DIXON MR IMAGING USING A MULTI-GRADIENT-ECHO SEQUENCE

Abstract

The invention relates to a method of MR imaging of an object. It is an object of the invention to provide a multi-gradient echo imaging technique with increased acquisition speed and intrinsic suppression of artefacts from Bo inhomogeneities, T.sub.2* decay, chemical shift, motion, and/or flow, in particular in combination with radial or spiral k-space trajectories. The method of the invention comprises the steps of: —subjecting the object (10) to an imaging sequence comprising RF excitation pulses and switched magnetic field gradients, wherein multiple echo signals are generated at different echo times after each RF excitation pulse, —acquiring the echo signal data along radial or spiral k-space trajectories, wherefore the imaging sequence comprises magnetic field gradient blips in the x-/y- and/or z-directions; —separating signal contributions from water and fat to the echo signals and estimating a B.sub.0 map and/or an apparent transverse relaxation time map (T.sub.2* map) using a Dixon algorithm; and —synthesizing an image of a specified contrast from the echo signal data, the Bo map and/or the T.sub.2* map. 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 an examination volume of a MR device, the method comprising: subjecting the object to an imaging sequence comprising RF excitation pulses and switched magnetic field gradients, wherein multiple echo signals are generated at different echo times after each RF excitation pulse, acquiring echo signal data along radial or spiral k-space trajectories, wherein the imaging sequence comprises magnetic field gradient blips in at least on an x-/y- or z-directions, such that different echo times (TE.sub.1, TE.sub.2, . . . , TE.sub.N) are provided, separate from the single echo data signal contributions from water and fat, estimate an apparent transverse relaxation time map (T.sub.2*), synthesizing T.sub.2*-weighted signals of a specified contrast from the acquired echo signal data, wherein the effective echo time is indirectly determined by the selected number of echoes, the first echo time TE.sub.1, and the echo spacing, and by T.sub.2*, and reconstructing an image of said specified contrast from the synthesised T.sub.2*-weighted signals, and the apparent transverse relaxation time map (T.sub.2*, map).

2. The method of claim 1, wherein a rotation angle of the radial or spiral k-space trajectories is incremented during acquisition by the golden angle.

3. The method of claim 1, wherein phase-encoding of the echo signals is varied in the z-direction and/or the rotation angle of the radial or spiral k-space trajectories is incremented in the k.sub.x-/k.sub.y-directions.

4. The method of claim 1, wherein the sampling density in the k.sub.x-/k.sub.y-directions varies as a function of k.sub.z such that a central portion of k-space is sampled more densely than the peripheral portions.

5. The method of claim 1, wherein one or more shots of the multi-echo acquisition is used to extract an intrinsic image navigation signal which is used for motion correction.

6. (canceled)

7. The method of claim 1, wherein a k-space weighted image contrast (KWIC) filter is used for reconstructing the single echo images.

8. The method of claim 1, wherein compressed sensing is used for reconstructing the single echo images or within the water/fat separation.

9. The method of claim 1, wherein a subset of the echo signals is generated at an ultra-short echo time (UTE).

10. The method of claim 1, wherein synthesizing the image of a specified contrast involves: computing a zero echo time magnitude image from the acquired echo signal data and applying a weighting to each voxel of the zero echo time magnitude image, which weighting is derived from the T.sub.2* map.

11. The method of claim 1, wherein the imaging sequence is a field echo sequence or a spin echo sequence.

12. The method of claim 1, wherein motion of the object is detected during the acquisition of the echo signals, wherein a corresponding motion-compensation is applied in the step of reconstructing the single echo images, in the step of separating the signal contributions from water and fat, or in the step of synthesizing the image of a specified contrast.

13. The method of claim 1, wherein the synthesized image resembles an image which is generated by magnitude reconstruction of single echo images from the acquired echo signal data and combination of the single echo images by a sum of squares algorithm.

14. The method of claim 1, wherein a flow map is derived from the acquired echo signal data, wherein the flow map is used in the step of synthesizing the image.

15. 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 configured to: subject the object to an imaging sequence comprising RF excitation pulses and switched magnetic field gradients, wherein multiple echo signals are generated at different echo times after each RF excitation pulse, acquire the echo signal data along radial or spiral k-space trajectories, wherefore the imaging sequence comprises magnetic field gradient blips in the x-/y- and/or z-directions, such that different echo times (TE.sub.1, TE.sub.2, . . . , TE.sub.N) are provided; separate from the single echo data signal contributions from water and fat and estimate an apparent transverse relaxation time map (T.sub.2* map), synthesize T.sub.2*-weighted signals of a specified contrast from the acquired echo signal data, wherein the effective echo time is indirectly determined by the selected number of echoes, the first echo time TE.sub.1, and the echo spacing, and by T.sub.2*, and reconstruct an image of said specified contrast from the synthesised T.sub.2*-weighted signals, and the apparent transverse relaxation time map (T.sub.2* map).

