WO2009047690A2

WO2009047690A2 - Segmented multi-shot mri involving magnetization preparation

Info

Publication number: WO2009047690A2
Application number: PCT/IB2008/054081
Authority: WO
Inventors: Gabriele M. Beck; Jan F. L. De Becker; Alun C. R. Jones
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-10-09
Filing date: 2008-10-06
Publication date: 2009-04-16
Also published as: WO2009047690A3

Abstract

The invention relates to a device for magnetic resonance imaging of at least a portion of a body placed in an examination volume. The device is arranged to perform the following successive steps: a) acquiring two or more differently phase-encoded MR signals (a, b, c, d) by subjecting the portion to a shot of an imaging pulse sequence comprising at least one RF pulse and switched magnetic field gradients; b) repeating step a) to scan k-space completely by applying a plurality of shots and by using a phase encoding scheme, according to which k-space is divided into at least two segments (17, 18, 19, 20), each segment (17, 18, 19, 20) being further divided into sub-segments, wherein MR signals (a, b, c, d) are acquired from different sub-segments of a single segment during each shot, which sub- segments are successively arranged in k-space in the order of the temporal succession of the MR signals (a, b, c, d), and c) reconstructing an MR image from the acquired MR signals.

Description

Magnetic resonance device and method

FIELD OF THE INVENTION

The present invention relates to the field of magnetic resonance (MR). It finds application in conjunction with magnetic resonance imaging (MRI) methods and MR scanners for diagnostic purposes.

BACKGROUND OF THE INVENTION

In MRI, pulse sequences consisting of RF and magnetic field gradient pulses are applied to an object (a patient) to generate magnetic resonance signals, which are scanned in order to obtain information therefrom and to reconstruct images of the object. Since its initial development, the number of clinical relevant fields of application of MRI has grown enormously. MRI can be applied to almost every part of the body, and it can be used to obtain information about a number of important functions of the human body. The pulse sequence which is applied during an MRI scan determines completely the characteristics of the reconstructed images, such as location and orientation in the object, dimensions, resolution, signal-to-noise ratio, contrast, sensitivity for movements, etcetera. An operator of a MRI device has to choose the appropriate sequence and has to adjust and optimize its parameters for the respective application.

3D Ti weighted magnetization prepared sequences have emerged as a useful technique for visualization of the uptake of a contrast agent in tumors within body organs. Such sequences produce isotropic images with high spatial resolution, excellent anatomic detail and complete coverage of the respective organ. A magnetization preparation sequence is employed according to these known techniques in order to control image contrast. One or more RF pulses are applied to prepare magnetization before the actual acquisition of MR signals begins. For example, a 180° RF excitation pulse is applied during the preparation phase and, at a predetermined time interval thereafter, the generation of the actual imaging pulse sequence is begun when the magnetization is in the desired state due to Ti relaxation. Immediately before the signal acquisition is started, an additional saturation RF pulse may be applied which is tuned to suppress the MR signals from fat spins (so-called fat suppression). In this way, the contrast to noise ratio (CNR) is improved and the depiction of vascular structures is enhanced. The time available for the acquisition of MR signals is critical since the characteristic enhancement patterns generated during the uptake of contrast agent within and around the tumors need to be captured and also artifacts due to respiratory and other motion of the examined body have to be avoided. Because of this, abdominal MR imaging is conventionally performed during a breathold using partial Fourier scanning together with parallel signal acquisition (e.g. according to the known SENSE technique). For time efficient signal acquisition, fast (turbo) methods are employed in which a large number of MR signals are acquired after generation of a single magnetization preparation sequence. A problem of this technique is that the acquired MR signals have different preparation delays. The prepared magnetization decays as the acquisition of MR signals proceeds such that the desired contrast fades during each scan. Usually, a segmented k-space ordering scheme is applied corresponding to the order of preparation delay values of the acquired signals. An optimal magnetization contrast is achieved in this way since the MR signals acquired from the central k-space regions that mainly determine image contrast are sampled at an optimal point in time after each magnetization preparation pre-pulse.

