JP4908638B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus that images a three-dimensional form of a flow (mainly blood flow) by a fast spin echo (FSE) method.

  Recently, `` Hennig J, Multiecho imaging sequences with a low flip angles, JMR, 78: 397-407. 1988 '' and `` Keifer B, et.al, Image acquisition in a second with half Fourier acquisition single shot turbo spin echo. Magnetic resonance imaging equipment 1994: 4 (P): 1986, etc. As can be seen from many literatures, as a method of cross-sectional imaging of magnetic resonance imaging equipment, multiple echoes are generated by applying refocusing RF pulses repeatedly. The fast spin echo method (Fast Spin Echo method), which increases the speed of imaging by adding different phase encoding information to each echo, is generally used, and "Y. Kassai, et. Al, 3D half-Fourier fast SE for heavy T2-weighted imaging,, In "Proceedings, ISMRM, 4th Annual Meeting", p736, 1996. and `` Yumori Kasai, Fast ASE and its clinical application, Medical Review No. 69, p. 28-34, 1998, etc. Performing 3D shooting has also been put to practical use as one of the options. FIG. 6 shows a typical example of a pulse sequence by the 3D fast spin echo method.

  Recent trends are also described in “RC Selmeka, et.al, HASTE imaging: Description of technique and preliminary results in the abdomen, J Magnetic Resonance Imaging, 6: 698-699, 1996”. In addition, Echo Train Spacing (ETS) has become possible to be shortened, and objects with movements such as real organs and blood flow have become targets for visualization (“Y. Kassai , et.al, 3D Half-Fourier RARE with MTC for Cardiac Imaging, ISMRM, p806, 1998.``Miyazaki M, et al, A Novel MR Angiography Technique: SPEED Acquisition Using Half-Fourier RARE, J Magnetic Resonance Imaging System 8: p. 505-507, 1998 ”,“ Miyazaki M, et al, Fresh Blood Imaging at 0.5T: Natural Blood Contrast 3D MRA within Single Breathhold, ISMRM, p. 780, 1998 ”). It is becoming increasingly important as a non-invasive method for drawing blood flow without using contrast agents.

  On the other hand, gradient moment nulling (GMN method) has been proposed for a long time to zeroize the “phase shift” that occurs in moving spins by zeroing the “moment” defined for the gradient magnetic field waveform. Yes. As shown in FIG. 7, a gradient moment nulling method (“Hinks RS, et al, Gradient Moment Nulling in Fast Spin Echo, MRM 32) as a technique for suppressing flow artifacts in FSE, as shown in FIG. : 698-706 (1994))), but 3D photography is not fully disclosed. In FSE, the gradient magnetic field waveform was devised so as to “rephase only the Hahn spin echo component”. There is a velocity-independent phase-shift stabilization method (VIPS method, initially VISS method), but since only a part of the signal is rephased, its effectiveness is limited ("Machida Y , et al. Velocity Independent Phase-Shift Stabilization (VIPS) Technique in FSE Flow Imaging, In "Proceedings, ISMRM. 7th Annual Meeting", p1910, 1999. ", Japanese Patent Application No. 11-4734").

  In such a conventional method, there is a phenomenon that ghost artifacts are generated from the blood flow or a blood flow signal is lost. In order to improve the reliability in clinical diagnosis, it is necessary to sufficiently suppress these artifacts.

  In view of the above circumstances, an object of the present invention is to obtain a flow image (blood flow image) with reduced artifacts, particularly in a magnetic resonance imaging apparatus using three-dimensional high-speed spin echo imaging. In particular, it is possible to provide a highly reliable image for clinical diagnosis.

  In a magnetic resonance imaging apparatus that collects data necessary for imaging by a pulse sequence according to a three-dimensional fast spin echo method, the pulse sequence has an area with respect to the readout gradient magnetic field pulse before and after the readout gradient magnetic field pulse. A gradient magnetic field pulse of half and opposite polarity is applied, and a spoiler gradient magnetic field pulse is applied in a slice encoding direction different from the application direction of the readout gradient magnetic field pulse.

The figure which shows the structure of the magnetic resonance imaging device which concerns on embodiment of this invention. FIG. 5 is a diagram showing a first pulse sequence (using GMN in 3D and using CFS (constant flop spoiler) in the slice direction) according to the present embodiment. The figure which shows the 2nd pulse sequence (it uses GMN by 3D) by this embodiment. The figure which shows the waveform of the gradient magnetic field for a read of the 3rd pulse sequence (3D uses GMN and uses VIPS in a slice direction) by this embodiment. The figure which shows the 4th pulse sequence (missing gradient method application) by this embodiment. The figure which shows the pulse sequence (it does not use GMN by 3D) of a prior art example. The figure which shows the pulse sequence (it uses GMN by 2D) of a prior art example.

