Zero echo time MR imaging with water-fat separation

Abstract

A method of magnetic resonance (MR) imaging to enable ‘silent’ zero echo time (ZTE) imaging in combination with water/fat separation. The method includes subjecting the object to a first self-refocusing zero echo time imaging sequence, wherein a first sequence of gradient echo signals is acquired as a first number N.sub.1 of radial k-space spokes at a first repetition time TR.sub.1; subjecting the object to a second self-refocusing zero echo time imaging sequence, wherein a second sequence of gradient echo signals is acquired as a second number N.sub.2 of radial k-space spokes at a second repetition time TR.sub.2, wherein N.sub.2≠N.sub.1 and/or TR.sub.2≠TR.sub.1; and reconstructing a MR image from the acquired gradient echo signals. Signal contributions of chemical species (e.g., water and fat) may be separated exploiting the different echo times attributed to the gradient echo signals.

Claims

1. A method of magnetic resonance (MR) imaging of an object positioned in an examination volume of a MR device, the method comprising: subjecting the object to a first self-refocusing zero echo time imaging sequence, wherein a first sequence of gradient echo signals is acquired as a first number N.sub.1 of radial k-space spokes at a first repetition time TR.sub.1, which first number N.sub.1 of radial k-space spokes forms a first closed trajectory in k-space; subjecting the object to a second self-refocusing zero echo time imaging sequence, wherein a second sequence of gradient echo signals is acquired as a second number N.sub.2 of radial k-space spokes at a second repetition time TR.sub.2, which second number N.sub.2 of radial k-space spokes forms a second closed trajectory in k-space, wherein N.sub.2 is not equal to N.sub.1 and/or TR.sub.1 is not equal to TR.sub.2 such that different echo times are attributed to the gradient echo signals of the first and second sequences of gradient echo signals respectively; and reconstructing a MR image from the acquired gradient echo signals.

2. The method of claim 1, wherein signal contributions of two or more chemical species to the acquired gradient echo signals are separated in the step of reconstructing the MR image exploiting the different echo times attributed to the gradient echo signals of the first and second sequences of gradient echo signals respectively.

3. The method of claim 1, wherein the different echo times attributed to the gradient echo signals of the first and second sequences of gradient echo signals are exploited for reconstructing an effective transverse relaxation time (T.sub.2*)-weighted MR image and/or a T.sub.2* map.

4. The method of claim 1, wherein the first zero echo time imaging sequence comprises: an FID acquisition loop comprising the following steps: i) setting a readout magnetic field gradient to define a readout direction; ii) radiating an RF pulse in the presence of the readout magnetic field gradient; iii) acquiring an FID signal as a radial k-space spoke in the presence of the readout magnetic field gradient, wherein k-space is sampled along said first closed trajectory by repeating steps i) through iii) N.sub.1 times with repetition time TR.sub.1 under gradual variation of the readout direction from repetition to repetition, followed by one or more gradient echo acquisition loops comprising the following steps: iv) setting the readout magnetic field gradient again to define the readout direction; v) acquiring a gradient echo signal as a radial k-space spoke in the presence of the readout magnetic field gradient, wherein k-space is sampled in the gradient echo acquisition loop again along the first closed trajectory by repeating steps iv) and v) N.sub.1 times with repetition time TR.sub.1 under gradual variation of the readout direction from repetition to repetition.

5. The method of claim 4, wherein the second zero echo time imaging sequence comprises: an FID acquisition loop comprising the following steps: vi) setting a readout magnetic field gradient to define a readout direction; vii) radiating an RF pulse in the presence of the readout magnetic field gradient; viii) acquiring an FID signal as a radial k-space spoke in the presence of the readout magnetic field gradient, wherein k-space is sampled along said second closed trajectory by repeating steps vi) through viii) N.sub.2 times with repetition time TR.sub.2 under gradual variation of the readout direction from repetition to repetition, followed by one or more gradient echo acquisition loops comprising the following steps: ix) setting the readout magnetic field gradient again to define the readout direction; x) acquiring a gradient echo signal as a radial k-space spoke in the presence of the readout magnetic field gradient, wherein k-space is sampled in the gradient echo acquisition loop again along the second closed trajectory by repeating steps ix) and x) N.sub.2 times with repetition time TR.sub.2 under gradual variation of the readout direction from repetition to repetition.

6. The method of claim 4, wherein a correction for motion occurring between the first, second and, where applicable, further self-refocusing zero echo time imaging sequences is derived from the acquired FID signals.

