MR imaging with water / fat / B0 mapping

Abstract

The invention relates to a method of MR imaging of an object (10) placed in a main magnetic field (B0) within an examination volume of an MR system (1). An object of the present invention is to provide an improved MR imaging technique in which B0 mapping is performed without or with only a minimum additional scan time. The method of the invention comprises the steps of: subjecting the object (10) to a first imaging sequence to generate a first set of MR signals at a first echo time; acquiring the first set of MR signals from the subject (10); subjecting the object (10) to a second imaging sequence to generate a second set of MR signals at a second echo time different from the first echo time, where the first imaging sequence and the second imaging sequence are further different in at least one of: field of view, image resolution, repetition time, k-space sampling pattern, relaxation time weighting; acquiring the second set of MR signals from the subject (10); and deriving a water map, a fat map and a B0 map from the first set of MR signals and the second set of MR signals using a Dixon method. Furthermore, the invention relates to an MR system (1) and to a computer program to be run on an MR system (1).

Description

Technical Field

[0001] The present invention relates to the field of magnetic resonance (MR) imaging. It relates to a method for MR imaging of an object placed in a main magnetic field within an examination volume of an MR system. The present invention also relates to an MR system and a computer program running on the MR system. Background Art

[0002] MR imaging methods that exploit the interaction between magnetic fields and nuclear spins to form two-dimensional or three-dimensional images are nowadays widely used, especially in the field of medical diagnostics, because they are superior to other imaging methods in many aspects for imaging of soft tissues, do not require ionizing radiation and are generally non-invasive.

[0003] Ensuring the geometric accuracy of MR images is critical for many applications. Geometric distortions of clinical MR images need to be evaluated and corrected, if necessary. In state-of-the-art MR systems, system-level distortions are adequately corrected, but object-induced distortions due to sensitivity and chemical shift effects still need to be considered and corrected.

[0004] A common approach for quantifying and correcting for the geometric distortion caused by the object is to acquire a map of the main magnetic field (e.g., a B0 map) in a separate pre-scan (i.e., before acquiring the actual MR image). The B0 map is used to assess the inhomogeneity of the main magnetic field based on the phase difference of two gradient echo images acquired at different echo times.

[0005] The B0 map can be used to correct for main field inhomogeneities during processing of the acquired MR signals in order to reduce or eliminate geometric distortions in the reconstructed MR images. This is particularly applicable to imaging techniques such as EPI (echo planar imaging) and techniques using non-Cartesian k-space trajectories such as spiral k-space scanning.

[0006] Alternatively (or additionally), minimization of B0 inhomogeneity can be achieved in a process known as B0 shimming. Shimming uses current applied to a dedicated coil that generates a correction to the main magnetic field in the examination volume of the MR system, thereby making the field within the volume as uniform as possible. These corrections are generated in the form of field gradients of different geometries. The process of adjusting the current through the shimming coil is called shimming and can be performed at any time. In particular, the determined B0 map can be used to derive appropriate shimming currents to compensate for any detected main field inhomogeneities. JP Stockmann et al. (Neuroimage, 168: 71-87, 2018) discuss several hardware methods for static and dynamic B0 shimming using high-order spherical harmonic shimming coils or multi-coil shimming arrays, and adaptive current networks are used for active shimming. This shimming technique can also achieve significant improvements in signal-to-noise ratio (SNR). However, shimming has inherent limitations and cannot correct all B0 inhomogeneities. Therefore, the determined B0 map can help evaluate any remaining inhomogeneities after shimming. This residual B0 map is still useful for distortion and blur correction during reconstruction.

[0007] The estimation of the B0 map faces several challenges, some of which are common to all body parts, such as poor signal-to-noise ratio and phase kinks at locations with large magnetic susceptibility effects, while others are more relevant to specific body parts, such as chemical shift. Due to the signal contribution from fat and water protons, the B0 map can be corrupted by chemical shift artifacts, especially for body parts with large fat content.

[0008] More advanced methods for B0 determination estimate the B0 map as well as the fat and water contributions, thereby reducing or substantially eliminating chemical shift artifacts. If the information from two or more corresponding echoes acquired at different echo times is combined, the relative contributions of fat and water to the MR signal can be calculated. This can be considered as chemical shift encoding, where an extra dimension (the chemical shift dimension) is defined and encoded by acquiring two or more MR images at slightly different echo times. For water / fat separation, these types of experiments are often referred to as Dixon type measurements. With the aid of 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. Typically, this separation is possible because there is a known difference in the precession frequencies of hydrogen in fat and water.

