Dixon-style water / fat separation MR imaging

Abstract

The present invention relates to a Dixon-type MR imaging method, in which an imaging sequence of at least two shots is performed on an object 10, each shot including an excitation RF pulse and a subsequent series of refocusing RF pulses, and at least one pair of phase-encoded echoes, i.e., a first echo at a first echo time and a second echo at a second echo time, is generated in each time interval between two successive refocusing RF pulses. Two sets of echo signal pairs, i.e., a first set and a second set, are acquired in two respective shots of the imaging sequence using a bipolar pair of readout magnetic gradients. The bipolar pair of readout magnetic gradients in the acquisition of the second set has a polarity opposite to that of the bipolar pair of readout magnetic gradients in the acquisition of the first set. Alternatively or additionally, the time trajectory of the readout magnetic gradients in the acquisition of the second set is reversed with respect to that of the readout magnetic gradients in the acquisition of the first set. Alternatively or additionally, the first and second sets of acquisitions differ from each other in terms of the gradient areas of the magnetic field gradients in the readout direction M that precede and follow the bipolar pair of readout magnetic field gradients, respectively. Finally, an MR image is reconstructed from the first and second sets of acquired echo signal pairs, thereby separating signal contributions from water protons and fat protons. Furthermore, the present invention relates to an MR apparatus 1 and a computer program executed on the MR apparatus 1.

Description

Technical Field

[0001] The present invention relates to the field of magnetic resonance (MR) imaging. It relates to MR imaging of an object placed within the examination volume of an MR device. The present invention also relates to an MR device and a computer program executed on the MR device.

Background Art

[0002] For forming two-dimensional or three-dimensional images, the imaging MR method that utilizes the interaction between a magnetic field and nuclear spins is superior to other imaging methods in many respects for soft tissue imaging, does not require ionizing radiation, and is usually non-invasive. Therefore, it is currently widely used, especially in the field of medical diagnosis.

[0003] According to a general MR method, the body of the patient to be examined is placed within a strong and uniform magnetic field B0, and the direction of the magnetic field B0 simultaneously defines the axis of a coordinate system (usually the z-axis), and measurements are made based on this coordinate system. The magnetic field B0 generates different energy levels for individual nuclear spins depending on the intensity of the magnetic field, and these can be excited (spin resonance) by applying an electromagnetic field (RF electromagnetic field) with a frequency (Larmor frequency) defined in the radio frequency region. From a macroscopic perspective, the overall magnetization is generated by the distribution of individual nuclear spins, and this magnetization can be deflected from the equilibrium state by applying an electromagnetic pulse (RF pulse) with an appropriate frequency perpendicular to the z-axis. As a result, the magnetization performs a precession motion centered on the z-axis. The precession motion traces the surface of a cone, and the opening angle of this cone is called the flip angle. The magnitude of the flip angle depends on the intensity and time width of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spin is deflected from the z-axis to the transverse plane (90° flip angle).

[0004] After the RF pulse ends, the magnetization relaxes and returns to its original equilibrium state. The magnetization in the z-direction is rebuilt with a first time constant T1 (spin-lattice relaxation time or longitudinal relaxation time), and the magnetization perpendicular to the z-direction relaxes with a second time constant T2 (spin-spin relaxation time or transverse relaxation time). The magnetization fluctuations are measured in a direction perpendicular to the z-axis and can be detected by an oriented receiving RF coil placed within the inspection volume of the MR device. The decay of transverse magnetization results in, for example, after applying a 90° pulse, a transition (diffusing) of nuclear spins (induced by local magnetic field inhomogeneity) from an ordered state with the same phase to a state with uniformly dispersed phases. Diffusing can be compensated for by a refocusing pulse (e.g., a 180° pulse). This generates an echo signal in the receiving coil.

[0005] To achieve spatial resolution within the body, constant magnetic field gradients extending along three principal axes are superimposed on a uniform magnetic field B0, resulting in a linear spatial dependence of the Larmor frequency. Therefore, the signal picked up by the receiving coil contains components of different frequencies that can be associated with different locations within the body. The signal data acquired via the receiving coil corresponds to the spatial frequency domain and is called k-space data. k-space data typically includes data from multiple lines in k-space acquired with different phase encodings. Each k-space line is digitized by collecting several samples. A set of samples from multiple lines in k-space is then transformed into an MR image, for example, by a Fourier transform.