16. A computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions stored in a non-transistors computer readable medium, such that when the instructions are executed causes the MR device to: generate an imaging sequence comprising RF excitation pulses and switched magnetic field gradients, wherein multiple echo signals are generated at different echo times after each RF excitation pulse, acquire the echo signal data along radial or spiral k-space trajectories, wherefore the imaging sequence comprises magnetic field gradient blips in the x-/y- and/or z-directions, such that different echo times (TE.sub.1, TE.sub.2, . . . , TE.sub.N) are provided; separate from the single echo images signal contributions from water and fat, estimating an apparent transverse relaxation time map (T.sub.2* map) synthesize T.sub.2*-weighted signals of a specified contrast from the acquired echo signal data, wherein the effective echo time is indirectly determined by the selected number of echoes, the first echo time TE.sub.1, and the echo spacing, and by T.sub.2*, reconstructing an image of said specified contrast from the synthesised T.sub.2*-weighted signals and the apparent transverse relaxation time map (T.sub.2*map).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] 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:

[0040] FIG. 1 shows a MR device for carrying out the method of the invention;

[0041] FIG. 2 shows a schematic (simplified) pulse sequence diagram of a multi-gradient echo MR imaging sequence according to the invention;

[0042] FIG. 3 illustrates an example of a k-space sampling pattern of the invention;

[0043] FIG. 4 shows an example of a merged multi-gradient echo image synthesized according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] 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.

[0045] 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.

[0046] 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 resonance, 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.

[0047] 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.

[0048] 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.

[0049] 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 signals 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 sample 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.

[0050] 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. 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.

[0051] The host computer 15 is programmed to execute the method of the invention described herein above and in the following.

[0052] In FIG. 2, a schematic pulse sequence diagram of an imaging sequence according to the invention is depicted. The diagram shows switched magnetic field gradients in the read-out and phase-encoding directions x and y and in the slice-selection direction z. Moreover, the diagram shows an RF excitation pulse as well as the time intervals during which a number of echo signals are acquired, designated by ACQ.sub.1, ACQ.sub.2 and ACQ.sub.N. The diagram covers the acquisition of N echo signals. A plurality of sets of N echo signals is acquired by multiple repetitions (shots) of the depicted sequence using different gradient waveforms in the x-/y-directions and/or in the z-direction in order to completely cover the required region of k-space by a radial sampling pattern. The timing and amplitudes of the readout gradients in the x-/y-directions are chosen such that different echo times TE.sub.1, TE.sub.2, . . . , TE.sub.N are provided. Short magnetic field gradient blips are applied in addition to the actual read-out magnetic field gradients between the echo signal acquisition time intervals ACQ.sub.1-ACQ.sub.N in the x-/y-directions. Blips are also applied in the z-direction for achieving a Cartesian k-space sampling pattern in this direction (stack of stars). A very fast acquisition is achieved in this fashion with an optimum distribution of the acquired echo signal data in k-space.

[0053] As further illustrated in FIG. 3, the phase-encoding of the echo signals is varied in the z-direction in a single shot while the rotation angle of the radial k-space trajectories is incremented by the golden angle increment in the k.sub.x-/k.sub.y-directions. In the example of FIG. 3, the magnetic field gradient blips in the z-direction (as shown in FIG. 2) result in k-space being sampled alternatingly in two adjacent k.sub.z-planes (designated by k.sub.z1 and k.sub.z2). The sequence of echo signals acquired in a single shot of the imaging sequence is designated as E.sub.1, . . . , E.sub.6. Echo signal E.sub.1 is acquired initially from plane k.sub.z1, the rotation angle of the radial k-space profile is then incremented by the tiny golden angle α and the next echo signal E.sub.2 is sampled from plane k.sub.z2. The rotation angle is again incremented by alpha, the next echo signal E.sub.3 is acquired again from plane k.sub.z1, and so forth. The rotation angle increment between two consecutive k-space profiles acquired from the same plane is thus 2α. A sufficiently high sampling density in the k.sub.x-k.sub.y-plane may thus not be achieved for every single k.sub.z-plane, but for a number of adjacent k.sub.z-planes. The acquisition speed can be increased significantly in this manner without relevant negative impact on final image quality.

[0054] As an intermediate step, single echo images may be reconstructed from the acquired echo signal data: A first single echo image attributed to the first echo time TE.sub.1, a second single echo image attributed to the second echo time TE.sub.2, and so forth. The contributions from water and fat to the respective voxel values are separated by application of a Dixon algorithm of known type on the basis of the different echo times TE.sub.1, . . . , TE.sub.N. At the same time, a T.sub.2* map is estimated by including the T.sub.2* decay in the signal model employed in the Dixon algorithm. Alternatively, the separation of water and fat and the estimation of T.sub.2* may be performed directly on the acquired echo signal data without explicitly reconstructing single echo images.