[0009] Several Dixon-type MR imaging methods have been proposed in recent years. In addition to different strategies for water / fat separation, the known techniques are mainly characterized by the specific number of echoes (or "points") they acquire and the constraints they impose on the echo times used. Conventional methods require in-phase and anti-phase echo times at which the water and fat signals are parallel and anti-parallel in the complex plane, respectively. Three-point and two-point methods have been gradually generalized to allow flexible echo times. Therefore, they no longer restrict the angle or phase between the water and fat signals at the echo time to certain values. In this way, they provide greater freedom in imaging sequence design and, in particular, achieve a trade-off between the signal-to-noise ratio (SNR) gain from acquisition and the SNR loss in separation. On the other hand, it is desirable to sample only two rather than three echoes to reduce scan time (see Eggers et al., Magn. Reson. Med., 65:96-107, 2011).

[0010] These known methods are mainly successful in correcting / suppressing artifacts related to main field inhomogeneities and chemical shifts. However, acquiring a B0 map in a pre-scan (i.e., a separate scan performed before the main scan) requires additional scan time, and inhomogeneity-induced artifacts cannot be corrected / suppressed if motion occurs between the pre-scan and the main scan. Summary of the invention

[0011] It is an object of the present invention to provide an improved MR imaging technique, wherein B0 mapping is performed with no or only minimal additional scan time.

[0012] According to the present invention, a method for MR imaging of an object placed in a main magnetic field in an examination volume of an MR system is proposed, wherein the method comprises the following steps:

[0013] - subjecting the object to a first imaging sequence to generate a first set of MR signals at a first echo time,

[0014] - acquiring said first set of MR signals from said object,

[0015] - subjecting the object to a second imaging sequence to generate a second set of MR signals at a second echo time different from the first echo time, wherein the first imaging sequence and the second imaging sequence further differ with respect to at least one of the following: field of view, image resolution, repetition time, k-space sampling mode, relaxation time weighting,

[0016] - acquiring said second set of MR signals from said subject, and

[0017] - deriving a water map, a fat map and a B0 map based on the first set of MR signals and the second set of MR signals.

[0018] As mentioned above, the known Dixon method is based on acquiring MR signal data at at least two different echo times and using calculations to separate water and fat signal contributions and determine a B0 map. The present invention generalizes this concept without an additional acquisition step, i.e. by utilizing MR signal data acquired at two or more different echo times, but also using two (or more) different (first and second) imaging sequences. The first imaging sequence and the second imaging sequence correspond to different (gradient echo) acquisitions. They may differ with respect to field of view, image resolution, repetition time, k-space sampling mode, relaxation time weighting, etc. Thus, the present invention enables different gradient echo scans to be used for water / fat separation and B0 mapping. In this way, the method of the present invention saves scan time by eliminating the need for an explicit B0 acquisition in a separate pre-scan. According to the present invention, the concept is realized in that the method of MR imaging involves a preparation phase and a diagnostic phase. The preparation phase includes: a spatially low-resolution survey scan, which is used to generate a geometric plan; and a spatially low-resolution calibration scan to determine the coil sensitivity distribution of a plurality of coil elements of an RF receiver array. The diagnostic phase includes a diagnostic imaging scan, in which k-space data is acquired with spatially high-resolution encoding using a geometric plan. Thus, the spatial low-resolution survey scan and the spatial low-resolution calibration scan form a first imaging scan and a second imaging scan at respective different echo times, from which the Dixon method can be used, and a water map, a fat map and a B0 map can be derived. The B0 map can be used for accurate B0 shimming and compensation for B0 inhomogeneities during the diagnostic phase. The water and / or fat images can be used for water-fat separation or fat suppression during the diagnostic phase.

[0019] The first imaging sequence and the second imaging sequence are associated with a survey sequence and a calibration sequence, respectively, wherein the survey sequence is used to provide an overview image of the object, and wherein the calibration sequence is used to determine the spatial sensitivity distribution of an RF receiver array of multiple receiving antenna (coil) elements used to acquire MR signals (e.g., for parallel imaging). Such survey scans and calibration scans are usually performed at the beginning of each imaging session anyway, so they do not involve any additional scanning time. They use different imaging sequences (in terms of field of view and image resolution). The present invention proposes to use the MR signal data acquired in these pre-scans to derive a B0 map (as well as a water map and a fat map to avoid chemical shift artifacts). Since the B0 map varies smoothly in space, there is no obstacle to the coarse resolution of the survey scan and the calibration scan.

[0020] In the case of brain imaging, since there is no inherent water / fat confusion in brain regions, B0 mapping can be reduced to a simple subtraction technique. Although this is typically done using a dedicated acquisition ((B0 pre-scan), the present invention eliminates this need and instead uses already acquired data (from the survey scan and calibration scan) to calculate the B0 map. In addition, since the imaging sequences used for the survey scan and calibration scan are typically factory-set protocols, they can be customized to improve B0 mapping.