SUMMARY OF THE INVENTION

A drawback of this known technique is that the individual MR signals are acquired at different points in time with respect to the arrival of the bolus of the contrast agent at the location of the organ of interest. The result is that the contrast determining MR signals are distributed over the entire set of acquired MR signals leading to a non-distinct contrast enhancement characteristic. Consequently, the reconstructed MR images show a non-predictable behaviour regarding the enhancement of the applied contrast agent. This results in potentially missed lesions. It is known that problems related to time dependent contrast enhancement also exist in the field of magnetic resonance angiography (MRA). In contrast-enhanced MRA, the timing of contrast agent application and MR signal acquisition is of utmost importance for obtaining optimal arterial contrast while avoiding venous vessel overlay. For example from US 2001/0033162 Al, an MRA technique is known according to which a central region of k- space is sampled during the early arterial phase, i.e., when the contrast agent arrives in the area of interest. The central k-space region is sampled in a random order to minimize artifacts caused by unstable contrast agent opacification. The peripheral regions of k-space are sampled before and after the maximum of contrast enhancement. This known technique, which is referred to as centric k-space ordering or - more specifically - contrast-enhanced time-robust angiography (CENTRA), relates to the long-term ordering of k-space sampling. It does not address fast MR imaging techniques in which a single magnetization preparation sequence is shared by a plurality of acquired MR signals having different preparation delays.

Therefore, it is readily appreciated that there is a need for an improved MR device and method in order to enable fast and reliable visualization of contrast agent uptake in anatomic structures of an examined body. It is consequently the primary object of the present invention to provide a device for MR imaging which combines fast signal acquisition with segmented k-space sampling and with long-term (centric) k-space ordering. In accordance with the present invention, a device for magnetic resonance imaging of at least a portion of a body placed in an examination volume is disclosed. The device is arranged to perform the following successive steps: a) acquiring two or more differently phase-encoded MR signals by subjecting the portion to a shot of an imaging pulse sequence comprising at least one RF pulse and switched magnetic field gradients; b) repeating step a) to scan k-space completely by applying a plurality of shots and by using a phase encoding scheme, according to which k-space is divided into at least two segments, each segment being further divided into sub-segments, wherein MR signals are acquired from different sub-segments of a single segment during each shot, which sub-segments are successively arranged in k-space in the order of the temporal succession of the MR signals, and c) reconstructing an MR image from the acquired MR signals.

According to one aspect of the invention, several MR signals are acquired during each shot of the applied imaging sequence. This enables time efficient signal acquisition, e.g. by means of a (balanced) turbo field echo (TFE) or a turbo spin echo (TSE) imaging sequence. Furthermore, a phase encoding scheme is proposed by the invention, according to which k-space is divided into two or more segments, which enables the long term ordering of k-space sampling. During each shot of the imaging sequence, MR signals are acquired from a single k-space segment, which is selected, e.g., to obtain optimal contrast enhancement after application of a contrast agent. Each k-space segment is further divided into sub-segments. The number of sub-segments corresponds to the number of MR signals acquired during each shot. In accordance with the phase encoding scheme proposed by the invention, a single MR signal is acquired from each sub-segment of the respectively selected k-space segment during each shot. A plurality of shots of the imaging sequence is applied until k-space is sampled completely and a full MR image can be reconstructed. An essential feature of the invention is that the MR signals are successively acquired from sub-segments which are successively arranged in k-space corresponding to the temporal order of the MR signals. Image artifacts arising from signal amplitude and phase discontinuities are efficiently avoided in this way. The k-space segments may be congruently divided into sub-segments. In this case, signal amplitude and phase discontinuities are avoided not only between the sub- segments which are scanned successively during each shot. The congruent division of the k- space segments into sub-segments has the advantage that signal discontinuities are minimized also between the different segments. This results in further reduced artifacts during reconstruction and improved image quality. As mentioned above, the division of k-space into segments relates to the long- term ordering of k-space sampling. In this respect it is advantageous to scan each segment completely by a plurality of shots of the imaging pulse sequence before the next segment is selected for acquisition by a plurality of further shots. In order to enable optimal long term k- space ordering for contrast control, the k-space segments should be associated with central and peripheral areas of k-space, respectively. The image contrast is mainly determined by MR signals sampled from the central areas of k-space. The central k-space area may for example be selected in accordance with the time of arrival of a bolus of a contrast agent at the location of the examined organ in order to obtain optimal contrast of the reconstructed MR image. The division of k-space into central and peripheral segments in combination with a corresponding contrast bolus timing method enables a precise timing of selection of the corresponding k-space segment during selected periods of contrast enhancement. This is an important prerequisite for the detection of lesions being visible only during certain short periods of enhancement as well as for lesion characterization. A known contrast bolus timing or tracking method may be used in combination with the device of the invention in order to provide a triggering signal for the selection of the appropriate k-space segment and for the acquisition of the contrast determining MR signals.

