main magnetic field inhomogeneity based on data obtained using the pulse sequence.

52. The low-field MRI system of claim 37 or any other preceding claim, wherein the at least one controller is further configured to generate an image of an imaged subject based on data obtained during the pulse sequence and an image of inhomogeneity of the main magnetic field.

53. The low-field MRI system of claim 52 or any other preceding claim, wherein the at least one controller is configured to generate the image of the subject by: generating a warped image of the object based on data obtained during the pulse sequence, and recovering the warped image of the object by using the image of the inhomogeneity to obtain an image of the object.

54. The low-field MRI system of claim 37 or any other preceding claim, wherein the series of RF pulses comprises frequency-modulated RF pulses.

55. The low-field MRI system of claim 37 or any other preceding claim, wherein the pulse sequence is a low-field zero-echo temporal pulse sequence.

56. The low-field MRI system of claim 37 or any other preceding claim, wherein the pulse sequence is a low-field refocusing pulse sequence.

57. The low-field MRI system of claim 37, wherein the pulse sequence is a low-field equilibrium steady state free precession (LF-bSSFP) pulse sequence.

58. The low-field MRI system of claim 37 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having an intensity equal to or less than about 0.2T and greater than or equal to about 0.1T₀A field.

59. The low-field MRI system of claim 37 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 0.1T and greater than or equal to about 50mT₀A field.

60. The low-field MRI system of claim 37 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 50mT and greater than or equal to about 20mT₀A field.

61. The low-field MRI system of claim 37 or any other preceding claim, wherein the at least one first magnetic component is configured to produce B having a strength equal to or less than about 20mT and greater than or equal to about 10mT₀A field.

62. A method for operating a low-field magnetic resonance imaging system, the system comprising a plurality of magnetic components configured to generate a magnetic resonance signal comprising a low-field main magnetic field B₀The plurality of magnetic components comprising a plurality of magnetic fields configured to generate the low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated, the method comprising:

operating the at least one first magnetic component to generate the low-field main magnetic field B₀(ii) a And

controlling one or more of the plurality of magnetics components according to a pulse sequence designed to compensate for non-uniformities in one or more of the plurality of magnetic fields at least in part by causing the plurality of magnetics components to apply a series of RF pulses having at least one parameter that varies during a respective series of pulse repetition periods of the pulse sequence.

63. At least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by a low-field MRI system having a plurality of magnetic components, cause the low-field MRI system to perform operations comprisingThe linear member is configured to generate a magnetic field including a low-field main magnetic field B₀The plurality of magnetic components comprising a plurality of magnetic fields configured to generate the low-field main magnetic field B₀And at least one second magnetic component configured to acquire magnetic resonance data when operated, the plurality of magnetic components generating at least one magnetic field:

operating the at least one first magnetic component to generate the low-field main magnetic field B₀(ii) a And

operating one or more of the plurality of magnetic components according to a pulse sequence designed to compensate for non-uniformity of one or more of the plurality of magnetic fields at least in part by causing the plurality of magnetic components to apply a series of RF pulses having at least one parameter that varies during a respective series of pulse repetition periods of the pulse sequence.