[0021] According to the present invention, the calculations involved in B0 mapping essentially correspond to the conventional (two-point) Dixon method (see Eggers et al., J. Magn. Reson. Imaging 2014; 40: 251-268). A signal model is used, which includes signal contributions from water and fat at each voxel position, a phase shift between water and fat caused by the difference in chemical shift (resulting from the selected spectral model of fat and the two different echo time values ​​of the first imaging sequence and the second imaging sequence), and a phase error (caused by the main field inhomogeneity). The essential ambiguity of the three unknowns (water contribution, fat contribution, phase error) of deriving the signal model from the MR signal data associated with the two echo times can be resolved by assuming spatial smoothing of the main field inhomogeneity. Therefore, instead of directly calculating the water signal contribution and the fat signal contribution, the two-point Dixon method usually performs a separation in three steps: First, a potential solution for the phase error is calculated. This is done independently for each voxel. Then, a solution is selected for each voxel by maximizing the spatial smoothing of the phase error, and finally the water signal contribution and the fat signal contribution are estimated given the phase error.

[0022] In a possible embodiment where the first imaging sequence and the second imaging sequence differ in field of view, the water map, fat map and B0 map are derived for an overlapping region of the fields of view of the first imaging sequence and the second imaging sequence.

[0023] In another embodiment, the object is subjected to a third imaging sequence to generate a third set of MR signals, wherein the image resolution of the third imaging sequence is higher than the image resolution of the first imaging sequence and / or the second imaging sequence, wherein the third set of MR signals is acquired and reconstructed into an MR image, the reconstruction involving compensating for B0 inhomogeneities based on a B0 map derived from the first set of MR signals and the second set of MR signals. In this embodiment, the first imaging sequence and the second imaging sequence may represent pre-scans, such as the mentioned survey scans and calibration scans, which are run at a lower image resolution, wherein the third imaging sequence represents a main scan run for actual image acquisition at a higher image resolution. The B0 map derived from the MR signal data acquired using the first imaging sequence and the second imaging sequence may be used to correct or suppress artifacts associated with primary field inhomogeneities and chemical shifts in the MR images reconstructed from the MR signal data acquired using the third imaging sequence. In a possible embodiment, the B0 inhomogeneities may be compensated for during application of the third imaging sequence by shimming the main magnetic field based on the B0 map derived according to the present invention. If the first imaging sequence and the second imaging sequence differ from each other in terms of the field of view, the field of view of the third imaging sequence (or at least the portion of the field of view that requires B0 inhomogeneity compensation) should be contained in the overlapping region of the fields of view of the first imaging sequence and the second imaging sequence so as to have available reliable B0 data in the entire relevant field of view of the third imaging sequence.

[0024] The method of the invention described so far can be performed with the aid of an MR system comprising: at least one main magnet coil for generating a substantially uniform static main magnetic field B0 within an examination volume; a plurality of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume; at least one body RF coil for generating RF pulses within the examination volume and / or for receiving MR signals from a body of a patient located in the examination volume; a control unit for controlling the temporal continuity of the RF pulses and the switched magnetic field gradients; and a reconstruction unit for reconstructing an MR image from the received MR signals. The method of the invention can be implemented by corresponding programming of the reconstruction unit and / or the control unit of the MR system.

[0025] The method of the present invention can be advantageously performed on most MR systems currently in clinical use. To this end, it is only necessary to use a computer program to control the MR system so that it performs the above method steps of the present invention. The computer program can exist on a data carrier or in a data network so as to be downloaded and installed in the control unit of the MR system. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed only for purposes of illustration and not as a definition of the limits of the present invention.

[0027] In the attached picture:

[0028] Figure 1 An MR system for performing the method of the present invention is shown;

[0029] Figure 2 An embodiment of the method of the present invention is shown in a flow chart;

[0030] Figure 3 A B0 map derived from survey scan data and calibration scan data according to the present invention is shown. DETAILED DESCRIPTION

[0031] refer to Figure 1 , an MR system 1 is shown as a block diagram. The device comprises a superconducting or resistive main magnet coil 2 such that a substantially uniform and time-constant main magnetic field B0 is created along a z-axis through an examination volume. The device further comprises a collection of (first-order, second-order and, where appropriate, third-order) shim coils 2', wherein the current flowing through each shim coil 2' of the collection is controllable to achieve the purpose of minimizing B0 deviations within the examination volume.

