HK1248821B - Magnetic coil power methods and apparatus - Google Patents

Description

Magnetic coil power supply method and device

Technical Field

The present disclosure relates generally to electrical components for magnetic coils and, more particularly, to electrical components for driving one or more gradient coils in a magnetic resonance imaging system.

Background Art

Magnetic resonance imaging (MRI) provides an important imaging modality for many applications and is widely used in clinical and research settings to produce images of the interior of the human body. Generally, MRI is based on detecting magnetic resonance (MR) signals, which are electromagnetic waves emitted by atoms in response to changes in state caused by an applied electromagnetic field. For example, nuclear magnetic resonance (NMR) technology involves detecting MR signals emitted by the nuclei of excited atoms when the nuclear spins of atoms in the object being imaged (e.g., atoms in the tissues of the human body) rearrange or relax. The detected MR signals can be processed to produce images that, in the context of medical applications, allow investigation of internal structures and/or biological processes in 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 subject to ionizing radiation such as x-rays or without introducing radioactive materials into the body). Furthermore, MRI is particularly well suited to providing soft tissue contrast, which can be used to image subjects that cannot be satisfactorily imaged by other imaging modalities. Furthermore, MR technology can capture information about structures and/or biological processes that cannot be acquired by other modalities. However, MRI has a number of disadvantages: for a given imaging application, MRI may involve relatively high equipment costs, limited availability and/or difficulty in obtaining clinical MRI scanners, and/or the length of the image acquisition process.

The trend in clinical MRI is to increase the field strength of MRI scanners to improve one or more of scan time, image resolution, and image contrast, which in turn increases costs. The vast majority of installed MRI scanners operate at 1.5 Tesla or 3 Tesla (T), referring to the field strength of the primary field strength, _B0 . A rough cost estimate for a clinical MRI scanner is approximately $1 million per Tesla, which does not take into account the substantial operating, service, and maintenance costs involved in operating such an MRI scanner.

In addition, conventional high-field MRI systems typically require large superconducting magnets and associated electronics to generate a strong uniform static magnetic field ( _B0 ) in which an object (e.g., a patient) is imaged. The size of such systems is quite large, with typical MRI installations comprising multiple spaces for magnets, electronics, thermal management systems, and console areas. The size and expense of MRI systems generally limit their use to facilities with ample space and resources to purchase and maintain MRI systems, such as hospitals or academic research centers. The high cost and large space requirements of high-field MRI systems result in limited availability of MRI scanners. Therefore, there are often clinical scenarios in which MRI scanning is beneficial, but due to one or more of the above-mentioned limitations, MRI scanning is impractical or impossible, as discussed in further detail below.

Summary of the Invention

Some embodiments relate to a device for providing power for operating at least one gradient coil of a magnetic resonance imaging system. The device includes a plurality of power terminals configured to supply different voltages of a first polarity. The device also includes a linear amplifier configured to provide current to the at least one gradient coil according to a pulse sequence to generate a magnetic field. The linear amplifier is configured to be powered by one or more of the plurality of power terminals. The one or more power terminals of the plurality of power terminals that power the linear amplifier can be changed to generate different linear amplifier output voltages.

Some embodiments include an apparatus for providing power for operating at least one gradient coil of a magnetic resonance imaging system, the apparatus comprising a plurality of power terminals configured to supply different voltages of a first polarity and a linear amplifier configured to provide at least one output to drive the at least one gradient coil to generate a magnetic field according to a pulse sequence, the linear amplifier configured to be powered by one or more of the plurality of power terminals, wherein the one or more of the plurality of power terminals that power the linear amplifier are selected based at least in part on the at least one output.

Some embodiments include a method of providing power to at least one gradient coil of a magnetic resonance imaging system using a linear amplifier, the linear amplifier configured to provide current to the at least one gradient coil according to a pulse sequence to generate a magnetic field, the linear amplifier configured to be powered by one or more power terminals of a plurality of power terminals configured to supply different voltages of a first polarity, the method comprising: when a magnitude of an output voltage generated by the linear amplifier is below a threshold, supplying power to the linear amplifier from at least a first power terminal of the plurality of power terminals to provide current to the at least one gradient coil, and when the magnitude of the output voltage generated by the linear amplifier is above the threshold, supplying power to the linear amplifier from at least a second power terminal of the plurality of power terminals to provide current to the at least one gradient coil.

Some embodiments include a magnetic resonance imaging system comprising: a _Bo magnet configured to generate a _Bo magnetic field; at least one gradient coil; and at least one power component configured to provide power to operate the at least one gradient coil, the at least one power component comprising a plurality of power terminals configured to supply different voltages of a first polarity and a linear amplifier, the plurality of power terminals configured to supply current to the at least one gradient coil according to a pulse sequence to generate a magnetic field, the linear amplifier configured to be powered by one or more of the plurality of power terminals, wherein the one or more of the plurality of power terminals powering the linear amplifier can be changed to generate different linear amplifier output voltages.

Some embodiments include a magnetic resonance imaging system comprising: a _Bo magnet configured to generate a _Bo magnetic field; at least one gradient coil; and at least one power component configured to provide power to operate the at least one gradient coil, the at least one power component comprising a plurality of power terminals configured to supply different voltages of a first polarity and a linear amplifier, the plurality of power terminals configured to supply current to the at least one gradient coil according to a pulse sequence to generate a magnetic field, the linear amplifier configured to be powered by one or more of the plurality of power terminals, wherein the one or more of the plurality of power terminals that power the linear amplifier are selected based at least in part on at least one output.

Some embodiments include an apparatus for providing power to operate at least one gradient coil of a magnetic resonance imaging system, the apparatus comprising: a linear amplifier configured to generate an output for driving the at least one gradient coil according to a pulse sequence, at least one power converter configured to generate a variable power supply voltage to power the linear amplifier, and at least one controller configured to control the at least one power converter to vary the variable power supply voltage based on the output of the linear amplifier.

Some embodiments include a method of providing power to at least one gradient coil of a magnetic resonance imaging system using a linear amplifier, the linear amplifier configured to provide current to the at least one gradient coil according to a pulse sequence to generate a magnetic field, the method comprising converting at least one fixed power supply to at least one variable power supply voltage to power the linear amplifier, varying the variable power supply voltage based on an output of the linear amplifier, and controlling the linear amplifier to generate an output to drive the at least one gradient coil according to the pulse sequence.

Some embodiments include a magnetic resonance imaging system comprising: a _Bo magnet configured to generate a _Bo magnetic field; at least one gradient coil; and at least one power component configured to provide power to operate the at least one gradient coil, the at least one power component comprising a linear amplifier configured to generate an output for driving the at least one gradient coil according to a pulse sequence, at least one power converter configured to generate a variable power supply voltage to power the linear amplifier, and at least one controller configured to control the at least one power converter to vary the variable power supply voltage based on the output of the linear amplifier.

Some embodiments include an apparatus for driving at least one gradient coil of a magnetic resonance imaging system, the apparatus comprising: a switching power converter configured to switch at a switching frequency higher than a Larmor frequency associated with a _B0 field strength of the magnetic resonance imaging system; and a controller configured to control the switching power converter to drive the at least one gradient coil according to a pulse sequence.

Some embodiments relate to a method of operating an apparatus for driving at least one gradient coil of a magnetic resonance imaging system according to the techniques described herein.

Some embodiments relate to at least one non-transitory computer-readable storage medium having stored thereon instructions that, when executed by a processor, perform such a method.

The foregoing summary has been provided by way of illustration and not limitation.

BRIEF DESCRIPTION OF THE 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 represented by the same reference numerals in all figures in which they appear.

1 is a block diagram of example components of a low-field MRI system in accordance with some implementations of the technology described herein.

2 illustrates a drive circuit for driving current through a coil to generate a magnetic field, according to some implementations of the technology described herein.

3A shows an example of a gradient coil current waveform in accordance with some implementations of the technology described herein.

3B shows waveforms of the current command, gradient coil current, and gradient coil voltage before, during, and after a rising transition of the gradient coil current waveform shown in FIG. 3A , according to some embodiments of the techniques described herein.

4A illustrates an example of a power component with a current feedback loop and a voltage feedback loop, according to some implementations of the technology described herein.

FIG. 4B illustrates an example of a voltage amplifier according to some implementations of the techniques described herein.

5A and 5B illustrate examples of output stages that can be powered by different supply terminals depending on the output voltage, according to some embodiments of the technology described herein.

6 shows an example of an output stage with multiple driver circuits for driving multiple transistor circuits connected to a high voltage supply terminal and a low voltage supply terminal according to some implementations of the technology described herein.

7 illustrates a driver circuit including a bias circuit and a timing circuit according to some implementations of the technology described herein.

FIG. 8 shows an example implementation of the driver circuit of FIG. 7 , in accordance with some implementations of the techniques described herein.

FIG. 9 illustrates another example of a technique for implementing sequential circuits in accordance with some embodiments.

FIG. 10 illustrates an example of a sequential circuit implemented by RC circuits and transistors, according to some embodiments.

FIG. 11 illustrates an example of an output stage including a single-ended linear amplifier according to some embodiments.