7. The method of claim 4, wherein the acquired FID signals are used in the reconstruction of the MR image to reduce noise and/or the T.sub.2*-weighting.

8. The method of claim 1, further comprising: subjecting the object to at least one further self-refocusing zero echo time imaging sequence, wherein a further sequence of gradient echo signals is acquired as a further number of radial k-space spokes at a further repetition time TR.sub.1, which further number of radial k-space spokes forms a further closed trajectory in k-space, wherein N.sub.1 is different from both N.sub.1 and N.sub.2 and/or TR.sub.i is different from both TR.sub.1 and TR.sub.2; and reconstructing the MR image from the acquired gradient echo signals, wherein signal contributions of the two or more chemical species to the gradient echo signals are separated exploiting the different echo times attributed to the gradient echo signals of the first, second and further sequences of gradient echo signals respectively.

9. The method of claim 1, wherein the differences between N.sub.1 and N.sub.2 and, where applicable, N.sub.i and/or the differences between TR.sub.1 and TR.sub.2 and, where applicable, TR.sub.i are determined such that the differences between the echo times attributed to the gradient echo signals of the first, second and, where applicable, further sequences are on the order of one millisecond.

10. 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 configured to: subject the object to a first self-refocusing zero echo time imaging sequence, wherein a first sequence of gradient echo signals is acquired as a first number N.sub.1 of radial k-space spokes at a first repetition time TR.sub.1, which first number N.sub.1 of radial k-space spokes forms a first closed trajectory in k-space; subject the object to a second self-refocusing zero echo time imaging sequence, wherein a second sequence of gradient echo signals is acquired as a second number N.sub.2 of radial k-space spokes at a second repetition time TR.sub.2, which second number N.sub.2 of radial k-space spokes forms a second closed trajectory in k-space, wherein N.sub.2 is not equal to N.sub.1 and/or TR.sub.2 is not equal to TR.sub.1; so that different echo times are attributed to the gradient echo signals of the first and second sequences of gradient echo signals respectively; and reconstruct a MR image from the acquired gradient echo signals.

11. 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: generating a first self-refocusing zero echo time imaging sequence, wherein a first sequence of gradient echo signals is acquired as a first number N.sub.1 of radial k-space spokes at a first repetition time TR.sub.1, which first number N.sub.1 of radial k-space spokes forms a first closed trajectory in k-space; generating a second self-refocusing zero echo time imaging sequence, wherein a second sequence of gradient echo signals is acquired as a second number N.sub.2 of radial k-space spokes at a second repetition time TR.sub.2, which second number N.sub.2 of radial k-space spokes forms a second closed trajectory in k-space, wherein N.sub.2 is not equal to N.sub.1 and/or TR.sub.2 is not equal to TR.sub.1; so that different echo times are attributed to the gradient echo signals of the first and second sequences of gradient echo signals respectively; and reconstructing a MR image from the acquired gradient echo signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 schematically shows a MR device for carrying out the method of the invention;

(3) FIG. 2 provides an example of the k-space sampling of the repeated self-refocusing ZTE imaging approach according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) 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.

(5) 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.

(6) 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 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.

(7) 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 to receive MR signals induced by body-coil RF transmissions.

(8) The resultant MR signals are picked up by the RF body coil 9 and/or by the RF array 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.

(9) A host computer 15 controls the current flow through the shimming coils 2′ as well as the gradient pulse amplifier 3 and the transmitter 7 to generate a ZTE imaging sequence according to the invention. The receiver 14 receives 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 the acquisition of raw image data.

(10) 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.

(11) The essence of the ‘silent’ ZTE technique as applied by the invention is that 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 pulses have to be extremely short (typically in the order of 1 μs or 10 μs) to achieve sufficient excitation bandwidth. Alternatively, RF pulses with a frequency sweep may be applied. The readout of FID signals takes place during intervals immediately after the RF 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. After each excitation/readout cycle, the direction is varied only gradually, e.g. by a few degrees (e.g. 2°). For a full sampling of k-space the readout magnetic field direction is varied until a spherical volume is covered with sufficient density.

(12) 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.

(13) The invention proposes that the self-refocusing ZTE imaging sequence is repeated by application of first and second self-refocusing ZTE imaging sequences, wherein the number of radial k-space spokes and/or the repetition time used in the individual loops of the second self-refocusing ZTE imaging sequence (N.sub.2, TR.sub.2) differ from the number of radial k-space spokes and/or the repetition time used in the individual loops of the first self-refocusing ZTE imaging sequence (N.sub.1, TR.sub.1).