[0032] Magnetic resonance generation and manipulation systems apply a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode magnetic resonance, saturate spins, etc. to perform MR imaging.

[0033] More specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of the 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 into the examination volume via a transmit / receive switch 8 and a body RF coil 9. A typical MR imaging sequence consists of packets of RF pulse segments of short duration, which together with any applied magnetic field gradients achieve selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate resonance, excite resonance, invert magnetization, refocus resonance or manipulate resonance and select a portion of a body 10 located in the examination volume. MR signals are also picked up by the body RF coil 9.

[0034] To generate MR images of limited regions of the body 10, a set of local array RF coils 11, 12, 13 are placed adjacent the region selected for imaging.The array coils 11, 12, 13 may be used to receive MR signals induced by RF transmissions via the body RF coil.

[0035] The resulting MR signals are picked up by the body RF coil 9 and / or by the array RF coils 11,12,13 and demodulated by a receiver 14 which preferably comprises a preamplifier (not shown). The receiver 14 is connected to the RF coils 9,11,12 and 13 via a transmit / receive switch 8.

[0036] The main computer 15 controls the shim coils 2' as well as the gradient pulse amplifiers 3 and the transmitter 7 to generate the imaging sequence of the present invention. For the selected sequence, the receiver 14 receives single or multiple MR data lines in rapid succession after each RF excitation pulse. The data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line into a digital format suitable for further processing. In modern MR systems, the data acquisition system 16 is a separate computer dedicated to acquiring raw image data.

[0037] Ultimately, 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 SENSE. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, etc. The image is then stored in an image memory where it can be accessed to convert a slice, projection, or other portion of the image representation into an appropriate format for visualization, such as via a video monitor 18 that provides a human-readable display of the resulting MR image.

[0038] The host computer 15 and reconstruction processor 17 are programmed to carry out the method of the present invention as described above and below.

[0039] exist Figure 2In the method of the present invention, a flowchart is illustrated. In step 21, a conventional survey scan is performed using a first imaging sequence. The survey scan provides a first set of MR signals at a first echo time, wherein an overview image of the body 10 is reconstructed based on the first set of MR signals. In step 22, a conventional calibration scan is performed to determine the spatial sensitivity distribution of the RF coils 11, 12, 13 used to acquire MR signals. A second set of MR signals is acquired at a second echo time different from the first echo time, wherein the first imaging sequence and the second imaging sequence of the survey scan and the calibration scan, respectively, also differ in image resolution and field of view, and they apply different k-space sampling modes, and they are different, independent gradient echo sequences. The (first and second sets) of MR signal data acquired in the two pre-scans are used in step 23 to derive a B0 map, a water map, and a fat map using a conventional (generalized) Dixon method. Derivation of the B0 map involves resampling the complex MR signal data acquired in at least one of the two (first and second) acquisitions to the same common k-space grid. In addition, amplitude scaling factors are applied to at least one of the (resampled) first and second sets of MR signals to account for different relaxation time weights or other amplitude effects associated with the two different imaging sequences. Such scaling factors can be estimated by simulation or by calibration. These are specific to the imaging sequence used (they are not dependent on the object being imaged) and therefore only need to be predetermined once before actually running the method of the present invention. Thereafter, the B0 map, water map and fat map are calculated by the conventional (two-point) Dixon method (see above). In step 24, a third imaging sequence is applied to generate a third set of MR signals, wherein the image resolution of the third imaging sequence is higher than the image resolution of the first and second imaging sequences in steps 21 and 22. The third imaging sequence represents a main scan for an actual image acquisition run at a higher image resolution. The B0 map derived in step 23 is used to correct or suppress artifacts associated with main magnetic field inhomogeneities and chemical shifts in MR images reconstructed from MR signal data acquired in the main scan in step 24. The B0 map can also be used for B0 shimming during acquisition in step 24 (to reduce distortion and / or improve SNR). Conventional B0 eddy current corrections, adjoint field corrections etc. may be applied in all three acquisition steps 21 , 22 , 24 .

[0040] Figure 3B0 maps (in three planes) derived according to the present invention from a survey scan and a calibration scan in a brain imaging session are shown. MR signal data were acquired at 3T. The echo time of the calibration scan was 0.78 ms, while the echo time of the survey scan was 1.4 ms. The calibration and survey scans had different fields of view and different image resolutions. During processing, i.e., before applying the Dixon algorithm, the MR signal data of the two pre-scans were brought to the same field of view and resolution. The determined B0 values ​​varied within a range of approximately 600 Hz and followed the pattern expected in the brain.

