Dixon-type water / fat separation MR image

Abstract

The invention relates to a method of MR imaging of the Dixon type, in which an object 10 is subjected to a dual or multi-acquisition imaging sequence comprising a series of time-equally spaced RF pulses. Echo signals are generated in the presence of a readout magnetic field gradient in each time interval TR between successive RF pulses, the echo time varying between at least a first value TE1 associated with a first acquisition ACQ1 and a second value TE2 associated with a second acquisition ACQ2. The invention proposes that at least one of the magnetic field gradients preceding and / or following the readout magnetic field gradient in each time interval TR is shifted in time, changed in duration and / or changed in amplitude between the time intervals TR. In this way, a reduction in the acoustic noise generated by the multi-acquisition Dixon sequence is achieved, and thus a reduction in the discomfort of the patient undergoing the corresponding examination. Echo signals are recorded and an MR image is reconstructed by separating the water and fat signal contributions based on the recorded echo signals of at least two acquisitions ACQ1, ACQ2. Furthermore, the invention relates to an MR device 1 and to a computer program executed on the MR device 1.

Description

[Technical Field]

[0001] This invention relates to the field of magnetic resonance (MR) imaging. The invention relates to a method for MR imaging an object placed within an examination volume of an MR apparatus. The invention also relates to an MR apparatus and a computer program that runs on the MR apparatus. [Background technology]

[0002] MRI (Magnetic Resonance) imaging methods, which utilize the interaction between a magnetic field and nuclear spin to form two-dimensional or three-dimensional images, are superior in many ways to other imaging methods for soft tissue imaging. Because they do not require ionizing radiation and are usually non-invasive, they are widely used today, particularly in the field of medical diagnostics. [Overview of the project] [Problems that the invention aims to solve]

[0003] Generally, in the MR method, the subject's body is placed in a strong, uniform magnetic field B0, whose orientation simultaneously defines the axis (usually the z-axis) of the coordinate system on which the measurement is based. The magnetic field B0 generates various energy levels for individual nuclear spins depending on the magnetic field strength, which can be excited (spin resonated) by applying an electromagnetic field (RF field) of a frequency defined in the radio frequency range (Larmor frequency). From a macroscopic perspective, the distribution of individual nuclear spins generates an overall magnetization that can be deflected out of equilibrium by applying an electromagnetic pulse (RF pulse) of an appropriate frequency perpendicular to the z-axis, resulting in the magnetization precessing around the z-axis. The precession represents the surface of a cone, with the aperture angle called the flip angle. The magnitude of the flip angle depends on the intensity and duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected in a cross section transverse to the z-axis (90° flip angle).

[0004] After the termination of the high-frequency pulse, the magnetization returns to its original equilibrium state, with the magnetization in the z-direction being rebuilt with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization perpendicular to the z-direction relaxing with a second time constant T2 (spin-spin or transverse relaxation time). Magnetization variations can be detected by a receiving RF coil positioned and oriented within the inspection volume of the MR apparatus, such that the magnetization variations are measured in the direction perpendicular to the z-axis. Transverse magnetization decay involves, for example, the application of a 90° pulse, a transition of nuclear spins (induced by local magnetic field inhomogeneity) from an ordered state with the same phase to a state where the phase is uniformly distributed (defasing). Defasing can be compensated for by a refocusing pulse (e.g., a 180° pulse), which generates an echo signal within 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. The signal picked up within the receiving coil then contains different frequency components that can be associated with different locations within the body. The signal data obtained through 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 using different phase encodings. Each k-space line is digitized by acquiring a large number of samples. The collection of samples from multiple lines in k-space is 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 of them or to analyze both contributions 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 refocus). This can be considered chemical shift encoding, in which case an additional dimension, namely 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 measurements are often called Dixon-type measurements. By Dixon MR imaging or Dixon water / fat MR imaging, water / fat separation is achieved 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 known frequency difference in the precession of hydrogen in water and fat. In its simplest form, water and fat images are generated by either adding or subtracting in-phase and out-of-phase data sets.

