WO2018114554A1

WO2018114554A1 - Dixon-type water/fat separation mr imaging

Info

Publication number: WO2018114554A1
Application number: PCT/EP2017/082736
Authority: WO
Inventors: Holger Eggers; Christian Stehning; Gabriele Marianne Beck; Gerrit Hendrik VAN IJPEREN
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2016-12-20
Filing date: 2017-12-14
Publication date: 2018-06-28
Anticipated expiration: 2019-06-20

Abstract

The invention relates to a method of Dixon-type MR imaging. The method comprises the steps of: subjecting the object (10) to an imaging sequence (31) comprising a series of refocusing RF pulses, wherein two echo signals are generated in each time interval between two consecutive refocusing RF pulses, acquiring the echo signals from the object (10) using a bipolar pair of readout magnetic field gradients, wherein the two readout magnetic field gradients have different strengths such that the corresponding two echo signals are acquired at different receive bandwidths, and reconstructing a MR image from the acquired echo signals, whereby signal contributions from water protons and fat protons are separated. Moreover the invention relates to a MR device (1) and to a computer program to be run on a MR device (1).

Description

Dixon-type water/fat separation MR imaging

FIELD OF THE INVENTION

The invention relates to the field of magnetic resonance (MR) imaging. It concerns a method of MR imaging of a portion of a body placed in the examination volume of a MR device. The invention also relates to a MR device and to a computer program to be run on a MR device.

BACKGROUND OF THE INVENTION

Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.

According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field Bo whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field Bo produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are defiected from the z-axis to the transverse plane (flip angle 90°).

After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant Ti (spin-lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T₂ (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the

magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal in the receiving coils.

In order to realize spatial resolution in the body, constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field Bo, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils correspond to the spatial frequency domain and are called k-space data. The k-space data usually include multiple lines acquired with different phase encoding. Each k-space line is digitized by collecting a number of samples. A set of k-space data is converted to an MR image, e.g., by means of Fourier transformation.

In MR imaging, it is often desired to obtain information about the relative contribution of water and fat to the overall signal, either to suppress the contribution of one of them or to separately or jointly analyse the contribution of both of them. These contributions can be calculated if information from two or more corresponding echoes, acquired at different echo times, is combined. This may be considered as chemical shift encoding, in which an additional dimension, the chemical shift dimension, is defined and encoded by acquiring two or more MR images at slightly different echo times. For water- fat separation, these types of experiments are often referred to as Dixon-type of measurements. By means of Dixon MR imaging or Dixon water/fat MR imaging, a water- fat separation is achieved by calculating contributions of water and fat from two or more corresponding echoes, acquired at different echo times. In general such a separation is possible because there is a known precessional frequency difference of hydrogen in fat and water. In its simplest form, water and fat images are generated by either addition or subtraction of the 'in- phase' and 'out-of-phase' datasets. Several Dixon-type MR imaging methods have been proposed in recent years. Apart from different strategies for the water/fat separation, the known techniques are mainly characterized by the specific number of echoes (or 'points') they acquire and by the constraints that they impose on the used echo times. Conventional so-called two- and three- point methods require in-phase and opposed-phase echo times at which the water and fat signals are parallel and anti-parallel in the complex plane, respectively. Three-point methods have gradually been generalized to allow flexible echo times. Thus, they do not restrict the angle or phase between the water and fat signals at the echo times to certain values anymore. In this way, they provide more freedom in imaging sequence design and enable in particular a trade-off between signal-to-noise ratio (SNR) gains from the acquisition and SNR losses in the separation. Sampling only two instead of three echoes is desirable to reduce scan time. However, constraints on the echo times may actually render dual-echo acquisitions slower than triple-echo acquisitions. Eggers et al. (Magnetic Resonance in Medicine, 65, 96-107, 2011) have proposed a dual-echo flexible Dixon-type MR imaging method which enables the elimination of such constraints. Using such Dixon-type MR imaging methods with more flexible echo times, in-phase and opposed-phase images are no longer necessarily acquired, but optionally synthesized from water and fat images.