Claims

1. A method for MR imaging of an object (10) placed in a main magnetic field (B0) within an examination volume of an MR system (1), the MR system comprising a radio frequency (RF) receiver array, the method comprising: (i) The preparation phase, which includes: (a) A spatial low-resolution survey scan that forms the basis for geometric planning; and, (b) a spatial low resolution calibration scan to determine the coil sensitivity distribution of the RF receiver array; and (ii) a diagnostic imaging phase comprising a diagnostic imaging sequence in which k-space data are acquired with spatially high resolution encoding using the geometric planning, The method comprises the following steps: - subjecting the object (10) to the spatial low-resolution survey scan to generate a first set of MR signals at a first echo time, - acquiring the first set of MR signals from the object (10), - subjecting the object (10) to the spatial low-resolution calibration scan to generate a second set of MR signals at a second echo time different from the first echo time, wherein the first imaging sequence and the second imaging sequence also differ in at least one of the following: field of view, image resolution, repetition time, k-space sampling mode, relaxation time weighting, - acquiring the second set of MR signals from the object (10), - deriving a water map, a fat map and optionally a B0 map from said first set of MR signals and said second set of MR signals using the Dixon method.

2. The method according to claim 1, wherein: The water map, the fat map and the B0 map are derived for an overlapping region of the field of view of a survey imaging sequence and a calibration sequence.

3. The method according to claim 1 or 2, wherein: The object (10) is subjected to the diagnostic imaging sequence to generate a set of diagnostic MR signals, wherein the image resolution of the diagnostic imaging sequence is higher than the image resolution of the first imaging sequence and / or the second imaging sequence, wherein a third set of MR signals is acquired and reconstructed into an MR image, the reconstruction involving compensating for B0 inhomogeneity based on the B0 map derived from the first set of MR signals and the second set of MR signals.

4. The method according to claim 1 or 2, wherein: The object (10) is subjected to a third imaging sequence to generate a third set of MR signals, wherein the third set of MR signals are acquired and reconstructed into an MR image, wherein B0 inhomogeneity is compensated by shimming the main magnetic field based on the B0 map derived from the first set of MR signals and the second set of MR signals.

5. The method according to any one of claims 2 to 4, wherein: The field of view of the third imaging sequence is contained in the overlapping region of the fields of view of the first imaging sequence and the second imaging sequence.

6. The method according to any one of claims 1 to 5, wherein: Deriving the water map, the fat map, and the B0 map involves resampling at least one of the first set of MR signals and the second set of MR signals to a same common k-space grid.

7. The method according to any one of claims 1 to 6, wherein: Deriving the water map, the fat map, and the B0 map involves rescaling at least one of the first set of MR signals and the second set of MR signals.

8. An MR system, comprising: at least one main magnet coil (2) for generating a uniform static main magnetic field B0 in an examination volume; A plurality of gradient coils (4, 5, 6) for generating switched magnetic field gradients in different spatial directions within the examination volume; at least one RF coil (9) for generating RF pulses within the examination volume; and an RF receiver array for receiving MR signals from an object (10) located in the examination volume; a control unit (15) for controlling the temporal continuity of the RF pulses and the switched magnetic field gradients; and a reconstruction unit (17) for reconstructing an MR image based on the received MR signals, wherein the MR system (1) is configured to perform the following steps in a preparation phase: - subjecting the object (10) to a spatial low-resolution survey scan to generate a first set of MR signals at a first echo time, thereby forming a basis for geometric planning, - acquiring the first set of MR signals from the object (10), - subjecting the object (10) to a spatial low-resolution calibration scan to generate a second set of MR signals at a second echo time to determine a coil sensitivity distribution of the RF receiver array, and the second echo time is different from the first echo time, wherein the first imaging sequence and the second imaging sequence also differ with respect to at least one of the following: field of view, image resolution, repetition time, k-space sampling mode, relaxation time weighting, - acquiring the second set of MR signals from the object (10), and - deriving a water map, a fat map and a B0 map from the first set of MR signals and the second set of MR signals using the Dixon method.

9. A computer program comprising instructions for performing the following steps when executed on a computer in a preparation phase: - running a spatial low-resolution survey scan to generate a first set of MR signals at a first echo time, forming the basis for geometric planning, - acquiring the first set of MR signals, - running a spatial low resolution calibration scan to generate a second set of MR signals at a second echo time to determine the coil sensitivity distribution of the RF receiver array, and the second echo time is different from the first echo time, wherein, The first imaging sequence and the second imaging sequence also differ in at least one of the following: field of view, image resolution, repetition time, k-space sampling mode, relaxation time weighting, - acquiring the second set of MR signals, and - deriving a water map, a fat map and a B0 map from the first set of MR signals and the second set of MR signals using the Dixon method.

Images