[0007] In recent years, several Dixon-type MR imaging methods have been proposed. Apart from their respective different strategies for water / fat separation, known techniques are primarily characterized by the specific number of echoes (points) acquired and the constraints imposed on 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 are gradually becoming more generalized to allow for more flexible echo times. Thus, 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 offer more degrees of freedom in imaging sequence design, particularly allowing for a trade-off between signal-to-noise ratio (SNR) gain from acquisition and SNR loss in separation. To reduce scan time, it is desirable to sample only two echoes instead of three. However, constraints on echo time can, in practice, make dual-echo acquisition 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 eliminates these constraints. When 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 and can optionally be synthesized from water and fat images.

[0008] Dixon-type MR imaging methods are often applied in combination with gradient echo imaging. These are typically performed using dual echo sequences, which are particularly preferred for high-resolution imaging (see Eggers et al., Journal of Magnetic Resonance Imaging, 40, 251-268, 2014).

[0009] Figure 2 shows a schematic pulse sequence diagram of conventional dual-acquisition gradient echo imaging. The diagram shows the switching magnetic field gradients in the frequency encoding (readout) direction M, the phase encoding direction P, and the slice selection direction S. Furthermore, the diagram shows RF pulses applied within a fixed time interval of duration TR and acquisition windows specified by ACQ1 and ACQ2, during which gradient echo signals are acquired. This diagram shows the acquisition of the first two echo signals. In subsequent iterations, the phase encoding magnetic field gradients in directions P and S are scaled to sample a predetermined region of k-space. As shown in Figure 2, the dual-acquisition Dixon gradient echo imaging sequence generates two echoes at two different echo times TE1 and TE2 after two RF pulses by shifting the readout magnetic field gradient within a fixed TR while keeping the phase encoding magnetic field gradient constant. The shift in the readout magnetic field gradient yields different phase offsets of the contributions from water and fat to the overall signal, based on which Dixon-type water / fat separation is performed. As can be seen further in Figure 2, the diffusing lobes of the readout magnetic field gradient in direction M and the phase encoding magnetic field gradients in directions P and S are shifted along with the refacing lobe of the readout magnetic field gradient in the second illustrated time interval (second acquisition). However, this conventional approach is chosen to keep the adverse effects of eddy currents in the two acquisitions as similar as possible, limiting the degrees of freedom remaining in the design of the imaging sequence to achieve a reduction in acoustic noise caused by switching of the magnetic field gradients.

[0010] The object of the present invention is to provide a method that enables further improved Dixon water / fat separation when combined with dual or multi-acquisition imaging techniques. [Means for solving the problem]

[0011] The present invention discloses a method for MR imaging of an object placed within the examination volume of an MR apparatus. The method comprises the steps of: subjecting the object to a dual or multi-acquisition imaging sequence comprising a series of temporally equal-interval RF pulses, wherein an echo signal is generated in the presence of a readout magnetic field gradient at each time interval between consecutive RF pulses, the echo time varying between at least a first value associated with a first acquisition and a second value associated with a second acquisition, and based on an established relationship between (i) the arrangement of magnetic field gradients preceding and / or following the readout magnetic field gradient at each time interval (TR) and (ii) the acoustic noise level caused by switching of the magnetic field gradients, at least one of the magnetic field gradients preceding and / or following the readout magnetic field gradient at each time interval (TR) is temporally shifted, its duration changed and / or its amplitude changed between time intervals (TR) to reduce the acoustic noise level; recording the echo signal; and reconstructing an MR image by separating signal contributions from water and fat based on the recorded echo signals of the at least two acquisitions.

[0012] According to the present invention, dual or multi-acquisition imaging sequences are used to acquire two or more sets of echo signals in separate acquisitions. Generally, two or more acquisitions use separate RF excitations. However, it is also possible to acquire two or more echo signals alternately after a single RF excitation pulse, for example, after a series of RF refocusing pulses. In the latter case, the time interval in the sense of the present invention is the interval between the series of RF refocusing pulses.

[0013] The timing and intensity of the readout magnetic field gradient are selected to shift the acquisition window of the two echo signals so that an appropriate phase offset is obtained for the contributions from water and fat to the overall signal, and based on this, Dixon type separation of these contributions is performed in the reconstruction step.