[0006] In MR imaging, it is often desirable to obtain information about the relative contributions of water and fat to the overall signal, either to suppress the contribution of one or the other, or to analyze the contributions of both water and fat separately or together. These contributions can be calculated by combining information from two or more corresponding echoes acquired at different echo times (with respect to excitation or spin-echo reconvergence). This is considered chemical shift encoding, where 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 called Dixon measurements. Water / fat separation is achieved by Dixon MR imaging or Dixon water / fat MR imaging by calculating the contributions of water and fat from two or more corresponding echoes acquired at different echo times. Generally, such separation is possible because there is a difference in the known precessional frequencies of hydrogen in water and fat. In its simplest form, images of water and fat are generated by either adding or subtracting in-phase and non-in-phase datasets.

[0007] In recent years, several Dixon-type MR imaging methods have been proposed. Besides various strategies for water / fat separation, known techniques are primarily characterized by constraints on the specific number of echoes (or points) acquired and the echo time used. Conventional so-called two-point and three-point methods require in-phase and out-of-phase echo times, where the water and fat signals are parallel and antiparallel, respectively, in the complex plane. Three-point methods have been gradually generalized to allow for more flexible echo times. Therefore, they no longer restrict the angle or phase between the water and fat signals in the echo time to a specific value. In this way, they increase the design flexibility of the imaging sequence, particularly allowing for a trade-off between the signal-to-noise ratio (SNR) gain from acquisition and the SNR loss in separation. To reduce scan time, it is desirable to sample only two echoes instead of three. However, due to constraints on echo time, dual-echo acquisition is actually slower than triple-echo acquisition. Eggers et al. (Magnetic Resonance in Medicine, 65, 96-107, 2011) propose a flexible dual-echo Dixon-type MR imaging method that enables the elimination of such constraints. Using such a Dixon-type MR imaging method with more flexible echo times, in-phase and out-of-phase images are no longer necessarily acquired but can be optionally synthesized from water and fat images.

[0008] Dixon-type MR imaging is often applied in combination with high-speed (turbo) spin-echo sequences using a multiple-repetition technique. Typically, two or three interleaved measurements with shifted readout magnetic field gradients and acquisition windows are employed. Figure 2 shows a schematic pulse sequence diagram of a conventional turbo-spin-echo (TSE) Dixon method sequence. This diagram shows the switching magnetic field gradients in the frequency encoding direction (M), phase encoding direction (P), and slice selection direction (S). Furthermore, this diagram shows the RF excitation pulse and refocusing pulse, as well as the time interval in which the echo signal is acquired, indicated as ACQ. This diagram covers the acquisition of the first three echo signals in one shot of the imaging sequence. Double-headed arrows indicate the shift in the readout magnetic field gradient (top row) and acquisition window ACQ (bottom row) during multiple repetitions of one shot with the same phase encoding. According to the shift in the readout magnetic field gradient, different phase offsets of the signal contributions from water protons and fat protons are acquired, respectively, and Dixon-type water / fat separation is based on this phase offset.

[0009] Compared to standard (non-Dixon) TSE sequences, the Dixon TSE technique provides superior fat suppression and multiple contrasts in a single acquisition. However, it increases scan time because each shot needs to be repeated multiple times with the same phase encoding. Furthermore, scan efficiency decreases because a dead time is introduced to allow for shifts in the readout magnetic field gradient and acquisition window. Alternatively, the echo placement interval increases, requiring longer or more echo trains. As a result, the coverage area is reduced, the reconstructed MR image becomes blurrier, or again, scan time increases.

[0010] Alternatively, multi-echo techniques are pursued, where two or three echoes are acquired instead of one after each RF refocusing pulse. Ma et al. (Magnetic Resonance in Medicine, 58, 103-109, 2007) describe a triple-echo Dixon TSE technique in which three echo signals are generated at each time interval between two consecutive refocusing RF pulses. To shorten the temporal spacing between these three echo signals, and thus reduce the time interval between two consecutive refocusing RF pulses and the T2 decay across the echo train, the echo signals are acquired using a bipolar triple pulse of the readout magnetic field gradient. Figure 3 shows an example of this technique. A pulse sequence diagram of the triple-echo Dixon TSE sequence is drawn, covering the acquisition of the first two triple echo signals in one shot. Three gradient-recalled echoes are acquired after each RF refocusing pulse. The bipolar triple pulse of the readout magnetic field gradient is preceded and followed by a spoiler readout magnetic field gradient. The vertical dashed lines indicate different echo times.

