PARALLEL MR IMAGING WITH SPECTRAL FAT SUPPRESSION

Abstract

The invention relates to a method of magnetic resonance (MR) imaging of an object positioned in an examination volume of a MR device. One aspect of the invention provides a method that enables parallel imaging in combination with fat suppression at an increased image quality, notably in combination with EPI. The method includes: acquiring reference MR signal data from the object in a pre-scan, acquiring imaging MR signal data from the object in parallel via one or more receiving coils having different spatial sensitivity profiles, wherein the MR signal data are acquired with sub-sampling of k-space and with spectral fat suppression, and reconstructing an MR image from the imaging MR signal data, wherein sub-sampling artefacts are eliminated using sensitivity maps indicating the spatial sensitivity profiles of the two or more RF receiving coils. A B.sub.0 map is derived from the reference MR signal data and the spatial dependence of the effectivity of the spectral fat suppression is determined using the B.sub.0 map. In the image reconstruction step, signal contributions from water and fat are separated using regularisation taking the spatial dependence of the effectivity of the spectral fat suppression into account. Moreover, the invention relates to a MR device for carrying out the method, and to a computer program to be run on a MR device.

Claims

1. A method of magnetic resonance (MR) imaging of an object positioned in an examination volume of a MR device, the method comprises: acquiring reference MR signal data from the object; deriving a B.sub.0 map from the reference MR signal data; acquiring imaging MR signal data from the object in parallel via more than one receiving coils having different spatial sensitivity profiles, wherein the MR signal data are acquired with sub-sampling of k-space and with spectral fat suppression; determining the spatial dependence of the effectivity of the spectral fat suppression using the B.sub.0 map; and reconstructing an MR image from the imaging MR signal data, wherein sub-sampling artefacts are eliminated using sensitivity maps indicating the spatial sensitivity profiles of the two or more RF receiving coils wherein signal contributions from water and fat to the imaging MR signal data are separated in the step of reconstructing the MR image using regularisation which takes the spatial dependence of the effectivity of the spectral fat suppression into account.

2. The method of claim 1, wherein the reference MR signal data are acquired using a multi-point Dixon technique.

3. The method of claim 2, wherein signal contributions from water and fat to the reference MR signal data are separated, whereby a water sensitivity map and a fat sensitivity map are derived from the reference MR signal data.

4. The method of claim 3, wherein the fat sensitivity map is computed from the water sensitivity map by a translation in the direction of a fat shift.

5. The method of claim 1, wherein the reference MR signal data are acquired at an image resolution that is lower than the image resolution of the imaging MR signal data.

6. The method of claim 1, wherein the reference MR signal data and/or the imaging MR signal data are acquired by using echo planar imaging.

7. The method of claim 1, wherein the imaging MR signal data are diffusion-weighted.

8. A magnetic resonance device (MR) device comprising: at least one main magnet coil for generating a uniform, steady magnetic field B.sub.0 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 RF coil for generating RF pulses within the examination volume, more than one receiving coils 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, wherein the MR device is configured to: acquire reference MR signal data from the object; derive a B.sub.0 map from the reference MR signal data; acquire imaging MR signal data from the object in parallel via one or more receiving coils having different spatial sensitivity profiles, wherein the MR signal data are acquired with sub-sampling of k-space and with spectral fat suppression; determine the spatial dependence of the effectivity of the spectral fat suppression using the B.sub.0 map; and reconstruct an MR image from the imaging MR signal data, wherein sub-sampling artefacts are eliminated using sensitivity maps indicating the spatial sensitivity profiles of the two or more RF receiving coils wherein signal contributions from water and fat to the imaging MR signal data are separated in the step of reconstructing the MR image using regularisation which takes the spatial dependence of the effectivity of the spectral fat suppression into account.

9. A computer program stored on a non-transitory computer readable medium to be run on a magnetic resonance (MR) device, which computer program comprises instructions for: acquiring reference MR signal data from an object; deriving a B.sub.0 map from the reference MR signal data; acquiring imaging MR signal data from the object in parallel more than one receiving coils having different spatial sensitivity profiles, wherein the MR signal data are acquired with sub-sampling of k-space and with spectral fat suppression; determining the spatial dependence of the effectivity of the spectral fat suppression using the B.sub.0 map; and reconstructing an MR image from the imaging MR signal data, wherein sub-sampling artefacts are eliminated using sensitivity maps indicating the spatial sensitivity profiles of the two or more RF receiving coils wherein signal contributions from water and fat to the imaging MR signal data are separated in the step of reconstructing the MR image using regularisation which takes the spatial dependence of the effectivity of the spectral fat suppression into account.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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:

[0028] FIG. 1 shows a MR device for carrying out the method of the invention;

[0029] FIG. 2 schematically illustrates the method of the invention as a flow chart.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] 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 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.

[0031] 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.

[0032] 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 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.

