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] The present invention relates to the field of magnetic resonance (MR) imaging. The present invention relates to a method for MR imaging of an object placed in an examination volume of an MR device. The present invention also relates to an MR device and to a computer program executed on the MR device. [Background technology]

[0002] MR imaging methods, which utilize the interaction between a magnetic field and atomic nuclear spins to form two- or three-dimensional images, are widely used today, especially in the field of medical diagnostics, since they are superior in many ways to other imaging methods for imaging soft tissues, do not require ionizing radiation, and are usually non-invasive. Summary of the Invention [Problem to be solved by the invention]

[0003] Generally, according to the MR method, the subject's body is placed in a strong and uniform magnetic field B0, the direction of which at the same time defines the axis (usually the z-axis) of the coordinate system on which the measurements are based. The magnetic field B0 generates various energy levels for the individual nuclear spins, depending on the magnetic field strength, which can be excited (spin resonance) by applying an electromagnetic field (RF field) of a frequency defined in the radio frequency range (Larmor frequency). From a macroscopic point of view, the distribution of the individual nuclear spins generates a global magnetization that can be deflected out of equilibrium by the application of an electromagnetic pulse (RF pulse) of a suitable frequency perpendicular to the z-axis, so that the magnetization precesses around the z-axis. The precession represents the surface of a cone whose opening angle is 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 transverse section from the z-axis (90° flip angle).

[0004] After the end of the radio frequency pulse, the magnetization returns to its original equilibrium state, the magnetization in the z direction builds up again with a first time constant T1 (spin-lattice or longitudinal relaxation time) and the magnetization in a direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The magnetization variations can be detected by a receiving RF coil that is placed in the examination volume of the MR device and oriented so that the magnetization variations are measured in a direction perpendicular to the z axis. The decay of the transverse magnetization involves, for example, 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 the phases are uniformly distributed (dephasing) after the application of a 90° pulse. The dephasing can be compensated by a refocusing pulse (for example 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 the three principal axes are superimposed on the uniform magnetic field B0, resulting in a linear spatial dependence of the Larmor frequency. The signals picked up in the receiving coils then contain different frequency components that can be associated with different locations within the body. The signal data obtained through the receiving coils corresponds to the spatial frequency domain and is called k-space data. The k-space data typically includes data from multiple lines in k-space, acquired with different phase encodings. Each k-space line is digitized by acquiring a number of samples. The collection of samples from multiple lines in k-space is transformed into an MR image, for example by Fourier transformation.

[0006] In MR images, it is often desired to obtain information about the relative contributions of water and fat to the overall signal in order to suppress the contribution of one of them or to analyze both contributions separately or together. These contributions can be calculated when information from two or more corresponding echoes acquired at different echo times (with respect to excitation or spin echo refocusing) is combined. This can be considered as chemical shift encoding, where an additional dimension, namely the dimension of chemical shift, is defined and encoded by acquiring two or more MR images at slightly different echo times. Due to the water / fat separation, these types of measurements are often called Dixon type measurements. With Dixon MR images or Dixon water / fat MR images, the water / fat separation is achieved by calculating the water and fat contributions from two or more corresponding echoes acquired at different echo times, respectively. In general, such a separation is possible since there exists a known precession frequency difference of hydrogen in water and fat. In its simplest form, the water and fat images are generated by either addition or subtraction of in-phase and out-of-phase data sets.

[0007] In recent years, several Dixon-type MR imaging methods have been proposed. Besides different strategies for water / fat separation, the known techniques are mainly characterized by the constraints imposed on the specific number of echoes (points) acquired and on the echo time used. The traditional so-called two-point and three-point methods require in-phase and anti-phase echo times, where the water and fat signals are parallel and anti-parallel, respectively, in the complex plane. The three-point methods are gradually generalized to allow flexible echo times. Thus, they no longer restrict the angle or phase between the water and fat signals at the echo time to specific values. In this way, they offer more degrees of freedom in the imaging sequence design, in particular allowing a trade-off between the signal-to-noise ratio (SNR) gain from the acquisition and the SNR loss in the separation. To reduce the scan time, it is desirable to sample only two echoes instead of three. However, the constraints on the echo time can actually make dual-echo acquisitions slower than triple-echo acquisitions. Eggers et al. (Magnetic Resonance in Medicine, 65, 96-107, 2011) propose a flexible dual-echo Dixon-type MR imaging method that allows the elimination of such 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, but are optionally synthesized from water and fat images.