[0014] An essential feature of the present invention is the utilization of the effect of shifting and / or stretching specific magnetic field gradient lobes on acoustic noise in a Dixon multi-acquisition sequence. Based on this, the acoustic noise generated by the multi-acquisition Dixon sequence can be minimized, and therefore the discomfort of the patient undergoing the corresponding examination can be minimized. To this end, the present invention proposes that some of the magnetic field gradients preceding and / or following the readout magnetic field gradient in each time interval are temporally shifted, their durations changed, and / or their amplitudes changed between time intervals (e.g., between one time interval and the next), so that the acoustic noise caused by switching magnetic field gradients is reduced compared to conventional approaches. The insight of the present invention is that there is a clear relationship between the manner in which the magnetic field gradients preceding and / or following the readout magnetic field gradient are arranged and the level of acoustic noise caused by switching magnetic field gradients. This relationship can be established by measurement or calculation, and calibration based on an acoustic model representing the acoustic behavior of gradient switching. Therefore, this established relationship is the basis for controlling the acoustic noise level by (re)arranging the magnetic field gradients preceding and / or following the readout magnetic field gradient. In particular, (re)arrangement can be performed by reducing or minimizing the acoustic noise level as a function of the arrangement of magnetic field gradients per iteration time interval (TR). As an alternative, the arrangement of magnetic field gradients preceding and / or following the readout magnetic field gradient corresponding to a preset (minimum) acoustic noise level can also be returned by a neural network trained using a limited set of imaging parameters, such as iteration time and echo time for a multi-acquisition imaging sequence, as input. The neural network can be trained for a number of magnetic field gradient arrangements and corresponding acoustic noise levels, determined based on acoustic noise measurements or model-based simulations of acoustic behavior.

[0015] A magnetic resonance imaging (MMRI) system is configured to reconstruct a set of MMRI images from echo signals, in which case the reconstruction software is installed on the MMRI system's computing system, or the MMRI system's computing system can access a remote reconstruction facility. The reconstruction software may also be installed, for example, on a remote server in a healthcare facility with access to a data network, in which case the reconstruction software may be available in the "cloud". In these remote configurations, the computing system has the capability to reconstruct the set of MMRI images at a remotely located reconstruction facility. Furthermore, the reconstruction of MMRI images can be performed by machine learning, for example, by a trained neural network that may be incorporated into the computing system or accessible from a remote location.

[0016] In a preferred embodiment, a systematic approach is employed to reduce acoustic noise based on acoustic spectrum modeling, utilizing an MR-specific acoustic transfer function that correlates the time-course frequency components of the switched magnetic field gradient with the acoustic frequency components. In this way, for example, acoustic resonances of the specific MR device used for imaging can be avoided, and thus the noise level can be significantly reduced.

[0017] In another preferred embodiment, the sequence of echo signals associated with the first and second acquisitions is selected to minimize acoustic noise caused by switching of the magnetic field gradient. This means that, in addition to the shifting and stretching of the specific magnetic field gradient lobes described above, the order in which echo signals associated with two or more acquisitions are acquired is optimized to reduce the patient's noise level. Options include alternation between two acquisitions after each time interval, or transitions between two acquisitions after multiple time intervals, and a single switchover from one acquisition to another after one of multiple acquisitions has been completed.

[0018] The shifting and stretching of the magnetic field gradient lobes according to the present invention corrects the effects of eddy currents in two or more acquisitions. However, it is an insight of the present invention that the corresponding corrected phase error can be compensated for and does not need to be proactively avoided, for example, by always positioning each diffusing lobe immediately before the refacing lobe of the readout magnetic field gradient. Techniques for determining and compensating for phase errors caused by eddy currents induced by magnetic field gradient switching in Dixon imaging are known in the art. For example, such phase errors can be estimated in a separate calibration measurement and compensated for in either the acquisition or the reconstruction.