Dixon-type MR imaging methods are often applied in combination with fast (turbo) spin echo sequences using multiple repetition or multiple acquisition approaches. Typically, two or three interleaved measurements with shifted readout magnetic field gradients and acquisition windows are employed. In Fig. 2, a schematic pulse sequence diagram of a conventional turbo spin echo (TSE) Dixon sequence is depicted. The diagram shows switched magnetic field gradients in the frequency-encoding direction (M), the phase- encoding direction (P) and the slice-selection direction (S). Moreover, the diagram shows RF excitation and refocusing pulses as well as the time intervals during which echo signals are acquired, designated by ACQ. The diagram covers the acquisition of the first three echo signals of one shot of the imaging sequence. The double arrows indicate the shifting of the readout magnetic field gradients (top) and the acquisition windows ACQ (bottom) between multiple repetitions of one shot with identical phase encoding. According to the shifting of the readout magnetic field gradients, different phase offsets of the signal contributions from water protons and fat protons, respectively, are obtained on which the Dixon-type water/fat separation is based.

In comparison to standard (non-Dixon) TSE sequences, Dixon TSE techniques provide superior fat suppression and multiple contrasts in a single acquisition. However, because of the required multiple repetitions of each shot with identical phase encoding, scan time increases. Moreover, scan efficiency decreases due to the dead times introduced to permit shifting the readout magnetic field gradients and the acquisition windows.

Alternatively, the echo spacing increases, and longer or more echo trains are needed. This results in less coverage and more blurring in the reconstructed MR images, or again in longer scan time.

US 2016/0033605 Al discloses a dual-echo Dixon TSE technique, in which two echo signals are generated at an opposed-phase and an in-phase echo time, respectively, in each time interval between two consecutive refocusing RF pulses. The echo signals are only partially acquired using a bipolar pair of readout magnetic field gradients in order to reduce the temporal spacing between these two echo signals and thus the time interval between two consecutive refocusing RF pulses and the T₂ decay over the echo trains. The US-patent application US2016/033606 mentions a respective imaging sequence with a low- bandwidth and a high-bandwidth for acquisition of the in-phase echo and the partially out of phase second echo.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method that enables a further improved Dixon water/fat separation in combination with a TSE acquisition.

In accordance with the invention, a method of MR imaging of an object placed in an examination volume of a MR device is disclosed. The method comprises the following steps:

subjecting the object to an imaging sequence comprising a series of refocusing RF pulses, wherein two echo signals are generated in each time interval between two consecutive refocusing RF pulses,

acquiring the echo signals from the object using a bipolar pair of readout magnetic field gradients, wherein the two readout magnetic field gradients have different strengths such that the corresponding two echo signals are acquired at different receive bandwidths, and

reconstructing a MR image from the acquired echo signals, whereby signal contributions from water protons and fat protons are separated.

According to the invention, a dual-echo TSE imaging sequence is used to acquire two echo signals in each interval between two refocusing RF pulses. The timing and strength of the bipolar readout gradients is chosen to shift the acquisition windows of the two echo signals such that appropriate phase offsets of the signal contributions from water protons and fat protons are obtained, on which the Dixon-type separation of these signal contributions is based in the reconstruction step.

The essential feature of the invention is that the two readout magnetic field gradients of the bipolar pair have different strengths such that the corresponding two echo signals are acquired at different receive bandwidths. The different strengths of the two readout magnetic field gradients introduce asymmetric echo shifts, which is favourable in terms of robustness of the water-fat separation, while preserving the total sampling duration, which is favourable in terms of SNR Asymmetric echo shifts mean that the two echo signals are shifted in time by different amounts relative to the spin echo that would normally appear in the centre of the interval between two consecutive refocusing RF pulses. The echo shifts can be selected individually for each echo signal as required by appropriately choosing the respective magnetic field gradient strength.

The acquisition of one of the two echo signals can be similar as in a standard TSE sequence, except that water and fat signals can be partially out-of-phase. The acquisition of the other echo signal can be performed at a higher receive bandwidth. Ideally, both acquisitions together cover a symmetric time interval around the spin echo to maximize scan efficiency for a given time interval between two consecutive refocusing RF pulses.

