MR imaging using a multi-point dixon technique

Abstract

At least a portion of a body (10) of a patient is positioned in an examination volume of a MR device (1). The portion of the body (10) is subject to a calibration sequence including RF pulses and switched magnetic field gradients controlled in such a manner that a calibration signal data set is acquired by a multi-point Dixon technique at a first image resolution. Calibration parameters are derived from the calibration signal data set. The portion of the body (10) is subject to an imaging sequence including RF pulses and switched magnetic field gradients controlled in such a manner that a diagnostic signal data set is acquired at a second image resolution which is higher than the first image resolution A diagnostic MR image is reconstructed from the diagnostic signal data set. The MR device (1) is operated according to the derived calibration parameters with fat saturation during acquisition of the diagnostic signal data set and/or during reconstruction of the diagnostic MR image.

Claims

1. A method of MR imaging of at least a portion of a body of a patient positioned in an examination volume of a MR device, the method comprising: with an MR controller, controlling at least one RF coil and gradient field coils in order to implement a multi-point Dixon imaging technique that generates calibration MR data for the portion of the body in the examination volume at a first image resolution; with one or more computer processors: reconstructing the calibration MR data into a B.sub.0 field map, a water map, and a fat map, identifying fat and water regions of the portion of the body in the image volume from the water and fat maps, and selecting a frequency and bandwidth for each of a plurality of fat suppression pulses, with the MR controller, controlling the at least one RF coil and the gradient field coils that generate RF pulses including the plurality of fat suppression pulses and the switched magnetic field gradients in order to generate diagnostic image data at a second image resolution which is higher than the first image resolution; with the one or more processors: reconstructing a diagnostic MR image from the generated diagnostic image data at the second image resolution which is higher than the first image resolution, and having the one or more processors, identify pixels or voxels of the reconstructed diagnostic MR image in which the fat suppression is incomplete or has failed, by using the reconstructed B.sub.0 field map and the reconstructed fat map along with the frequency and bandwidth of the plurality of fat suppression pulses.

2. The method of claim 1, wherein fat suppression RF pulses effect a spectrally selective suppression of fat signals in the diagnostic image data.

3. The method of claim 2, further including with the one or more computer processors, determining shim settings from the B.sub.0 field map and at least one of the water and fat maps in order to control a plurality of shim coils and thereby improve linearity of the existing B.sub.0 field occurring in at least one of water and fat regions within the portion of the body that is in the examination volume.

4. The method of claim 3, wherein the frequency and bandwidth of the one or more fat suppression RF pulses is determined from the improved linearity of the B0 field whereby a number of pixels or voxels within a given region of interest having a B.sub.0 field strength outside of a pre-determined range of spectral selectivity, of the one or more fat suppression RF pulses is minimized.

5. The method of claim 2, wherein one or more computer processors generates a prediction derived from the calibration MR data about the pixels or voxels within the reconstructed diagnostic MR image, in which the spectrally selective suppression of fat signals is incomplete or has failed.

6. The method of claim 5, wherein the diagnostic MR image is corrected in a post-processing according to the prediction by regenerating the calibration MR data using a higher order multi-point Dixon imaging technique.

7. The method of claim 1, further comprising: determining shim settings that maximize magnetic field homogeneity in the water region and/or in the fat region.

8. The method of claim 7, wherein the shim settings are computed with the one or more processors by optimizing a cost function depending on the B.sub.0 field deviation within the water region and the fat region.

9. A magnetic resonance (MR) device that images at least a portion of a body of a patient positioned in an examination volume comprising: at least one main magnet coil configured to generate a uniform, steady magnetic field B.sub.0 within an examination volume; a number of gradient coils configured to generate switched magnetic field gradients in different spatial directions within the examination volume; at least one body RF coil configured to generate RF pulses within the examination volume and/or configured for receiving MR signals from a portion of a body of a patient positioned in the examination volume; shimming coils configured to adjust the homogeneity of the a steady magnetic field B.sub.0; a MR controller configured to control the at least one RF body coil and the number of gradient coils in order to generate the temporal succession of RF pulses and switched magnetic field gradients which then subject the portion of the body of the patient positioned in the examination volume to a multi-point Dixon imaging technique in order to generate calibration data at a first image resolution; one or more computer processors configured to: segment the generated calibration data into a water region and a fat region select B.sub.0 shim settings that maximize B0 homogeneity in each of the water region and the fat region, cause the MR controller to subject the portion of the body to a multi-point Dixon magnetic resonance imaging sequence comprising RF pulses and switched magnetic field gradients controlled in such a manner that diagnostic MR image data is acquired at a second image resolution which is higher than the first image resolution, and reconstruct a diagnostic MR image from the diagnostic MR image data, wherein the MR device is operated according to the derived calibration parameters during acquisition of the diagnostic MR data and/or during reconstruction of the diagnostic MR image.