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

[0009] FIG. 2 shows a schematic pulse sequence diagram of conventional dual-acquisition gradient echo imaging. The diagram shows switched magnetic field gradients in the frequency encoding (readout) direction M, the phase encoding direction P, and the slice selection direction S. The diagram further shows RF pulses applied within a fixed time interval of duration TR and an acquisition window designated by ACQ1 and ACQ2 during which gradient echo signals are acquired. The diagram shows the acquisition of the first two echo signals. In subsequent repetitions, the phase encoding magnetic field gradients in the directions P and S are scaled to sample a predefined region of k-space. As shown in FIG. 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 unchanged. The shift of the readout magnetic field gradient results in different phase offsets of the contributions from water and fat to the total signal, based on which Dixon-type water / fat separation is performed. As can further be seen in Figure 2, the dephasing lobes of the readout magnetic field gradient in direction M and the phase encoding magnetic field gradient in directions P and S are shifted together with the rephasing lobes of the readout magnetic field gradient in the second illustrated time interval (second acquisition). However, this conventional approach, chosen to keep the deleterious effects of eddy currents in the two acquisitions as similar as possible, limits the freedom remaining in the design of the imaging sequence to achieve a reduction in acoustic noise caused by magnetic field gradient switching.

[0010] It is an object of the present invention to provide a method that allows for a more improved Dixon water / fat separation in combination with dual or multi-acquisition imaging techniques. [Means for solving the problem]

[0011] According to the invention, a method for MR imaging of an object placed in an examination volume of an MR device is disclosed, comprising the steps of: subjecting the object to a dual or multi-acquisition imaging sequence comprising a series of time-equally spaced RF pulses, in which echo signals are generated in the presence of readout magnetic field gradients in each time interval between successive 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 in which 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) to reduce the acoustic noise level based on an established relationship between (i) the placement of the magnetic field gradients preceding and / or following the readout magnetic field gradient in each time interval (TR) and (ii) the acoustic noise level caused by the switching of the magnetic field gradients, recording the echo signals, 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 invention, a dual or multi-acquisition imaging sequence is used to acquire two or more sets of echo signals in separate acquisitions. Typically, the two or more acquisitions use separate RF excitations. However, it is also feasible to acquire the echo signals of two or more acquisitions after a single RF excitation pulse, for example after successive RF refocusing pulses, alternating therewith. In the latter case, the time interval in the sense of the invention is the interval between successive RF refocusing pulses.

[0013] The timing and strength of the readout magnetic field gradient are selected to shift the acquisition window of the two echo signals so as to obtain an appropriate phase offset of the contributions from water and fat to the total signal, based on which a Dixon-type separation of these contributions is performed in the reconstruction step.

[0014] An essential feature of the present invention is to exploit the effect of shifting and / or stretching of certain magnetic field gradient lobes in a Dixon multi-acquisition sequence on acoustic noise. On this basis, the acoustic noise generated by a multi-acquisition Dixon sequence can be minimized, and thus the discomfort of the patient undergoing the corresponding examination can be minimized. For this purpose, 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 shifted in time, changed in duration and / or changed in amplitude between the time intervals (e.g. between one time interval and the next), so that the acoustic noise caused by the switching of the 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 acoustic noise level caused by the switching of the magnetic field gradient. This relationship can be established by measurements or calculations and calibration based on an acoustic model that represents the acoustic behavior of gradient switching. This established relationship is therefore the basis for controlling the acoustic noise level by (re)positioning the magnetic field gradients preceding and / or following the readout magnetic field gradient. In particular, the (re)positioning can be performed by reducing or minimizing the acoustic noise level as a function of the positioning of the magnetic field gradients per repetition time interval (TR). Alternatively, the positioning of the magnetic field gradients preceding and / or following the readout magnetic field gradients corresponding to a pre-set (minimum) acoustic noise level can also be returned by a neural network trained with a limited set of imaging parameters as input, such as repetition time and echo time for a multi-acquisition imaging sequence. The neural network can be trained for a large number of positions of the magnetic field gradients and the corresponding acoustic noise levels, determined based on acoustic noise measurements or model-based simulations of the acoustic behavior.

[0015] The magnetic resonance imaging system is configured to reconstruct a set of magnetic resonance images from the echo signals, in which case the reconstruction software is installed in a computing system of the magnetic resonance imaging system or the computing system of the magnetic resonance imaging system has access to a remote reconstruction facility. The reconstruction software can also be installed, for example, in a remote server of a medical institution accessible to a data network, in which case the reconstruction software can be available in the "cloud". In these remote configurations, the computing system comprises the functionality for reconstructing the set of magnetic resonance images at a remotely located reconstruction facility. Furthermore, the reconstruction of the magnetic resonance images can be performed by machine learning, for example by a trained neural network, which can be embedded in the computing system or can be accessible from a remote location.