[0011] U.S. Patent Application No. 2016 / 0033605 discloses a dual-echo Dixon TSE technique in which only two echo signals are generated at each time interval between two consecutive refocusing RF pulses. Furthermore, to reduce the temporal spacing between these two echo signals and to further decrease the time interval between the two consecutive refocusing RF pulses and the T2 decay across the echo train, a bipolar pair of readout gradients is used so that the echo signals are only partially acquired. The bipolar pair of readout gradients is preceded by a spoiler readout gradient and followed by a flyback and spoiler readout gradient.

[0012] To improve efficiency, the multi-echo technique must rely on the bipolar readout magnetic field gradient, as shown in Figure 3. This inevitably leads to off-resonance effects due to chemical shifts or heterogeneity of the main magnetic field B0, resulting in opposite-direction distortion in the resulting single-echo image. In addition, eddy currents cause phase errors between single-echo images. Furthermore, the multi-echo technique often requires the use of partial echo sampling, especially at higher main magnetic field strengths, making correction difficult or impossible and negatively impacting image quality.

[0013] U.S. Patent Application No. 2016 / 0033605 relates to a dual-echo Dixon TSE technique in which only two echoes are acquired in the time interval between consecutive refocusing pulses. [Overview of the project] [Problems that the invention aims to solve]

[0014] The objective of the present invention is to provide a method that enables a further improved Dixon water / fat separation method in combination with TSE acquisition. [Means for solving the problem]

[0015] According to the present invention, a method for MR imaging of an object placed within the inspection volume of an MR device is disclosed. A step of performing an imaging sequence of at least two shots on an object (10), wherein each shot includes an excitation RF pulse and a subsequent series of refocusing RF pulses, and at least one pair of phase-encoded echoes, i.e., a first echo at a first echo time and a second echo at a second echo time, are generated at each time interval between two consecutive refocusing RF pulses. The steps include: acquiring a first set of echo signal pairs from the object (10) in the first shot of the imaging sequence using a bipolar pair of readout magnetic field gradients at each iteration interval; A step of acquiring a second set of echo signal pairs from object (10) in a second shot of the imaging sequence, using a bipolar pair of readout magnetic field gradients at each iteration interval, The dipole pair of readout field gradients in the acquisition of the second set has the opposite polarity to the dipole pair of readout field gradients in the acquisition of the first set. The temporal trajectory of the readout magnetic field gradient in the acquisition of the second set is reversed with respect to the temporal trajectory of the readout magnetic field gradient in the acquisition of the first set, and / or The acquisition of the first and second sets is a step in which the gradient areas of the magnetic field gradients in the preceding and succeeding readout directions (M) differ from each other in relation to the bipolar pair of readout magnetic field gradients, respectively. A step of reconstructing an MR image from first and second sets of acquired echo signal pairs, wherein the signal contributions from water protons and lipid protons are separated; It has.

[0016] According to the present invention, two instances (shots) of a dual-echo TSE Dixon imaging sequence are used, and each instance (shot) acquires two echo signals at each interval between two refocusing RF pulses using a bipolar pair of readout magnetic field gradients. Generally, each instance may consist of multiple shots, and each shot may acquire more than two echo signals. However, for the sake of brevity, we will hereafter assume that each instance consists of one shot and that each shot acquires two echo signals. The phase encoding of the echo signals is selected so that each of the acquired first and second sets covers the required region of k-space. Thus, the present invention proposes combining the above multiple-repetition method with the multi-echo method. High acquisition duty cycles are achieved and high image quality is maintained by performing two instances of bipolar dual-echo acquisition, where the polarities of the bipolar pair of readout gradients are opposite, the temporal trajectory of the readout gradients is reversed, and the gradient area (the area under the temporal trajectory of the magnetic field gradient) is transitioned between spoiler readout gradients, flyback readout gradients, and / or defasing gradients that precede and follow the bipolar pair of readout gradients. In other words, reconstruction involves suppressing or removing artifacts arising from bipolar acquisition. More specifically, repeated readouts at opposite polarities of the bipolar gradient lobes provide phase information due to eddy current effects. This phase information is utilized in reconstruction to at least partially remove eddy current effects due to gradient switching of bipolar readouts. Reversing the temporal trajectory of the readout magnetic field pulses improves the conditioning of water / fat separation with different echo times, and can even reduce or eliminate water / fat separation artifacts and / or noise amplification. These embodiments may be incorporated into the reconstruction. Differences in the gradient area of ​​the magnetic field gradient in the preceding and succeeding readout directions (M) for each bipolar pair improve the overall coverage of k-space, particularly for partial echo acquisition. This improves the effective coverage of k-space and improves the conditioning of the conjugation symmetry problem in the reconstruction.