[0055] Depending on the type of combination, the images resulting from the known MERGE or MEDIC methods can be described by:

[00001]$\begin{array}{cc}{S}_1=\underset{n=1}{\overset{N}{.Math.}}.Math..Math.S.Math..Math.e^{-{\mathrm{TE}}_n.Math.\text{/}.Math.T_2^{*}},& \left(1\right)\\ S_2=\underset{n=1}{\overset{N}{.Math.}}.Math..Math.{\left(S.Math..Math.e^{-{\mathrm{TE}}_n.Math.\text{/}.Math.T_2^{*}}\right)}^2,& \left(2\right)\\ S_3=\sqrt{\underset{n=1}{\overset{N}{.Math.}}.Math..Math.{\left(S.Math..Math.e^{-{\mathrm{TE}}_n.Math.\text{/}.Math.T_2^{*}}\right)}^2},& \left(3\right)\\ S_4=\frac{1}{N}.Math.\sqrt{\underset{n=1}{\overset{N}{.Math.}}.Math..Math.{\left(S.Math..Math.e^{-{\mathrm{TE}}_n.Math.\text{/}.Math.T_2^{*}}\right)}^2},& \left(4\right)\end{array}$

[0056] wherein N denotes the number of echoes and S denotes a voxel value of the resulting images.

[0057] Assuming a constant echo spacing ΔTE, Eq. (1) can be rewritten as:

[00002]$\begin{array}{cc}{S}_1=S.Math..Math.e^{-{\mathrm{TE}}_1.Math.\text{/}.Math.T_2^{*}}.Math.\underset{n=1}{\overset{N}{.Math.}}.Math..Math.e^{-\left(n-1\right).Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}},& \left(5\right)\\ S_1=S.Math..Math.e^{-{\mathrm{TE}}_1.Math.\text{/}.Math.T_2^{*}}.Math.\frac{1-e^{-N.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}{1-e^{-\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}.& \left(6\right)\end{array}$

[0058] This allows introducing an effective echo time TE.sub.1e:

S.sub.1=Se.sup.−TE.sup.1e.sup./T.sup.2.sup.*,  (7)

[0059] given by:

[00003]$\begin{array}{cc}{e}^{-{\mathrm{TE}}_{1.Math.e}.Math.\text{/}.Math.T_2^{*}}=e^{-{\mathrm{TE}}_1.Math.\text{/}.Math.T_2^{*}}.Math.\frac{1-e^{-N.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}{1-e^{-\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}},& \left(8\right)\\ {\mathrm{TE}}_{1.Math.e}={\mathrm{TE}}_1-T_2^{*}.Math..Math.\ln \left(\frac{1-e^{-N.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}{1-e^{-\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}\right).& \left(9\right)\end{array}$

[0060] Similarly, the combined images S.sub.2, S.sub.3, S.sub.4 can be computed using effective echo times given by:

[00004]$\begin{array}{cc}{\mathrm{TE}}_{2.Math.e}=2.Math..Math.{\mathrm{TE}}_1-T_2^{*}.Math..Math.\ln \left(\frac{1-e^{-2.Math.N.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}{1-e^{-2.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}\right),& \left(10\right)\\ {\mathrm{TE}}_{3.Math.e}={\mathrm{TE}}_1-\frac{T_2^{*}}{2}.Math..Math.\ln \left(\frac{1-e^{-2.Math.N.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}{1-e^{-2.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}\right),& \left(11\right)\\ {\mathrm{TE}}_{4.Math.e}={\mathrm{TE}}_1+T_2^{*}.Math..Math.\ln \left(N\right)-\frac{T_2^{*}}{2}.Math..Math.\ln \left(\frac{1-e^{-2.Math.N.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}{1-e^{-2.Math..Math.\Delta .Math..Math.\mathrm{TE}.Math.\text{/}.Math.T_2^{*}}}\right),& \left(12\right)\end{array}$

[0061] It is thus the insight of the invention that the images resulting from the MERGE or MEDIC methods can be considered as T.sub.2*-weighted images, wherein the effective echo time is indirectly determined by the selected number of echoes, the first echo time, and the echo spacing, but also, and most remarkably, by T.sub.2*.

[0062] To overcome the only indirect determination of the effective echo time and any constraints on the imaging sequence, the invention proposes to first estimate a T.sub.2* map from the echo signal data and to then synthesize an image from the estimated zero echo time magnitude image and the T.sub.2* map. The separation of water and fat may be included optionally to provide even more flexibility in optimizing the contrast in the resulting images depending on the particular diagnostic purposes.

[0063] To overcome the sensitivity of the known MERGE/MEDIC methods to motion, it is proposed to employ radial or spiral k-space trajectories instead of conventional Cartesian k-space trajectories. Preferably, a projection or interleaf order based on the golden angle is used, and an incremental rotation by a tiny golden angle is applied between successive radial or spiral acquisitions. Additionally, inconsistent data may optionally be rejected and motion may optionally be detected and corrected between the individual acquisitions.

[0064] FIG. 4 shows an exemplary image of the spinal cord synthesized by the method of the invention. The image allows excellent differentiation between gray and white matter.