ENHANCED 3D RADIAL MR IMAGING

Abstract

The invention relates to a method of MR imaging of an object (10) positioned in an examination volume of a MR device (1). It is an object of the invention to provide an arrangement and ordering of the radial k-space spokes for 3D radial imaging that achieves an efficient and uniform k-space coverage. The method of the invention comprises the steps of: —specifying a set of radial k-space spokes to cover a spherical k-space volume, which set is subdivided into a number of subsets, wherein the end points of the spokes of each subset are distributed along a trajectory forming a spherical spiral in k-space with subsampling along the trajectory and wherein the trajectories of the different subsets are rotated relative to each other about an axis passing through the k-space origin, generating MR signals by subjecting the object (10) to an imaging sequence, wherein the MR signals are acquired to sample the spokes of one of the subsets, executing step b) for each of the subsets until the full set of spokes is sampled, reconstructing an MR image from the acquired MR signals. Moreover, the invention relates to a MR device and to a computer program for 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 comprising: specifying a set of radial k-space spokes to cover a spherical k-space volume, which set is subdivided into a number of subsets, wherein the end points of the spokes of each subset are distributed along a trajectory forming a spherical spiral in k-space with subsampling along the trajectory and the trajectories of the different subsets are rotated relative to each other about an axis passing through the k-space origin, such that the distance between adjacent end points of the spokes along each trajectory equals or approximates the distance between the windings of the spiral such that the spokes of each subset are uniformly distributed over the spherical k-space volume, generating MR signals by subjecting the object to an imaging sequence, wherein the MR signals are acquired to sample the spokes of one of the subsets, rotating the trajectories of the different subsets for each of the subsets until the full set of spokes is sampled, and reconstructing an MR image from the acquired MR signals.

2. The method of claim 1, wherein the trajectories of subsequently sampled subsets are rotated relative to each other by the golden angle, the small golden angle, any of the tiny golden angles, or fractions thereof.

3. The method of claim 1, wherein the imaging sequence is a zero echo time or ultra-short echo time imaging sequence, wherein each of the subsets is sampled as a sequence of free induction decay signals.

4. The method of claim 1, wherein the imaging sequence is a spoiled or refocused gradient echo imaging sequence, wherein each of the subsets is sampled as a sequence of gradient echo signals.

5. The method of claim 1, wherein the imaging sequence is a combination of a zero echo time or ultra-short echo time imaging sequence and of a spoiled or refocused gradient echo imaging sequence, wherein each of the subsets is sampled as a sequence of free induction decay signals and gradient echo signals.

6. The method of claim 1, wherein each trajectory reaches from one pole to the other pole of the spherical k-space volume.

7. The method of claim 1, wherein each trajectory starts at the pole the preceding trajectory ended at.

8. The method of claim 1, wherein the spokes are sampled in the sequence in which they are aligned along each trajectory.

9. A magnetic resonance (MR) device comprising at least one main magnet coil for generating a uniform, static magnetic field 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, wherein the MR device is arranged to perform a method, the method comprising: specifying a set of radial k-space spokes to cover a spherical k-space volume, which set is subdivided into a number of subsets, wherein the end points of the spokes of each subset are distributed along a trajectory forming a spherical spiral in k-space with subsampling along the trajectory and the trajectories of the different subsets are rotated relative to each other about an axis passing through the k-space origin, such that the distance between adjacent end points of the spokes along each trajectory equals or approximates the distance between the windings of the spiral such that the spokes of each subset are uniformly distributed over the spherical k-space volume, generating MR signals by subjecting the object to an imaging sequence, wherein the MR signals are acquired to sample the spokes of one of the subsets, rotating the trajectories of the different subsets for each of the subsets until the full set of spokes is sampled, reconstructing an MR image from the acquired MR signals.

10. A computer program to be run on a magnetic resonance (MR) device, wherein the computer program is stored on a non-transitory computer readable medium and the computer program comprises instructions for: specifying a set of radial k-space spokes to cover a spherical k-space volume, which set is subdivided into a number of subsets, wherein the end points of the spokes of each subset are distributed along a trajectory forming a spherical spiral in k-space with subsampling along the trajectory and the trajectories of the different subsets are rotated relative to each other about an axis passing through the k-space origin, such that the distance between adjacent end points of the spokes along each trajectory equals or approximates the distance between the windings of the spiral such that the spokes of each subset are uniformly distributed over the spherical k-space volume, generating an imaging sequence and acquiring MR signals to sample the spokes of one of the subsets, rotating the trajectories of the different subsets for each of the subsets until the full set of spokes is sampled, and reconstructing an MR image from the acquired MR signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] 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:

[0039] FIG. 1 schematically shows a MR device for carrying out the method of the invention;

[0040] FIG. 2 illustrates the conventional arrangement of the end points of the spokes of one subset on a spherical spiral trajectory;

[0041] FIG. 3 illustrates the conventional arrangement of the end points of the spokes of one subset on a spiral phyllotaxis trajectory;

[0042] FIG. 4 illustrates the arrangement of the end points of the spokes of one subset according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] With reference to FIG. 1, a MR device 1 which can be used for carrying out the method of the invention 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, 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.

