Parallel multi-slice MR imaging

Abstract

A method of magnetic resonance (MR) imaging of an object includes: generating MR signals by subjecting the object to a number N of shots of a multi-echo imaging sequence comprising multi-slice RF pulses for simultaneously exciting two or more spatially separate image slices, with a phase offset in the slice direction being imparted to the MR signals; acquiring the MR signals that are received in parallel via a set of at least two RF coils having different spatial sensitivity profiles; and reconstructing a MR image for each image slice from the acquired MR signals using a parallel reconstruction algorithm, wherein the MR signal contributions from the different image slices are separated on the basis of the spatial encodings of the MR signals according to the spatial sensitivity profiles of the RF coils and of the phase offsets attributed to the respective image slices and shots.

Claims

1. A method of magnetic resonance (MR) imaging of an object placed in an examination volume of a MR device, the method comprising: generating MR signals by subjecting the object to a number N of shots of a multi-echo imaging sequence comprising multi-slice RF pulses for simultaneously exciting two or more spatially separate image slices, with a phase offset being imparted to the MR signals of each image slice, wherein the phase offset is varied from shot to shot, acquiring the MR signals, 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 a MR image for each image slice from the acquired MR signals using a parallel reconstruction algorithm, wherein the MR signal contributions from the different image slices are separated on the basis of the spatial encodings of the MR signals according to the spatial sensitivity profiles of the RF coils and on the basis of the phase offsets attributed to the respective image slices and shots.

2. The method of claim 1, wherein k-space data under-sampled in a phase-encoding direction are acquired in each shot of the imaging sequence.

3. The method of claim 1, wherein k-space data under-sampled in a phase-encoding direction imparted by the varying phase offsets are acquired in each shot of the imaging sequence.

4. The method of claim 1, wherein the phase offset is imparted by means of a phase modulation of the RF pulses.

5. The method of claim 1, wherein the phase offset is imparted by means of a magnetic field gradient applied in a slice-selection direction.

6. The method of claim 1, wherein an inverse problem of MR image reconstruction is solved using an encoding matrix, with matrix elements of the encoding matrix being determined by the spatial sensitivity profiles of the RF coils, k-space sampling of each shot of the imaging sequence, and the phase offsets attributed to the image slices and shots.

7. The method of claim 6, wherein a prior known phase errors of the MR signals are taken into account by incorporating the corresponding phase error values into the encoding matrix.

8. The method of claim 1 wherein a motion weighting based on motion information is applied in the MR image reconstruction.

9. The method of claim 8, wherein navigator signals are generated by subjecting the object to a navigator sequence between the shots of the imaging sequence, wherein the motion information is derived from the navigator signals.

10. The method of claim 6, wherein navigator signals are generated by subjecting the object to a navigator sequence, wherein motion-induced phase errors are taken into account by incorporating phase information derived from the navigator signals into the encoding matrix.

11. The method of claim 1, wherein a number M<N of shots of the imaging sequence is performed two or more times for the purpose of improving the signal-to-noise ratio.

12. A magnetic resonance (MR) device for carrying out the method of claim 1, wherein the 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 for generating switched magnetic field gradients in different spatial directions within the examination volume, a set of at least two RF coils having different spatial sensitivity profiles, a control unit for controlling temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit, wherein the MR device is configured to: generate MR signals by subjecting the object to a number N of shots of a multi-echo imaging sequence comprising multi-slice RF pulses for simultaneously exciting two or more spatially separate image slices, with a phase offset being imparted to the MR signals of each image slice, wherein the phase offset is varied from shot to shot, acquire the MR signals, wherein the MR signals are received in parallel via the set of RF coils, and reconstruct a MR image for each image slice from the acquired MR signals using a parallel reconstruction algorithm, wherein the MR signal contributions from the different image slices are separated on the basis of the spatial encodings of the MR signals according to the spatial sensitivity profiles of the RF coils and on the basis of the phase offsets attributed to the respective image slices and shots.

13. A computer program comprising executable instructions stored on a non-transitory computer readable storage medium, which when executed by a magnetic resonance (MR) device, causes the MR device to: generate a number N of shots of a multi-echo imaging sequence comprising multi-slice RF pulses for simultaneously exciting two or more spatially separate image slices, with a phase offset being imparted to MR signals of each image slice, wherein the phase offset is varied from shot to shot, acquire the MR signals, and reconstruct a MR image for each image slice from the acquired MR signals using a parallel reconstruction algorithm, wherein the MR signal contributions from the different image slices are separated on the basis of the spatial encodings of the MR signals according to spatial sensitivity profiles of a set of at least two RF coils and on the basis of the phase offsets attributed to the respective image slices and shots.

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 shows a MR device for carrying out the method of the invention;

(3) FIG. 2 illustrates one shot of the multi-shot multi-echo imaging sequence employed according to the invention;

(4) FIG. 3 schematically illustrates the k-space encoding scheme applied in accordance with the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) 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.

(6) 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.

(7) 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.

(8) 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 having different spatial sensitivity profiles are placed contiguous to the region selected for imaging.

(9) 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.

(10) 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.

(11) 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.

(12) The host computer 15 and the reconstruction processor 17 are programmed to execute the method of the invention as described above and in the following.

(13) With continuing reference to FIG. 1 and with further reference to FIGS. 2 and 3 an embodiment of the imaging technique of the invention is explained.

