MR IMAGING USING A STACK-OF-STARS ACQUISITION WITH INTRINSIC MOTION CORRECTION

Abstract

The invention relates to a method of MR imaging of an object (10). It is an object of the invention to enable MR imaging using the stack-of-stars or stack-of-spirals acquisition scheme providing an enhanced image quality in the presence of motion. The method of the invention comprises the steps of:—generating MR signals by subjecting the object to an imaging sequence comprising RF pulses and switched magnetic field gradients;—acquiring signal data according to a stack-of-stars or stack-of-spirals scheme, wherein the MR signals are acquired as radial or spiral k-space profiles from a number of parallel slices arranged at adjacent positions along a slice direction, wherein a central portion (20) of k-space is more densely sampled during the acquisition than peripheral portions (21) of k-space;—reconstructing an intermediate MR image (22-25) from sub-sampled signal data for each of a number of successive time intervals;—deriving motion induced displacements and/or deformations by registering the intermediate MR images (22-25) with each other; and—combining the sub-sampled signal data and reconstructing a final MR image therefrom, wherein a motion correction is applied according to the derived motion induced displacements and/or deformations. Moreover, the invention relates to a MR device (1) and to a computer program for a MR device (1).

Claims

1. A method of magnetic resonance (MR) imaging of an object positioned in the examination volume of a MR device, the method comprising: generating MR signals by subjecting the object to an imaging sequence comprising RF pulses and switched magnetic field gradients; acquiring signal data according to a stack-of-stars or stack-of-spirals scheme, wherein the MR signals are acquired as radial or spiral k-space profiles from a number of parallel slices arranged at adjacent positions along a slice direction, sampling an ellipsoidal or spherical volume of k-space, wherein a three-dimensional ellipsoidal or spherical central portion of k-space is more densely sampled during the acquisition than peripheral portions of k-space; reconstructing respective intermediate MR images from sub-sampled signal data for each of a number of successive time intervals; determining by way of elastic image registration geometric transformations which transform each of the different intermediate MR images into one common coordinate system, to compensate for the occurred motion resulting in motion-corrected intermediate MR images applying the geometric transformations to the intermediate MR images to compensate for the occurred motion resulting in motion-corrected intermediate MR images and combining the corrected intermediate MR images into a high-resolution final MR image.

2. The method of claim 1, wherein the central portion of k-space is sampled in accordance with the Nyquist criterion while the peripheral portions of k-space are sub-sampled in each time interval.

3. The method of claim 2, wherein the central portion of k-space is sampled more closely in time as compared to the peripheral portions.

4. The method of claim 3, wherein the motion corrected intermediate MR images are combined either in image space or in k-space into the final MR image.

5. Method The method of claim 1, wherein the intermediate MR images are reconstructed using a k-space weighted image contrast (KWIC) filter.

6. The method of claim 4, wherein the intermediate MR images are corrected according to the derived motion induced displacements and/or deformations attributed to the respective time interval.

7. The method of claim 1, wherein the signal data is weighted in the reconstruction of the final MR image corresponding to the extent of the derived motion induced displacements and/or deformations.

8.The method of claim 7, wherein the weighting is derived from a measure of similarity of the motion corrected intermediate MR images.

9. The method of claim 2, wherein the imaging sequence is a turbo field echo (TFE) or a balanced (turbo) field echo sequence or an echo planar imaging (EPI) sequence or a turbo spin echo (TSE) sequence or a GRASE sequence.

10. The method of claim 1, wherein the rotation angle of the radial k-space profiles is incremented according to a golden angle scheme during the acquisition of successive k-space profiles.

11. A magnetic 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 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 a method, the method comprising: generating MR signals by subjecting the object to an imaging sequence comprising RF pulses and switched magnetic field gradients; acquiring signal data according to a stack-of-stars or stack-of-spirals scheme, wherein the MR signals are acquired as radial or spiral k-space profiles from a number of parallel slices arranged at adjacent positions along a slice direction, sampling an ellipsoidal or spherical volume of k-space, wherein a three-dimensional ellipsoidal or spherical central portion of k-space is more densely sampled during the acquisition than peripheral portions of k-space; reconstructing respective intermediate MR images from sub-sampled signal data for each of a number of successive time intervals; determine by way of elastic image registration geometric transformations which transform each of the different intermediate MR images into one common coordinate system, to compensate for the occurred motion resulting in motion-corrected intermediate MR images deriving motion induced displacements and/or deformations by registering the intermediate MR images with each other; and combining the corrected intermediate MR images into a high-resolution final MR image.

