DIXON-TYPE WATER/FAT SEPARATION MR IMAGING

Abstract

The invention relates to a method of Dixon-type MR imaging. The object (10) is subjected to at least two shots of an imaging sequence, each shot comprising an excitation RF pulse followed by a series of refocusing RF pulses, wherein at least a pair of phase encoded echoes, a first echo at a first echo time and a second echo at a second echo time, is generated in each time interval between two consecutive refocusing RF pulses. Two sets of echo signal pairs, a first set and a second set, are acquired using in bipolar pairs of readout magnetic gradients in two respective shots of the imaging sequence. The bipolar pair of readout magnetic field gradients in the acquisition of the second set has an opposite polarity to that of the bipolar pair of readout magnetic field gradients in the acquisition of the first set. Alternatively or additionally the temporal course of the readout magnetic field gradients in the acquisition of the second set is reversed with respect to the temporal course of the readout magnetic field gradients in the acquisition of the first set. Alternatively or additionally the acquisitions of the first and second sets are different from each other with respect to the gradient areas of magnetic field gradients in the readout direction (M) preceding respectively succeeding the bipolar pair of readout magnetic field gradients. Finally, an MR image is reconstructed from the acquired first and second sets of echo signal pairs, whereby signal contributions from water protons and fat protons are separated. Moreover the invention relates to an MR device (1) and to a computer program to be run on an MR device (1).

Claims

1. A method of magnetic resonance (MR) imaging of an object placed in an examination volume of an MR device, the method comprising: subjecting the object to at least two shots of an imaging sequence, each shot comprising an excitation RF pulse followed by a series of refocusing RF pulses, wherein at least a pair of phase encoded echoes, a first echo at a first echo time and a second echo at a second echo time, is generated in each time interval between two consecutive refocusing RF pulses, acquiring a first set of echo signal pairs from the object in a first shot of the imaging sequence using a bipolar pair of readout magnetic field gradients in each repetition interval, acquiring a second set of echo signal pairs from the object in a second shot of the imaging sequence using a bipolar pair of readout magnetic field gradients in each repetition interval, wherein the bipolar pair of readout magnetic field gradients in the acquisition of the second set has an opposite polarity to that of the bipolar pair of readout magnetic field gradients in the acquisition of the first set, and/or the temporal course of the readout magnetic field gradients in the acquisition of the second set is reversed with respect to the temporal course of the readout magnetic field gradients in the acquisition of the first set, and/or the acquisitions of the first and second sets are different from each other with respect to the gradient areas of magnetic field gradients in the readout direction (M) preceding respectively succeeding the bipolar pair of readout magnetic field gradients, and arranging to reconstruct an MR image from the acquired first and second sets of echo signal pairs, whereby signal contributions from water protons and fat protons are separated and wherein the reconstruction includes suppression or elimination of artefacts that may arise from the bipolar acquisitions.

2. The method of claim 1, wherein at least one of the echo signals of the first set or the second set are acquired only partially.

3. The method of claim 1, wherein the reconstruction of the MR image involves the reconstruction of single-echo images from the acquired echo signal pairs, namely a first single-echo image attributed to the first echo time and a second single-echo image attributed to the second echo time, for each of the first and second sets.

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

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

6. The method of claim 5, wherein the fat shift and/or B.sub.0 distortions are corrected.

7. The method of claim 5 wherein the first and second water images are combined into a final water image, and/or the first and second fat images are combined into a final fat image.

8. A magnetic resonance (MR) device comprising at least one main magnet coil for generating a uniform, static magnetic field B.sub.0 within an examination volume, a number 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 within the examination volume and/or for receiving MR signals from an object positioned 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 MR images from the received MR signals, wherein the MR device is configured to perform the method of claim 1.