FIG. 12 illustrates an example of a power component that may include a switching power converter, according to some embodiments.

13 illustrates an embodiment of an output stage that may be powered by a variable voltage positive supply terminal and a variable voltage negative supply terminal in accordance with some embodiments.

FIG. 14A shows an embodiment similar to FIG. 5A with a variable low voltage supply terminal.

FIG. 14B shows an embodiment in which the high voltage supply terminal is the same as the power supply terminal that supplies power to the power converter.

15A-15D illustrate gradient coil current waveforms, gradient coil voltage waveforms, and power supply terminal voltage waveforms according to some embodiments.

FIG. 16A shows an embodiment similar to FIG. 11 with a variable low voltage supply terminal.

FIG. 16B shows an embodiment in which the high voltage supply terminal is the same as the power supply terminal that supplies power to the power converter.

DETAILED DESCRIPTION

The MRI scanner market is mainly occupied by high field systems, particularly for medical or clinical MRI applications. As mentioned above, the general trend in medical imaging is to produce MRI scanners with increasingly stronger field strengths, with the vast majority of clinical MRI scanners operating at 1.5T or 3T, and using higher field strengths of 7T and 9T in research environments. As used herein, "high field" generally refers to the MRI systems currently used in clinical environments, more particularly, to MRI systems operating with a main magnetic field (i.e., _B0 field) of 1.5T or higher than 1.5T, although clinical systems operating between 0.5T and 1.5T are also commonly referred to as "high field". In contrast, "low field" generally refers to MRI systems operating with a _B0 field lower than or equal to approximately 0.2T, although systems with a _B0 field between 0.2T and approximately 0.3T are sometimes referred to as low field.

Low-field MRI presents an attractive imaging solution, providing a relatively low-cost and highly available alternative to high-field MRI. In particular, low-field MRI systems can be implemented as self-contained systems that can be deployed in many clinical settings where high-field MRI systems cannot be deployed due to cost, size, and/or the need for specific facilities. However, due to the lower field strength, low-field MRI systems generally also have a relatively low signal-to-noise ratio. Therefore, the design of low-noise components may play an important role in the development of low-field MRI systems.

Aspects of the technology development described herein arise from the inventors' recognition of the need to provide relatively low-noise and efficient power to one or more magnetic coils of an MRI system, particularly (but not limited to) low-field MRI systems where noise in the power source may be particularly problematic. In this regard, the inventors have developed a power component for driving one or more magnetic coils suitable for relatively low-noise operation. In addition, the inventors have recognized that during intervals when a relatively high voltage is not required to operate the corresponding components, conventional power sources are often powered by a single relatively high-voltage power terminal, and in this regard, such solutions are relatively inefficient. The inventors have developed a power component with substantially improved efficiency in the following manner: according to some embodiments, by providing a plurality of power terminals that can be connected in a desired combination to power an amplifier, wherein each power terminal is configured to supply a different voltage level to power the amplifier. In this way, the intervals during which the amplifier draws significantly more power than required can be reduced.

Briefly, MRI involves placing the object to be imaged (e.g., all or part of a patient) in a static, homogeneous magnetic field, _B0 , so that the net magnetization of the object's atoms (typically represented by a net magnetization vector) is aligned in the direction of _B0 . One or more transmit coils are then used to generate a pulsed magnetic field, _B1 , having a frequency related to the rate of precession of the atomic spins of the atoms in the magnetic field, _B0 , so that the net magnetization of the atoms produces a component in a direction transverse to the direction of the _B0 field. After the _B1 field is turned off, the transverse component of the net magnetization vector precesses, its magnitude decaying over time until the net magnetization is realigned with the direction of the _B0 field. This process generates an MR signal that can be detected, for example, by a voltage induced in one or more receive coils of the MRI system.

MRI also involves the use of gradient coils to induce gradients in the main magnetic field, _B0 , so that MR signals radiated by specific spatial locations within the subject can be identified (i.e., the gradient coils are used to spatially encode the detected MR signals). An MR image is formed in part by pulsing one or more transmit coils and/or gradient coils in a specific sequence, called a pulse sequence, and sensing the MR signals induced by the pulse sequence using one or more receive coils. The detected MR signals can then be processed (e.g., "reconstructed") to form an image. A pulse sequence generally describes the order and timing of the operation of the transmit/receive coils and gradient coils to prepare for magnetization of the subject and obtain the resulting MR data. For example, a pulse sequence may indicate the order of transmit pulses, gradient pulses, and the acquisition times of the receive coils to acquire MR data.

In order to generate a pulse sequence for MRI, an electrical component is generally provided to drive a magnetic component to generate a magnetic field according to a prescribed pulse sequence. There are many considerations that make conventional high-field electrical solutions unfavorable and/or unsuitable for low-field MRI in the context of low-field MRI. For example, although the cost of conventional high-field electrical components may be acceptable given their relative insignificance compared to the total cost of a high-field MRI installation, this cost may be unacceptable in the context of a low-field MRI system designed as a lower-cost alternative. Therefore, the cost of conventional electrical components for high-field MRI can be disproportionately large and, therefore, unsatisfactory for some lower-cost low-field MRI systems.

In addition, the challenge in low-field MRI is the relatively low signal-to-noise ratio. In particular, the signal-to-noise ratio of the MR signal is related to the intensity of the main magnetic field _B0 and is one of the factors that drive clinical systems to operate in high-field schemes. Therefore, the MR signal intensity is relatively weak under low-field backgrounds due to the low field strength, so that any additional noise in the system may have a relatively significant impact on image quality. In this respect, the inventors have recognized that conventional power components for driving the coils of high-field MRI systems may not be suitable for low-field MRI systems because they are not designed to drive the coils with sufficiently low noise. Although the noise injected with such power components may be acceptable in the high SNR scheme of high-field MRI systems, such components generally do not provide sufficiently low noise levels to provide acceptable image quality in low-field MRI systems. For example, conventional power components may present unsatisfactory variations (such as ripples) in the output for use under low-field backgrounds, injecting relatively large noise into the gradient coil system of low-field MRI systems.

The inventors have developed one or more low-noise power components suitable for driving one or more magnetic components (e.g., coils) of a low-field MRI system, and according to some embodiments, have developed one or more relatively low-noise power components that are implemented using relatively high-efficiency linear amplifier designs, some examples of which are described in further detail below. Although the low-noise power components described herein are suitable for low-field MRI, they are not limited to use with low-field MRI systems and may be used with any suitable MRI system.

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

FIG1 is a block diagram of exemplary components of an MRI system 100 (e.g., a low-field MRI system). In the illustrative example of FIG1 , 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 understood that the system 100 is exemplary and that the MRI system may have one or more other components of any suitable type in addition to or in place of the components shown in FIG1 .

As shown in FIG1 , the magnetic component 120 includes a _Bo magnet 122, a shim coil 124, an RF transmit and receive coil 126, and a gradient coil 128. The magnet 122 can be used to generate a main magnetic field _B0 . The magnet 122 can be any suitable type of magnetic component or combination thereof that can generate a desired main magnetic field _B0 (e.g., any one or combination of one or more electromagnets, printed magnetic materials, or one or more permanent magnets). Therefore, the _Bo magnet herein refers to any one or combination of any type of magnetic component configured to generate a _Bo field. According to some embodiments, the _Bo magnet 122 can generate or bring about a _Bo field that is greater than or equal to about ₂₀ mT and less than or equal to about 50 mT, greater than or equal to about 50 mT and less than or equal to about 0.1 T, greater than or equal to about 0.1 T and less than or equal to about 0.2 T, greater than or equal to about 0.2 T and less than or equal to about 0.3 T, greater than or equal to about 0.3 T and less than or equal to about 0.5 T, and the like. Shim coils 124 may be used to act on the magnetic field to improve the homogeneity of the B ₀ field generated by the magnet 122 .

The gradient coils 128 can be arranged to provide gradient fields and, for example, can be arranged to generate gradients in three substantially orthogonal directions (X, Y, Z) within the _B0 field. The gradient coils 128 can be configured to encode the transmitted MR signals by systematically varying _{the B0} field (generated by the magnet 122 and/or the shim coils 124) to encode the spatial position of the received MR signals as a function of frequency or phase. For example, the gradient coils 128 can be configured to vary the frequency or phase as a linear function of spatial position along a particular direction, although more complex spatial decoding profiles can also be provided by using nonlinear coils. For example, a first gradient coil can be configured to selectively vary the _B0 field in a first (X) direction to perform frequency decoding in that direction, a second gradient coil can be configured to selectively vary the _B0 field in a second (Y) direction substantially orthogonal to the first direction to perform phase encoding, and a third gradient coil can be configured to selectively vary the _B0 field in a third (Z) direction substantially orthogonal to the first and second directions to enable slice selection for volumetric imaging applications.