  In the following, a preferred embodiment of the device according to the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of a magnetic resonance imaging apparatus according to this embodiment. The static magnetic field magnet 1 generates a static magnetic field in, for example, a substantially cylindrical imaging region where a subject is inserted. Inside this static magnetic field magnet 1, a shim coil 3, a gradient coil 2, and a probe (RF coil) 4 are arranged. The shim coil power supply 6 drives the shim coil 3 to generate a magnetic field in order to improve the magnetic field uniformity. The gradient coil power source 5 drives the gradient coil 2 in order to generate three types of gradient magnetic field pulses having different directions in which the magnetic field intensity changes, from the gradient coil 2 under the control of the sequencer 10. Note that a slice selection gradient magnetic field, a phase encoding gradient magnetic field, and a read gradient magnetic field are formed by individually or appropriately combining these three types of gradient magnetic fields. The transmission unit 7 drives the probe 4 in order to generate a high-frequency magnetic field pulse from the probe 4 under the control of the sequencer 10. The receiving unit 9 receives a magnetic resonance signal (echo here) generated from the transverse magnetization component via the probe 4, amplifies it, performs phase detection, and converts it into a digital signal, and then the data collecting unit 11. Output to. The computer system 12 reconstructs image data by 2D or 3D Fourier transform processing based on the digital signal. The image data is sent to the display 14 and displayed.

  Next, in this embodiment, a pulse sequence realized by controlling the gradient coil power supply 5, the transmission unit 7, and the reception unit 9 of the sequencer 10 will be described. Note that the sequencer 10 can selectively execute at least four types of pulse sequences, which will be described below in order, in accordance with designation by the user.

  The first pulse sequence shown in FIG. 2 is obtained by applying gradient moment nulling (GMN) to 3D fast spin echo imaging. As is well known, the high-speed spin echo method uses a high frequency pulse for excitation (90 °) to excite magnetization once, and then applies a high frequency pulse for refocus (180 °) continuously at regular intervals. Echoes are generated continuously, and these multiple echoes are used to generate one image. In this way, a large amount of data necessary to create one image is collected with one excitation. This makes it possible to reduce the shooting time. In order to make such a high-speed spin echo method into 3D, phase encoding is performed in a direction (slice direction) orthogonal to both the reading direction and the phase encoding direction.

  Gradient moment nulling was originally studied for the purpose of suppressing flow (mainly blood flow) artifacts, and in particular, for example, taking a general image of a cervical sagittal by a 2D fast spin echo method. It is used for the purpose. On the other hand, in recent years, it has been found that the high-speed spin echo method can be used as a so-called MR angiography to obtain a blood vessel morphological image by converting it into 3D.

  Therefore, GMN is applied to imaging by the 3D fast spin echo method. As the GMN method, various pulse waveforms can be taken. Among them, as a typical pulse sequence, as shown in FIG. Then, a gradient magnetic field pulse (shaded portion) having a reverse polarity is applied. As a result, the magnetization spin of the blood flow causing the phase shift in the echo time is rephased before the next refocusing gradient magnetic field pulse is applied. As a result, the signal from the blood flow maintains a high signal over a plurality of echoes, and a flow enhanced image (blood flow enhanced image) is obtained.

  Of course, in order to finish collecting 3D data necessary for blood flow rendering in a short imaging time, a technique in which imaging using a single shot half Fourier method is extended to 3D is useful as 2D. Here, the half Fourier method is a method for speeding up by creating data necessary for imaging from collected data of a little more than half by utilizing the objectivity of MRI data. It is also possible to apply the GMN method to the 3D fast spin echo method combined with the half Fourier method.

The second pulse sequence shown in FIG. 3 uses both the GMN method and the spoiler method for 3D fast spin echo imaging.
There are a plurality of types of GMN waveforms, but the waveform shown in FIG. 2 forms a signal when the FID signal generated from the refocus pulse (flop pulse) collects echo signals necessary for video. . Therefore, a characteristic of the second pulse sequence shown in FIG. 3 is to apply a spoiler gradient magnetic field pulse for eliminating this signal. In the second pulse sequence, in the 3D high-speed spin echo imaging using the GMN shown in FIG. 2, the GMN performs flow enhancement for a specific direction, and the FID signal is not signaled by a spoiler pulse for other directions. It is to become.

  In particular, as a method of applying a spoiler, a constant flop spoiler (CSF) indicated by hatching in FIG. 3 is applied (Japanese Patent Application No. 11-353168) in which a fixed amount of gradient magnetic field after application of an RF pulse is applied. Since this CSF method can obtain a spoiling effect very efficiently, it is very good in compatibility with the GMN method in which artifacts due to FID signals are likely to occur. Therefore, it is possible to obtain a flow image free from artifacts due to the FID signal.