10. A non-transitory computer-readable medium carrying software to control one or more computer processors in order to control a MR device and perform the method of claim 1.

11. A magnetic resonance (MR) imaging apparatus configured for imaging a portion of a body in an examination volume, the MR imaging apparatus comprising: at least one main magnet coil configured to generate a uniform, steady B.sub.0 magnetic field within the examination volume; gradient coils configured to generate magnetic field gradients within the examination volume; RF coil windings configured to generate RF pulses within the examination volume and receive MR signals from the portion of the body positioned in the examination volume; shim coils configured to shim the B.sub.0 magnetic field; one or more computer processors configured to: control the RF coil windings and the gradient coils in order to implement a multi-point Dixon imaging sequence and generate calibration data at a first image resolution, reconstruct, from the generated calibration data, a B.sub.0 field map, a water image, and a fat image at the first image resolution, identify water and fat regions within the portion of the body that is in the examination volume from the reconstructed water and fat images, determine a B.sub.0 magnetic field homogeneity in each of the water regions and in each of the fat regions, determine settings for each of the shim coils in order to adjust the B.sub.0 magnetic field homogeneity that is present in the water and fat regions, select a frequency and a bandwidth of one or more fat suppression RF pulses, based on the adjusted B.sub.0 magnetic field homogeneity, the reconstructed water image, and the reconstructed fat image, whereby a number of pixels or voxels, located within a given region of interest of the portion of the body in the examination region, that have a B.sub.0 magnetic field homogeneity outside of a pre-determined range of spectral selectivity, of the one or more fat suppression RF pulses, is minimized, control the gradient coils, the RF coil windings, and the shim coils in order to shim the B.sub.0 magnetic field and adjust the B.sub.0 magnetic field homogeneity and apply a magnetic resonance imaging sequence that uses the one or more fat suppression pulses of the selected frequency and bandwidth in order to generate diagnostic MR image data at a second image resolution, the second image resolution being higher than the first image resolution, and reconstruct the diagnostic MR image data into a diagnostic MR image.

12. The MR imaging apparatus of claim 11, wherein the one or more computer processors are further configured to adjust the B.sub.0 magnetic field homogeneity such that the linearity of the B.sub.0 magnetic field in at least one of the water and fat regions is optimized.

13. The MR imaging apparatus of claim 11, wherein the one or more processors are further configured to: Identify pixels or voxels in the reconstructed diagnostic MR image in which fat suppression is projected to be incomplete or fail, by comparing a frequency, and/or bandwidth of the B.sub.0 fat saturation RF pulses with the B.sub.0 homogeneity at each pixel or voxel; and perform a post-processing on at least the identified pixels or voxels in order to compensate for signal contribution from the fat saturation RF pulses.

14. The MR imaging apparatus of claim 13, wherein the post-processing on at least the identified pixels or voxels further includes: using the B.sub.0 field map in order to minimize any undesired suppression of water signals of at least those identified pixels or voxels.

15. The MR imaging apparatus of claim 11, wherein the one or more computer processors are further configured to: determine the pixels or voxels in which the suppression of the fat signals is incomplete or fails by comparing the frequency and the bandwidth of the fat suppression RF pulses with the adjusted B.sub.0 magnetic field at the pixels or voxels being analyzed.

16. The MR imaging apparatus according to claim 11, wherein the multi-point Dixon imaging sequence is one of a three-point or a four-point Dixon imaging technique.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows a MR device for carrying out the method of the invention;

(3) FIG. 2 shows low-resolution MR images reconstructed from calibration signal data according to the invention;

(4) FIG. 3 shows water and fat regions in a MR image reconstructed from calibration signal data according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) With reference to FIG. 1, a MR device 1 is shown. The device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field B.sub.0 is created along a z-axis through an examination volume. The device further comprises a set of (1.sup.st, 2.sup.nd, andwhere applicable3.sup.rd order) shimming coils 2, wherein the current flow through the individual shimming coils of the set 2 is controllable for the purpose of minimizing B.sub.0 deviations within the examination volume.

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

(7) Most specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send/receive switch 8, to a body RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the body RF coil 9.

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

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

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

(11) 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 algorithms, such like SENSE or SMASH. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

(12) A first practical embodiment of the method of the invention is described as follows:

(13) After positioning the patient 10 in the iso-centre of the main magnet coil 2, a calibration sequence is started as a three-dimensional SENSE reference scan. The calibration scan uses several signal averages performed sequentially in time and is designed in the conventional way with interleaving signal acquisition via the body RF coil 9 and the array RF coils 11, 12, 13 using a Fast Field Echo (FFE) sequence. This sequence is modified into a multi-echo gradient echo sequence generating two or three gradient echoes after one excitation RF pulse, thereby facilitating chemical shift encoding. The calibration sequence is applied at a low image resolution such that the whole calibration scan can be performed within a couple of seconds.

