MR imaging with dixon-type water/fat separation

Abstract

A Dixon water/fat separation technique, in particular in combination with a single-point acquisition scheme, avoids swaps of water and fat signals in the reconstructed MR images due to imperfections of the main magnetic field B.sub.0. Echo signals are generated and acquired in a pre-scan by subjecting an object (10) to a two or more point imaging sequence. A fat fraction map is derived from the echo signals of the pre-scan. Echo signals are generated and acquired in a clinical scan by subjecting the object (10) to a single-point imaging sequence. A field map estimate is derived from the fat fraction map and from the echo signals of the clinical scan. An MR image is reconstructed from the echo signals of the clinical scan. Signal contributions from fat and water are separated on the basis of the field map estimate.

Claims

1. A method of magnetic resonance (MR) imaging, the method comprising: generating and acquiring chemical shift encoded echo signals in a pre-scan by subjecting an object to a lower resolution first imaging sequence having two or more different echo times; deriving a lower resolution fat fraction map from the echo signals of the pre-scan; generating and acquiring echo signals in a clinical scan by subjecting the object to a higher resolution second imaging sequence having a single echo time; deriving a field map estimate from the fat fraction map and from the echo signals of the clinical scan; and reconstructing a higher resolution MR image from the echo signals of the clinical scan, wherein signal contributions from fat and water to the echo signals of the clinical data are separated using a single-point Dixon technique and on the basis of the field map estimate.

2. The method of claim 1, wherein the fat fraction map is derived using a two- or multi-point Dixon technique.

3. The method of claim 1, wherein the spatial resolution of the pre-scan is lower than the spatial resolution of the clinical scan.

4. The method of claim 1, wherein the field map estimate is derived at the spatial resolution of the pre-scan.

5. The method of claim 4, wherein the field map estimate is computed at the resolution of the clinical scan by interpolation prior to separating the signal contributions from fat and water in the step of reconstructing the MR image.

6. The method of claim 1, wherein smoothing is applied to the field map estimate prior to separating the signal contributions from fat and water in the step of reconstructing the MR image.

7. The method of claim 1, wherein the fat fraction map is corrected for differences in contrast weighting between the pre-scan and the clinical scan, using knowledge of tissue type and sequence parameters of the pre-scan and the clinical scan.

8. A magnetic resonance (MR) device comprising: at least one main magnet coil for generating a steady main 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 arranged to perform the following steps: generating and acquiring chemical shift encoded lower resolution echo signals in a pre-scan by subjecting the object to a first imaging sequence; deriving a lower resolution fat fraction map from the lower resolution echo signals of the pre-scan using a two or multi-point Dixon technique; generating and acquiring higher resolution echo signals in a clinical scan by subjecting the object to a second imaging sequence; deriving a field map estimate from the fat fraction map and from the echo signals of the clinical scan; and reconstructing a higher resolution MR image from the echo signals of the clinical scan, wherein signal contributions from fat and water are separated using a single-point Dixon technique and on the basis of the field map estimate.

9. A computer program stored on a non-transitory computer readable medium to be executed on a magnetic resonance (MR) device, wherein the computer program comprises instructions for: generating and acquiring chemical shift encoded echo signals in a pre-scan by subjecting an object to a first imaging sequence with two or more echo times; deriving a fat fraction map from the echo signals of the pre-scan; generating and acquiring echo signals in a clinical scan by subjecting the object to a second imaging sequence with a single echo time; deriving a field map estimate from the fat fraction map and from the echo signals of the clinical scan; and reconstructing an MR image from the echo signals of the clinical scan, wherein signal contributions from fat and water are separated in the clinical scan using a single point Dixon technique and on the basis of the field map estimate.

10. A method of magnetic resonance (MR) imaging, the method comprising: generating and acquiring chemical shift encoded echo signals from a central region of k-space in a pre-scan by subjecting an object to a first imaging sequence with two or more different echo times; deriving a lower resolution fat fraction map with a coarse grid from the echo signals of the pre-scan; generating and acquiring echo signals in a clinical scan by subjecting the object to a second imaging sequence that generates the echo signals of the clinical scan at a single echo time; deriving a field map estimate indicative of a phase shift for each voxel of the coarse grid due to inhomogeneity of the main magnetic field B.sub.0 from the fat fraction map and from the echo signals of the clinical scan; interpolating the field map to a finer grid; separating signal contributions from fat and water to the echo signals of the clinical scan using a single-point Dixon technique; and reconstructing a higher resolution MR image with the finer grid from the signal contributions from water and fat and on the basis of the field map estimate.