9. A computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions stored on a non-transitory computer readable medium, which when executed performs the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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:

[0030] FIG. 1 shows an MR device for carrying out the method of the invention;

[0031] FIG. 2 shows a schematic (simplified) pulse sequence diagram of a conventional multi-repetition TSE Dixon imaging sequence;

[0032] FIG. 3 shows a schematic (simplified) pulse sequence diagram of a conventional multi-echo TSE Dixon imaging sequence using bipolar readout magnetic field gradients;

[0033] FIG. 4 shows a schematic (simplified) pulse sequence diagram according to a first embodiment of the invention;

[0034] FIG. 5 shows a schematic (simplified) pulse sequence diagram according to a second embodiment of the invention; and

[0035] FIG. 6 shows a schematic (simplified) pulse sequence diagram according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] With reference to FIG. 1, an MR device 1 is shown as a block diagram. 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 and-where applicable-3.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.

[0037] 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 and otherwise encode the magnetic resonance to perform MR imaging.

[0038] More 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, together with any applied magnetic field gradients, achieve a selected manipulation of nuclear magnetic resonance. In particular, the RF pulses 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.

[0039] For generation of MR images of limited regions of the body 10, 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 transmissions of the body RF coil.

[0040] 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 the send/receive switch 8.

[0041] A host computer 15 controls the shimming coils 2′ as well as the gradient pulse amplifier 3 and the transmitter 7 to generate the imaging sequences of the invention. For the selected sequence, the receiver 14 receives signal data from a single or a plurality of 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 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.

[0042] Ultimately, the digital raw image data are reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such as SENSE. 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.

[0043] The host computer 15 and the reconstruction processor 17 are arranged, by corresponding programming, to perform the method of the invention described herein above and in the following.

[0044] According to the invention, two instances (shots) of a dual-echo TSE Dixon imaging sequence are used, each acquiring two echo signals in each interval between two refocusing RF pulses using a bipolar pair of readout magnetic field gradients. FIG. 4 shows a pulse sequence diagram of a dual-echo TSE sequence constituting an imaging sequence according to the invention. The diagram shows switched magnetic field gradients in the frequency-encoding direction (M), the phase-encoding direction (P) and the slice-selection direction (S). Moreover, the diagram shows the RF excitation and refocusing pulses as well as the time intervals during which echo signals are acquired, designated by ACQ. A pair of echo signals is acquired in each time interval between two consecutive refocusing RF pulses using a bipolar pair of readout magnetic field gradients. FIG. 4 covers the first two pairs of echo signals of one shot of the imaging sequence. By performing two shots of the imaging sequence with the polarity of the bipolar readout magnetic field gradients reversed between the shots (indicated by the double arrow in FIG. 4) a high acquisition duty cycle is attained, and high image quality is retained by facilitating suppression of artefacts arising from the bipolar acquisitions (for details see description above). In general, the phase encoding in the two shots does not need to be identical. Advanced parallel imaging and/or compressed sensing subsampling and reconstruction techniques can advantageously be applied in this case.

[0045] Alternatively, or additionally, the temporal course of the readout magnetic field gradients (i.e. the sequence or order of the individual readout magnetic field gradient pulses) can be reversed in the second shot. This is illustrated in FIG. 5. The pulse sequence diagram of FIG. 5 is identical to that FIG. 4, except for the temporal course of the readout magnetic field gradients in the frequency-encoding direction (M) which is reversed in the second shot as shown in FIG. 5 with respect to the first shot as shown in FIG. 4.

[0046] Alternatively, or additionally, the gradient areas of the readout magnetic field gradients other than the bipolar pair can be varied between the first and the second shot, while keeping the sum of these gradient areas fixed. This is illustrated in FIG. 6. The pulse sequence diagram of FIG. 6 is identical to that of FIG. 4, except for the gradient area being shifted from the spoiler and fly-back readout magnetic field gradient succeeding the bipolar pair of readout magnetic field gradients to the spoiler readout magnetic field gradient preceding the bipolar pair of readout magnetic field gradients. Moreover, an optional, minor optimization of the timing was carried out, leading to slightly longer acquisition windows.