As described above, MRI is performed by energizing and detecting transmitted MR signals using transmit and receive coils (commonly referred to as radio frequency (RF) coils), respectively. The transmit/receive coils may include separate coils for transmission and reception, multiple coils for transmission and/or reception, or the same coil for transmission and reception. Transmit/receive coils are also commonly referred to as Tx/Rx or Tx/Rx coils to generally refer to various configurations of transmit and receive magnet components used in MRI systems. These terms are used interchangeably herein. In FIG1 , RF transmit and receive coils 126 include one or more transmit coils that can be used to generate RF pulses to induce an oscillating magnetic field _B1 . The one or more transmit coils can be configured to generate any suitable type of RF pulse. For example, the one or more transmit coils can be configured to generate any pulse sequence described in U.S. patent application serial number 14/938,430, entitled “Pulse Sequences For Low Field Magnetic Resonance,” filed on November 11, 2015 (the '430 application).

Each of the magnet assemblies 120 can be constructed in any suitable manner. For example, in some embodiments, one or more (e.g., all) of the magnet assemblies 120 can be fabricated, constructed, or manufactured using the techniques described in U.S. patent application Ser. No. 14/845,652, filed on Sep. 4, 2015, and entitled “Low-Field Magnetic Resonance Imaging Methods and Apparatus” (the '652 application). However, the techniques described herein are not limited in this respect, and any suitable technique can be used to provide the magnet assemblies 120.

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 components, transmit coil components, and/or any other suitable power electronics required to provide suitable operating power to energize and operate the components of the low-field MRI system 100.

As shown in FIG1 , the power management system 110 includes a power supply 112, one or more power components 114, a transmit/receive switch 116, and a thermal management component 118. The power supply 112 includes electronics that provide operating power to the magnet component 120 of the MRI system 100. For example, the power supply 112 may include electronics that provide operating power to one or more _Bo coils (e.g., _Bo magnet 122) to generate the main magnetic field for a low-field MRI system. In some embodiments, the power supply 112 is a unipolar 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 or the RF receive coil is being operated.

The one or more power components 114 may include: one or more RF receive (Rx) pre-amplifiers that amplify MR signals detected by one or more RF receive coils (e.g., coil 126); one or more RF transmit (Tx) power components configured to provide power to one or more RF transmit coils (e.g., coil 126); one or more gradient power coils configured to provide power to one or more gradient coils (e.g., gradient coil 128); and one or more shim power components 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 for components of the low-field MRI system 100 by facilitating the transfer of heat generated by one or more components of the low-field MRI system 100 away from these components. The 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 in close proximity to heat-generating MRI components, including, but not limited to, _Bo coils, 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 liquid coolants (e.g., water), to transfer heat away from components of the low-field MRI system 100.

As shown in FIG1 , the MRI system 100 includes a controller 106 (also referred to as a console) having control electronics for sending instructions to and receiving information from a power management system 110. The controller 106 can be configured to implement one or more pulse sequences, which are used to determine the instructions sent to the power management system 110 to operate the magnetic component 120 in a desired sequence. For example, for embodiments in which the MRI system 100 operates at a low field, the controller 106 can be configured to control the power management system 110 to operate the magnetic component 120 according to the following pulse sequences: a zero echo time (ZTE) pulse sequence, a balanced steady-state free precession pulse sequence (LF-bSSFP), a gradient echo pulse sequence, a spin echo pulse sequence, an inversion recovery pulse sequence, arterial spin labeling, diffusion-weighted imaging (DWI), and/or any other pulse sequence designated for operation in a low-field context. The pulse sequence for low-field MRI can be applied to different contrast types, such as T1-weighted imaging and T2-weighted imaging, diffusion-weighted imaging, arterial spin labeling (perfusion imaging), Overhauser imaging, etc. However, any pulse sequence can be used, as the aspects are not limited to this aspect. The controller 106 can be implemented as hardware, software, or any suitable combination of hardware and software, as the aspects of the disclosure provided herein are not limited to this aspect.

In some embodiments, the controller 106 can be configured to implement a pulse sequence by obtaining information about the pulse sequence from a pulse sequence memory 108, which stores information for each of one or more pulse sequences. The information for a particular pulse sequence stored by the pulse sequence memory 108 can be any suitable information that enables the controller 106 to implement the particular pulse sequence. For example, the information for the pulse sequence stored in the pulse sequence memory 108 can 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 memory 108 can be stored in one or more non-transitory storage media.

As shown in FIG1 , the controller 106 also interacts with a computing device 104 that 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. The controller 106 may provide information about one or more pulse sequences to the computing device 104 for processing the data by the computing device. For example, the controller 106 may provide information about one or more pulse sequences to the computing device 104, and the computing device may perform an image reconstruction process based at least in part on the provided information.

The computing device 104 can be any electronic device that can process the acquired MR data and generate one or more images of the object being imaged. In some embodiments, the computing device 104 can be a fixed electronic device, such as a desktop computer, a server, a rack computer, a workstation, or any other suitable fixed electronic device configured to process MR data and generate one or more images of the object being imaged. Alternatively, the computing device 104 can be a portable device, such as a smart phone, a personal digital assistant, a laptop computer, a tablet computer, or any other portable device that can be configured to process MR data and generate one or more images of the object being imaged. In some embodiments, the computing device 104 can include multiple computing devices of any suitable type, as aspects of the disclosure provided herein are not limited to this aspect. The user 102 can interact with the computing device 104 to control various aspects of the low-field MR system 100 (e.g., programming the system 100 to operate according to a specific pulse sequence, adjusting one or more parameters of the system 100, etc.), and/or view images obtained by the low-field MR system 100.

As described above, the inventors have recognized that conventional power components used to drive the coils of high-field MRI systems are generally not suitable for low-field MRI systems because they are not designed to drive the coils with low noise. Although the noise injected by such power components may be acceptable in high-field MRI systems with high SNR, such power components do not provide a sufficiently low noise level to provide acceptable image quality in low-field MRI systems. The low SNR of low-field MRI increases the demand for low-noise power components to drive one or more coils of low-field MRI systems. The design of one or more low-noise power components can improve the SNR of low-field MRI systems.

Some high-field MRI systems use power components with switching power converters to drive current through the coils. While switching power converters can offer high efficiency, the inventors have recognized and appreciated that conventional switching converters can introduce significant switching noise into the system because they switch at frequencies in a range (e.g., in the tens to hundreds of kHz range) that can affect the transmission of pulse sequences and the detection of MR signals emitted in response to the pulse sequences. For example, the switching frequency of conventional switching power converters and/or their harmonics can overlap with the frequencies to which the transmit and/or receive coils are tuned, resonating and thereby adding noise to the transmit/receive channel system of low-field MRI. While the noise injected by such power converters may not be significant in high-field MRI systems, the injected noise level may be unacceptable in low-field MRI systems and may degrade imaging quality. Furthermore, the difference in transmit/receive frequencies in high-field MRI often allows for easier filtering of switching noise because the switching noise is typically out-of-band relative to the transmit/receive frequencies (the switching frequency and/or its harmonics are much lower than the _B1 frequency (the transmit frequency), making it easier to filter).

An alternative to using a switching power converter is to use a linear amplifier. Unlike a switching power converter, which switches its transistors between fully on and fully off states, a linear amplifier operates its transistors over a continuous range to produce an amplified output. In a linear amplifier, a control signal can be provided to the control terminal (e.g., gate or base) of one or more transistors, and the current flowing through the one or more transistors is controlled based on the magnitude of the control signal. Because linear amplifiers generate their output by varying the current through the transistors over a continuous range, they can avoid the injection of switching noise, as opposed to switching the transistors at a switching frequency.

However, the inventors have recognized that it may be necessary to provide a wide range of output currents and/or voltages to coils of an MRI system, such as gradient coils, such that, for example, a single positive voltage terminal is used to provide a positive output voltage and a single negative voltage terminal is used to provide a negative output voltage, resulting in inefficient power components. In particular, linear amplifiers may dissipate significant amounts of power when generating relatively low-magnitude output voltages. For example, providing relatively low voltages and high currents at the output of a linear amplifier may require reducing the voltage across one or more amplifier transistors between the voltage supply terminal and the amplifier's output terminal. Consequently, such linear amplifiers may be inefficient when operated to generate low output voltages, and thus may consume significant amounts of power and cause significant heat dissipation. Although cooling systems can be used to cool the system, for some MRI systems, such as low-field MRI systems designed to be relatively small, low-weight, and/or low-cost, providing significant cooling capacity for the amplifier circuitry may be unacceptable.

The inventors have recognized that the efficiency of power components using linear amplifiers can be improved by powering the amplifier from different supply voltages (e.g., multiple supply rails at different fixed voltages) based on the output voltage generated by the amplifier. By providing the ability to connect the amplifier to different supply voltages, a suitable supply voltage can be selected that is closer to the amplifier's output voltage, which can reduce the voltage drop across one or more transistors of the amplifier. Thus, the efficiency of the amplifier can be improved, and the need for cooling the amplifier can be significantly reduced or eliminated. Such amplifiers may be particularly advantageous in low-field MRI systems, as described above, which can benefit from efficient, low-noise power components.

2 shows a drive circuit for driving current through coils 202 of an MRI system to generate a magnetic field according to a desired pulse sequence, according to some embodiments. The power component 114 drives the current through the coils 202 based on a control signal from the controller 106. As described above, the controller 106 can generate a control signal to drive the power component 114 based on a pulse sequence implemented by the controller 106 (or provided by one or more other controllers). In some embodiments, the coils 202 can be gradient coils 128. However, the technology described herein is not limited in this respect, as the coils 202 can be coils 122 of a magnet, shim coils 124, or RF transmit/receive coils 126.