The division of k-space into central and peripheral segments is further advantageous since it allows combining the technique of the invention with known dynamic (4D) MR imaging methods, such like the so-called Keyhole or TRICKS methods. These methods enable a fast acquisition and reconstruction of a temporal succession of two or more MR images in order to visualize motions of examined body organs. It is straightforward to realize such fast dynamic MRI studies in accordance with the invention by sharing MR signals acquired from peripheral segments between different MR images during reconstruction. In accordance with a preferred embodiment of the invention, the peripheral k- space segments are ring-shaped such that the sub-segments of the peripheral k-space segments are successively arranged along concentric rings in k-space. The k-space ordering achieved in this way has the advantage that acoustic noise and eddy current effects caused by large phase encoding steps are effectively minimized.

According to a further aspect of the invention, the imaging sequence encompasses a magnetization preparation sequence comprising at least one RF pulse. Because of the careful ordering of k-space sampling, the technique of the invention can advantageously combine the application of turbo sequences, such like the above-mentioned TFE or TSE sequences, with a magnetization preparation pre-pulse. According to the invention, several MR signals are acquired during each shot. In other words, a single magnetization preparation sequence is shared by a plurality of differently phase encoded MR signals. The number of sub-segments corresponds to the number of MR signals acquired during each shot. Thus, a specific preparation delay value is fixedly associated with each sub- segment. The contrast generated by the magnetization preparation is well controlled by the arrangement of the sub-segments in k-space. The magnetization preparation sequence may be a Ti preparation sequence. This is particularly advantageous in case of the application of Gadolinium based contrast agents. For the suppression of background tissue and for enhanced CNR, the magnetization preparation sequence may additionally include fat suppression.

According to a further aspect of the invention, the acquisition of MR from the sub-segments of the central area of k-space is successive from a start sub-segment to an end segment. The start sub-segment and the end-sub-segment are located such that (i) neither the start sub-segment nor the end sub-segment contain the centre of k-space and (ii) the start sub- segment and the end sub-segment are located at opposite sides of the centre of k-space. In other words, the scanning of the central area of k-space is unidirectional through k-space, e.g. along a so-called low-high phase-encoding order. Thus, the MR-signals are acquired from the centre of k-space after several sub-segments of the central area have already been scanned. These sub-segments that are scanned before the centre of k-space is reached while the magnetisation preparation sequence generates its optimum effect, e.g. fat suppression or nulling of the signal of moving blood of which the spins have been inverter (e.g. black blood imaging). Thus it is achieved to scan the centre of k-space at optimum magnetization preparation. Also scanning time is used more efficiently, because sub-segments in the central are but outside the centre of k-space are already scanned during the magnetisation preparation. It appears the MR-signals from the sub-segments that do no contain the centre of k-space are less sensitive to the optimisation of the magnetisation preparation than the MR- signals from the centre of k-space.

The invention not only relates to a device but also to a method for magnetic resonance imaging of at least a portion of a body placed in a stationary and substantially homogeneous main magnetic field, the method comprising the following successive steps: a) acquiring two or more differently phase-encoded MR signals by subjecting the portion to a shot of an imaging pulse sequence comprising at least one RF pulse and switched magnetic field gradients; b) repeating step a) to scan k-space completely by applying a plurality of shots and by using a phase encoding scheme, according to which k-space is divided into at least two segments, each segment being further divided into sub-segments, wherein MR signals are acquired from different sub-segments of a single segment during each shot, which sub-segments are successively arranged in k-space in the order of the temporal succession of the MR signals, and c) reconstructing an MR image from the acquired MR signals.

A computer program with instructions for carrying out the MR procedure of the invention can advantageously be implemented on any common computer hardware, which is presently in clinical use for the control of magnetic resonance scanners. The computer program can be provided on suitable data carriers, such as CD-ROM or diskette. Alternatively, it can also be downloaded by a user from an Internet server.

