EFFICIENT SELF-REFOCUSING ZERO ECHO TIME MR IMAGING

Abstract

The invention relates to a method of MR imaging of an object positioned in an examination volume of a MR device (1). It is an object of the invention to enable efficient silent ZTE imaging with self-refocusing. The method of the invention comprises the steps of:—specification of a set of radial k-space spokes to cover a spherical k-space volume;—selection of subsets of a predetermined number of spokes from the specified set so that the concatenation of the spokes contained in each of the subsets forms a closed trajectory in k-space, wherein the selection of the subsets involves optimizing a cost function;—subjecting the object (10) to a zero echo time imaging sequence, wherein each of the subsets of spokes is acquired as a sequence of gradient echo signals; and—reconstructing an MR image from the acquired spokes. Moreover, the invention relates to a MR device and to a computer program for a MR device.

Claims

1. The method of magnetic resonance (MR) imaging of an object positioned in an examination volume of a MR device (1), the method comprising: specification of a set of radial k-space spokes to cover a spherical k-space volume; selection of subsets, with each subset comprising a predetermined number of spokes from the specified set so that the concatenation of the spokes contained in each of the subsets forms a closed trajectory in k-space, wherein the selection of the subsets is based on optimizing a cost function favouring (i) minimizing the maximum angle between subsequently acquired spokes within the same subset, (ii) maximizing the uniformity of the distribution of all spokes end points on the surface of the covered spherical k-space volume, while ensuring that the concatenation of the spokes of each subset forms a closed trajectory in k-space; subjecting the object to a zero echo time imaging sequence, wherein each of the subsets of spokes is acquired as a sequence of gradient echo signals; and reconstructing an MR image from the acquired spokes.

2. The method of claim 1, wherein the selection of the subsets involves determining the sequence in which the spokes contained in each of the subsets are acquired.

3. The method of claim 2, wherein the cost function depends on the relative orientations of subsequently acquired spokes from the same subset.

4. The method of claim 1, wherein the cost function penalizes the level of acoustic noise generated during the acquisition of the subsets of spokes.

5. The method of claim 1, wherein the set of radial k-space spokes consists of pairs of spokes having anti-parallel orientations.

6. The method of claim 5, wherein the subsets are selected to consist of pairs of spokes having anti-parallel orientations.

7. The method of claim 1, wherein a greedy selection strategy is used in a sequential selection of the individual subsets of spokes.

8. The method of claim 1, wherein a greedy selection strategy is used in a sequential selection of the individual or pairs of spokes for each of the subsets.

9. The 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 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 configured to perform the following steps: specification of a set of radial k-space spokes to cover a spherical k-space volume; selection of subsets of a predetermined number of spokes from the specified set so that the concatenation of the spokes contained in each of the subsets forms a closed trajectory in k-space, wherein the selection of the subsets is based on a cost function is based on optimizing a cost function favouring (i) minimizing the maximum angle between subsequently acquired spokes within the same subset, (ii) maximizing the uniformity of the distribution of all spokes end points on the surface of the covered spherical k-space volume, while ensuring that the concatenation of the spokes of each subset forms a closed trajectory in k-space; subjecting the object a zero echo time imaging sequence, wherein each of the subsets of spokes is acquired as a sequence of gradient echo signals; and reconstructing an MR image from the acquired spokes.

10. A computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions stored on a non-transitory computer readable medium for: specification of a set of radial k-space spokes to cover a spherical k-space volume; selection of subsets of a predetermined number of spokes from the specified set so that the concatenation of the spokes contained in each of the subsets forms a closed trajectory in k-space, wherein the selection of the subsets is based on a cost function favouring (i) minimizing the maximum angle between subsequently acquired spokes within the same subset, (ii) maximizing the uniformity of the distribution of all spokes end points on the surface of the covered spherical k-space volume, while ensuring that the concatenation of the spokes of each subset forms a closed trajectory in k-space; generation of a zero echo time imaging sequence, wherein each of the subsets of spokes is acquired as a sequence of gradient echo signals; and reconstructing an MR image from the acquired spokes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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:

[0027] FIG. 1 schematically shows a MR device for carrying out the method of the invention;

[0028] FIG. 2 illustrates the specification of a set of radial k-space spokes according to the invention;

[0029] FIG. 3 shows a diagram indicating the relative orientation angles between successively acquired spokes according to the invention;

[0030] FIG. 4 shows a further diagram indicating the relative orientation angles between successively acquired spokes according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] 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 Bo 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.o deviations within the examination volume.

[0032] 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.

[0033] 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 nuclear 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.

[0034] 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.

[0035] 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.

[0036] 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 a ZTE 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 convert 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.

[0037] Ultimately, the digital raw image 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.