  Next, the third pulse sequence will be described. In the above method, it is inevitable that dephasing occurs in the flow in the slice direction. Moreover, the application of a spoiler is essential. Therefore, the VIPS method is further applied to the gradient magnetic field pulse for the spoiler. The VIPS method will be described in detail with reference to FIG. FIG. 4A is equivalent to the gradient magnetic field pulse train for slice selection combined with CFS in FIG.

  This gradient magnetic field for slice selection is formed by combining the three actions of slice encoding (2) shown in FIG. 4B, slice selection (1) and spoiler (3) shown in FIG. 4C. Among these, the VIPS method is used in combination with slice selection (1) and a spoiler gradient magnetic field (3) that are not variable parts for phase encoding (slice encoding) in the slice direction. In the VIPS method, as shown by the arrow in FIG. 4 (c ′), the application time of the spoiler pulse is shifted, and the initial gradient moment (GM), for example, the first-order GM up to the second echo is changed between the subsequent echoes. It is intended to be half of the GM. Therefore, the waveform of the gradient magnetic field for slice selection finally formed is obtained by synthesizing the waveform of FIG. 4C ′ with the waveform for slice encoding of FIG. 4B. The waveform is as shown in a ').

  Thereby, the dephasing of the slice direction used as a spoiler can be suppressed to the minimum, and the artifact can be suppressed.

  Next, the fourth pulse sequence will be described. This pulse sequence is obtained by applying the Missing Gradient (MG) method to the fast spin echo method combined with the GMN method.

  In the GMN high-speed spin echo system imaging, in the typical GMN technique as shown in FIG. 2, the area of the readout gradient magnetic field between periodically applied refocus pulses (180 °) becomes zero. Is set to Further, in the fastest spin echo method, in the most general imaging method, the area of the phase encoding gradient magnetic field is generally set to be zero. Therefore, some of these gradient magnetic field pulses can be omitted.

  In particular, in imaging such as MRCP with a long effective echo time (TE), data near the center of k-space necessary for half reconstruction (half-Fourier method) remains, and echoes up to the middle other than near the center of k-space An image can be created without necessarily obtaining it.

  As shown by a dotted line in FIG. 5, a method in which a section in which no gradient magnetic field application is applied is provided in either or both of the direction using GMN and the direction in which phase encoding is applied, that is, the missing gradient method (general The MIGHT method (abbreviated as Missing Gradient Half-Fourier Fast Spin Echo Technique) is applied.

  For example, assuming that the echo interval is 6 ms and the effective echo time is 240 ms, the 40th echo is usually an echo corresponding to the effective echo time. Therefore, it is possible to omit the application of the gradient magnetic field up to the portion where the 34th echo is originally generated, and to acquire data centering on the 40th echo for the first time from the 35th echo. In this case, the S / N ratio is reduced by not acquiring data, but the blood flow rendering ability is improved.

  Although the gradient magnetic field in the slice direction is not shown, any of the methods shown so far may be used, and a simple spoiler combined method, CFS combined method, CFS + VIPS combined method, and the like are possible.

  Note that the MIGHT method is not limited to 3D. Even in 2D, it can be realized by using a simple spoiler technique in the slice direction.

  The present invention is not limited to the embodiment described above, and can be implemented with various modifications.

  DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet, 2 ... Gradient coil, 3 ... Shim coil, 4 ... Probe (RF coil), 5 ... Gradient coil power supply, 6 ... Shim coil power supply, 7 ... Transmission part, 9 ... Reception part, 10 ... Sequencer, 11 ... Data collection unit, 12 ... calculator system, 13 ... console, 14 ... display.

Claims (4)

By applying a plurality of high frequency pulses for refocusing at regular intervals after applying a high frequency pulse for excitation, data necessary for imaging can be obtained by a pulse sequence corresponding to a three-dimensional fast spin echo method that generates a plurality of echoes. In the magnetic resonance imaging device to collect,
In the pulse sequence, before and after the read gradient magnetic field pulse, a gradient magnetic field pulse having a half area and a reverse polarity with respect to the read gradient magnetic field pulse is applied, and the slice direction is different from the application direction of the read gradient magnetic field pulse. A magnetic resonance imaging apparatus, wherein a plurality of spoiler gradient magnetic field pulses respectively corresponding to a plurality of refocusing high-frequency pulses are applied with the same polarity . The magnetic resonance imaging apparatus according to claim 1, wherein the spoiler gradient magnetic field pulse in the slice direction is applied so that an initial gradient moment is half of a gradient moment between subsequent echoes. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence corresponding to the three-dimensional fast spin echo method is combined with a half Fourier method. The pulse sequence according to the three-dimensional fast spin echo method does not apply the readout gradient magnetic field pulse to echoes other than the echo used for the imaging in the half Fourier method. Magnetic resonance imaging device.

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