The following 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.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings Fig.l shows an embodiment of an MRI scanner according to the invention;

Fig.2 shows a diagram illustrating the acquisition of MR signals in accordance with the present invention;

Fig.3 illustrates the phase encoding scheme applied in accordance with the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In Fig.1 a magnetic resonance imaging device 1 in accordance with the present invention is shown as a block diagram. The apparatus 1 comprises a set of main magnetic coils 2 for generating a stationary and homogeneous main magnetic field and three sets of gradient coils 3, 4 and 5 for superimposing additional magnetic fields with controllable strength and having a gradient in a selected direction. Conventionally, the direction of the main magnetic field is labelled the z-direction, the two directions perpendicular thereto the x- and y-directions. The gradient coils are energized via a power supply 11. The apparatus 1 further comprises a radiation emitter 6, an antenna or coil, for emitting radio frequency (RF) pulses to a body 7, the radiation emitter 6 being coupled to a modulator 8 for generating and modulating the RF pulses. Also provided are receiving antennas 10a, 10b, 10c for receiving the MR signals, the receiving antennas can for example be separate surface coils with different spatial sensitivity profiles. The received MR signals are input to a demodulator 9. The modulator 8, the emitter 6 and the power supply 11 for the gradient coils 3, 4 and 5 are controlled by a control system 12 to generate the actual imaging sequence for MR imaging in accordance with the above-described invention. The control system is usually a microcomputer with a memory and a program control. For the practical implementation of the invention it comprises a programming with a description of an imaging procedure as described above. The demodulator 9 is coupled to a data processing unit 14, for example a computer, for transformation of the received magnetic resonance signals into an image in accordance with a combination of the method of the invention with the known SENSE unfolding algorithm (see for example Pruessmann et al, Magnetic Resonance in Medicine, volume 42, page 952, 1999). In this case, the spatial sensitivity profiles of the antennas 10a, 10b, 10c would have to be taken into account. The final image can be made visible, for example, on a visual display unit 15.

A sequence design in accordance with the method of the present invention is depicted in Fig.2. The diagram shows the temporal succession of magnetization preparation pre-pulses 16 (e.g. fat suppression RF pulses) and the acquisition of four phase encoded MR signals a, b, c, and d. Each MR signal a, b, c, and d has a different preparation delay. The magnetization preparation and signal acquisition is repeated until k-space is sampled completely. The magnetization preparation pre-pulses 16 are shared by the four MR signals a, b, c, and d in order to minimize the total acquisition time. For fast MR signal acquisition, a 3D Ti weighted fat suppressing turbo gradient echo sequence (THRIVE) combined with a parallel imaging technique (SENSE) may be employed. For fat suppression, a spectrally selective inversion pre-pulse (SPAIR) may be used. Dynamic studies are possible, e.g., by using a corresponding Keyhole technique (4D THRIVE).

Fig. 3 illustrates the phase encoding scheme applied according to the invention. k_y-k_z-space is divided into four concentrically arranged ring-shaped segments 17, 18, 19, 20. The segment 17 covers the central k-space area while segments 18, 19, 20 are associated with the peripheral areas of k-space. One of the segments 17, 18, 19, 20 is selected for acquisition after each magnetization preparation pre-pulse 16. This implies that MR signals are acquired from only one k-space segment after each pre-pulse 16. This is illustrated by the dashed arrows depicted in Fig. 3. The arrows indicate the progression of the phase encoding during each repetition of the imaging sequence. The form of the concentric segments 17, 18, 19, 20 is defined by the required field of view (FOV), slice thickness, image resolution, etc. The segments 17, 18, 19, 20 are further divided into sub-segments associated with the MR signals a, b, c, and d of each shot. As can be seen in Fig. 3, the k-space segments 17, 18, 19, 20 are congruently divided into sub-segments in the order of the MR signals a, b, c, and d. Signal amplitude and phase discontinuities are hereby avoided during signal acquisition. Eddy current effects are minimized by sampling the MR signals along the concentric ring-shaped segments 18, 19, 20. In the depicted embodiment, positive k_y-k_z-space is subdivided into sub-segments forming sectors corresponding to the temporal order of the MR signals a, b, c, and d. The corresponding negative quadrants of k_y-k_z-space are point symmetrical copies with respect to the origin. The (long term) order in which the segments 17, 18, 19, 20 are selected during image acquisition determines the contrast enhancement due to an applied contrast agent. In case of a CENTRA like long term ordering, the central k- space segment 17 is acquired in the initial phase and then moves towards the peripheral segments 18, 19, 20. The magnetization preparation contrast is determined by the preparation delay of the MR signals acquired from the central k-space segment 17.

As the dashed arrows in the central area 17 indicate, the progress of the phase encoding in the central area 17 may be form the centre to the edge of the central area along sub-segments a,b and c in alternate positive and negative phase encoding direction. Alternatively, the progression of phase encoding in the central area 17 is from low (negative) ky-values to high (positive) k_y- values. That is, the central area is scanned from the start sub- segment c at negative k_y- values, through the sub-segment a that contains the centre of k- space, to the end sub-segment c at positive k_y-values. The magnetization preparation is being built-up during the scanning of the first few sub-segments in the central area, then reaches optimum during scanning of the sub-segment that contains the centre of k-space and than to some degree fades away as the remaining sub-segments of the central area are scanned.

