PARALLEL MR IMAGING USING WAVE-ENCODING

Abstract

The invention relates to a method of MR imaging of an object (10) placed in an examination volume of a MR device (1). The method comprises the steps of: —generating MR signals by subjecting the object to an imaging sequence, —acquiring MR signal profiles in a Cartesian k-space sampling scheme, wherein each MR signal profile is acquired in the presence of a temporally constant magnetic field gradient along a readout direction and a sinusoidally modulated magnetic field gradient along a phase encoding direction, and —reconstructing an MR image from the acquired MR signal profiles taking the modulation scheme of the magnetic field gradients into account. The invention proposes that the frequency of the sinusoidal modulation of the magnetic field gradient is varied during acquisition of each MR signal profile. Moreover, the invention relates to a MR device for carrying out this method as well as to a computer program to be run on a MR device.

Claims

1. Method of MR imaging of an object placed in an examination volume of a MR device, the method comprising the steps of: generating MR signals by subjecting the object to an imaging sequence, acquiring MR signal profiles in a superposition of a Cartesian readout employing a linear readout magnetic field gradient and one or more sinusoidally varying magnetic field gradients along one or more phase encoding directions so that each MR signal profile is acquired in the presence of a temporally constant magnetic field gradient along a readout direction and said sinusoidally modulated magnetic field gradients along said phase encoding direction, and reconstructing an MR image taking the modulation scheme of the magnetic field gradient into account, wherein the frequency of the sinusoidal modulation of the magnetic field gradient is varied during acquisition of each MR signal profile.

2. Method of claim 1, wherein the imaging sequence is a two-dimensional, three-dimensional or higher-dimensional spin echo sequence, preferably a turbo spin echo sequence.

3. Method of claim 1, wherein the imaging sequence is a two-dimensional, three-dimensional or higher-dimensional gradient echo sequence, preferably a turbo field echo sequence.

4. Method of claim 1, wherein the instantaneous frequency of the sinusoidal modulation of the magnetic field gradient is increasing during the first half of the acquisition time interval of each MR signal profile and decreasing back to its initial value during the second half of the acquisition time interval.

5. Method of claim 1, wherein the amplitude of the magnetic field gradient modulation is varied during the acquisition of the MR signals.

6. Method of claim 1, wherein the MR signals are acquired with subsampling via at least two RF coils having different spatial sensitivity profiles, wherein the MR images are reconstructed using a parallel image reconstruction algorithm, like SENSE, SMASH or GRAPPA.

7. Method of claim 1, wherein the imaging sequence comprises multi-slice RF pulses for simultaneously exciting two or more spatially separate image slices, wherein MR signal contributions from the different image slices are separated on the basis of the spatial sensitivity profiles of the at least two RF coils.

8. MR device for carrying out the method claimed in claim 1, which MR device includes 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 or more RF coils 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 the following steps: generating MR signals by subjecting the object to an imaging sequence, acquiring MR signal profiles in a superposition of a Cartesian readout employing a linear readout magnetic field gradient and one or more sinusoidally varying magnetic field gradients along one or more phase encoding directions such that each MR signal profile is acquired in the presence of a temporally constant magnetic field gradient along a readout direction and a sinusoidally modulated magnetic field gradient along said phase encoding directions, and reconstructing an MR image from the acquired MR signal profiles taking the modulation scheme of the magnetic field gradients into account, wherein the frequency of the sinusoidal modulation of the magnetic field gradients is varied during acquisition of each MR signal profile.