[0044] A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to excite, invert, or saturate nuclear magnetic spins, to induce, refocus, and manipulate magnetic resonance, to spatially and otherwise encode magnetic resonance, and the like to perform MR imaging.

[0045] 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 transmitter 7 transmits RF pulses, 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 RF pulses of short duration which, taken together with any applied magnetic field gradients, achieve a selected manipulation of magnetic resonance, including the selection of a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the body RF coil 9.

[0046] For generation of MR images of a limited region of the body 10 by means of parallel imaging, a set of local array RF coils 11, 12, 13 is placed contiguous to the region to be imaged. The array coils 11, 12, 13 can be used to receive MR signals induced by RF transmissions with the body RF coil.

[0047] 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.

[0048] A host computer 15 controls the current flow through the shimming coils 2′ as well as the gradient pulse amplifier 3 and the RF transmitter 7 to generate, e.g., a ZTE or a refocused gradient echo imaging sequence according to the invention. The receiver 14 receives the MR signal from the individual radial k-space spokes after the RF excitation pulses in rapid succession. A data acquisition system 16 performs analog-to-digital conversion of the received MR signal and converts it 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 the acquisition of raw image data.

[0049] Ultimately, the digital raw data is reconstructed into an image representation by a reconstruction processor 17 which applies an appropriate reconstruction algorithm. The MR image represents a three-dimensional volume. The image is then stored in an image memory where it may be accessed for converting projections or other portions of the image representation into an appropriate format for visualization, for example via a video monitor 18 which provides a human-readable display of the resultant MR image.

[0050] According to the invention, 3D radial MR imaging is performed, wherein a number of radial k-space spokes is acquired to cover a spherical volume in k-space. The radial k-space spokes are defined by polar and azimuthal rotation angles that are incremented independently during the acquisition, wherein the end points of the radial k-space spokes are distributed on the surface of the spherical k-space volume. The acquired MR signal is finally gridded onto a Cartesian k-space grid and then reconstructed into an MR image via Fourier transformation.

[0051] Trajectories are specified that determine both the positions of the end points of the spokes on the sphere and the temporal order in which the spokes are sampled.

[0052] It is essential to arrange and order the spokes in k-space and time properly to achieve a uniform and efficient k-space coverage.

[0053] A nearly uniform distribution of the end points of the spokes on the surface of the sphere and a minimal distance between consecutively sampled end points are achieved by aligning the end points along a spherical spiral trajectory traversing the surface of the sphere from one pole to the other pole. This is illustrated in FIG. 2. The two k-space diagrams show the end points of the spokes of one subset each, following a spherical spiral trajectory, wherein the end points of consecutively sampled spokes are interconnected by a straight line. In the depicted embodiment, the full set of spokes covering the spherical k-space volume is subdivided into 36 subsets. The subset shown in the left diagram comprises all spokes of 1 out of 36 trajectories. To fully cover the spherical k-space volume, the shown trajectory is rotated around the k.sub.z-axis. This subdivision into subsets does obviously not provide a uniform distribution of the end points on the surface of the sphere. The distance between the end points along the trajectory is much smaller than the distance of the windings of the spiral. The subset shown in the right diagram comprises every 36th spoke of 1 out of 1 trajectory. To fully cover the spherical k-space volume, the shown trajectory is basically kept, as is the selection of every 36.sup.th spoke, but the selection of the first spoke is changed. This subsampling along the trajectory leads to a large distance between the end points along the trajectory.

[0054] Another way of specifying the distribution of the end points of the spokes on the surface of the sphere is based on spiral phyllotaxis. As shown in FIG. 3, the spiral phyllotaxis trajectory achieves a better, but still far from uniform distribution of the end points of the spokes on the surface of the sphere. In the left diagram, the end points of the spokes of 1 out of 34 trajectories are plotted. Choosing a Fibonacci number, such as 34, for the number of subsets, leads to a more uniform distribution of the end points of the spokes on the surface of the sphere than in FIG. 2. For other numbers, such as 36, however, the distance between the end points of consecutively sampled spokes is again very large, as seen in the right diagram.

[0055] To obtain a uniform distribution of the end points of the spokes, for both the entire set and each subset, and a minimal distance between the end points of consecutively sampled spokes, the invention proposes a modified spherical spiral trajectory. In addition to the subdivision into subsets, in which the trajectories are rotated relative to each other, a matching subsampling along the trajectories is introduced. A uniform distribution of the end points of the spokes is thus achieved for each subset, as evident from FIG. 4. While the left diagram shows the end points of the spokes of 1 out of 34 subsets, the right diagram shows those of 1 out of 36 subsets. By choosing the number of subsets equal to the subsampling factor along the trajectories, the distance between the end points of consecutively sampled spokes along the trajectory approximates the distance between adjacent windings of the spiral. Notably, the overall reduction factor does not have to be an integer. Thus, the choice of the overall acceleration is much more flexible with the proposed trajectory than with the spiral phyllotaxis trajectory described above.

[0056] As with the spherical spiral trajectory and the spiral phyllotaxis trajectory, the choice of the rotation angles between the spiral trajectories of the different subsets is unrestricted in the method of the invention, since the distance between end points vanishes at the poles of the sphere. Therefore, in particular a rotation according to the golden angle is still possible.