(14) The body 10 is subjected to multiple shots of a multi-echo imaging sequence as illustrated in FIG. 2. The imaging sequence is a modified multi-shot EPI sequence. The sequence comprises a slice-selective multi-slice RF pulse 21 for simultaneously exciting two or more spatially separate image slices. RF pulse 21 is radiated in the presence of a corresponding slice selection magnetic field gradient 22 in order to produce transverse nuclear magnetization in a number of imaging slices. Following excitation RF pulse 21 MR signals are acquired by sampling a series of gradient-recalled echo signals in the presence of an alternating readout magnetic field gradient 23. Phase-encoding of the MR signals is performed by a series of phase-encoding magnetic field gradient ‘blips’ 24. During the application of each phase encoding gradient blip 24, a further magnetic field gradient blip 25 is also applied in the slice-encoding direction. The slice encoding gradient blips 25 are used according to the invention to impart a phase offset to the MR signals of each image slice. While gradient blips 25 can be used to apply the different phase offsets the phase offsets can also be imparted by corresponding phasing of the slice-selective RF pulses 21 of the successive shots. The MR signal acquisition is accelerated by SENSE encoding. The MR signals are acquired in parallel via RF receiving coils 11, 12, 13 having different spatial sensitivities, and the MR signals are acquired with under-sampling in the phase-encoding direction. One single shot of the imaging sequence is depicted in FIG. 2. A plurality of (N) shots of the multi-echo sequence is applied for completely sampling k-space in order to be able to reconstruct full MR slice images from the acquired imaging echo signal data using SENSE reconstruction, wherein the phase offset imparted to the MR signals of the different is varied from shot to shot according to the invention. In other words, the k.sub.z-phase encoding is varied from shot to shot and various ky-phsae encodings are made for each k.sub.z-phase encoding. At each (k.sub.z, k.sub.y) position in k-space an entire k.sub.x-line is sampled in the frequency encoding direction.

(15) An MR image for each image slice is reconstructed from the acquired MR signal data using a parallel reconstruction algorithm, wherein the MR signal contributions from the different image slices are separated on the basis of the spatial encodings of the MR signals according to the spatial sensitivity profiles of the RF coils and on the basis of the phase offsets attributed to the respective image slices and shots. This is explained in more detail in the following:

(16) In a multi-shot sequence with N shots, wherein each shot represents a regular under-sampling of k-space, a set of SENSE equations can be set up as:
S{right arrow over (p)}={right arrow over (m)}.sub.sh

(17) Therein vector {right arrow over (p)} comprises the image pixel values that need to be calculated from the aliased pixel values measured via the separate RF coils (‘channels’). The aliased pixel values are described by vector {right arrow over (m)}. S is the coil sensitivity matrix which is determined by the spatial sensitivity profiles of the used RF coils. The size of the sensitivity matrix S is determined by the under-sampling of the shots, the number of used RF coils and the number of slices excited by the multi-slice RF pulses. The phase offset applied in each shot of the imaging sequence according to the invention can be taken into account by a matrix Φ.sub.sh which is a diagonal matrix describing the resulting extra phase encoding in the slice direction. The SENSE equations can be written on this basis as:
SΦ.sub.sh{right arrow over (p)}={right arrow over (m)}.sub.sh

(18) Herein Φ.sub.sh is a diagonal matrix containing the phase encoding per location:

(19) ${\Phi }_{\mathrm{sh}}=\left(\begin{array}{ccc}{e}^{\phi \left(r_1\right)}& 0& 0\\ 0& O& 0\\ 0& 0& e^{\phi \left(r_N\right)}\end{array}\right)$

(20) Not only the different phase encodings in the slice direction (k.sub.z) have to be taken into account, but also the different phase encodings in the phase-encoding direction (k.sub.y) have to be considered. Hence, φ(r) describes the phase imparted to the respective pixel values by both the phase encoding in the regular phase-encoding direction (k.sub.y) and the phase offset in the slice direction (k.sub.z). The coil sensitivity encoding and the phase encoding (y and z) can be combined into one encoding matrix E.sub.sh:
SΦ.sub.sh=E.sub.sh

(21) Finally, the equations for all N shots can be combined in one generalized SENSE reconstruction kernel:

(22) $\left[\begin{array}{c}{E}_1\\ M\\ E_N\end{array}\right]\stackrel{.fwdarw.}{p}=\left[\begin{array}{c}{m}_1\\ M\\ m_N\end{array}\right].fwdarw.E_{\mathrm{all}}\stackrel{.fwdarw.}{p}={\stackrel{.fwdarw.}{m}}_{\mathrm{all}}$

(23) Therein {right arrow over (p)} comprises the pixel values of all final slice images resulting from the multi-slice multi-shot SENSE reconstruction including phase encoding in the slice direction. The least squares solution (without noise de-correlation and regularization) of {right arrow over (p)} is:
{right arrow over (p)}=(E.sub.all.sup.HE.sub.all).sup.−1E.sub.all{right arrow over (m)}.sub.all

(24) The multi-slice acquisition approach of the invention can be regarded as a three-dimensional scan in which k.sub.y-k.sub.z space is sampled by a corresponding phase-encoding, wherein the number of k.sub.z lines is equal to the number of simultaneously excited slices. With the multi-shot approach of the invention it becomes possible to acquire a given k.sub.y encoding step multiple times with different k.sub.z encodings. This makes it possible to optimally sample the three-dimensional k-space (using under-sampling). FIG. 3 illustrates an example of k.sub.y-k.sub.z-sampling according to the invention, wherein eight slices are simultaneously excited. Four shots A, B, C, and D of an EPI sequence are applied with four different phase offsets applied in the slice direction. Under-sampling is applied in both the k.sub.y and k.sub.z directions with interleaved sampling in the k.sub.g-direction. In the depicted embodiment, each shot applies a constant k.sub.z encoding, such that no extra gradient blips need to be applied in the slice direction within the EPI sequence of a single shot.