[0038] The essence of the virtually silent ZTE technique as applied by the invention is that the RF excitation pulses are transmitted while a frequency encoding readout magnetic field gradient is switched on. The readout magnetic field gradient is not intended as a slice-selection gradient, which implies that the RF excitation pulses have to be short (typically in the order of 1 μs or 10 μs) to achieve sufficient excitation bandwidth. Alternatively, RF excitation pulses with a frequency sweep may be applied. The readout of FID signals takes place during intervals immediately after the RF excitation pulses in the presence of the readout magnetic field gradient. These intervals are also preferably short (typically in the order of 100 μs or 1 ms). The readout magnetic field gradient has a strength and a direction that both stay substantially constant over each excitation/readout cycle to acquire the MR signal from one radial k-space spoke. After each excitation/readout cycle, the direction is varied. For a full sampling of k-space, this is repeated until a spherical volume is fully covered with sufficient density.

[0039] According to the invention, self-refocusing ZTE imaging is achieved by a gradient echo refocusing mechanism. The pulse sequence is organized in a number of (two or more) segments, and each segment is divided into a number of loops. Each loop includes the acquisition of a number of radial k-space spokes. RF excitation is active only for the first loop (the FID acquisition loop) and turned off afterwards for the subsequent second and further loops (the gradient echo acquisition loops). The radial k-space spokes of each loop form a closed trajectory in k-space. In this way, the later loops form gradient echoes of the initial FIDs excited in the initial loop. With regard to the details of the self-refocusing ZTE imaging sequence adopted by the invention reference is made to US 2017/0307703 A1.

[0040] The invention proposes that, initially, a set of radial k-space spokes is specified to cover the spherical k-space volume to be acquired. In a possible embodiment, the set of spokes is specified as follows:

[0041] A sufficient number N.sub.P of parallels are defined on the surface of the k-space sphere, for example

[00001]$N_P=2.Math.\frac{\pi N}{4}.Math.+1,$

[0042] where N corresponds to the diameter of the k-space sphere, discretized according to the acquired spatial resolution.

[0043] A sufficient number of end points NE are equidistantly placed along each parallel, for example

[00002]$N_E\left(p\right)=\max \left\{2.Math.\frac{\pi N\cos \left(\pi \left(p-.Math.\frac{N_P}{2}.Math.\right)/N_P\right)}{2}.Math.,1\right\}.$

[0044] By choosing an odd value for Np and an even value for NE, the existence of pairs of anti-parallel radial spokes is ensured. One such predefined set of end points of radial k-space spokes is plotted in the diagram of FIG. 2.

[0045] In the next step, pairs of anti-parallel spokes are selected from the specified set in an optimization. In this way, the k-space trajectory resulting from the concatenation of the radial k-space spokes of one subset, which corresponds to one acquisition segment, is guaranteed to be self-refocusing.

[0046] In the optimization, the following operations are repeated until all radial k-space spokes are assigned to one subset:

[0047] One previously unselected spoke is picked systematically or randomly from the specified set.

[0048] A cost function is evaluated for this spoke and all permutations of N.sub.S/2−1 unselected spokes, where Ns denotes the number of spokes in each subset.

[0049] Applying a greedy selection strategy, the optimum permutation is chosen (for which, e.g., the cost function takes on its minimum value), and the respective spokes, as well as their anti-parallel counterparts, are assigned to the current subset.

[0050] As cost function, e.g., the squared relative orientation angle between subsequent spokes can be employed in order to achieve a minimum level of acoustic noise during the ZTE acquisition.

[0051] The per se known greedy selection strategy is designed to make the locally optimum selection for each subset with the intent of finding a global optimum for the whole specified set. The greedy optimization strategy may not actually find the true globally optimum solution, but nonetheless it yields a locally optimum solution for the subsets that at least approximates the globally optimum solution in a reasonable amount of computing time. The globally optimum solution minimizes the level of acoustic noise not only for each subset, or acquisition segment, but globally for the whole specified set, or the entire acquisition.

[0052] However, any known heuristic algorithm for an efficient, but approximate search for the optimal permutation may be applied instead of the greedy selection strategy. In particular, the search may be stopped whenever a predefined maximum of the relative orientation angle between subsequent spokes within a segment is met.

[0053] A representative result obtained with the afore-described procedure is provided in FIG. 3. Eight spokes per subset were chosen in this example, with an optimal angle between subsequent spokes of 45°. Using the greedy selection strategy, the actual angle between subsequent spokes is close to the optimum for most of the spokes. Only towards the end of the procedure, the choice of remaining unselected spokes becomes very limited, and the angle between subsequent spokes substantially exceeds the optimum.

[0054] If a predefined maximum of the relative orientation angle between subsequent spokes within a subset cannot be met anymore, spokes may be added, at the expense of a small increase in the required number of spokes and a corresponding small increase of scan time. This is demonstrated in FIG. 4, where a re-acquisition of spokes was permitted, i.e. some spokes were assigned to a subset that had already been selected for a previous subset. This re-selection of spokes was allowed if the relative orientation angle exceeded 50°, leading to an increase of 13% of the total number of spokes.