[0017] In a preferred embodiment, some or all of the echo signals from the first set and / or the second set are acquired only partially. That is, the k-space is sampled only partially in either the positive or negative readout direction of the k-space. Both echoes may be sampled only partially between two consecutive refocusing RF pulses, toward the center of the repetition interval, meaning, for example, that the first echo is sampled ahead of the echo, i.e., only in a portion of the positive readout direction, and the second echo is sampled behind the echo, i.e., only in a portion of the negative readout direction of the k-space. In this way, the difference in echo shift can be reduced, which is particularly advantageous at higher principal magnetic field strengths (3T or more). Furthermore, the interval between two consecutive refocusing RF pulses can be shortened to reduce scan time and T2 attenuation over the echo train.

[0018] In a more preferred embodiment, the reconstruction of the MR image includes the reconstruction of single-echo images from the acquired echo signal pairs for each of the first and second sets, i.e., a first single-echo image belonging to a first echo time and a second single-echo image belonging to a second echo time. Thus, phase errors induced by eddy currents can be eliminated by aligning the pixel-level or voxel-level phase of the first single-echo images of the first and second sets, and by aligning the pixel-level or voxel-level phase of the second single-echo images of the first and second sets, respectively. This is mainly applied when the bipolar pair of readout magnetic field gradients in the acquisition of the second set has the opposite polarity to the bipolar pair of readout magnetic field gradients in the acquisition of the first set, since the first and second echo times are identical in the two sets in this case. The single-echo images thus corrected can be used for water / fat separation in a further image reconstruction procedure. This preferably includes a first water / fat separation based on a first set of first single-echo images and one single-echo image from a second set, yielding a first water image and a first fat image, and a second water / fat separation based on a second set of second single-echo images and the other single-echo image from a second set, yielding a second water image and a second fat image. Advantageously, the water / fat separation is performed based on single-echo images in acquisitions where the readout magnetic field gradients have the same polarity. Thus, fat shift and B0 distortion can be ignored in this step. Finally, the first and second water images are combined to form a final water image, and / or the first and second fat images are combined to form a final fat image. For this purpose, for example, the fat image is aligned with the water image and B0 distortion is corrected using a B0 map estimated by the water / fat separation.

[0019] Furthermore, reversing the temporal trajectory of the readout magnetic field gradient in the acquisition of the second set relative to the temporal trajectory of the readout magnetic field gradient in the acquisition of the first set, and / or shifting the gradient area between spoiler readout magnetic field gradients, flyback readout magnetic field gradients, and / or defacing readout magnetic field gradients that precede and follow the bipolar pair of readout magnetic field gradients, makes it possible to correct the echo shift between the acquisition of the first and second sets of echo signal pairs. In this way, the conditioning of the inverse problem associated with water / fat separation can be improved, and artifacts and / or noise amplification in water / fat separation can also be reduced or eliminated. This is mainly applicable when performing water / fat separation in k space to correct for fat shift.

[0020] The method of the present invention as described herein can be carried out by an MR apparatus comprising: at least one main magnet coil for generating an essentially uniform static magnetic field B0 within an examination volume; several gradient coils for generating a switching magnetic field gradient in different spatial directions within the examination volume; at least one bio-RF coil for generating RF pulses within the examination volume and / or receiving MR signals from the patient's body located within the examination volume; a control unit for controlling the temporal transitions of the RF pulses and the switched magnetic field gradient; and a reconstruction unit for reconstructing an MR image from the received MR signal. The method of the present invention can be carried out by corresponding programming of the reconstruction unit and / or control unit of the MR apparatus.

[0021] The method of the present invention can be advantageously carried out with most MR devices currently in clinical use. For this purpose, it is only necessary that the MR device utilizes a computer program that controls it to perform the method steps described above of the present invention. The computer program may reside on a data carrier or within a data network so that it is downloaded for installation on the control unit of the MR device.

[0022] The attached drawings disclose preferred embodiments of the present invention. However, it should be understood that the drawings are for illustrative purposes only and are not intended to define limitations of the present invention.