[0033] 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 as receiving coils to receive MR signals induced by body-coil RF transmissions.

[0034] 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.

[0035] 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). 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.

[0036] 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. 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.

[0037] A practical embodiment of the method of the invention is described with reference to FIG. 2 and with further reference to FIG. 1 as follows:

[0038] After positioning the body 10 in the iso-centre of the main magnet coil 2, a pre-scan is started in step 21 for acquiring reference MR signal data. The pre-scan uses interleaving signal acquisition via the body RF coil 9 and the array RF coils 11, 12, 13. Sensitivity maps CSM.sub.w and CSM.sub.f indicating the spatial sensitivity profiles of the array RF coils 11, 12, 13 for fat and water are derived from the reference MR signal data as well as a B.sub.0 map. A multi-point Dixon technique is employed for this purpose. The reference MR signal data are acquired at low resolution, i.e. from a limited central portion of k-space. The whole pre-scan can thus be performed within a couple of seconds.

[0039] After the pre-scan, an EPI imaging scan with spectral fat suppression and diffusion weighting is performed in step 22 at a higher image resolution, i.e. an image resolution that is sufficient for the respective diagnostic imaging task. Imaging MR signal data are acquired from the body 10 via the array RF coils 11, 12, 13 in parallel with sub-sampling of k-space. Finally, SENSE reconstruction is applied in step 23 to the acquired imaging MR signal data. Therein, sub-sampling artefacts (aliasing) are suppressed using the sensitivity maps obtained from the pre-scan and, finally, an ADC map is derived indicating the spatially resolved apparent diffusion coefficient of water protons in the imaged tissue region.

[0040] According to the invention, the B.sub.0 map is used in step 24 to determine the fat suppression quality, i.e., the spatial dependence of the effectivity of the spectral fat suppression. A too large non-uniformity of B.sub.0 will cause fat suppression failure which is taken into account by the method of the invention. A spatially dependent function f(B.sub.0) is derived from the B.sub.0 map indicating the quality of the fat suppression. In a simple case, f(B.sub.0) may be a binary function as:

[00001]$f\left(B_0\right)=\{\begin{array}{c}\mathrm{.Math.},B_{0}D\\ 1,B_0>D\end{array},\mathrm{.Math.}1$

[0041] In this embodiment, D is a threshold value which is determined in step 24 to indicate from which deviation of B.sub.0 on the spectral fat suppression has to be considered as ineffective.

In step 23, a water image and a fat image are reconstructed simultaneously from the acquired MR signal data m by solving a set of linear equations as follows:

[00002]$\left[\begin{array}{cc}{\mathrm{CSM}}_w& {\mathrm{CSM}}_f\\ R_w^{-1/2}& 0\\ 0& {\left(R_f.Math.f\left(B_0\right)\right)}^{-1/2}\end{array}\right]\left[\begin{array}{c}{p}_w\\ p_f\end{array}\right]=\left[\begin{array}{c}m\\ 0\\ 0\end{array}\right]$

[0042] Therein, m represents the vector of imaging MR signal data for all used receiving coils, p.sub.w and p.sub.f are the vectors of the image values of the water image and the fat image respectively. CSM.sub.w and CSM.sub.f are matrix representations of the sensitivity maps for water and fat for all used receiving coils 11, 12, 13. A regularised SENSE reconstruction scheme is applied. R.sub.w and R.sub.f are the corresponding (diagonal) regularisation matrices for water and fat respectively. The method of the invention may be extended by determining separate functions f.sub.f(B.sub.0) indicating, as f(B.sub.0) above, the quality of the fat suppression, and f.sub.w(B.sub.0) indicating the (unintentional) B.sub.0 inhomogeneity-induced suppression of water. In this case, the set of linear equations describing the reconstruction problem reads:

[00003]$\left[\begin{array}{cc}{\mathrm{CSM}}_w& {\mathrm{CSM}}_f\\ {\left(R_w.Math.f_w\left(B_0\right)\right)}^{-1/2}& 0\\ 0& {\left(R_f.Math.f_f\left(B_0\right)\right)}^{-1/2}\end{array}\right]\left[\begin{array}{c}{p}_w\\ p_f\end{array}\right]=\left[\begin{array}{c}m\\ 0\\ 0\end{array}\right]$

[0043] Water and fat maps derived from the reference MR signal data acquired in step 21 may be used advantageously as regularisation maps R.sub.w and R.sub.f for the reconstruction of the water and fat images respectively. These maps provide valuable information regarding the location of water and fat signal. Hence, using the multi-point Dixon acquisition as a pre-scan enhances conditioning significantly. It has to be noted that the conditioning of the SENSE reconstruction can be improved considerably by the method of the invention. Any signal contribution from fat protons which is not (fully) suppressed can be removed using the proposed technique, thus leading to a very robust fat suppressed SENSE reconstruction and very accurate ADC maps.