Power components configured to power gradient coils typically provide relatively high power and are typically required to provide precise control of the current supplied to the gradient coils so that a desired pulse sequence can be accurately delivered. Inaccuracies in delivering commanded currents to the gradient coils result in a reduction in the signal-to-noise ratio due to differences between the gradient pulse sequence being delivered and the expected (and projected) pulse sequence. Power components configured to drive the gradient coils should also respond promptly when delivering the commanded currents to the gradient coils, including rapid transitions between the commanded current levels, to accurately produce the current waveforms required by the desired pulse sequence. Accordingly, the inventors have developed power components that can be controlled to accurately and precisely deliver current with relatively low noise and relatively high efficiency to one or more gradient coils to accurately reproduce the desired pulse sequence, some embodiments of which are discussed in further detail below.

In some embodiments, the power component 114 may be a "current mode" power component that drives a desired current through the coils 202. This desired current may be generated by the power component 114 in response to a current command from the controller 106. In this regard, the power component 114 may operate as a current source controlled by a current command (which may be provided by the controller as a voltage level indicating the current to be provided to the coils 202). The controller 106 may vary the current command so that the power component 114 generates a current value that varies depending on the selected pulse sequence. For example, the controller 106 may command the power component to drive one or more gradient coils according to a pulse sequence comprising a plurality of gradient pulses. For each gradient pulse, the power component may need to ramp up the current provided to the corresponding gradient coil on the rising edge of the gradient pulse and ramp down the current provided to the gradient coil on the falling edge of the gradient pulse. Exemplary operation of a power component configured to drive a gradient coil to provide a plurality of such gradient pulses is described in further detail below.

FIG3A illustrates an example gradient coil current waveform according to some embodiments. In this example, the gradient coil current rapidly ramps from 0 A to +20 A within a 0.2 ms interval at the rising edge of the gradient pulse, remains at +20 A for a period of time, and then rapidly ramps down to -20 A at the falling edge of the gradient pulse and remains at -20 A for a period of time. It should be understood that exemplary currents for generating gradient pulses are provided by way of illustration, and that different pulse sequences may include gradient pulses with different current and/or voltage requirements. The controller 106 and the power component 114 may be configured to drive one or more gradient coils according to any suitable pulse sequence.

FIG3B shows the waveforms of the current command, gradient coil current, and gradient coil voltage before, during, and after the rising edge of the gradient coil current shown in FIG3A . The gradient coil current is the current flowing through the gradient coil. The gradient coil voltage is the voltage across the gradient coil. The current command is a signal indicating the amount of current driven through the gradient coil by the power component 114. In response to the current command at time 0 ms, the current flowing through the gradient coil begins to increase toward the command current of +20 A. Because the gradient coil is an inductive load, a relatively high voltage must be supplied to the gradient coil to rapidly increase the current flowing through the gradient coil. Providing a rapid increase in the current flowing through the gradient coil is desirable in MRI applications because providing rapid transitions between gradient coil current values can reduce acquisition time and may be required for implementing certain pulse sequences. As can be appreciated from the exemplary voltages and currents shown in FIG3A and FIG3B , the power component 114 may be capable of driving the gradient coil at relatively high power.

As an example, a gradient coil may have an inductance of 200 μH and a resistance of 100 mΩ. Since the rate of change of the current through the gradient coil is proportional to its inductance, a voltage of 100 V is required to increase its current at a rate of 100 A/ms. However, once the gradient coil current reaches equilibrium at 20 A, the voltage requirement drops significantly. At this point, since the current no longer changes, the required voltage depends on the resistance of the gradient coil. Since the resistance of the gradient coil is 100 mΩ, the voltage required to maintain the current at a stable 20 A is 2 V, which is significantly lower than the voltage (100 V) required during transitions between current values. However, these values for current, voltage, inductance, and resistance are provided by way of example only, as any suitable gradient coil design with different inductance and/or resistance values may be used. Furthermore, other suitable values for current, voltage, transition time, etc. may be used and/or required to implement a given pulse sequence.

Because the resistance of the gradient coils can be relatively low (e.g., less than 500 mΩ), in some embodiments, the power component 114 has a relatively low output impedance to efficiently provide the command current. For example, in some embodiments, the power component 114 includes a linear amplifier configured to power one or more gradient coils (e.g., to provide current to the one or more gradient coils according to a desired pulse sequence). To implement a linear amplifier with low output impedance, suitably sized transistors with low equivalent series resistance can be used, and/or multiple transistors can be connected in parallel to collectively produce a low resistance. The interconnects can be designed to have a relatively low resistance. In some embodiments, the output impedance of the linear amplifier can, for example, be less than twice the impedance of the gradient coils. In some embodiments, the voltage drop across the transistors of the linear amplifier can be relatively low during operation, e.g., less than 5V, less than 2V, or less than 1V (and greater than 0V). Using an amplifier with a relatively low output impedance can be particularly useful for driving current through the gradient coils, which can have a large DC current. Low output impedance can improve amplifier efficiency and limit heat generation. Details of an exemplary linear amplifier implementation are discussed in further detail below.

FIG4A shows a power component 114 having a current feedback loop and a voltage feedback loop according to some embodiments. The power component 114 is configured to provide the current required to drive one or more gradient coils according to a desired pulse sequence. Therefore, the power component 114 is designed as a low-noise current source that can be precisely controlled to provide the command current waveform required to drive one or more gradient coils to accurately generate the desired gradient magnetic field. The power component 114 includes a comparator 301, which receives a current command from the controller 106 at its non-inverting input and a current feedback signal FB from a current sensor 401 at its inverting input. The current command can be a voltage value representing the commanded current. The current feedback signal FB can be a voltage value representing the measured current. In some embodiments, a high-quality current sensor can be used to provide an accurate feedback signal FB, which can improve the accuracy of the gradient coil current pulses.

The comparator 301 generates an error signal (e.g., a voltage) representing the difference between the current command and the current feedback signal FB. The amplifier circuit 302 amplifies the error signal to generate an amplified error signal that is provided to the output stage 303. The output stage 303 drives the coil 202 based on the amplified error signal. As described above, the current through the coil 202 is measured by the current sensor 401, and the feedback signal FB is fed back to the comparator 301. The current feedback loop thus ensures that the current through the coil 202 is equal to the current commanded by the controller 106. In this regard, the power component 114 can operate as a voltage-controlled current source. According to some embodiments, a highly accurate and precise current sensor 401 is used to ensure that the current output provided to the gradient coil accurately tracks the current commanded by the controller 106. Thus, the current provided to power the gradient coil can be maintained as close to the commanded current as possible. The power component 114 also has a voltage feedback loop that provides the output voltage of the output stage 303 to the input of the voltage amplifier circuit 302.

As shown in FIG4B , the voltage amplifier circuit 302 may include an operational amplifier OA that receives an error signal E at its non-inverting input and a voltage feedback signal V_FB at its inverting input. The voltage feedback signal may be provided to the inverting input of the operational amplifier via a resistor divider (e.g., including resistors R1 and R2), which causes the operational amplifier to amplify the input voltage based on the ratio of the resistor values in the voltage divider. For the voltage amplifier, any suitable voltage gain may be used, such as a gain of 5 to 15. In some embodiments, the voltage gain of the output stage may be unity (unit-one).

As shown in FIG4A , in some embodiments, the controller 106 can provide commands to the output stage 303. The controller 106 can command the output stage 303 to generate a power supply voltage suitable for supplying the current required to perform the corresponding portion of the pulse sequence. For example, the command can cause the power converter of the output stage to begin ramping up the power supply voltage before the gradient coil current pulse. Such commands will be discussed further below with reference to FIG15D .

In some embodiments, the output stage 303 is configured to be selectively powered by a plurality of power supply terminals at different voltages. The power supply terminal selected for powering the output stage 303 can be selected according to the output voltage generated by the voltage amplifier. For example, when the power component is ordered to generate a relatively high (positive) output voltage, the power component can be powered from a relatively high (positive) voltage supply terminal, and when the power component is ordered to generate a relatively low (positive) output voltage, the power component can be powered from a relatively low (positive) voltage supply terminal. Therefore, the efficiency of the power component can be improved by reducing the voltage drop on one or more transistors of the power component when generating a relatively low voltage. It should be understood that any number of supply terminals and voltage levels can be used because these aspects are not limited to this aspect. For example, high, medium and low voltage supply terminals (both positive and negative) can be used, or even larger numbers suitable for a specific design and/or implementation.

FIG5A illustrates an example of an output stage 303A having outputs Vout and Iout suitable for powering one or more gradient coils of a magnetic resonance imaging system. To improve power supply efficiency when powering the one or more gradient coils, the output stage 303 can be powered by different supply terminals depending on the output voltage Vout. For example, the output stage 303 can be powered by multiple supply terminals of a first polarity (e.g., multiple different positive voltages) and/or multiple supply terminals of a second polarity (e.g., multiple different negative voltages). To facilitate low-noise operation, in some embodiments, the output stage 303A can include a linear amplifier 304. In some embodiments, each of the different supply terminals provides a different fixed supply voltage. In some embodiments, one or more of the different supply terminals generates a variable supply voltage, as discussed in further detail below.