(14) The acquired calibration signal data are then reconstructed corresponding low-resolution images involving a water/fat separation step according to the per se known multi-point Dixon technique. The resulting images are shown in FIG. 2. FIG. 2 shows the images reconstructed after the water/fat resolved SENSE calibration scan according to the invention. The reconstructed calibration images include water images 21, fat images 22, and B.sub.0 maps 23. Only a subset of the complete calibration data is shown in FIG. 2. In the practical embodiment, 64 slices are acquired with a voxel volume of 8812 mm.sup.3. A 3-point Dixon technique is applied. The repetition time is 3.5 ms. The low-resolution images reconstructed from the calibration signal data set are transferred to image processing while the acquisition of the next three-dimensional block of calibration signal data is acquired to improve the signal-to-noise ratio or to acquire a different motion state.

(15) Shim settings are derived from the B.sub.0 maps 23, which are used for controlling currents through the shimming coils 2 of the MR device 1 in order to optimize B.sub.0 homogeneity.

(16) After the calibration sequence, an imaging sequence is applied at higher resolution, wherein diagnostic signal data are acquired in parallel via the array RF coils 11, 12, 13. The imaging sequence comprises one or more fat suppression RF pulses effecting a spectrally selective suppression of fat signals in the diagnostic signal data set. For example, a per se known SPAIR technique (spectrally adiabatic inversion recovery) is used for fat suppression. The B.sub.0 maps 23 are used for determining the correct frequency and/or bandwidth of the fat suppression RF pulses. To this end, simulated B.sub.0 maps (i.e. B.sub.0 maps 23 corrected according to the used shim settings) reflecting the true B.sub.0 within the examination volume are computed as a basis for estimating the frequency and bandwidth of the fat suppression RF pulses. The frequency and bandwidth of the fat suppression RF pulses are determined in such a manner that the number of voxels having a B.sub.0 outside the range of spectral selectivity of the fat suppression RF pulses is minimized.

(17) Finally, a diagnostic MR image (not depicted) is reconstructed from the acquired diagnostic signal data set by the reconstruction processor 17 which applies the per se known SENSE reconstruction algorithm.

(18) According to yet another practical embodiment of the invention, again a B.sub.0 map, a water image and a fat image is acquired by means of the calibration sequence at a low image resolution. An automated segmentation is then performed in order to identify regions containing water, regions containing fat tissue and regions containing both water and fat on the basis of the B.sub.0 map, the water map, and the fat map as delivered by the multi-point Dixon technique. FIG. 3 shows the result of the segmentation. The water region is designated by reference number 31. The segment 32 represents subcutaneous fat. The region 33 represents a region containing a combination of water and internal fat. On the basis of this segmentation, the shim settings for the shimming coils 2 of the used MR device 1 are computed by optimizing a cost function depending on the B.sub.0 deviation within the regions 31 and 32. It is postulated that in order for the fat segment 32 to be suppressed optimally by means of frequency-selective saturation, the number of voxels in the segment 32 with a frequency offset larger than 160 Hz needs to be minimized. It has to be noted that this is an asymmetric criterion since large negative frequency offsets are not relevant. For the water segment 31, it is assumed that shimming is sufficient if the number of pixels outside of a frequency offset interval of +120 Hz is minimized. Both criteria are summed in order to form the cost function. The cost function is minimized by means of an appropriate algorithm. The resulting parameters are then used to derive the shim settings accordingly.

(19) Since a B.sub.0 map, a water map, and a fat map are obtained from the calibration signal data, a prediction of voxels of the diagnostic MR image, in which the fat suppression of fat signals will be incomplete or even fails totally, are derived in accordance with an embodiment of the invention. This is achieved by comparing the frequency and bandwidth of the fat suppression RF pulses with the B.sub.0 at the location of a given voxel. If the local B.sub.0 field is lower or higher than the spectral coverage of the fat suppression RF pulses, fat suppression will fail for the respective voxel. Since the fat and water contribution to the individual voxels are known from the fat and water maps, voxels for which an incomplete or entirely failing fat suppression is predicted are corrected in the final diagnostic MR image in a post-processing step in order to eliminate the signal contribution from fat. To this end, the signal contribution from fat in the respective voxel is derived from the fat map, and the image intensity of the voxel is reduced proportionally. The spectral profile of the fat suppression RF pulses can be taken into account in order to estimate an analogous value reflecting how much the fat suppression will fail for the respective voxel. This value can then be used as a correction factor in order eliminate the fat contribution as precisely as possible. An unwanted suppression of water signal can be processed and corrected for in an analogous fashion. A considerable increase of image quality is achieved in this way by mere post-processing of the reconstructed diagnostic MR image.

(20) The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.