Method and apparatus for rapid magnetic resonance imaging

Abstract: A method and apparatus for magnetic resonance imaging employing a pulse sequence for rapid imaging called Mosaic Scan or Interleave Scan in which a magnetic resonance image is reconstructed from a plurality of trajectories. In each trajectory a plurality of echo signal are induced by an excitation pulse. Sampled data are selected and ordered at the corresponding location on the Fourier space. The ordered data are reconstructed to produce magnetic resonance images with high frame rate.

Description: The present invention relates to an apparatus and a method for obtaining magnetic resonance images, more specifically, an apparatus employing a pulse sequence for rapid imaging. A number of investigations have been done to reduce the imaging time of a magnetic resonance imaging apparatus especially for imaging the heart or abdominal organs. A pulse sequence called "Echo Planar Imaging (EPI)" method is well-known as a rapid imaging sequence which enable to obtain an image in sub-seconds. In addition, there are some other rapid sequences, for example, "Rapid Acquisition with Relaxation Enhancement (RARE)" method and "Gradient and Spin Echo (GRASE)" method disclosed respectively 3, 828-833, 1986 and 20, 344-349, 1991, both in Magnetic Resonance in Medicine. A method for improving the spatial resolution in EPI method has been shown by Rzedzian in U.S. Patent No. 4,767,991, which method is called "Mosaic Scan" or "Interleave Scan". The Fourier space is divided into a plurality of trajectories. In each trajectory a plurality of magnetic resonance signals are obtained by one excitation pulse. Trajectories fill the Fourier space similar to a mosaic work in Mosaic Scan, or interleaving each other in Interleave Scan, and these trajectories produce a magnetic resonance image. In this method, the more trajectories in the Fourier space, the more the spatial resolution is improved, since the spatial resolution is proportional to the number of the sampling points when the sampling pitch is constant. A similar method is disclosed in EP-A-0414318. However, the frame rate, which indicates the number of images obtained per unit time, is worse since the whole sampling time is longer. Accordingly, it is an object of the present invention to provide an improved magnetic resonance imaging apparatus with an improved frame rate employing Mosaic Scan or Interleave Scan. In accordance with the present invention, the foregoing object is achieved by a method for magnetic resonance imaging comprising the steps of: (a) applying a static magnetic field; (b) applying an excitation pulse with a gradient field to excite nuclear spins in a region to be examined; (c) reversing the direction of said excited nuclear spins repeatedly to induce a plurality of echo signals, said echo signals corresponding to a predetermined trajectory on the Fourier space, each of said echo signals having a different phase encoding; (d) sampling said echo signals; characterised by the steps of: (e) repeating the steps (b)-(d) to obtain sets of echo signal data of a plurality of different trajectories which are interleaved with one another; (f) repeating a cycle of the steps (b)-(e) to obtain further sets of echo signal data on the same trajectories as the previous cycle; (g) replacing each set of trajectory data obtained in the step (e) in turn, by the corresponding set of trajectory data obtained in step (f), both sets of trajectory data being located on the same trajectory; and (h) reconstructing a fresh magnetic resonance image each time one of the sets of data has been replaced. The invention also extends to an apparatus for magnetic resonance imaging comprising: static field generating means for generating a static magnetic field in a region to be examined; gradient field generating means for generating a gradient magnetic field according to a predetermined pulse sequence; transmitting means for transmitting an RF pulse to excite nuclear spins in an object according to said predetermined pulse sequence; receiving means for receiving a magnetic resonance echo signal from said region; characterised by: sequence control means for carrying out said predetermined pulse sequence, said pulse sequence inducing a plurality of echo signals on a predetermined trajectory, each of said echo signals having a different phase encoding, said echo signals from a plurality of different trajectories being obtained, said trajectories interleaving one another, a cycle of obtaining said echo signals from the trajectories being repeated; data ordering means for ordering said echo signals according to a predetermined order, wherein one of the trajectory echo signals obtained in the cycle is replaced by a trajectory echo signal obtained in another cycle and the replaced trajectory echo signal is ordered to be located on a predetermined location on the Fourier space; and reconstruction means for reconstructing a magnetic resonance image from echo signals from a first cycle and a fresh magnetic resonance image including some echo signals from the first cycle, with echo signals from a replaced trajectory from a subsequent cycle each time one set of echo signals is replaced; whereby magnetic resonance images are continuously reconstructed. Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figures 1a and 1b show a block diagram of a first embodiment of the present invention. Figures 2 and 3 are a pulse sequence chart and a locus of trajectories, respectively, of Interleave Scan method used in the first embodiment of the present invention. Figure 4 shows a block diagram of a second embodiment of the present invention. Figure 5 shows a locus of trajectories according to the second embodiment of the present invention. Figure 6 is a timing chart showing an electrocardiogram and sampling points according to the second embodiment of the present invention. Referring now to Figs. 1 (A), (B), 2 and 3, a first embodiment of the present invention will be described. A magnet assembly 1 driven by a static field controller 2 provides a static magnetic field to the area to be examined. A sequencer 3 memorizes pulse sequences, for example, as shown in Fig. 2 and provides control signals to a gradient amplifier 4, consisting of three elements 4x, 4y and 4z, and an RF transmitter circuit 5 according to the pulse sequence. The gradient amplifier 4 drives a gradient coil system 6 for generating a gradient magnetic field to provide position information of a region in which magnetic resonance signals are induced. The gradient coil system 6 also consists of three elements corresponding to x-, y- and z-axis, respectively (not shown in Fig. 1 (A)). The transmitter circuit 5 provides RF signals to and receives induced echo signals from an RF probe 7. The RF probe 7, which may consists of a transmitter coil and a receiver coil and/or a transmitter-receiver coil, transmits RF pulses and detects induced echo signals. The detected echo signals are provided into a data ordering unit 8 via a receiver circuit 9 to be ordered. The ordered echo signal data are provided into a reconstruction unit 10 to reconstruct a magnetic resonance image of a patient P by carrying out a reconstruction process, for example, two-dimensional Fourier transform. The reconstructed image is displayed on a display unit 11. A system control unit 12 provides control signals to the sequence 3 and the data ordering unit 8. Fig. 1 (B) shows an example of a detailed block diagram of the data ordering unit 8. Echo data from the receiver circuit 9 are temporarily stored in a memory unit 20. Then data selected according to control signals from the system control unit 12 are provided into a memory unit 21. The memory unit 21 has address corresponding to the coordinate system of the Fourier space. According to the signal magnitude, the frequency and the phase information of the echo signal data, the memory unit 21 recognises the address for the input data and the data are stored in the corresponding address. Fig. 2 shows an example of a pulse sequence used in the present invention. In Fig. 2, Gr, Ge and Gs correspond to read, phase-encode and slice gradient, respectively. A 90° RF pulse excites spins in a selected slice and an echo signal is obtained after a period T during the supply of the read gradient Gr. Repeatedly supplying the phase-encode gradient pulse signals Ge and reversing the direction of the read gradient Gr, a train of echo signals are induced and sampled along Trajectory A as shown in Fig. 3. Trajectory B, which interleaves Trajectory A, is sampled with a pulse sequence in which the duration period T' of the initial phase-encode Ge is longer than T of Trajectory A to shift the trajectory along the ky axis on the Fourier space. Otherwise, the magnitude of the initial phase-encode gradient Ge may be larger to shift the trajectory. Trajectories C and D are also sampled in the similar manner. Consequently, data A1, B1, C1 and D1 are obtained along Trajectories A, B, C and D, respectively, and stored in the memory unit 20. The stored data A1, B1, C1 and D1 are read out and provided into the memory unit 21 in the respective address corresponding to the coordinates of the Fourier space. Data A1, B1, C1 and D1 are read out from the memory unit 21 and input into the reconstruction unit 10. The reconstruction unit 10 reconstructs a first magnetic resonance image from data A1, B1, C1 and D1. The cycle of sampling trajectories A, B, C and D is repeated a plurality of times to observe