In a preferred embodiment of the invention, the second readout magnetic field gradient of the bipolar pair is stronger than the first readout magnetic field gradient of the bipolar pair. In this way, the acquisition window is longer for the first echo signal, and the first echo signal is closer to the spin echo.

In another preferred embodiment of the invention, at least one of the two echo signals is acquired only partially. This means that k-space is sampled only partially in either the positive or negative readout direction of k-space for the respective echo. Both echo signals may be sampled only partially towards the centre of the interval between two successive refocusing RF pulses. In this way, smaller differences in the echo shifts can be obtained, which is particularly beneficial at higher main magnetic field strengths (3 Tesla or more). Moreover, the interval between two successive refocusing RF pulses can be shortened to reduce scan time and T₂ decay over the echo trains.

The dual-echo TSE imaging sequence according to the invention is preferably complemented by a phase-preserving partial echo reconstruction that corrects, as part of the water/fat separation, for the fat shift and the geometric distortion due to main magnetic field inhomogeneities in opposite directions with unequal magnitude in the two single-echo images.

In a further preferred embodiment of the invention, a phase-encoding magnetic field gradient is switched between the acquisitions of the two echo signals. In this way, cor- responding echoes, acquired with the same phase-encoding but different chemical shift encoding, do not have to be sampled in immediate succession. This can be favourable in terms of sensitivity to motion and to free induction decay (FID) signal.

In yet another preferred embodiment of the invention, the strengths of the two readout magnetic field gradients are determined automatically. For this purpose, an optimization of the expected signal-to-noise ratio (SNR) in a MR image reconstructed from the acquired echo signals is performed. This exploits that the SNR in the single-echo images, i.e. the MR images reconstructed from the acquired first and second echo signals, respectively, simply scales with the inverse of the square root of the corresponding receive bandwidth. Moreover, it relies on a description of the noise propagation through the water/fat separation by the noise co variance matrix for the water and fat images, which can be derived analytically in simple cases and can be estimated numerically otherwise. Starting from a given time interval for the acquisition between two successive refocusing RF pulses, it then involves systematically varying the strengths of the two readout magnetic field gradients, calculating the expected SNR in the water image, the fat image, or in any combination of them, and recording the maximum. Optionally, the partial echo factor(s) can be included in this optimization. The partial echo factor can be defined as the ratio of the full k-space size (associated with the respective field-of-view and required resolution) and the reduced k-space size covered by the partial acquisition of the echo signals.

In a further preferred embodiment of the invention, the echo signals are acquired as a plurality of k-space blades according to a PROPELLER scheme, each k-space blade comprising a number of substantially parallel k-space lines, wherein the k-space blades are rotated about the center of k-space, so that the total acquired data set of MR signals spans at least part of a circle in k-space, a common central circular region of k-space being covered by all k-space blades. PROPELLER imaging offers several advantages for clinical MR imaging like robustness against patient motion and intrinsic motion compensation

capabilities. In the PROPELLER concept (Periodically Rotated Overlapping ParalEL Lines, see James G. Pipe: "Motion Correction With PROPELLER MRI: Application to Head Motion and Free-Breathing Cardiac Imaging", Magnetic Resonance in Medicine, vol. 42, 1999, pages 963-969), MR signal data are acquired in k-space in N strips, each consisting of L parallel k-space lines, corresponding to the L lowest frequency phase-encoding lines in a Cartesian-based k-space sampling scheme. Each strip, which is also referred to as k-space blade, is rotated in k-space by an angle of, for example, 180°/N, so that the total MR data set spans a circle in k-space. If a full k-space data matrix having a diameter M is desired, then L and N may be chosen so that L*N=M*n/2. One essential characteristic of PROPELLER is that a central circular portion in k-space, having a diameter L, is acquired for each k-space blade. This central portion can be used to reconstruct a low-resolution MR image for each k- space blade. The low-resolution MR images can be compared to each other to remove in- plane displacements and phase errors, which are due to patient motion. The PROPELLER technique makes use of oversampling in the central portion of k-space in order to obtain an MR image acquisition technique that is robust with respect to motion of the examined patient during MR signal acquisition. Moreover, due to the (weighted) averaging of k-space blades PROPELLER 'averages out' further imaging artefacts resulting from, for example, Bo inhomogeneities or inaccurate coil sensitivity maps when parallel imaging techniques like SENSE are used for MR data acquisition. PROPELLER imaging is well- suited to be combined with the TSE acquisition scheme of the invention, wherein a complete k-space blade can be acquired after a single RF excitation.