Brief Description of the Drawings

[0023] [Figure 1] It is a diagram showing an MR apparatus for executing the method of the present invention. [Figure 2] It is a schematic (simplified) pulse sequence diagram of a conventional multiple repetition TSE Dixon method imaging sequence. [Figure 3] It is a schematic (simplified) pulse sequence diagram of a conventional multi-echo TSE Dixon method imaging sequence using a bipolar readout magnetic field gradient. [Figure 4] It is a schematic (simplified) pulse sequence diagram according to the first embodiment of the present invention. [Figure 5] It is a schematic (simplified) pulse sequence diagram according to the second embodiment of the present invention. [Figure 6] It is a schematic (simplified) pulse sequence diagram according to the third embodiment of the present invention.

Modes for Carrying Out the Invention

[0024] Referring to FIG. 1, an MR apparatus 1 is shown as a block diagram. The apparatus includes a superconducting or resistive main magnet coil 2 such that a substantially uniform and temporally constant main magnetic field B0 is generated along the z-axis through the examination volume. The apparatus further includes a set of (first, second, and, if appropriate, third) shim coils 2', and the currents through the individual shim coils of the set 2' are controllable for the purpose of minimizing B0 deviations within the examination volume.

[0025] The magnetic resonance generation and operation system applies a series of RF pulses and switched magnetic field gradients to invert, excite, saturate, refocus magnetic resonance, and encode it spatially and in other ways to perform MR imaging.

[0026] More specifically, the gradient pulse amplifier 3 applies current pulses to selected whole-body gradient coils 4, 5, and 6 along the x, y, and z axes of the examination volume. The digital RF frequency transmitter 7 transmits RF pulses to the examination volume by sending RF pulses or pulse packets to the bio-RF coil 9 via the transmit / receive switch 8. A typical MR imaging sequence consists of packets of short-duration RF pulse segments that achieve selected operation of nuclear magnetic resonance with any applied magnetic field gradient. In particular, the RF pulses select a portion of the body 10 located within the examination volume. The MR signal is also picked up by the bio-RF coil 9.

[0027] To generate an MR image of a limited area of ​​body 10, a set of local array RF coils 11, 12, and 13 are positioned adjacent to the area selected for imaging. The array coils 11, 12, and 13 can be used to receive MR signals induced by the transmission of the bio-RF coils.

[0028] The resulting MR signal is picked up by the bio-RF coil 9 and / or array RF coils 11, 12, 13 and demodulated by a receiver 14, which preferably includes a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12, and 13 via a transmit / receive switch 8.

[0029] The host computer 15 controls the shim coil 2', as well as the gradient pulse amplifier 3 and the transmitter 7, to generate the imaging sequence of the present invention. For the selected sequence, the receiver 14 rapidly and sequentially receives signal data from one or more k-space lines, following each RF excitation pulse. The data acquisition system 16 performs analog-to-digital conversion of the received signals, converting each k-space line into a digital format suitable for further processing. In modern MR devices, the data acquisition system 16 is a separate computer dedicated to acquiring raw image data.

[0030] Finally, the digital raw image data is reconstructed into an image display by a reconstruction processor 17 that applies a Fourier transform or other appropriate reconstruction algorithm such as SENSE. The MR image is a display of planar slices, an array of parallel planar slices, a three-dimensional volume, etc., passing through the patient. The image is then stored in image memory, which can be accessed to convert slices, projections, or other parts of the image display into an appropriate format for visualization, for example, via a video monitor 18 that provides a human-readable display of the resulting MR image.

[0031] The host computer 15 and the reconfiguration processor 17 are configured, by corresponding programming, to perform the methods of the present invention as described above and below herein.

[0032] According to the present invention, two instances (shots) of a dual-echo TSE Dixon imaging sequence are used, and each instance (shot) acquires two echo signals at each interval between two refocusing RF pulses using a bipolar pair of readout magnetic field gradients. Figure 4 shows a pulse sequence diagram of the dual-echo TSE sequence that constitutes the imaging sequence according to the present invention. This diagram shows the switched magnetic field gradients in the frequency encoding direction (M), phase encoding direction (P), and slice selection direction (S). Furthermore, this diagram shows the RF excitation pulse and refocusing pulse, as well as the time intervals at which the echo signals are acquired, with the time intervals indicated as ACQ. At each time interval between two consecutive refocusing RF pulses, a pair of echo signals is acquired using a bipolar pair of readout magnetic field gradients. Figure 4 covers the first two pairs of echo signals in one shot of the imaging sequence. By performing two shots of the imaging sequence, where the polarity of the bipolar readout magnetic field gradient is reversed between shots (indicated by double-headed arrows in Figure 4), a high acquisition duty cycle is achieved, and high image quality is maintained by facilitating the suppression of artifacts arising from bipolar acquisition (see the explanation above for details). In general, the phase encoding of the two shots does not need to be identical. In this case, advanced parallel imaging and / or compressed sensing subsampling and reconstruction techniques can be advantageously applied.