[0016] In a preferred embodiment, a systematic approach is taken to reduce acoustic noise based on modeling of the acoustic spectrum, utilizing MR system specific acoustic transfer functions that relate the frequency content of the time course of the switched magnetic field gradients to the acoustic frequency content. In this way, for example, acoustic resonances of the particular MR system 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 such that the acoustic noise caused by switching of the magnetic field gradient is minimized. This means that in addition to the shifting and stretching of the specific magnetic field gradient lobes described above, the order in which the echo signals associated with the two or more acquisitions are acquired is optimized to reduce the patient noise level. Options include alternation between the two acquisitions after each time interval, or transition between the two acquisitions after multiple time intervals, and a single switchover from one acquisition to another after one of the multiple acquisitions is completed.

[0018] The shifting and stretching of the magnetic field gradient lobes according to the invention corrects the effects of eddy currents in two or more acquisitions. However, it is an insight of the invention that the corresponding corrected phase errors can be corrected and do not have to be preemptively avoided, for example, by always placing the respective dephasing lobe immediately before the rephasing 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 invention is a turbo spin echo (TSE) sequence, which includes an RF excitation pulse followed by multiple RF refocusing pulses, each of the echo signals being generated at the time interval between successive RF refocusing pulses. This means that the approach of the invention can be combined with dual or multi-acquisition TSE sequences, just 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 magnetic field gradients and acquisition windows are used.

[0020] The inventive method as described above can be performed by an MR device having at least one main magnet coil for generating an essentially uniform static magnetic field B0 in an examination volume, a plurality of gradient coils for generating switched magnetic field gradients in different spatial directions in the examination volume, at least one body RF coil for generating RF pulses in the examination volume and / or for receiving MR signals from a patient's body arranged 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 an MR image from the received MR signals. The inventive method can be realized by a corresponding program of the MR device reconstruction unit and / or the control unit.

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

[0022] The accompanying drawings disclose preferred embodiments of the invention, but it is to be understood that the drawings are designed for illustrative purposes only and are not designed as a definition of the limits of the invention. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 shows an MR apparatus for implementing the method of the present invention. [Diagram 2] Schematic (simplified) pulse sequence diagram of a conventional multi-acquisition Dixon imaging sequence. [Diagram 3] FIG. 1 is a schematic (simplified) pulse sequence diagram according to a first embodiment of the present invention. [Figure 4] FIG. 4 is a schematic (simplified) pulse sequence diagram according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Referring to Fig. 1, an MR apparatus 1 is shown as a block diagram. The apparatus comprises superconducting or resistive main magnet coils 2 such that a substantially uniform and time-constant main magnetic field B0 is generated along a z-axis through an examination volume. The apparatus further comprises a set 2' of (first, second and, if applicable, third) shimming coils, the current through each individual shimming coil of the set 2' being controllable to minimize deviations of B0 within the examination volume.

[0025] A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert, excite, saturate, refocus, and spatially encode magnetic resonance to perform MR imaging.

[0026] More specifically, gradient pulse amplifiers 3 apply current pulses to selected ones of whole body gradient coils 4, 5 and 6 along the x, y and z axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets via a transmit / receive switch 8 to a body RF coil 9 which transmits RF pulses into the examination volume. A typical MR imaging sequence consists of packets of short duration RF pulse segments which, together with the application of optional magnetic field gradients, achieve selected manipulations of nuclear magnetic resonance. In particular, the RF pulses select a portion of a body 10 located within the examination volume. MR signals are also picked up by the body RF coil 9.

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

[0028] The resulting MR signals are picked up by the body 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, 13 via a transmit / receive switch 8.

[0029] A host computer 15 controls the shimming coils 2' as well as the gradient pulse amplifiers 3 and the transmitter 7 to generate the imaging sequences of the present invention. For a selected sequence, the receiver 14 receives signal data from single or multiple k-space 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 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, which applies a Fourier transform or other appropriate reconstruction algorithm, such as detection. The MR image may represent a planar slice across the patient, an array of parallel planar slices, a three-dimensional volume, etc. The image is then stored in an image memory, which can be accessed to convert the image representation into an appropriate format for visualization, for example, via a video monitor 18, which provides a human-readable display of the resulting MR image.

[0031] The host computer 15 and the reconstruction processor 17 are configured by corresponding programs to carry out the methods of the invention described above and below.