Claims

CLAIMS:

1. Device for magnetic resonance (MR) imaging of at least a portion of a body (7) placed in an examination volume, the device (1) being arranged to perform the following consecutive steps: a) acquiring two or more differently phase-encoded MR signals (a, b, c, d) by subjecting the portion to a shot of an imaging pulse sequence comprising at least one

RF pulse and switched magnetic field gradients; b) repeating step a) to scan k-space completely by applying a plurality of shots and by using a phase encoding scheme, according to which k-space is divided into at least two segments (17, 18, 19, 20), each segment (17, 18, 19, 20) being further divided into sub-segments, wherein MR signals (a, b, c, d) are acquired from different sub-segments of a single segment during each shot, which sub-segments are successively arranged in k-space in the order of the temporal succession of the MR signals (a, b, c, d), and c) reconstructing an MR image from the acquired MR signals.

2. Device of claim 1, wherein the device is further arranged to scan each segment

(17, 18, 19, 20) completely by a plurality of shots of the imaging pulse sequence before the next segment (17, 18, 19, 20) is selected for acquisition by a plurality of further shots.

3. Device of claim 1 or 2, wherein the imaging sequence encompasses a magnetization preparation sequence (16) comprising at least one RF pulse.

4. Device of claim 3, wherein the magnetization preparation sequence (16) is a Ti preparation sequence.

5. Device of claim 3 or 4, wherein the magnetization preparation sequence (16) includes fat suppression.

6. Device of any one of claims 1-5, wherein the imaging pulse sequence is a

Turbo Spin Echo (TSE) or a Turbo Field Echo (TFE) sequence.

7. Device of any one of claims 1-6, wherein the k-space segments (17, 18, 19, 20) are associated with central and peripheral areas of k-space, respectively, the peripheral k- space segments (18, 19, 20) being ring-shaped such that the sub-segments of the peripheral k- space segments (18, 19, 20) are successively arranged along concentric rings in k-space.

8. Device as claimed in claim 7, wherein the acquisition of MR from the sub- segments of the central area of k-space is successive from a start sub-segment to an end segment such that neither the start sub-segment nor the end sub-segment contain the centre of k- space and the start sub-segment and the end sub-segment are located at opposite sides of the centre of k-space.

9. Device of any one of claims 1-8, wherein the k-space segments (17, 18, 19,

20) are congruently divided into sub-segments.

10. Device of any one of claims 1-8, wherein the device is further arranged to apply a centric ordering of k-space segments in accordance with the time of arrival of a bolus of a contrast agent or in accordance with a triggering signal.

11. Device of any one of claims 1-9, wherein the device is arranged to reconstruct a temporal succession of two or more MR images by sharing acquired MR signals (a, b, c, d) between different MR images.

12. Method for magnetic resonance imaging of at least a portion of a body placed in a stationary and substantially homogeneous main magnetic field, the method comprising the following steps: a) acquiring two or more differently phase-encoded MR signals (a, b, c, d) by subjecting the portion to a shot of an imaging pulse sequence comprising at least one

13. Method of claims 11, wherein the k-space segments (17, 18, 19, 20) are associated with central and peripheral areas of k-space, respectively, the peripheral k-space segments (18, 19, 20) being ring-shaped such that the sub-segments of the peripheral k-space segments (18, 19, 20) are successively arranged along concentric rings in k-space.

14. Method of claim 11 or 12, wherein the k-space segments (17, 18, 19, 20) are congruently divided into sub-segments.

15. Computer program for an MR device with instructions for performing the following successive steps: a) acquiring two or more differently phase-encoded MR signals (a, b, c, d) by generating a shot of an imaging pulse sequence comprising at least one RF pulse and switched magnetic field gradients; b) repeating step a) to scan k-space completely by generating a plurality of shots and by using a phase encoding scheme, according to which k-space is divided into at least two segments (17, 18, 19, 20), each segment (17, 18, 19, 20) being further divided into sub-segments, wherein MR signals (a, b, c, d) are acquired from different sub-segments of a single segment during each shot, which sub-segments are successively arranged in k-space in the order of the temporal succession of the MR signals (a, b, c, d), and c) reconstructing an MR image from the acquired MR signals.