The method of the invention described thus far can be carried out by means of a MR device including at least one main magnet coil for generating an essentially uniform, static magnetic field Bo within an examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one body RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from a body of a patient positioned in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit for reconstructing MR images from the received MR signals. The method of the invention can be implemented by a corresponding programming of the reconstruction unit and/or the control unit of the MR device.

The method of the invention can be advantageously carried out on most MR devices in clinical use at present. To this end it is merely necessary to utilize a computer program by which the MR device is controlled such that it performs the above-explained method steps of the invention. The computer program may be present either on a data carrier or be present in a data network so as to be downloaded for installation in the control unit of the MR device. BRIEF DESCRIPTION OF THE DRAWINGS

The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

Figure 1 shows a MR device for carrying out the method of the invention;

Figure 2 shows a schematic (simplified) pulse sequence diagram of a conventional TSE Dixon imaging sequence;

Figure 3 shows a schematic (simplified) pulse sequence diagram according to a first embodiment of the invention;

Figure 4 shows a schematic (simplified) pulse sequence diagram according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference to Figure 1, a MR device 1 is shown as a block diagram. The device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field Bo is created along a z-axis through an examination volume. The device further comprises a set of (1^st, 2^nd, and - where applicable - 3^rd order) shimming coils 2', wherein the current flow through the individual shimming coils of the set 2' is controllable for the purpose of minimizing Bo deviations within the examination volume.

A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.

More specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the

examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send/receive switch 8, to a body RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which, together with any applied magnetic field gradients, achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate resonance, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the body RF coil 9.

For generation of MR images of limited regions of the body 10, a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by transmissions of the body RF coil.

The resultant MR signals are picked up by the body RF coil 9 and/or by the array RF coils 11, 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via the send/receive switch 8.

A host computer 15 controls the shimming coils 2' as well as the gradient pulse amplifier 3 and the transmitter 7 to generate the imaging sequences of the invention. For the selected sequence, the receiver 14 receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

Ultimately, the digital raw image data are reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such as SENSE. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

The host computer 15 and the reconstruction processor 17 are arranged, by corresponding programming, to perform the method of the invention described herein above and in the following.

According to the invention, efficient TSE Dixon imaging is achieved by acquiring the echo signals using a bipolar pair of readout magnetic field gradients, wherein the two readout magnetic field gradients have different strengths such that the corresponding two echo signals are acquired at different receive bandwidths.

This is illustrated in Figure 3. Figure 3 shows a pulse sequence diagram of a dual-echo TSE sequence constituting an imaging sequence according to the invention. The diagram shows switched magnetic field gradients in the frequency-encoding direction (M), the phase-encoding direction (P) and the slice-selection direction (S). Moreover, the diagram shows the RF excitation and refocusing pulses as well as the time intervals during which echo signals are acquired, designated by ACQ. A pair of echo signals is acquired in each time interval between two consecutive refocusing RF pulses. The pairs of echo signals are acquired using bipolar readout magnetic field gradients. Each pair of echo signals is acquired using a corresponding pair of readout magnetic field gradients having opposed polarities and different (absolute) strengths. The first echo signal is acquired during each interval ACQ at a first (low) receive bandwidth to obtain a high SNR. To this end, a comparatively weak readout magnetic field gradient (in the M direction) is chosen. The second readout magnetic field gradient is stronger (and has an opposed polarity) such that the receive bandwidth is correspondingly higher. The different strengths of the two opposed readout magnetic field gradients introduce asymmetric echo shifts. These echo shifts can be selected individually for each echo signal as required by appropriately choosing the respective magnetic field gradient strength. In the depicted embodiment, dead times are avoided by choosing different strengths for the two readout magnetic field gradients. In this way, the sampling duration and thus the scan efficiency is maximized. At the same time, asymmetric echo shifts are obtained, which are essential to ensure a sufficient robustness of the water-fat separation.