In operation, if a positive output voltage is generated at Vout, the switch circuit S1 connects the high-side power input of the linear amplifier 304 to the high-voltage terminal +Vhigh or the low-voltage terminal +Vlow according to the magnitude of the output voltage. If a relatively high output voltage is to be generated (e.g., if the output voltage to be generated exceeds a certain threshold), the switch circuit S1 connects the high-side power input of the linear amplifier 304 to the high-voltage terminal +Vhigh. If a relatively low output voltage is to be generated (e.g., if the output voltage to be generated remains below a certain threshold), the switch circuit S1 connects the high-side power input of the linear amplifier 304 to the low-voltage terminal +Vlow. Similarly, as described above, if a negative output voltage is generated, the switch circuit S2 connects the low-side power input of the linear amplifier 304 to the high-voltage terminal -Vhigh or the low-voltage terminal -Vlow according to the magnitude of the output voltage. Any suitable switch circuits S1 and S2 can be used. Such a switch circuit can include a passively switched diode and/or an actively switched transistor.

In some embodiments, the high voltage terminal or the low voltage terminal can be directly connected to the linear amplifier 304 without an intermediate switch S1 or S2. For example, as shown in the exemplary output stage 303A' in Figure 5B, the high voltage terminals +Vhigh and -Vhigh can be directly connected to the linear amplifier 304, while the low voltage terminals +Vlow and -Vlow can be connected to the linear amplifier 304 through corresponding switches S1 and S2. The linear amplifier 304 can be designed so that it is powered by the low voltage supply terminal unless its voltage is insufficient to supply the output current, in which case the linear amplifier 304 is powered by the high voltage supply terminal. It should be understood that the use of +-Vhigh and +-Vlow is only exemplary, and any number of voltage levels can be used to provide the desired output voltage. For example, one or more intermediate voltage levels between +-Vhigh and +-Vlow can be used to generate the desired voltage level.

6 shows an example of an output stage 303A having a plurality of driver circuits 601 to 604. Driver circuits 601 to 604 drive a linear amplifier 304 including a plurality of transistor circuits 605 to 608, each of which includes one or more transistors. Linear amplifier 304 can be connected to a high voltage supply terminal or a low voltage supply terminal depending on the output voltage to be generated.

When a low positive output voltage is to be generated, one or more transistors 606 are connected to the low voltage terminal +Vlow via the switch circuit S3. One or more transistors 605 are turned off by the driver circuit 601 to isolate the transistors 606 from the high voltage terminal +Vhigh. The driver circuit 602 drives the one or more transistors 606 as linear amplification elements based on the input to generate an amplified output using the low voltage terminal +Vlow as a current source.

To provide a high positive output voltage, the driver circuit 601 turns on one or more transistors 605 to connect the high voltage terminal +Vhigh to the transistor 606. The switch circuit S3 can be turned off to isolate the one or more transistors 606 from the low voltage terminal +Vlow. The driver circuit 602 can drive the one or more transistors 606 to be fully turned on so that the one or more transistors 605 are connected to the output of the output stage 303A. The driver circuit 601 drives the one or more transistors 605 as linear amplification elements based on the input to generate an amplified output using the high voltage terminal +Vhigh.

Therefore, the low voltage terminal +Vlow can be used to provide a low output voltage, and the high voltage terminal +Vhigh can be used to provide a high output voltage. A negative output voltage can similarly be provided by the driver circuits 603 and 604, one or more transistors 607 and 608, and the switch circuit S4. When generating a negative output voltage, the driver circuits 601 and 602 can turn off one or more transistors 605 and 606. Similarly, when generating a positive output voltage, the driver circuits 603 and 604 can turn off one or more transistors 607 and 608.

For low output voltages, one or more transistors 606 can operate as linear amplifying elements of linear amplifier 304, while for high output voltages, one or more transistors 605 can operate as linear amplifying elements. In some embodiments, one or more transistors 606 and 605 can be biased so that for the transition region between the low positive output voltage and the high positive output voltage, one or more transistors 605 and 606 both function as linear amplifying elements of linear amplifier 304, i.e., they are neither fully turned on nor fully turned off. Operating both transistors 605 and 606 as linear elements during such transitions can facilitate a linear amplifier 304 having a smooth and continuous transfer function. Transistors 607 and 708 can operate similarly to transistors 605 and 606 to generate a range of negative output voltages.

In some embodiments, the switch circuits S3 and S4 can be implemented by diodes that automatically turn on and off depending on whether the high voltage terminal is being used. For example, if the switch circuit S3 includes a diode, the anode can be connected to the terminal +Vlow, and the cathode can be connected to one or more transistors 606, so that current can only flow from the terminal +Vlow to the output stage 303A. However, the technology described herein is not limited in this respect, as the switch circuits S3 and S4 can be implemented using controlled switches such as transistors or any other suitable switching circuits.

In some embodiments, the circuit of FIG6 can drive the gradient coil using a pulse sequence such as that shown in FIG3. When the output current is constant, an output voltage (e.g., 2V) can be generated by supplying current from the low voltage terminal +Vlow. During a transition period when the current changes rapidly, a high output voltage (e.g., 100V) can be generated by supplying current from the high voltage terminal +Vhigh. Thus, the high voltage terminal can be used to provide a high output voltage during a transition period of the output current, while the low voltage terminal can be used to provide a low output voltage to achieve high efficiency.

According to some embodiments, for example, according to some pulse sequences, one or more high voltage terminals may only need to be used for relatively short periods of time, so that one or more transistors 605 (and 608) may be turned on for only a relatively small duty cycle. Therefore, in some embodiments, the size of one or more transistors 605 (and 608) can be reduced relative to transistor 606 (or 607), and/or the number of transistors connected in parallel can be reduced because one or more transistors 605 (and 608) will have time to dissipate heat between transitions in the gradient coil current.

In some embodiments, the driver circuits 601 and 604 can be designed to provide time-limited output signals. Providing time-limited output signals can ensure that one or more transistors 605 and/or 608 are only temporarily turned off and are not turned on to drive a steady-state current. If one or more transistors 605 or 608 are designed to be turned on only for relatively short periods of time, such a technique can be advantageous because it can prevent excessive power consumption of one or more transistors 605 or 608.

7 shows a block diagram of driver circuits 601 and 602 according to some embodiments. Driver circuit 601 includes a driver transistor 703A for driving one or more transistors 605. Driver circuit 602 includes a driver transistor 703B for driving one or more transistors 606.

The driver circuits 601 and 602 may include one or more bias circuits 701 for generating a DC bias on the input voltage provided to the driver transistors 703A and 703B. In some embodiments, the bias circuits 701 may bias the driver transistors 703A and/or 703B slightly below their turn-on voltage. The inventors have recognized and appreciated that biasing the driver transistors slightly below their turn-on voltage can reduce or eliminate thermal runaway. Advantageously, such biasing techniques may not degrade the linearity of the output stage 303A. If the operational amplifier OA of the voltage amplifier circuit 302 has a sufficiently high speed, the operational amplifier OA may respond quickly enough to accurately control the output voltage of the output stage, even when the driver transistors are biased slightly below their turn-on voltage.

In some embodiments, the driver circuit 601 can include a timing circuit that causes the driver circuit 601 to produce a time-limited output. Any suitable timing circuit can be used. In the example of FIG7 , the timing circuit 702 is connected to the input of the output stage 303A via the bias circuit 701 and limits the amount of time that the input can be provided to the driver transistor 703A.

In some embodiments, the sequential circuit 702 can be an RC circuit having an output voltage that decays over time and turns off the driver transistor 703A when the output of the sequential circuit 702 falls below the turn-on voltage of the driver transistor 703A. The time during which the one or more transistors 605 are turned on is limited based on the RC time constant of the RC circuit. However, the technology described herein is not limited to using RC circuits to implement sequential circuits, as any suitable sequential circuit can be used, including analog and/or digital circuits. In some embodiments, the driver circuits 603 and 604 can be implemented similarly to the driver circuits 602 and 601 for negative input and output voltages, respectively.

FIG8 shows an example implementation of the driver circuit of FIG7 according to some embodiments of the technology described herein. As shown in FIG8 , in some embodiments, the bias circuit 701 can be implemented by a Zener diode connected in series with a resistor R2 between a high voltage terminal +Vhigh and a lower voltage DC terminal (e.g., -Vhigh) having a voltage lower than +Vhigh. In some embodiments, the bias circuit 701 may include another circuit between the high voltage terminal +Vhigh and the lower voltage DC terminal to provide a DC path for current to flow therebetween and to establish a suitable bias voltage. In some embodiments, the bias circuit 701 may include another Zener diode and a resistor connected in series with the Zener diode and resistor shown in FIG8 to provide one or more bias voltages to the low-side driver circuits 603 and 604. However, this is merely an example, as any suitable bias circuit may be used. FIG8 also shows an example of a sequential circuit 702 implemented as an RC circuit with a capacitor C1 and a resistor R1. Again, this is merely an example of a sequential circuit, as other configurations of sequential circuits may be used. Driver transistors 703A and 703B are shown as being implemented by bipolar junction transistors. However, the technology described herein is not limited to this aspect, as the driver transistors can be implemented by any type of transistor. In this example, transistor circuits 605 and 606 are shown as MOSFETs. However, transistor circuits 605 and 606 can be implemented by any type of transistor. In some embodiments, transistor circuits 605 and/or 606 can have multiple transistors connected in parallel. As described above, switch circuit S3 can be implemented as a diode as shown in Figure 8. However, as described above, the technology described herein is not limited to this aspect, as in some embodiments, switch circuit S3 can be implemented by a transistor.