(14) In more detail, the first zero echo time imaging sequence encompasses an FID acquisition loop comprising: i) setting a readout magnetic field gradient to define a readout direction; ii) radiating an RF pulse in the presence of the readout magnetic field gradient; iii) acquiring an FID signal as a radial k-space spoke in the presence of the readout magnetic field gradient. K-space is sampled along a first closed trajectory by repeating steps i) through iii) N.sub.1 times with repetition time TR.sub.1 under gradual variation of the readout direction from repetition to repetition. The FID acquisition loop is followed by one or more gradient echo acquisition loops, each comprising: iv) setting the readout magnetic field gradient again to define the readout direction; v) acquiring a gradient echo signal as a radial k-space spoke in the presence of the readout magnetic field gradient. K-space is sampled in the gradient echo acquisition loop again along the first closed trajectory by repeating steps iv) and v) N.sub.1 times with repetition time TR.sub.1 under gradual variation of the readout direction from repetition to repetition. Similarly, the subsequent second zero echo time imaging sequence encompasses an FID acquisition loop comprising: vi) setting a readout magnetic field gradient to define a readout direction; vii) radiating an RF pulse in the presence of the readout magnetic field gradient; viii) acquiring an FID signal as a radial k-space spoke in the presence of the readout magnetic field gradient, wherein k-space is sampled along a second closed trajectory by repeating steps vi) through viii) N.sub.2 times with repetition time TR.sub.2 under gradual variation of the readout direction from repetition to repetition. This FID acquisition loop of the second zero echo time imaging sequence is followed by one or more gradient echo acquisition loops, each comprising: ix) setting the readout magnetic field gradient again to define the readout direction; x) acquiring a gradient echo signal as a radial k-space spoke in the presence of the readout magnetic field gradient. Again, k-space is sampled in the gradient echo acquisition loop along the second closed trajectory by repeating steps ix) and x) N.sub.2 times with repetition time TR.sub.2 under gradual variation of the readout direction from repetition to repetition. The difference between the numbers of spokes (N.sub.1, N.sub.2) and/or the difference between the repetition times (TR.sub.1, TR.sub.2) applied in the two instances of the ZTE imaging sequence entails that the echo time attributed to the gradient echo signals of the acquired first series of gradient echoes differs from the echo time attributed to the gradient echo signals of the acquired second series of gradient echo signals. If the numbers of k-space spokes (N.sub.1, N.sub.2) and the repetition time values (TR.sub.1, TR.sub.2) are selected to be similar, the resulting echo spacing can be on the order of one millisecond, which is well-suited for robust water-fat separation by a Dixon algorithm. Hence, a MR image is reconstructed from the acquired gradient echo signals, wherein signal contributions of two or more chemical species (such as, e.g., water and fat) to the gradient echo signals are separated exploiting the different echo times attributed to the gradient echo signals of the first and second sequences of gradient echo signals respectively.

(15) FIG. 2 provides an example of the k-space sampling according to the repeated self-refocusing ZTE imaging approach according to the invention. It schematically illustrates the case of segmented radial acquisitions with N.sub.1=8 k-space spokes per segment for the first self-refocusing ZTE imaging sequence (left diagram) and N.sub.2=9 k-space spokes per segment for the second self-refocusing ZTE imaging sequence (right diagram). The solid arrows indicate the k-space sampling of the FID signals and the dashed lines indicate the resulting cumulative closed k-space trajectories refocusing the first FID signal as a gradient echo after eight or nine repetitions respectively. The echo times of the gradient echo signals generated for N.sub.1=8 and N.sub.2=9 respectively differ by one repetition time TR=TR.sub.1=TR.sub.2 in this embodiment. TR is on the order of one millisecond such that robust water-fat separation is enabled.

(16) In a similar fashion, different echo times attributed to the gradient echo signals can be achieved by choosing different values for TR.sub.1 and TR.sub.2. For example, with N.sub.1=N.sub.2=8 and TR.sub.1=1.0 ms and TR.sub.2=1.125 ms, the echo times of the gradient echo signals generated with TR.sub.1=1.0 ms and TR.sub.2=1.125 ms respectively differ again by one millisecond. Different repetition times can be obtained by increasing the spoiling for the self-refocusing ZTE imaging sequence with the longer TR, or by decreasing the readout magnetic field gradient strength. In any case, the flip angles of the RF excitation pulses, which may systematically vary over each of the sequences to implement a flip angle sweep, may be chosen differently for the two self-refocusing ZTE imaging sequences, e.g. to minimize differences in contrast due to the difference in TR.