EPI MR IMAGING WITH DISTORTION CORRECTION

Abstract

The invention relates to a method of MR imaging of an object (10) positioned in an examination volume of a MR device (1). An object of the invention is to provide a method that enables EPI imaging with improved distortion correction. The method of the invention comprises the steps of: acquiring reference MR signal data from the object (10) using a multi-point Dixon method; deriving a B.sub.0 map from the reference MR signal data; acquiring a series of imaging MR signal data sets from the object (10), wherein an instance of an echo planar imaging sequence is used for acquisition of each imaging MR signal data set; and reconstructing an MR image from each imaging MR signal data set, wherein geometric distortions in each MR image are corrected using the B.sub.0 map. Moreover, the invention relates to a MR device (1) for carrying out the method, and to a computer program to be run on a MR device (1).

Claims

1. A method of magnetic resonance (MR) imaging of an object positioned in an examination volume of a MR device, the method comprising: acquiring reference MR signal data from the object using a multi-point Dixon method; deriving a B.sub.0 map from the reference MR signal data; acquiring a series of imaging MR signal data sets from the object, wherein an instance of an echo planar imaging sequence is used for acquisition of each imaging MR signal data set, and reconstructing a dynamic series of MR images from the imaging MR signal data sets, wherein one of the imaging MR signal data sets is acquired with a direction of the echo planar imaging's phase-encoding gradient blips which is opposite to the direction of the phase-encoding gradient blips used in the acquisition of the other imaging MR signal data sets and said MR signal data set(s) is acquired with said opposite phase-encoding gradient provides prior knowledge that is included in correction using the B.sub.0 map of geometric distortions in each MR image.

2. The method of claim 1, wherein the imaging MR signal data sets are diffusion weighted and acquired for different b-values, wherein a diffusion-weighted MR image is derived from the reconstructed MR images.

3. The method of claim 1, wherein a deformation model is derived from the imaging MR signal data set acquired with opposite phase-encoding gradient blips, which deformation model is used for correcting the geometric distortions in each MR image.

4. The method of claim 1, wherein a water map is derived from the reference MR signal data which is used as prior information in the reconstruction of the MR images.

5. The method of claim 3, wherein the correcting of the geometric distortions is performed by solving an inverse problem with a regularization scheme.

6. The method of claim 5, wherein the regularization scheme biases each corrected MR image towards a solution which is in congruency with a voxel shift map derived from the B.sub.0 map.

7. The method of claim 5, wherein the regularization scheme biases each corrected MR image towards a solution which is in congruency with the deformation model.

8. The method of claim 5, wherein the regularization scheme biases each corrected MR image towards a solution which is in congruency with the water map.

9. The method of claim 5, wherein the regularization scheme biases each corrected MR image towards a solution which is in congruency with a pixel shift map derived from the B.sub.0 map.

10. The method of claim 5, wherein the regularization scheme further biases each corrected MR image towards a solution which is spatially smooth.

11. A magnet resonance (MR) device including at least one main magnet coil for generating a uniform, steady 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, one or more 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 perform method comprising: acquiring reference MR signal data from the object using a multi-point Dixon method; deriving a B.sub.0 map from the reference MR signal data; acquiring a dynamic series of series of imaging MR signal data sets from the object, wherein an instance of an echo planar imaging sequence is used for acquisition of each imaging MR signal data set, and reconstructing an MR image from each imaging MR signal data set, wherein one of the imaging MR signal data sets is acquired with a direction of the echo planar imaging's phase-encoding gradient blips which is opposite to the direction of the phase-encoding gradient blips used in the acquisition of the other imaging MR signal data sets and said MR signal data set(s) is acquired with said opposite phase-encoding gradient provides prior knowledge that is included in correction using the B.sub.0 map of geometric distortions.