As can further be seen in Figure 3, the zeroth moment of the readout magnetic field gradient over the interval between two consecutive RF pulses equals zero. The zeroth moment of the readout magnetic field gradient is nulled by a preceding dephasing magnetic field gradient pulse and a subsequent re-phasing magnetic field gradient pulse in the readout direction (M), which both temporally coincide with the phase-encoding magnetic field gradient (P) and a spoiler magnetic field gradient in slice direction (S). In this way, the TSE echo spacing is kept as short as possible. Preferably, the two echo signals are acquired only partially in each interval. Both echo signals are sampled only partially towards the centre of the interval between two successive refocusing RF pulses in this case. In this way, the temporal spacing between these two echo signals and thus the time interval between two consecutive refocusing RF pulses and the T₂ decay over the echo trains is reduced.

In the embodiment shown in Figure 4, a phase-encoding magnetic field gradient ('blip') is introduced between the two opposed readout magnetic field gradients, such that two different k-space lines are acquired in each interval between two consecutive refocusing RF pulses. Corresponding echoes, acquired with the same phase-encoding but different chemical shift encoding, are thus not sampled in immediate succession. This is favourable in terms of sensitivity to motion artefacts and to free induction decay (FID) signal. Moreover, it allows to sample k-space differently for the two echo signals, which can be advantageous in combination with compressed sensing, among others.

Claims

CLAIMS:

1. Method of MR imaging of an object (10) placed in an examination volume of a MR device (1), the method comprising the steps of:

subjecting the object (10) to an imaging sequence (31) comprising a series of refocusing RF pulses, wherein two echo signals are generated in each time interval between two consecutive refocusing RF pulses,

acquiring the echo signals from the object (10) using a bipolar pair of readout magnetic field gradients, wherein the two readout magnetic field gradients have different strengths such that the corresponding two echo signals are acquired at different receive bandwidths, and

- reconstructing a MR image from the acquired echo signals, whereby signal contributions from water protons and fat protons are separated.

2. Method of claim 1 wherein

the readout magnetic field gradient's zeroth moment in each time interval between two consecutive refocusing RF pulses is nulled by a preceding dephasing magnetic field gradient pulse and a subsequent re-phasing magnetic field gradient pulse in the readout direction (M), which both temporally coincide with the phase-encoding magnetic field gradient (P) and a spoiler magnetic field gradient in slice direction (S) 3. Method of claim 1 or 2, wherein the second readout magnetic field gradient of the bipolar pair is stronger than the first readout magnetic field gradient of the bipolar pair.

4. Method of any one of claims lto 3, wherein at least one of the two echo signals is acquired only partially.

5. Method of any one of claims 1 to 4, wherein a phase-encoding magnetic field gradient is switched between the acquisitions of the two echo signals.

6. Method of any one of claims lto 5, wherein the different strengths of the two readout magnetic field gradients are determined automatically based on an optimization of the expected signal-to-noise ratio in a MR image reconstructed from the acquired echo signals.

7. Method of claims 4 and 6, wherein the partial echo factors are determined automatically based on the optimization.

8. Method of any one of claims lto 7, wherein the echo signals are acquired as a plurality of k-space blades according to a PROPELLER scheme, each k-space blade comprising a number of substantially parallel k-space lines, wherein the k-space blades are rotated about the center of k-space, so that the total acquired data set of MR signals spans at least part of a circle in k-space, a common central circular region of k-space being covered by all k-space blades.

9. MR device comprising at least one main magnet coil (2) for generating a uniform, static magnetic field Bo within an examination volume, a number of gradient coils (4, 5, 6) for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil (9) for generating RF pulses within the examination volume and/or for receiving MR signals from an object (10) positioned in the examination volume, a control unit (15) for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit (17) for reconstructing MR images from the received MR signals, wherein the MR device (1) is configured to perform the steps of the method of any one of claims lto 8.

10. Computer program to be run on a MR device, which computer program comprises instructions for executing the method of any one of claims 1-8.