11. The method of claim 10, wherein the fat fraction map is derived using a two or multi-point Dixon technique.

12. A magnetic resonance (MR) device including: at least one main magnet coil for generating a steady main 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, and one or more computer processors configured to control the gradient coils and the at least one RF coil to carry out the method of claim 10.

13. A computer program stored on a non-transitory computer readable medium to be executed on a magnetic resonance (MR) device, which computer program comprises instructions for controlling the MR device to perform the method of claim 10.

14. A method of magnetic resonance (MR) imaging comprising: generating at least one of a low resolution fat fraction map and a low resolution water fraction map from signals acquired from a central region of k-space; in a clinical scan, generating an MR image using MR echo signals acquired from all of k-space using a one-point Dixon technique, the one-point Dixon technique failing to accurately separate relative water and fat contributions in regions with mixed water and fat voxels; and, during generating the MR image, using the at least one of the fat fraction map and the water fraction map to separate the relative water and fat contributions in the regions with mixed water and fat voxels.

15. The method of claim 14, wherein the at least one of the low resolution fat fraction map and the water fraction map includes using a two- or multi-point Dixon technique.

16. The method of claim 15, further including: deriving a B.sub.0 field map estimate from the at least one of the fat fraction map and the water fraction map and from the MR echo signals of the clinical scan; and wherein the water and fat contributions are separated using the field map.

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 a practical example of a fat fraction map (left) derived from the pre-scan according to the invention, and the corresponding field map estimate (right) on a coarse spatial grid;

(4) FIG. 3: shows a water image (left) and a fat image (right) reconstructed according to the invention from the clinical scan data and the field map estimate 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) 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. 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, e.g. 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 pre-amplifier (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 are reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such as 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) According to the invention, echo signals are generating and acquired at a low spatial resolution in a pre-scan by application of a first imaging sequence. This first imaging sequence may be any conventional two- or multi-point spin echo or gradient echo sequence, such as, for example, a turbo spin echo sequence using appropriate specific echo times. The voxel size of the pre-scan is 5 mm5 mm5 mm or more. The duration of this pre-scan is approximately 5 seconds or less. A fat fraction map is derived from the echo signals of the pre-scan in conventional well-known manner on the coarse grid of the pre-scan. As a next step, a clinical scan is performed at a higher resolution as required for the respective diagnostic task (voxel size 1 mm1 mm1 mm or less) by application of a second imaging sequence. The second imaging sequence is a single-echo sequence, as conventionally used for single-point Dixon imaging, in which the echo signals are acquired at only a single specific echo time.

(13) The signals of the clinical scan can be modeled as:
s(t)=(w+fc(t))z(t)

(14) Wherein s(t) is the measured (complex) signal at time t for a given voxel; w denotes the water contribution (real number); f denotes the fat contribution (real number); c(t) denotes the amplitude and phase of the fat contribution at time t relative to the water contribution (complex number). c(t) is a priori known and is determined by the MR spectrum of the fat protons (the spectral model as conventionally used in Dixon water/fat separation algorithms), and z(t) is a phasor representing the field map estimate within the meaning of the present invention. z(t) denotes the phase evolution due to inhomogeneity of the main magnetic field B.sub.0 at the given voxel position.

(15) The above formula can be rewritten as:

(16) $s\left(t\right)=w\left(1+\mathrm{gc}\left(t\right)\right)z\left(t\right);g=\frac{f}{w}$

(17) The fat fraction FF, which is available for the given voxel from the fat fraction map, is computed as:

(18) $\mathrm{FF}=\frac{f}{w+f}.fwdarw.s\left(t\right)=f\left(\frac{1-\mathrm{FF}}{\mathrm{FF}}+c\left(t\right)\right)z\left(t\right)$

(19) On this basis, the phase of z(t) can be calculated as:

(20) $\arg \left(z\left(t\right)\right)=\arg \left(\frac{s\left(t\right)}{\frac{1-\mathrm{FF}}{\mathrm{FF}}+c\left(t\right)}\right)$

(21) Because only the phase is needed to compute the fieldmap estimate, the unknowns w or f can be ignored. Hence, the above equations show that the field map estimate is fully defined if the fat fraction map is known.