[0019] In yet another preferred embodiment, the imaging sequence applied according to the present invention is a turbo spin echo (TSE) sequence comprising an RF excitation pulse and a plurality of subsequent RF refocus pulses, each of which echo signals is generated at the time interval between consecutive RF refocus pulses. This means that the approach of the present invention can be combined with dual or multi-acquisition TSE sequences, as dual or multi-acquisition TSE sequences are often applied in combination with Dixon-type MR imaging. Typically, two or three interleaved acquisitions with shifted readout field gradients and acquisition windows are used.

[0020] The method of the present invention described so far includes at least one main magnet coil that generates an essentially uniform static magnetic field B0 within the examination volume, a plurality of gradient coils that generate magnetic field gradients switched in various spatial directions within the examination volume, at least one body RF coil that generates RF pulses within the examination volume and / or receives MR signals from the body of a patient placed within the examination volume, a control unit that controls the temporally successive RF pulses and the switched magnetic field gradients, and a reconstruction unit that reconstructs MR images from the received MR signals. The method of the present invention can be implemented by a corresponding program of the MR device reconstruction unit and / or the control unit.

[0021] The method of the present invention can advantageously be carried out on most MR devices currently in clinical use. For this purpose, it is only necessary to utilize a computer program by which the MR device is controlled to execute the above-described method steps of the present invention. The computer program may be present on a data carrier so as to be downloaded for installation in the control unit of the MR device, or may be present within a data network.

[0022] The accompanying drawings disclose preferred embodiments of the present invention. However, it should be understood that the drawings are designed only for the purpose of explanation and not as a definition of limitation of the present invention.

Brief Description of the Drawings

[0023] [Figure 1] A diagram showing an MR device for implementing the method of the present invention. [Figure 2] A schematic (simplified) pulse sequence diagram of a conventional multi-acquisition Dixon imaging sequence. [Figure 3] A schematic (simplified) pulse sequence diagram according to the first embodiment of the present invention. [Figure 4] A schematic (simplified) pulse sequence diagram according to the second embodiment of the present invention. [Modes for carrying out the invention]

[0024] Referring to Figure 1, the MR apparatus 1 is shown as a block diagram. The apparatus has a superconducting or resistive main magnet coil 2 such that a substantially uniform and time-constant main magnetic field B0 is generated along the z-axis through the test volume. The apparatus further has a set of shimming coils 2' (first, second, and third where applicable) such that the current through each shimming coil of set 2' is controllable to minimize the bias of B0 in the test volume.

[0025] The magnetic resonance generation and manipulation system applies a series of RF pulses and a switching magnetic field gradient to invert, excite, saturate, refocus, and spatially encode magnetic resonances 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 or pulse packets to the body RF coil 9 via the transmit / receive switch 8, and transmits RF pulses to the examination volume. A typical MR imaging sequence consists of packets of short-duration RF pulse segments that achieve a selected operation of nuclear magnetic resonance, along with the application of an arbitrary 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 body 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 body RF coils.

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

[0029] The host computer 15 controls the shimming coil 2', 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 continuously 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 today's 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 representation by a reconstruction processor 17 that applies a Fourier transform or other appropriate reconstruction algorithm such as detection. The MR image can represent a planar slice across the patient, an array of parallel planar slices, a three-dimensional volume, and so on. The image is then stored in image memory, which can be accessed to convert it into an appropriate format for visualizing slices, projections, or other parts of the image representation, 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 to perform the methods of the present invention described above and below by corresponding programs.