changes in the region in the patient P for a long period. In the second cycle, data A2, which is sampled along Trajectory A, is obtained first and stored in the memory unit 20. Data A2 is read out and provided to the memory unit 21 to be stored. In the memory unit 21, the stored data A1 is replaced by the newly input data A2. Then, data A2, B1, C1 and D1 are provided to the reconstruction unit 10. Subsequently obtained data B2 replace data B1 in the memory unit 21 and a third image is reconstructed from data A2, B2, C1 and D1. As shown above, new images are obtained continuously. The first embodiment of the present invention can obtain four images during the period between an excitation and the next excitation, while the prior art could obtain one during the same period. Thus, the first embodiment can provide a magnetic resonance imaging apparatus which is improved four times in the frame rate. Referring now to Figs. 4, 5 and 6, a second embodiment of the present invention will be described. This embodiment is applied especially to obtaining images of the organ moving frequently, for example, a heart. The structure in Fig. 4 is same as Fig. 1 except for an electrocardiograph 15 for detecting heartbeats and controlling the sequencer 3. This embodiment employs four-tile Mosaic Scan which divides the Fourier space into four rectangular tile trajectories and the four tiles produce a magnetic resonance image as shown in Fig. 5. Fig. 6 (A) shows output signals from the electrocardiograph 15 indicating cardiac cycles of a patient. Each cardiac cycle is divided into four small cardiac delay phases t1-t4 as shown in Fig. 6 (B). Data of each trajectory are sampled during one of the cardiac delay phases t1-t4 in synchronism with the output signal of the electrocardiograph 15 as shown in Fig. 6 (C). The order of sampling the trajectories is decided such that one of four trajectories is sampled at each cardiac delay phase and every trajectory at every cardiac delay phase is sampled in four cardiac cycles P1-P4. This order may be predetermined in the sequencer 3. During the first cardiac cycle P1 data of the tiles are sampled in order of E1-F1-G1-H1, the second cycle P2 H2-E2-F2-G2, the third cycle P3 G3-H3-E3-F3 and the fourth cycle P4 F4-G4-H4-E4, and each data is stored in the memory unit 20. Data E1, H2, G3 and F4, each corresponding to the cardiac delay phase p1, are read out from the memory unit 20 and stored in the corresponding address in the memory unit 21. Then, data E1, F4, G3 and H2 are read out from the memory unit 21 and an image corresponding to the cardiac phase p1 is obtained by reconstruction unit 10. When data E1, F4, G3 and H2 is read out from the memory unit 21, data E2, F1, G4 and H3, each corresponding to the cardiac phase p2, replace data formerly stored in the memory unit 21. Then an image corresponding to the phase p2 is obtained. In the similar manner, images corresponding to the phases p3 and p4 are obtained from data E3-F2-G1-H4 and E4-F3-G2-H1, respectively. Thus, by the second embodiment, data of tiles E, F, G and H at each cardiac phases are obtained during four cardiac cycles and four magnetic resonance images are obtained, while the conventional MRI apparatus produced only one image for the same period. This embodiment improves the frame rate four times comparing with the conventional system, while the spacial resolution is not lowered. In the above embodiment, after the fourth cardiac cycle, data of the fifth cardiac cycle may be obtained and the other four images can be obtained from data of the second to the fifth cycles. This enables the observation of the change in the patient for a longer period. Further, this embodiment can be applied to images of abdomen by providing a respiration sensor, for example a motion sensor attached to the patient's body, instead of an electrocardiograph. As described above, this embodiment improves the frame rate comparing with the conventional system, while the spatial resolution is not lowered. In addition, in the present invention the scan method is not limited to the above embodiments. Each embodiment may employ Mosaic Scan method or Interleave Scan method. As the pulse sequence used in the embodiments of the present invention, not only EPI method but also RARE or GRASE, etc. may be adopted. The number of trajectories dividing the Fourier space also is not limited to the above description. Further, more than two trajectory data may be replaced at once. Numerous modifications and variations of the present invention are possible in light of the above-described teachings. It is therefore to be understood that, within the scope of the appended claims, the present invention can be practiced in a manner other than as specifically described herein.