12. A computer program to be run on a MR device, which computer program comprises executable instructions stored on a non-transitory computer readable medium to perform a method, the method comprising: generating an imaging sequence comprising RF pulses and switched magnetic field gradients; acquiring signal data according to a stack-of-stars or stack-of-spirals scheme, wherein MR signals are acquired as radial or spiral k-space profiles from a number of parallel slices arranged at adjacent positions along a slice direction, sampling an ellipsoidal or spherical volume of k-space, wherein a three-dimensional ellipsoidal or spherical central portion of k-space is more densely sampled during the acquisition than peripheral portions of k-space; reconstructing an intermediate MR image from sub-sampled signal data for each of a number of successive time intervals; determining by way of elastic image registration geometric transformations which transform each of the different intermediate MR images into one common coordinate system, to compensate for the occurred motion resulting in motion-corrected intermediate MR images, deriving motion induced displacements and/or deformations by registering the intermediate MR images with each other; and combining the corrected intermediate MR images into a high-resolution final MR image.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] 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:

[0034] FIG. 1 shows a block diagram of a MR device for carrying out the method of the invention;

[0035] FIG. 2 schematically illustrates the acquisition and reconstruction scheme of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] With reference to FIG. 1, a 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 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.

[0038] More specifically, a gradient amplifier 3 applies current pulses or waveforms 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 signals. 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.

[0039] For generation of MR images of limited regions of the body 10 or for scan acceleration 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.

[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 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 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.

[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 or GRAPPA. 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 is programmed to execute the method of the invention described herein above and in the following.

[0044] FIG. 2 schematically illustrates an embodiment of the method of the invention. The left column shows sampling of k-space by stack-of-stars imaging in a number of successive time intervals (four in the depicted embodiment) according to the invention. Echo signals are acquired using multi-shot TFE imaging. In each of a number of shots comprising a spatially non-selective or slab-selective RF excitation, a train of echo signals is acquired, wherein each echo signal represents a k-space profile. The echo signals are acquired as radial k-space profiles from a number of parallel k-space slices (five in the depicted embodiment). In an alternative embodiment, the radial k-space profiles may be shifted or rotated for an improved volumetric distribution. The slices are arranged at different positions along slice direction k.sub.z. In the k.sub.z-direction Cartesian phase-encoding is performed, while the echo signals are acquired within each single slice along radial ‘spokes’ that are rotated around the center (k.sub.x=k.sub.y=0). This results in a cylindrical k-space coverage composed of stacked discs. A spherical volume is sampled in a variable density fashion, wherein the central region 20 is more frequently updated during the stack-of-stars acquisition than the peripheral k-space portions 21 (Nyquist representation). The four time intervals are selected such that the central volume 20 is fully sampled in each time interval, while the peripheral portions 21 of k-space are undersampled in each time interval. For the angular ordering of the k-space spokes, the golden angle-scheme is employed. The rotation angle of the spokes is incremented from echo signal to echo signal by ΔΦ=111.25°. Intermediate MR Images 22-25 (second column from the left in FIG. 2) are reconstructed from the sub-sampled k-space data for each of the four time intervals (e.g. on a high resolution sampling grid). The four reconstructed intermediate MR images are then registered with each other. An elastic image registration algorithm is used to determine a set of transformations T which transform each of the different intermediate MR images 22-25 into one common coordinate system. The set of transformations T reflects displacements and deformations induced by motion occurring between the time intervals. The set of transformations T is then applied to the intermediate MR images 22-25 to compensate for the occurred motion resulting in motion-corrected intermediate MR images 22-25 (third column from the left in FIG. 2). Finally, the corrected intermediate MR images 22-25 are combined into a high-resolution MR image 26 which is essentially free of motion artifacts.