[0032] Figure 3 shows a pulse sequence diagram of a dual acquisition gradient echo sequence that constitutes the imaging sequence according to the present invention. The diagram shows the switching magnetic field gradients in the frequency encoding (readout) direction M, the phase encoding direction P, and the slice selection direction S. Furthermore, the diagram shows RF pulses applied at fixed time intervals of duration TR, and acquisition windows indicated by ACQ1 and ACQ2, during which gradient echo signals are acquired in the presence of a readout magnetic field gradient in direction M. One gradient echo is generated at each time interval between consecutive RF excitations. This diagram shows the acquisition of the first two echo signals. In subsequent time intervals, the phase encoding magnetic field gradients in directions P and S are scaled to sample a predetermined region of k-space. Two echoes are generated at two different echo times TE1 and TE2 after the two RF pulses by shifting the readout magnetic field gradient within the fixed TR without changing the phase encoding magnetic field gradient. The shift in the readout magnetic field gradient and the resulting different echo times TE1 and TE2 provide different phase offsets of the contributions of water and fat to the overall signal, on which Dixon-type water / fat separation is performed. According to the present invention, as indicated by the arrows in Figure 3, a portion of the magnetic field gradient preceding and / or following the rephasing lobe of the readout magnetic field gradient (the defasing and rephasing lobes of the phase-encoding magnetic field gradient, and the spoil lobe of the magnetic field gradient) is shifted in time in relation to the pulse sequence diagram in Figure 2. The rephasing lobe of the phase-encoding magnetic field gradient and the spoiling lobe of the magnetic field gradient after the first acquisition are moved closer to the rephasing lobe of the readout magnetic field gradient of the first acquisition, and the readout and phase-inverse lobes of the readout magnetic field gradient of the second acquisition are moved away from the rephasing lobe of the readout magnetic field gradient of the second acquisition. The light gray rectangles in Figure 3 indicate the original temporal positions of each magnetic field gradient (corresponding to the situations shown in Figure 2).

[0033] In the pulse sequence diagram of FIG. 4 showing one embodiment of the present invention, the corresponding magnetic field gradients preceding and / or following the refocusing lobe of the readout magnetic field gradient are temporally stretched and simultaneously reduced in amplitude as shown by the arrows in FIG. 4 compared to the pulse sequence diagram of FIG. 2. The bright gray rectangles in FIG. 4 represent the original temporal shape and position of each magnetic field gradient again.

[0034] The present invention applies the illustrated shift and / or stretching of specific lobes of the magnetic field gradient to reduce the acoustic noise generated by the multi-acquisition Dixon sequence. To estimate the acoustic noise generated by the imaging sequence within the time interval TR, the frequency spectrum of the trapezoidal gradient lobe m s n can be modeled by the following equation TIFF0007862734000001.tif22137 where the harmonic f n =n / TR

[0035] G represents the strength (amplitude) and direction of the gradient lobe, t r represents the relative position of the center of the trapezoidal plateau within the time interval TR, t p is the relative length of the plateau, t sl and t st are the relative lengths of the rising slope and the falling slope.

[0036] Then, the acoustic noise is calculated, for example, as follows TIFF0007862734000002.tif38147

[0037] Here, A represents the acoustic transfer function specific to the MR device that associates the frequency components of the temporal evolution of the magnetic field gradient with the corresponding frequency components of the acoustic noise, and a0 is an appropriate reference value. The parameters of each gradient lobe of the imaging sequence G , t r , t p , t sl and t stThis is adapted according to the present invention such that the acoustic noise calculated based on this is reduced compared to the acoustic noise generated by the conventional sequence design shown in Figure 2. The embodiments of the present invention are described below. (Note 1) A method for MR imaging of an object placed within the examination volume of an MR device, A step of subjecting the subject to a dual or multi-acquisition imaging sequence having a series of temporally equal-interval RF pulses, wherein an echo signal is generated in the presence of a readout magnetic field gradient in each time interval between consecutive RF pulses, the echo time varies between at least a first value associated with a first acquisition and a second value associated with a second acquisition, and based on an established relationship between (i) the arrangement of magnetic field gradients preceding and / or following the readout gradient in each time interval and (ii) the acoustic noise level caused by the switching of the magnetic field gradients, at least one of the magnetic field gradients preceding and / or following the readout magnetic field gradient in each time interval is temporally shifted between time intervals, its duration is changed and / or its amplitude is changed, such that the acoustic noise level is reduced. The steps include recording the aforementioned echo signal, The steps include: reconstructing an MR image by separating signal contributions from water and fat based on the recorded echo signals of at least two acquisitions; A method of having. (Note 2) The method according to Appendix 1, wherein the magnetic field gradients preceding and / or succeeding the readout magnetic field gradient in each time interval are temporally shifted, their durations are changed, and / or their amplitudes are changed between time intervals, so as to minimize acoustic noise caused by switching of the magnetic field gradients. (Note 3) The method according to Appendix 2, wherein the minimization is based on acoustic spectrum modeling and includes an MR-specific acoustic transfer function that relates the time-course frequency components of the switched magnetic field gradient of the imaging sequence to acoustic frequency components. (Note 4) The method according to any one of appendices 1 to 3, wherein the sequence of echo signals associated with each of the second acquisitions is selected such that acoustic noise caused by switching of the magnetic field gradient is minimized. (Note 5) The method according to any one of Appendix 1 to 4, wherein the reconstruction of the MR image includes estimating and correcting the phase error of eddy current induction in the recorded echo signal. (Note 6) The imaging sequence is a turbo spin echo sequence comprising an RF excitation pulse and a plurality of subsequent RF refocusing pulses, wherein each of the echo signals is generated at the time interval between consecutive RF refocusing pulses, according to any one of Appendix 1 to 5. (Note 7) A test volume containing at least one main magnet coil that generates a uniform static magnetic field, Multiple gradient coils that generate magnetic field gradients that are switched in various spatial directions within the inspection volume, At least one RF coil that generates RF pulses within the inspection volume and / or receives MR signals from an object placed within the inspection volume, A control unit that controls time-continuous RF pulses and a switching magnetic field gradient, A reconstruction unit that reconstructs an MR image from the received MR signal, An MR apparatus having the following characteristics, configured to perform the method described in any one of the items 1 to 6. (Note 8) A computer program that runs on an MR device, having instructions for performing each step of the method described in any one of the appendices 1 to 6.