FIG9 shows another example of a technique for implementing a sequential circuit. The inventors have recognized and appreciated that if switch S3 is implemented as a diode, the voltage across the diode can serve as a trigger for the sequential circuit to limit the amount of time one or more transistors 605 are on. When the linear amplifier 304 produces a low output voltage, the diode is forward biased and conducts. When the linear amplifier 304 produces a high output voltage, one or more transistors 605 conduct and the diode switches from forward bias to reverse bias. The sequential circuit 902 can sense the reverse bias as an indication that one or more transistors 605 are conducting. In the example of FIG9 , the voltage across the diode is provided as an input to the sequential circuit 902, which generates a disable signal that disables the operation of the driver circuit 601 after a period of time, thereby limiting the amount of time one or more transistors 605 are on. The sequential circuit 904 can similarly operate to disable the operation of the driver circuit 604 after one or more transistors 608 have been conducting for a period of time.

FIG10 shows an example of sequential circuits 902 and 904 implemented using an RC circuit and bipolar transistors. For example, in sequential circuit 902, when the diode is reverse biased after a period of time, the output of the RC circuit rises to a level that turns on the bipolar transistor. When the bipolar diode turns on, the input of the driver circuit 601 is pulled down to +Vlow, which turns off the driver circuit 601 and one or more transistors 605.

Although Figures 6, 9, and 10 illustrate a "two-ended" linear amplifier 304 that can generate a positive output voltage or a negative output voltage, the technology described herein is not limited in this respect, as in some embodiments, a single-ended linear amplifier can be used. Figure 11 shows an example of an output stage 303B that includes a single-ended linear amplifier 305 that only generates a positive output voltage. Figure 11 schematically illustrates that the single-ended linear amplifier 305 can be connected to a high positive voltage terminal +Vhigh or a low positive voltage terminal +Vlow via a switch S1, depending on the output voltage to be generated. In some embodiments, the output stage 303B can be implemented using the driver circuits 601, 602, one or more transistors 605 and 606, and associated switching circuit S3 described above.

The output stage 303B can use a polarity switching circuit 1104 to provide a positive output voltage or a negative output voltage to the load. In the example of Figure 11, the polarity switching circuit 1104 is implemented using an H-bridge including switches S5 to S8. A positive voltage can be provided to the load by turning on switches S5 and S8 and turning off switches S6 and S7. A negative voltage can be provided to the load by turning on switches S6 and S7 and turning off switches S5 and S8. In some embodiments, a control circuit (not shown) can control switches S5 to S8 to generate an output voltage of appropriate polarity. The polarity can be determined by checking the polarity of the current command, error signal E, or any other appropriate signal.

As described above, conventional switching converters may introduce a large amount of switching noise into the system because they switch at frequencies in the range of tens to hundreds of kHz. Such switching noise can interfere with imaging because the switching noise is in the same frequency range as the MR signals that are desired to be detected. The inventors have recognized that power converters with switching frequencies higher than the Larmor frequency of interest do not interfere with imaging to a large extent. Therefore, in some embodiments, the power component 114 may include a switching power converter 1202 that is designed to switch at a relatively high switching frequency higher than the Larmor frequency of interest, as shown in FIG12 . In some embodiments, the switching frequency may be higher than 1 MHz, higher than 10 MHz, higher than 30 MHz, or higher than 300 MHz.

As described above, the inventors have appreciated that providing a variable voltage supply terminal facilitates efficient powering of one or more gradient coils of a magnetic resonance imaging system (e.g., a low-field MRI system). In some embodiments, the output stage can be powered by one or more variable voltage supply terminals that are controlled to produce a supply voltage close to a desired output voltage. Providing such a variable voltage supply terminal can improve the efficiency of the output stage by limiting the voltage drop across the linear amplifier.

FIG13 illustrates an embodiment of an output stage 303C that can be powered by a variable voltage positive supply terminal and a variable voltage negative supply terminal. The voltage of the supply terminal can be varied based on the output voltage to reduce the voltage drop across one or more transistors of the linear amplifier 306, thereby facilitating efficient powering of one or more gradient coils to generate a magnetic field according to a desired pulse sequence. In some embodiments, the voltages of the positive and/or negative voltage terminals can be provided by power converters 1304 and/or 1306, respectively. The variable output voltages of power converters 1304 and/or 1306 can be controlled by controller 1308 based on the desired output voltage of the output stage 303C to maintain the voltages of the positive and/or negative voltage terminals slightly above (or below, respectively) the output voltage of the output stage, thereby reducing the voltage drop across one or more transistors of the linear amplifier.

According to some embodiments, controller 1308 controls the variable output voltage of power converters 1304 and/or 1306 based on the output voltage of linear amplifier 306. However, the variable output voltage can be controlled in other ways and/or in a different relationship to the desired output voltage of output stage 303C. For example, the variable output voltage can be controlled based on a command (e.g., a current command) provided to linear amplifier 306. As previously discussed, the controller can be configured to command the linear amplifier to generate an output sufficient to drive one or more gradient coils of a magnetic resonance imaging system according to a desired pulse sequence. Thus, controller 1308 can be configured to control the variable output voltage of power converters 1304 and/or 1306 so that the output voltage provided to the linear amplifier is sufficient, but not excessive and therefore inefficient, to allow the linear amplifier to generate an output according to the desired pulse sequence to power the one or more gradient coils. Control of power converters 1304 and 1306 can be performed in any suitable manner, such as by controlling their duty cycle, their frequency, or any other control parameter that can control the output voltage of the power converter. 13 may be power converters designed to switch at a relatively high switching frequency, as described above, that is above the Larmor frequency of interest. However, any suitable power converter may be used, as aspects are not limited in this respect.

In some embodiments, both the high voltage supply terminal and the low voltage supply terminal (e.g., +Vhigh and +Vlow) can supply power to the linear amplifier, as shown in Figures 5, 6, and 11, and the voltage provided by the low voltage supply terminal, the high voltage supply terminal, the low voltage supply terminal, and the high voltage supply terminal, or any of the supply terminals can be variable. Figure 14A shows an embodiment of an output stage 303D with a variable low voltage supply terminal similar to Figure 5A. Instead of having low voltage terminals +Vlow and -Vlow at fixed voltages, Figure 14A shows that +Vlow and -Vlow can have variable voltages. In some embodiments, variable voltages of +Vlow and -Vlow can be provided by power converters 1403 and 1404, respectively. In some embodiments, power converters 1403 and 1404 can be switching power converters designed to switch at a relatively high switching frequency higher than the Larmor frequency of interest as described above. When a relatively low output voltage is to be generated (e.g., in a steady state), current is derived from the low voltage supply terminal +Vlow or -Vlow. The output voltage +Vlow or -Vlow of the power converter 1403 or 1404 can be controlled by the controller 1308 based on the desired output voltage Vout of the linear amplifier 304 to maintain the voltage of the low voltage supply terminal +Vlow or -Vlow slightly higher (or lower, respectively) than the output voltage of the output stage, thereby reducing the voltage drop across one or more transistors of the linear amplifier in the steady state and reducing power consumption. When a relatively high output voltage is to be generated, current can be sourced from the high voltage terminal +Vhigh or -Vhigh, which can have a fixed voltage.

+Vhigh may be a separate terminal from the power supply terminal Vhigh_Supply that provides power to the power converter 1403, as shown in FIG14A , or may be the same terminal as Vhigh_Supply, as shown in FIG14B . FIG14B shows an example of an output stage 303E in which +Vhigh is provided from the power supply terminal Vhigh_Supply and -Vhigh is provided from the power supply terminal Vlow_Supply that provides power to the power converter 1404. Providing +Vhigh and/or -Vhigh from existing power supply terminals can avoid the need to generate additional power supply voltages, which can simplify the design and implementation of the output stage.

FIG15A illustrates an example of a gradient coil current waveform according to some embodiments. The gradient coil current is initially zero, then rapidly ramps up to 10 A over 0.1 ms. The current is maintained at 10 A for a period of time, then drops back down to 0 A. The current is maintained at 0 A for a period of time, then rapidly ramps up to 20 A over 0.2 ms. The current is maintained at 20 A for a period of time, then drops back down to 0 A. It should be understood that the amp values and time intervals are examples for illustrative purposes only, and any suitable values may be used.