12. The computer program to be run on a MR device, which computer program comprises instructions stored in a non-transitory computer readable medium for: acquiring reference MR signal data using a multi-point Dixon method; deriving a B.sub.0 map from the reference MR signal data; acquiring a dynamic series series of imaging MR signal data sets, wherein an instance of an echo planar imaging sequence is used for acquisition of each imaging MR signal data set, and reconstructing an MR image from each imaging MR signal data set, wherein one of the imaging MR signal data sets is acquired with a direction of the echo planar imaging's phase-encoding gradient blips which is opposite to the direction of the phase-encoding gradient blips used in the acquisition of the other imaging MR signal data sets and said MR signal data set(s) is acquired with said opposite phase-encoding gradient provides prior knowledge that is included in correction using the B.sub.0 map of geometric distortions in each MR image.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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:

[0029] FIG. 1 shows a MR device for carrying out the method of the invention;

[0030] FIG. 2 schematically shows the method of the invention as a flow chart;

[0031] FIG. 3 shows examples of brain images illustrating the application of the method of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] A practical embodiment of the method of the invention is described with reference to FIGS. 2 and 3 and with further reference to FIG. 1 as follows:

[0040] 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. 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.

[0041] After the pre-scan, an EPI imaging scan with spectral fat suppression and diffusion weighting is performed at a higher image resolution, i.e. an image resolution that is sufficient for the respective diagnostic imaging task. In step 22, a single imaging MR signal data set is acquired with blip-down phase-encoding for a zero b-value. In step 23, a number of imaging MR signal data sets is acquired with blip-up phase-encoding, again for a zero b-value and further for a number of different b-values.

[0042] The reference MR signal data of the Dixon pre-scan are reconstructed in step 4 and a B0 map and a water map are derived. In step 25, a voxel shift map S.sub.0 is computed from the B0 map and distortion models corresponding to the blip-up and blip-down acquisitions are derived (D.sub.up and D.sub.down respectively) starting from the B.sub.0 map. A distorted reference image I.sub.down is reconstructed in step 25 from the blip-down imaging MR signal data. In step 26, a distorted blip-up MR image I.sub.up .sub.j is reconstructed for each b-value j. A distortion correction is applied in step 27 to produce the undistorted MR images I.sub.j. Finally, in step 28, an ADC map is derived indicating the spatially resolved apparent diffusion coefficient of water protons in the imaged tissue region.

[0043] The correcting of the geometric distortions is performed in step 27 for each MR image of the series by solving an inverse problem with a regularization scheme as follows:

[00001]$\hat{I}=\arg \underset{I}{\min }C\left(D_{\mathrm{up}}I-I_{\mathrm{up}}\right)+\alpha {.Math.\frac{I}{\mathrm{Water}\mathrm{Map}}.Math.}_2^2+\beta {.Math.\frac{D_{\mathrm{down}}I}{I_{\mathrm{down}}}.Math.}_2^2+\gamma {.Math.W{\nabla }_{y}I.Math.}_2^2,W=\exp \left(-{\nabla }_y{S}_0\right)$

[0044] Where C(D.sub.upI−I.sub.up) is a data consistency term of any kind, ∇.sub.3, performs the derivative operation along the phase encoding direction, α, β, γ are regularization parameters. The regularization scheme finds a solution for the distortion-corrected image Î which is in congruency with the deformation models resulting from both the blip-up and blip-down acquisitions and which is also in conformity with the water map.

[0045] In a possible variation of the described scheme, the distortion models D.sub.up and D.sub.down are derived in the step in step 25 only from the blip-up and blip-down acquisitions, i.e., without combining the information of the B.sub.0 map. The prior knowledge on the water signal distribution (water map) could be replaced in step 27 by a prior knowledge on the combination of the water and fat signals in case of a predictably non-successful fat suppression, or it can be omitted. The local smoothness enforcing term ∥W∇.sub.yI|.sub.2.sup.2, W=exp(−∇.sub.yS.sub.0), can have a different formulation, or it can be omitted. Any formulation penalizing negative slopes of the shift map S.sub.0 would be a valid alternative.

[0046] FIG. 3 shows examples of DWI brain images illustrating the application of the method of the invention. Each of the two depicted panels shows in the top row the distorted MR images and in the bottom row the corresponding distortion-corrected MR images. The true anatomy borders are shown as overlays in each image. As can be seen from FIG. 3, the EPI images acquired and corrected according to the invention have substantially reduced geometric distortions.