(22) In case the pre-scan has a different contrast weighting than the clinical scan, the fat fraction map derived from the echo signals of the pre-scan needs to be corrected prior to further processing. In general, given the parameters of the first and second imaging sequences (referred to as scan1 and scan2 in the following) and the properties of the tissue, one can correct for signal modifications due to T.sub.1, T.sub.2, T.sub.2* etc. using the well-known signal equations. In general, the following equation applies to relate the signal from a given tissue in one scan to another:

(23) $s_{\mathrm{scan}2}=s_{\mathrm{scan}1}\frac{f\left(T_1,T_2,T_2^{*},\mathrm{sequence}\mathrm{parameters}\mathrm{scan}2\right)}{f\left(T_1,T_2,T_2^{*},\mathrm{sequence}\mathrm{parameters}\mathrm{scan}1\right)}=s_{\mathrm{scan}1}{A}_{12}$

(24) Therein the function f depends on the type of sequence and the set of sequence parameters containing, e.g., repetition time, flip angle, inversion time, echo time, etc. The parameter A.sub.12 denotes the variation in signal between two scans for a given tissue type. The parameter A.sub.12 can hence be defined for water tissue (w) and for fat tissue (f):
w.sub.scan2=w.sub.scan1A.sub.12,w
f.sub.scan2=f.sub.scan1A.sub.12,f

(25) These relations can be used to correct the fat fraction (FF) map:

(26) ${\mathrm{FF}}_{\mathrm{scan}1}=\frac{f_{\mathrm{scan}1}}{w_{\mathrm{scan}1}+f_{\mathrm{scan}1}}=\frac{1}{\frac{w_{\mathrm{scan}1}}{f_{\mathrm{scan}1}}+1}=\frac{1}{g_{\mathrm{scan}1}+1}.fwdarw.g_{\mathrm{scan}1}=\frac{1-{\mathrm{FF}}_{\mathrm{scan}1}}{{\mathrm{FF}}_{\mathrm{scan}1}}$$g_{\mathrm{scan}2}=\frac{w_{\mathrm{scan}2}}{f_{\mathrm{scan}2}}=\frac{w_{\mathrm{scan}1}{A}_{12,w}}{f_{\mathrm{scan}1}{A}_{12,f}}=g_{\mathrm{scan}1}\frac{A_{12,w}}{A_{12,f}}=g_{\mathrm{scan}1}{A}_{12,g}$${\mathrm{FF}}_{\mathrm{scan}2}=\frac{1}{g_{\mathrm{scan}2}+1}=\frac{1}{g_{\mathrm{scan}1}{A}_{12,g}+1}$${\mathrm{FF}}_{\mathrm{scan}2}=\frac{1}{\frac{1-{\mathrm{FF}}_{\mathrm{scan}1}}{{\mathrm{FF}}_{\mathrm{scan}1}}{A}_{12,g}+1}={\mathrm{FF}}_{\mathrm{scan}1}\frac{1}{A_{12,g}+{\mathrm{FF}}_{\mathrm{scan}1}\left(1-A_{12,g}\right)}$

(27) Once the corrected fat fraction map FF.sub.scan2 is obtained, it can be readily used in the further process for water/fat separation as described above.

(28) FIG. 2 shows the fat fraction map derived from the pre-scan (left image). The corresponding field map estimate derived as described before is shown in the right image. Both the fat fraction map and the field map estimate are now available on the coarse spatial grid of the pre-scan.

(29) Optionally, spatial smoothing of the field map estimate may be applied before further processing. The field map estimate is required on the finer spatial grid of the clinical scan on which the water/fat separation needs to be performed. This can be achieved simply by interpolation.

(30) For the reconstruction of water and fat MR images, again the above signal model is applied:
s(t)=(w+fc(t))z(t)

(31) The real numbers w and f can now be derived straight forward as the complex values s(t) (the measured signal), c(t) (the spectral model), and also z(t) (the field map estimate) are known. w and f are obtained by solving the following system of equations:

(32) $\left[\begin{array}{c}\mathrm{Re}\left(s\left(t\right)z^{*}\left(t\right)\right)\\ \mathrm{Im}\left(s\left(t\right)z^{*}\left(t\right)\right)\end{array}\right]=\left[\begin{array}{cc}1& \mathrm{Re}\left(c\left(t\right)\right)\\ 0& \mathrm{Im}\left(c\left(t\right)\right)\end{array}\right]\left[\begin{array}{c}w\\ f\end{array}\right]$

(33) The inversion is possible if the fieldmap estimate is known and Im(c(t)) is not zero. The condition Im(c(t))=0 corresponds to an echo time at which the water and fat contributions are either out of phase or in phase. Hence, the described method based on a single-point acquisition works only works when these two cases do not apply. An in phase echo time or an out of phase echo time should not be used. The closer the acquisition gets to Im(c(t))=+/1, the better the system is conditioned and the better the quality of the water/fat separation will be. Im(c(t))=+/1 corresponds to a partially out of phase and partially in phase echo time.

(34) FIG. 3 shows the reconstructed water image (left) at the high resolution of the clinical scan and the corresponding fat image (right). As can be seen, the separation of water and fat is correct in the spine region (arrow) which is known to be a problematic region in conventional single-point Dixon imaging techniques in terms of water/fat swaps.

(35) 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.