FIG15B illustrates a ramp-up transition of the gradient coil current from 0 A to 10 A, the voltage 1502 required to drive the gradient coil, the voltage at the high voltage supply terminal +Vhigh, and the voltage at the low voltage supply terminal -Vlow. During the transition, current is drawn from the high voltage supply terminal +Vhigh to provide a high voltage to the gradient coil to rapidly ramp its current. As the transition occurs, the power converter 1403 begins ramping the voltage at the low voltage supply terminal +Vlow from approximately 0 V to a voltage slightly greater than the output voltage required to drive the gradient coil with a steady-state current of 10 A. When the steady-state current of 10 A is reached, current is drawn from the low voltage supply terminal +Vlow to provide high efficiency in the steady state.

FIG15C illustrates the gradient coil current rising from 0 A to 20 A, the gradient coil voltage, and the voltages at the high voltage supply terminal +Vhigh and the low voltage supply terminal +Vlow. During the transition to 20 A, as with the transition to 10 A, current is drawn from the high voltage supply terminal +Vhigh to provide a high voltage to the gradient coil to quickly ramp up its current. As the transition occurs, the power converter 1403 begins to ramp the voltage at the low voltage supply terminal +Vlow from approximately 0 V to a voltage slightly above the output voltage required to drive the gradient coil with a steady-state current of 20 A. When the steady-state current of 20 A is reached, current is drawn from the low voltage supply terminal +Vlow.

Because the voltage of the low-voltage supply terminal +Vlow can be varied, it can be set slightly higher than the output voltage required for different steady-state current levels. This improves efficiency compared to using a low-voltage supply terminal +Vlow with a fixed voltage, as the fixed voltage would need to be designed to handle the maximum steady-state current, which could be higher than the voltage required to drive a lower steady-state current, which could reduce efficiency. For example, if +Vlow is set high enough to supply a 20A steady-state gradient coil current, this voltage is higher than the voltage required to supply a 10A steady-state gradient coil current. This results in an increased voltage drop across the linear amplifier transistors when supplying 10A steady-state gradient coil current, resulting in higher than necessary power consumption. The variable voltage can be set at or near the minimum voltage required to supply the commanded steady-state gradient coil current, which improves efficiency.

FIG15D illustrates several different transition waveforms for the current command, the gradient coil current, the gradient coil voltage 1502 required to supply the current, and the voltage +Vlow. Transition waveform 1504 illustrates an ideal transition, in which the voltage of +Vlow begins ramping in response to the rising edge of the gradient coil current command and reaches the steady-state value (and voltage value) of +Vlow at the same time that the steady-state gradient coil current (and voltage value) is reached. However, the inventors have recognized and appreciated that there may be factors that prevent the voltage +Vlow from reaching a sufficient voltage level when supplying the steady-state current. Transition waveform 1506 illustrates a more realistic transition of +Vlow, which has a time delay (latency) in response to the gradient coil current command. As shown in FIG15D , transition waveform 1506 begins ramping only a period of time after the rising edge of the current command. The slope of transition waveform 1506 may be limited because the power converter 1403 may include an output filter (e.g., a capacitor) that limits the rate at which the power converter 1403 can change the voltage of +Vlow. Therefore, when the steady state gradient coil current and voltage are reached, the transition waveform 1506 may not have reached a sufficient voltage level, which may result in the low voltage supply terminal +Vlow being at least temporarily unable to supply the steady state current.

To address this, in some embodiments, power converter 1403 (or 1404) can begin ramping up the voltage magnitude of +Vlow (or -Vlow) prior to the rising edge of the gradient coil current command. FIG15D illustrates a transition waveform 1508 of +Vlow that begins ramping up prior to the rising edge of the gradient coil current command. To begin the transition prior to the rising edge of the gradient coil current command, controller 1308 can receive information from controller 106 regarding an upcoming gradient coil current pulse and begin ramping up the voltage magnitude of +Vlow (or -Vlow) in anticipation of the current pulse. This information can be provided from controller 106 to controller 1308 in any suitable manner. For example, controller 106 can analyze the currently selected gradient coil pulse sequence, determine a power supply voltage level suitable for supplying a steady-state gradient coil current for the next current pulse, and send a voltage command to controller 1308 prior to the anticipated current command. Power converter 1403 (or 1404) can then respond to the received voltage command and begin ramping +Vlow (or -Vlow) to the commanded voltage value. As another example of providing information to controller 1308, controller 106 can send the currently selected pulse sequence or a portion of the pulse sequence to controller 1308. Controller 1308 can then analyze the pulse sequence and send a command to power converter 1403 (or 1404) to begin ramping +Vlow (or -Vlow) before the gradient coil current pulse. In the example of FIG. 15D , power converter 1403 begins ramping the voltage of +Vlow before the rising edge of the current command in response to the voltage command provided by controller 106 to controller 1308. As a result, transition waveform 1508 reaches a +Vlow level sufficient to provide the steady-state current when the steady-state current level is reached.

FIG16A shows an embodiment of an output stage 303F having a single-ended linear amplifier with a variable low voltage supply terminal +Vlow, similar to FIG11. As with the embodiment of FIG14A, a power converter 1403 supplies a variable voltage to the low voltage supply terminal +Vlow that can be set slightly higher than the voltage required to supply the commanded steady-state gradient coil current.

As described above in conjunction with Figures 14A and 14B, the high voltage supply terminal +Vhigh can be separate from the power supply terminal Vhigh_Supply as shown in Figure 16A, or can be the same terminal as Vhigh_Supply as shown in Figure 16B. In Figure 16B, an example of an output stage 303G that provides +Vhigh from the power supply terminal Vhigh_Supply is shown. Providing the voltage +Vhigh from the existing power supply terminal Vhigh_Supply can avoid the need to generate an additional power supply voltage, which can simplify the design and implementation of the output stage.

In some embodiments, one or more low voltage supply terminals and one or more high voltage supply terminals can each have a variable voltage. For example, the embodiments of FIG. 14 or FIG. 11 can be modified so that the high voltage supply terminals +Vhigh and/or -Vhigh are variable voltages generated by a power converter. Such a power converter can be similar to power converters 1403 and 1404 and can also be controlled by controller 1308. Such embodiments can be used for any suitable type of imaging and can be particularly suitable for diffusion-weighted imaging, for example, where relatively large currents (e.g., 40A, 50A, 70A, 90A or greater, or any value therebetween) may be required.

In some embodiments, one or more additional power supply terminals can supply power to the linear amplifier. For example, a third power supply terminal can be provided that has a higher voltage (e.g., at least 5 times or at least 10 times higher, or even 20 times or 30 times higher, or higher, or any range between such values) than the high voltage supply terminal +Vhigh. Adding a third power supply terminal can help improve efficiency when a wide range of voltages needs to be generated. Any number of power supply terminals can be provided, as the technology described herein is not limited in this respect.

Having thus described several aspects and embodiments of the technology set forth in this disclosure, it will be understood that those skilled in the art will readily conceive of various changes, modifications, and improvements. Such changes, modifications, and improvements are intended to be within the spirit and scope of the technology described herein. For example, a person of ordinary skill in the art will readily come up with various other means and/or structures for performing functions and/or obtaining results and/or one or more advantages described herein, 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 determine many equivalents of the specific embodiments described herein using no more than routine experimentation. Therefore, it is to be understood that the foregoing embodiments are presented only as examples, and within the scope of the appended claims and their equivalents, inventive embodiments may be implemented in other ways than those specifically described. In addition, if two or more features, systems, articles, materials, equipment, and/or methods described herein do not contradict each other, any combination of such features, systems, articles, materials, equipment, and/or methods is included within the scope of this disclosure.

The above embodiments can be implemented in any of many ways. One or more aspects and embodiments of the execution of the process or method of the present disclosure can be performed or controlled by program instructions executable by a device (e.g., a computer, a processor or other device). In this regard, various inventive concepts can be embodied as a computer-readable storage medium (or multiple computer-readable storage media) (e.g., a computer memory, one or more floppy disks, compact disks, optical disks, magnetic tapes, flash memories, circuit configurations in field programmable gate arrays or other semiconductor devices, or other tangible computer storage media) encoded with one or more of the following programs, wherein, when executed on one or more computers or other processors, the one or more programs execute one or more methods implementing the various embodiments described above. The one or more computer-readable media can be portable so that the one or more programs stored thereon can be loaded onto one or more different computers or other processors to implement the various aspects described above. In some embodiments, the computer-readable medium can be a non-transient medium.

In this document, the term "program" or "software" is used in a general sense to refer to any type of computer code or set of computer-executable instructions that can be used to program a computer or other processor to implement the various aspects described above. In addition, it should be understood that, according to one aspect, one or more computer programs that, when executed, perform the methods of the present disclosure need not reside on a single computer or processor, but can be distributed in a modular form among multiple different computers or processors to implement various aspects of the present disclosure.

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

Furthermore, the data structure can be stored in a computer-readable medium in any suitable form. To simplify the description, the data structure can be shown as having fields that are related by their position in the data structure. Such relationships can also be achieved by allocating storage for the fields using positions in the computer-readable medium that convey the relationship between the fields. However, any suitable mechanism can be used to establish the relationship between the information in the fields of the data structure, including through the use of pointers, tags, or other mechanisms that establish relationships between data elements.