[0033] Alternatively or additionally, the temporal trajectory of the readout magnetic field gradient (i.e., the sequence or order of individual readout magnetic field gradient pulses) can be reversed in the second shot. This is shown in Figure 5. The pulse sequence diagram in Figure 5 is identical to the pulse sequence diagram in Figure 4, except that the temporal trajectory of the readout magnetic field gradient in the frequency encoding direction (M) is reversed in the second shot shown in Figure 5 compared to the first shot shown in Figure 4.

[0034] Alternatively or additionally, the gradient areas of readout magnetic field gradients other than the bipolar pairs can be varied between the first and second shots while keeping the sum of these gradient areas fixed. This is shown in Figure 6. The pulse sequence diagram in Figure 6 is identical to the pulse sequence diagram in Figure 4, except that the gradient area transitions from the spoiler readout magnetic field gradient and flyback readout magnetic field gradient following the bipolar pairs of readout magnetic field gradients to the spoiler readout magnetic field gradient preceding the bipolar pairs of readout magnetic field gradients. Furthermore, optional fine-tuning of the timing has been performed to slightly lengthen the acquisition window.

Claims

1. A method for MR imaging of an object placed within the inspection volume of an MR device, wherein the method is A step of performing an imaging sequence of at least two shots on the object, wherein each shot includes an excitation RF pulse and a subsequent series of refocusing RF pulses, and at least one pair of phase-encoded echoes, i.e., a first echo at a first echo time and a second echo at a second echo time, are generated at each time interval between two consecutive refocusing RF pulses. The steps include: obtaining a first set of echo signal pairs from the object in a first shot of the imaging sequence using a bipolar pair of readout magnetic field gradients at each repetition interval; A step of acquiring a second set of echo signal pairs from the object in a second shot of the imaging sequence, using a bipolar pair of readout magnetic field gradients at each iteration interval, The acquisition of the first and second sets is an acquisition step in which the gradient area of ​​the magnetic field gradient in the reading direction preceding and succeeding the bipolar pair of the read magnetic field gradient differs from that of the other, A step of configuring the reconstruction of an MR image from the first and second sets of acquired echo signal pairs, wherein the signal contributions from water protons and lipid protons are separated, and the reconstruction includes suppressing or removing artifacts arising from bipolar acquisition. A method having.

2. The method according to claim 1, wherein some or all of the echo signals of the first set and / or the second set are acquired only partially.

3. The method according to claim 1 or 2, wherein the reconstruction of the MR image includes the reconstruction of single echo images from the acquired echo signal pairs for each of the first and second sets, i.e., a first single echo image belonging to the first echo time and a second single echo image belonging to the second echo time.

4. The method according to claim 3, wherein eddy current-induced phase errors are eliminated by aligning the pixel-level or voxel-level phase of the first single-echo images of the first and second sets, and by aligning the pixel-level or voxel-level phase of the second single-echo images of the first and second sets.

5. The method according to claim 3 or 4, wherein the reconstruction of the MR image comprises a first water / fat separation based on the first single-echo image of the first set and one single-echo image of the second set, resulting in a first water image and a first fat image; and a second water / fat separation based on the second single-echo image of the first set and the other single-echo image of the second set, resulting in a second water image and a second fat image.

6. Fat shift and / or B ０ The method according to claim 5, wherein distortion is corrected.

7. The method according to claim 5 or 6, wherein the first and second water images are combined to form a final water image, and / or the first and second fat images are combined to form a final fat image.

8. A uniform static magnetic field B is present within the inspection volume. ０ An MR apparatus comprising: at least one main magnet coil for generating; several gradient coils for generating a switching magnetic field gradient in different spatial directions within the inspection volume; at least one RF coil for generating RF pulses within the inspection volume and / or receiving an MR signal from an object located within the inspection volume; a control unit for controlling the temporal transitions of the RF pulses and the switching magnetic field gradient; and a reconstruction unit for reconstructing an MR image from the received MR signal, wherein the MR apparatus is configured to perform steps of the method according to any one of claims 1 to 7.

9. A computer program that is executed on an MR device, wherein the computer program includes instructions necessary to perform the method according to any one of claims 1 to 7.

Images