Claims

1. A method for MR imaging of an object placed within the examination volume of an MR device, A step of subjecting the subject to a dual or multi-acquisition imaging sequence having a series of time-equally spaced RF pulses, wherein an echo signal is generated in the presence of a readout magnetic field gradient in each time interval between consecutive RF pulses, the echo time varies between at least a first value associated with a first acquisition and a second value associated with a second acquisition, and based on the relationship between (i) the arrangement of magnetic field gradients preceding and / or following the readout gradient in each time interval and (ii) the acoustic noise level caused by switching of the magnetic field gradients, at least one of the magnetic field gradients preceding and / or following the readout magnetic field gradient in each time interval is temporally shifted, its duration changed and / or its amplitude changed between the time intervals so as to reduce the acoustic noise level, The steps include recording the aforementioned echo signal, The steps include: reconstructing an MR image by separating signal contributions from water and fat based on the recorded echo signals of at least two acquisitions; It has, To minimize the acoustic noise caused by the switching of the magnetic field gradient, the magnetic field gradient preceding and / or following the readout magnetic field gradient in each time interval is temporally shifted, its duration changed, and / or its amplitude changed during the time interval. The method wherein the minimization is based on modeling of an acoustic spectrum, the modeling includes an acoustic transfer function specific to the MR apparatus that relates the frequency components of the time-course switching magnetic field gradient of the imaging sequence to acoustic frequency components, the frequency components being the frequencies of the magnetic field gradient that generate acoustic noise.

2. The method according to claim 1, wherein the reconstruction of the MR image includes estimating and correcting the phase error of eddy current induction in the recorded echo signal.

3. The method according to claim 1 or 2, wherein the imaging sequence is a turbo spin echo sequence comprising an RF excitation pulse and a plurality of subsequent RF refocusing pulses, and each of the echo signals is generated at a time interval between consecutive RF refocusing pulses.

4. A test volume containing at least one main magnet coil that generates a uniform static magnetic field, Multiple gradient coils that generate magnetic field gradients that are switched in various spatial directions within the inspection volume, At least one RF coil that generates RF pulses within the test volume and / or receives MR signals from an object placed within the test volume, A control unit that controls time-continuous RF pulses and a switching magnetic field gradient, A reconstruction unit that reconstructs an MR image from the received MR signal, An MR apparatus having, configured to perform the method described in any one of claims 1 to 3.

5. A computer program to be executed on an MR device, having instructions for performing each step of the method according to any one of claims 1 to 4.

Images