[0032] FIG. 3 shows a pulse sequence diagram of a dual acquisition gradient echo sequence constituting an imaging sequence according to the invention. The diagram shows switched magnetic field gradients in the frequency encoding (readout) direction M, the phase encoding direction P and the slice selection direction S. The diagram further shows RF pulses applied in 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 the direction M. One gradient echo is generated in each time interval between successive RF excitations. The diagram shows the acquisition of the first two echo signals. In the following time intervals, the phase encoding magnetic field gradients in the directions P and S are scaled to sample a given region of k-space. The two echoes are generated at two different echo times TE1 and TE2 after the two RF pulses by shifting the readout magnetic field gradient in a fixed TR while leaving the phase encoding magnetic field gradient unchanged. The shift of the readout magnetic field gradient and the resulting different echo times TE1 and TE2 result in different phase offsets of the contributions from water and fat to the total signal, on the basis of which Dixon-type water / fat separation is performed. According to the invention, as shown by the arrows in Fig. 3, parts of the magnetic field gradient preceding and / or following the rephasing lobe of the readout magnetic field gradient (the dephasing lobe and rephasing lobe of the phase encoding magnetic field gradient and the spoiling lobe of the magnetic field gradient) are shifted in time with respect to the pulse sequence diagram of Fig. 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 anti-phase lobes of the phase encoding magnetic field gradient 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 grey rectangles in FIG. 3 indicate the original time positions of each of the magnetic field gradients (corresponding to the situation shown in FIG. 2).

[0033] In the pulse sequence diagram of Fig. 4 illustrating one embodiment of the present invention, the corresponding magnetic field gradients preceding and / or following the rephasing lobes of the readout magnetic field gradient are stretched in time and simultaneously reduced in amplitude compared to the pulse sequence diagram of Fig. 2, as indicated by the arrows in Fig. 4. The light grey rectangles in Fig. 4 again indicate the original temporal shape and position of each of the respective magnetic field gradients.

[0034] The present invention applies the illustrated shifting and / or stretching of certain 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 is s n can be modeled by TIFF2024506575000002.tif22137 where harmonic f n =n / TR

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

[0036] The acoustic noise is then calculated, for example, as follows: TIFF2024506575000003.tif38147

[0037] Where: A represents the MR system-specific acoustic transfer function relating the frequency components of the time course of the magnetic field gradient to the corresponding frequency components of the acoustic noise, and a0 is a suitable reference value. The parameters of each gradient lobe of the imaging sequence G , t r , t p , t sl and t stis adapted in accordance with the present invention such that the acoustic noise calculated thereon is reduced relative to the acoustic noise produced by the conventional sequence design of FIG.

Claims

1. 1. A method for MR imaging of an object located in an examination volume of an MR device, comprising: subjecting the subject to a dual or multi-acquisition imaging sequence having a series of time-equally spaced RF pulses, where an echo signal is generated in the presence of a readout magnetic field gradient in each time interval between successive 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 where based on a relationship between (i) the placement of the 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 shifted in time, changed in duration and / or changed in amplitude between the time intervals to reduce the acoustic noise level; recording said echo signals; - reconstructing an MR image based on the recorded echo signals of at least two acquisitions, separating signal contributions from water and fat; The method according to claim 1,

2. 2. The method of claim 1, wherein the magnetic field gradients preceding and / or following the readout magnetic field gradient in each time interval are shifted in time, changed in duration and / or changed in amplitude between the time intervals such that acoustic noise caused by switching of the magnetic field gradients is minimized.

3. 3. The method of claim 2, wherein the minimization is based on modeling of an acoustic spectrum, including an MR device specific acoustic transfer function relating frequency components of a time course of the switched magnetic field gradients of the imaging sequence to acoustic frequency components, the frequency components being frequencies of the magnetic field gradients that generate acoustic noise.

4. 4. The method of claim 1, wherein the sequence of echo signals associated with each second acquisition is selected such that acoustic noise caused by switching of the magnetic field gradients is minimized.

5. 5. The method of claim 1, wherein the reconstruction of the MR image includes estimating and correcting eddy current induced phase errors in the recorded echo signals.

6. 6. The method of claim 1, wherein the imaging sequence is a turbo spin echo sequence including an RF excitation pulse followed by a number of RF refocusing pulses, each of the echo signals being generated in a time interval between successive RF refocusing pulses.

7. at least one main magnet coil for generating a uniform static magnetic field within the examination volume; a plurality of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume; at least one RF coil for generating RF pulses in the examination volume and / or receiving MR signals from a subject disposed in the examination volume; a control unit for controlling the succession of RF pulses and switched magnetic field gradients; a reconstruction unit for reconstructing an MR image from the received MR signals; 7. An MR apparatus configured to carry out the method according to any one of claims 1 to 6.

8. A computer program running on an MR device, comprising instructions for carrying out the steps of the method according to any one of claims 1 to 6.

Images