Claims: A method for magnetic resonance imaging comprising the steps of: (a) applying a static magnetic field; (b) applying an excitation pulse with a gradient field to excite nuclear spins in a region to be examined; (c) reversing the direction of said excited nuclear spins repeatedly to induce a plurality of echo signals, said echo signals corresponding to a predetermined trajectory on the Fourier space, each of said echo signals having a different phase encoding; (d) sampling said echo signals; characterised by the steps of: (e) repeating the steps (b)-(d) to obtain sets of echo signal data of a plurality of different trajectories which are interleaved with one another; (f) repeating a cycle of the steps (b)-(e) to obtain further sets of echo signal data on the same trajectories as the previous cycle; (g) replacing each set of trajectory data obtained in the step (e) in turn, by the corresponding set of trajectory data obtained in step (f), both sets of trajectory data being located on the same trajectory; and (h) reconstructing a fresh magnetic resonance image each time one of the sets of data has been replaced. A method for magnetic resonance imaging according to claim 1, wherein said step (e) comprises a first storing step for storing said sampled echo data, selecting step for selecting data to be reconstructed from said stored data and a second storing step for storing said selected data. A method for magnetic resonance imaging according to claim 2, wherein said selected data are stored according to the coordinates of the Fourier space in said second storing step. A method for magnetic resonance imaging according to claim 1, wherein said steps (b)-(d) are carried out in synchronism with motion of a patient and sampled data are selected according to said motion. A method for magnetic resonance imaging according to claim 4, wherein data corresponding to the same motion phase are selected and ordered. A method for magnetic resonance imaging according to claim 4, wherein at least one of said sets of trajectory data is replaced by data corresponding to the same trajectory at a different motion cycle. A method for magnetic resonance imaging according to claim 4, wherein said motion is caused by heartbeats and detected by an electrocardiograph. A method for magnetic resonance imaging according to claim 4, wherein said motion is caused by respiration and detected by a respiration sensor. An apparatus for magnetic resonance imaging comprising: static field generating means for generating a static magnetic field in a region to be examined; gradient field generating means for generating a gradient magnetic field according to a predetermined pulse sequence; transmitting means for transmitting an RF pulse to excite nuclear spins in an object according to said predetermined pulse sequence; receiving means for receiving a magnetic resonance echo signal from said region; characterised by: sequence control means for carrying out said predetermined pulse sequence, said pulse sequence inducing a plurality of echo signals on a predetermined trajectory, each of said echo signals having a different phase encoding, said echo signals from a plurality of different trajectories being obtained, said trajectories interleaving one another, a cycle of obtaining said echo signals from the trajectories being repeated; data ordering means for ordering said echo signals according to a predetermined order, wherein one of the trajectory echo signals obtained in the cycle is replaced by a trajectory echo signal obtained in another cycle and the replaced trajectory echo signal is ordered to be located on a predetermined location on the Fourier space; and reconstruction means for reconstructing a magnetic resonance image from echo signals from a first cycle and a fresh magnetic resonance image including some echo signals from the first cycle, with echo signals from a replaced trajectory from a subsequent cycle each time one set of echo signals is replaced; whereby magnetic resonance images are continuously reconstructed. An apparatus for magnetic resonance imaging according to claim 9, wherein said ordering means comprising a first storing means for storing said sampled echo data and a second storing means for storing data selected from data stored in said first storing means. An apparatus for magnetic resonance imaging according to claim 10, wherein said second storing means stores said selected data according to the coordinates of the Fourier space. An apparatus for magnetic resonance imaging according to claim 9, wherein said sequence control means operates in synchronism with motion of a patient; and further comprising motion detecting means for detecting said motion. An apparatus for magnetic resonance imaging according to claim 12, wherein said data ordering means selects and orders data corresponding to the same motion phase. An apparatus for magnetic resonance imaging according to claim 12, wherein at least one of said selected data is replaced by data corresponding to the same trajectory and the different motion cycle. An apparatus for magnetic resonance imaging according to claim 12, wherein said motion is caused by heartbeat and further comprises an electrocardiograph for detecting said heartbeat. An apparatus for magnetic resonance imaging according to claim 12, wherein said motion is caused by respiration and further comprises a respiration sensor for detecting said respiration.

Inventors: MORIYA EIJI, SEO YASUTSUGU, MORIYA, EIJI, SEO, YASUTSUGU, Moriya, Eiji, c/o Intellectual Property Div., Seo, Yasutsugu, c/o Intellectual Property Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: G01R33
IPC codes: G01R33