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.

Furthermore, it should be understood that the computer may be implemented in any of a variety of forms, such as, by way of non-limiting example, a rack-mounted computer, a desktop computer, a laptop computer, or a tablet computer. Furthermore, the computer may be embedded in a device not generally considered a computer but having suitable processing capabilities, including a personal digital assistant (PDA), a smart phone, or any other suitable portable or fixed electronic device.

In addition, the computer can have one or more input devices and output devices. Among other things, these devices can be used to present a user interface. Examples of output devices that can be used to provide a user interface include a printer or display screen for visual presentation output and a loudspeaker or other sound generating device for audible presentation output. Examples of input devices that can be used for a user interface include a keyboard and a pointing device, such as a mouse, a touchpad, and a digitizing tablet computer. As another example, the computer can receive input information through speech recognition or other audible formats.

Such computers may be interconnected in one or more networks of any suitable form, including local or wide area networks such as enterprise networks, 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.

Furthermore, as described, some aspects may be embodied as one or more methods. The actions performed as part of a method may be ordered in any suitable manner. Thus, embodiments may be constructed in which the actions are performed in a different order than that shown, which may include performing certain actions simultaneously, even when shown as sequential actions in illustrative embodiments.

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

Unless explicitly indicated to the contrary, the indefinite articles "a" and "an" as used in this specification and the claims should be understood to mean "at least one".

The phrase "and/or" as used in this specification and claims should be understood to refer to "one or both" of the elements so connected, i.e., in some cases present in conjunction and in other cases present separately. Multiple elements listed with "and/or" should be interpreted in the same manner, i.e., "one or more" of the elements so connected. Other elements may optionally be present in addition to the elements specifically identified by the "and/or" clause, whether related or unrelated to the specifically identified elements. Thus, as a non-limiting example, when used in conjunction with open language such as "comprising," a reference to "A and/or B" may, in one embodiment, refer to only A (optionally including elements in addition to B); in another embodiment, refer to only B (optionally including elements in addition to A); in another embodiment, refer to A and B (optionally including other elements); and so on.

As used in this specification and claims, the phrase "at least one" with respect 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 element of each and every element specifically listed in the list of elements and not excluding any combination of elements in the list of elements. This definition also allows for the optional presence of elements in addition to the elements specifically identified in the list of elements to which the phrase "at least one" refers, whether related or unrelated to those specifically identified elements. Thus, as a non-limiting example, in one embodiment, "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") can mean at least one, optionally including more than one A but no B (and optionally including elements other than B); in another embodiment, can mean at least one, optionally including more than one B but no A (and optionally including elements other than A); in another embodiment, can mean 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 so on.

In addition, the words and terms used herein are for descriptive purposes and should not be regarded as limiting. The use of "includes," "comprising," or "having," "containing," "involving," and variations thereof herein is intended to encompass the items listed thereafter and their equivalents as well as additional items.

In the claims and the foregoing description, all conjunctions such as "include," "comprising," "carrying," "having," "containing," "involving," "having," "including," etc. are to be understood as open-ended, meaning including but not limited to the following. Only the conjunctions "consisting of" and "consisting essentially of" are to be considered closed or semi-closed conjunctions, respectively.

Claims (23)

1. An apparatus for providing power to operate at least one gradient coil of a magnetic resonance imaging system, the apparatus comprising: A linear amplifier configured to generate an output that drives the at least one gradient coil according to a pulse sequence; At least one power converter, the at least one power converter being configured to generate a variable power supply voltage to power the linear amplifier; and At least one controller is configured to control at least one power converter to change the variable power supply voltage based on the output of the linear amplifier. The at least one power converter includes a switching power converter configured to switch at a switching frequency higher than the Larmor frequency associated with the _B0 field strength of the magnetic resonance imaging system. 2. The apparatus of claim 1, wherein the pulse sequence comprises a plurality of gradient pulses, and wherein the at least one controller is configured to change the variable power supply voltage in accordance with the power demand required to supply power to the at least one gradient coil according to the pulse sequence. 3. The apparatus of claim 2, wherein the at least one controller is configured to increase the variable power supply voltage in response to the rising edge of each of the plurality of gradient pulses. 4. The apparatus of claim 3, wherein the at least one controller is configured to decrease the variable power supply voltage in response to the falling edge of each of the plurality of gradient pulses. 5. The apparatus of claim 1, wherein the at least one power converter comprises a first power converter and a second power converter, the first power converter being configured to generate a variable positive power supply voltage to supply power to the linear amplifier, and the second power converter being configured to generate a variable negative power supply voltage to supply power to the linear amplifier. 6. The apparatus of claim 5, wherein the at least one controller is configured to control the second power converter to change the variable negative power supply voltage based on the output voltage of the linear amplifier. 7. The apparatus of claim 6, wherein the pulse sequence comprises a plurality of gradient pulses, and wherein the at least one controller is configured to change the variable positive power supply voltage and the variable negative power supply voltage in accordance with the power demand required to supply power to the at least one gradient coil according to the pulse sequence. 8. The apparatus according to claim 1, wherein the magnetic resonance imaging system is a low-field magnetic resonance imaging system. 9. A method for supplying power to at least one gradient coil of a magnetic resonance imaging system using a linear amplifier, the linear amplifier being configured to supply current to the at least one gradient coil according to a pulse sequence to generate a magnetic field, the method comprising: Convert at least one fixed power supply voltage to at least one variable power supply voltage to power the linear amplifier; The at least one variable power supply voltage is changed based on the output of the linear amplifier; The linear amplifier is controlled to generate an output that drives the at least one gradient coil according to the pulse sequence; and The linear amplifier is powered by at least one fixed power supply converted into at least one variable power supply voltage via a switching power converter configured to switch at a switching frequency higher than the Larmor frequency associated with the _B0 field strength of the magnetic resonance imaging system. 10. The method of claim 9, wherein the pulse sequence comprises a plurality of gradient pulses, and wherein changing the at least one variable power supply voltage comprises: changing the at least one variable power supply voltage in accordance with a change in the power demand required to supply power to the at least one gradient coil according to the pulse sequence. 11. The method of claim 10, wherein changing the at least one variable power supply voltage comprises: increasing the at least one variable power supply voltage in response to the rising edge of each of the plurality of gradient pulses. 12. The method of claim 11, wherein changing the at least one variable power supply voltage comprises: decreasing the at least one variable power supply voltage in response to the falling edge of each of the plurality of gradient pulses. 13. A magnetic resonance imaging system, comprising: _B0 magnet, the _B0 magnet being configured to generate a _B0 magnetic field; At least one gradient coil; and At least one electrical component, configured to provide power to operate the at least one gradient coil, the at least one electrical component comprising: A linear amplifier configured to generate an output that drives the at least one gradient coil according to a pulse sequence; At least one power converter, the at least one power converter being configured to generate a variable power supply voltage to power the linear amplifier; and At least one controller is configured to control at least one power converter to change the variable power supply voltage based on the output of the linear amplifier. The at least one power converter includes a switching power converter configured to switch at a switching frequency higher than the Larmor frequency associated with the _B0 field strength of the magnetic resonance imaging system. 14. The magnetic resonance imaging system of claim 13, wherein the _B0 magnet, when operated, is configured to generate a _B0 magnetic field having a field strength equal to or less than 0.2T and greater than or equal to 0.1T. 15. The magnetic resonance imaging system of claim 13, wherein the _B0 magnet, when operated, is configured to generate a _B0 magnetic field having a field strength equal to or less than 0.1 T and greater than or equal to 50 mT. 16. The magnetic resonance imaging system of claim 13, wherein the _B0 magnet, when operated, is configured to generate a _B0 magnetic field having a field strength equal to or less than 50 mT and greater than or equal to 20 mT. 17. The magnetic resonance imaging system of claim 13, wherein the _B0 magnet, when operated, is configured to generate a _B0 magnetic field having a field strength equal to or less than 20 mT and greater than or equal to 10 mT. 18. The magnetic resonance imaging system of claim 13, wherein the at least one gradient coil includes at least one first gradient coil, at least one second gradient coil, and at least one third gradient coil to provide three-dimensional encoding, and wherein the at least one power component is configured to provide power to the at least one first gradient coil, the at least one second gradient coil, and the at least one third gradient coil. 19. An apparatus for driving at least one gradient coil in a magnetic resonance imaging system, the apparatus comprising: A switching power converter configured to switch at a switching frequency higher than the Larmor frequency associated with the _B0 field strength of the magnetic resonance imaging system; and A controller configured to control the switching power converter to drive the at least one gradient coil according to a pulse sequence. 20. The apparatus of claim 19, wherein the _B0 field strength is equal to or less than 0.2T and greater than or equal to 0.1T. 21. The apparatus of claim 19, wherein the _B0 field strength is equal to or less than 0.1T and greater than or equal to 50mT. 22. The apparatus of claim 19, wherein the _B0 field strength is equal to or less than 50 mT and greater than or equal to 20 mT. 23. The apparatus of claim 19, wherein the _B0 field strength is equal to or less than 20 mT and greater than or equal to 10 mT.