9. Computer program to be run on a MR device, which computer program comprises instructions for: generating an imaging sequence, acquiring MR signal profiles in a superposition of a Cartesian readout employing a linear readout magnetic field gradient and one or more sinusoidally varying magnetic field gradients along one or more phased encoding directions so that each MR signal profile is acquired in the presence of a temporally constant magnetic field gradient along a readout direction and sinusoidally modulated magnetic field gradients along slice and said phase encoding direction, wherein the MR signals are received in parallel via a set of at least two RF coils having different spatial sensitivity profiles within the examination volume, and reconstructing an MR image from the acquired MR signal profiles taking the modulation scheme of the magnetic field gradients into account, wherein the frequency of the sinusoidal modulation of the magnetic field gradients is varied during acquisition of each MR signal profile.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] 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:

[0021] FIG. 1 shows a MR device for carrying out the method of the invention;

[0022] FIG. 2 shows diagrams illustrating variable frequency Wave-CAIPI gradient waveforms according to the invention in comparison to conventional constant frequency Wave-CAIPI gradient waveforms;

[0023] FIG. 3 shows a comparison of the point spread function PSF for constant frequency Wave-CAIPI and variable frequency Wave-CAIPI according to the invention;

[0024] FIG. 4 shows central and lateral slices of a three-dimensional brain scan comparing conventional Cartesian sampling with constant frequency Wave-CAIPI and variable frequency Wave-CAIPI according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] With reference to FIG. 1, a MR device 1 is shown. The device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field is created along a z-axis through an examination volume.

[0026] 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.

[0027] 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 whole-body volume 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 each other and 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.

[0028] For generation of MR images of limited regions of the body 10 by means of parallel imaging, a set of local RF coils 11, 12, 13 are placed contiguous to the region selected for imaging.

[0029] The resultant MR signals are picked up by the 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.

[0030] A host computer 15 controls 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.

[0031] Ultimately, the digital raw image data is 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.

[0032] With continuing reference to FIG. 1 and with further reference to FIGS. 2-4 the imaging approach of the invention is explained.

[0033] According to the invention, an imaging sequence (for example a spin-echo sequence, like the known TSE sequence) is applied. MR signals are received in parallel via the RF coils 11, 12, 13 having different spatial sensitivities. MR signal profiles are acquired in a Cartesian k-space scheme with subsampling in the phase encoding directions. According to the Wave-CAIPI technique, each MR signal profile is acquired in the presence of a temporally constant magnetic field gradient along the readout direction and sinusoidally modulated magnetic field gradients along the phase encoding directions. In other words, the MR signal acquisition is a superposition of a Cartesian readout employing a linear readout magnetic field gradient G.sub.x with sinusoidally varying magnetic field gradients along the phase encoding directions G.sub.y and G.sub.z.

[0034] According to the invention, the frequency of the sinusoidal modulation of the magnetic field gradients is varied during acquisition of each MR signal profile. The instantaneous frequency of the magnetic field gradient modulation can be generally defined as f(t)=f.sub.c+h(t), t∈[0,T.sub.acq], where f.sub.c and h(t) correspond to the constant frequency component and time-varying frequency component respectively. h(t) can be any kind of symmetric, asymmetric, piecewise linear and nonlinear function. In an embodiment of the invention, a symmetric and nonlinear function is used to modulate the instantaneous frequency, namely f (t)=f.sub.c−2πf.sub.m . . . cos (2πf.sub.mt), where parameter f.sub.m and β control the frequency varying pattern and waveform shape respectively. Typically, f.sub.m=1/T.sub.acq is chosen to achieve a monotonous increasing or decreasing property during the first or second half of the acquisition time interval T.sub.acq, while β is a tunable parameter to optimize the performance and adapt to the hardware constraints including maximum gradient strength and slew rate. With this, the wave encoding k-space trajectories (C.sub.1(t) and C.sub.2(t)) and corresponding gradient waveforms (g.sub.y (t) and g.sub.z (t)) can be formulated as follows:

[00001]$\{\begin{array}{c}{C}_1\left(t\right)=\frac{\gamma }{2\pi }\frac{G_y}{2\pi f_c}\left(1-\cos \left(2\pi \left(f_{c}t-\beta \sin \left(2\pi f_{m}t\right)\right)\right)\right)\\ C_2\left(t\right)=\frac{\gamma }{2\pi }\frac{G_y}{2\pi f_c}\sin \left(2\pi \left(f_{c}t-\beta \sin \left(2\pi f_{m}t\right)\right)\right)\end{array},t\in \left[0,T_{\mathrm{acq}}\right],\{\begin{array}{c}{g}_y\left(t\right)=G_y\left(1-2\pi \frac{f_m}{f_c}\beta \cos \left(2\pi f_{m}t\right)\right)\sin \left(2\pi \left(f_{c}t-\beta \sin \left(2\pi f_{m}t\right)\right)\right)\\ g_z\left(t\right)=G_z\left(1-2\pi \frac{f_m}{f_c}\beta \cos \left(2\pi f_{m}t\right)\right)\cos \left(2\pi \left(f_{c}t-\beta \sin \left(2\pi f_{m}t\right)\right)\right)\end{array},t\in \left[0,T_{\mathrm{acq}}\right],$

[0035] where G.sub.y and G.sub.z indicate the maximum gradient amplitude along the y- and z-direction respectively. The above ideal gradient waveforms may be corrected for effects of the gradient modulation transfer function (i.e. eddy current compensation) and then applied in the MR scan. Also, additional pre-phasing and re-winding gradients may be applied to maintain the Carr-Purcell-Meiboom-Gill (CPMG) condition for the 3D TSE scan.

[0036] For MR image reconstruction it is exploited that the encoding created by the sinusoidal gradients G.sub.y and G.sub.z can be captured through a separable point spread function (PSF). Each MR signal profile in the underlying three-dimensional image is convolved with the PSF that depends on the spatial position in the y- and z-directions to yield the acquired MR image. In a hybrid (k.sub.g, y, z) space, each k.sub.x line of the MR signal data at a given (y, z) position obtains a unique phase modulation contained in the PSF.

[0037] FIG. 2 illustrates an example implementation of the proposed variable frequency wave encoding magnetic field gradient waveform (indicated by letter V in FIG. 2) and compares it with a constant frequency wave encoding waveform (indicated by letter C). The gradient waveform is shown in FIG. 2a while the corresponding k-space trajectories are shown in FIG. 2b, each time for constant and variable frequency wave encoding schemes. The wave encoding k-space trajectories in FIG. 2b are calibrated from in vivo imaging. The corresponding gradient waveforms in FIG. 2a are simulated by numerical derivative and scaling.

[0038] With the calibrated wave encoding k-space trajectories and the employed subsampling mask, the (transformed) PSF can be synthesized. FIG. 3 shows a comparison of the PSF for constant (C) and variable frequency (V) wave encoding schemes. FIG. 3a shows the employed CAIPI subsampling mask with reduction factor 2 in both y- and z-directions and a fully sampled central area. FIGS. 3b and 3c show the resulting PSF for two different (y, z) positions. It can be seen from FIG. 3 that the variable frequency wave encoding scheme of the invention improves the aliasing propagation along the readout direction and significantly reduces the amplitude of side lobes of the PSF.

[0039] FIGS. 4a and 4b show central and lateral slices of a brain scan respectively. In the top row of FIGS. 4a and 4b (indicated by ‘Full’) slice images are shown that have been reconstructed from fully sampled MR signal data. A conventional Cartesian sampling scheme has been employed for the images in the left column (indicated by ‘Cartesian’). Constant frequency wave encoding has been employed for the images in the middle column (indicated by ‘Wave C’) while variable frequency wave encoding has been employed for the images in the right column (indicated by ‘Wave V’). The images in the bottom row of FIGS. 4a and 4b (indicated by ‘Sub’) are reconstructed from subsampled MR signal data with a reduction factor 3 in the phase encoding direction and a reduction factor 2 in the slice encoding direction. As can be seen from FIGS. 4a and 4b, for the 3×2 Wave-CAIPI in vivo brain acceleration experiment, the variable frequency wave encoding approach of the invention can provide better aliasing suppression results in both central and lateral image slices. It can clearly be seen in the bottom row of FIG. 4a that residual aliasing artifacts are reduced with variable frequency wave encoding. In addition, the variable frequency wave encoding can significantly reduce the signal loss due to eddy current induced slice profile degradation in lateral slices. A clear signal drop can clearly be seen in the middle column of FIG. 4b which is not present in the right column.