JP2001204708A - Magnetic resonance video equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a flow image (blood stream image) reduced in artifact and to finally provide an image for reliable clinical diagnosis in a magnetic resonance video equipment using especially photographing by a three-dimensional high-speed spin echo method. SOLUTION: In the magnetic resonance video equipment for collecting data required for visualizing by a pulse sequence corresponding to the three- dimensional high-speed spin echo method, a tilted magnetic field pulse corresponding to a gradient moment nulling method is added to the pulse sequence.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus for imaging a three-dimensional flow (mainly blood flow) by a fast spin echo (FSE) method.

[0002]

2. Description of the Related Art In recent years, 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. J Magnetic Resonance Imaging System 1994: 4 (P): 19
As can be seen from many documents such as "86", as a method of cross-sectional imaging of a magnetic resonance imaging apparatus, a plurality of echoes are generated by repeatedly applying a refocusing RF pulse,
A fast spin echo method (Fast Spin Ec) that speeds up imaging by adding different phase encoding information to each echo
ho method (high-speed spin echo method) is common, and "Y.
Kassai, et.al, 3D half-Fourier fast SE for heavy
T2-weighted imaging,, In "Proceedings, ISMRM, 4t
h AnnualMeeting ", p736, 1996." and "Kasai Yumori, Fast
ASE and its clinical application, Medical Review No. 69, p. 28-
34, 1998, etc., 3D imaging by the high-speed spin echo method has been put to practical use as one of the options. FIG. 6 shows a typical example of a pulse sequence based on the 3D fast spin echo method.

[0003] A recent trend is that "RC Selmek
a, et.al, HASTE imaging: Description of technique
and preliminary results in the abdomen, J Magnetic Resonance Imaging System, 6: 698-699, 1996 " Objects that have movement such as solid organs and blood flow have also been targets for depiction ("Y. Kassai, et. Al, 3D Half-Fourier RARE w
ith MTC for Cardiac Imaging, ISMRM, p806, 1998. '',
`` Miyazaki M, et al, A Novel MR Angiography Techni
que: 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 Bl
ood Contrast 3D MRA within Single Breathhold, ISMR
M, p. 780, 1998 ”). It is gaining importance as a non-invasive blood flow imaging method without a contrast agent.

[0004] On the other hand, by making the "moment" defined for the gradient magnetic field waveform zero, a gradient "phase shift" generated in the moving spin is made zero.
Moment nulling (GMN method) has been proposed for a long time. As a technique for suppressing flow artifacts in the FSE, as shown in FIG. 7, a gradient moment nulling method in a two-dimensional (2D) imaging method (“Hinks RS, et al, Gradient Moment Nulling in Fa
st Spin Echo, MRM 32: 698-706 (1994) "), but 3D imaging is not sufficiently disclosed. In the FSE, the gradient magnetic field waveform was devised so as to "rephase only the Hernspin echo component". Although there is a velocity independent phase shift stabilization method (VIPS method, initially VISS method), its effectiveness is limited because only a part of signals is rephased ("Machida Y , et a
l. VelocityIndependent Phase-Shift Stabilization
(VIPS) Technique in FSE Flow Imaging, In "Proce
edings, ISMRM. 7th Annual Meeting ", p1910, 199
9. ", and Japanese Patent Application No. 11-4732.

[0005] In such a conventional method, there is a phenomenon that ghost artifacts occur from the blood flow and blood flow signals are lost. In order to improve the reliability in clinical diagnosis, it is necessary to sufficiently suppress such artifacts.

[0006]

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

[0007]

According to the present invention, there is provided a magnetic resonance imaging apparatus for collecting data required for imaging by a pulse sequence according to a three-dimensional fast spin echo method.
A gradient magnetic field pulse according to the nulling method is added.

[0008]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an apparatus according to the present invention. FIG. 1 shows the configuration of the magnetic resonance imaging apparatus according to the present embodiment. The static magnetic field magnet 1 generates a static magnetic field in, for example, a substantially cylindrical imaging region into which a subject is inserted. Inside this static magnetic field magnet 1,
Shim coil 3, gradient coil 2, 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 supply 5 drives the gradient coil 2 in accordance with the control of the sequencer 10 so that the gradient coil 2 generates three types of gradient magnetic field pulses in which the magnetic field intensity changes in different directions. A slice selection gradient magnetic field, a phase encoding gradient magnetic field, and a readout gradient magnetic field are formed by individually or appropriately combining these three types of gradient magnetic fields. The transmitting unit 7 transmits the high frequency magnetic field pulse to the probe 4 according to the control of the sequencer 10.
The probe 4 is driven in order to generate from. The receiving unit 9 receives a magnetic resonance signal (here, an echo) generated from the transverse magnetization component via the probe 4, amplifies the signal, performs phase detection, converts the signal into a digital signal, and converts the signal into a digital signal. Output to The calculator system 12
Image data is reconstructed 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, the sequencer 1
A pulse sequence realized by controlling the zero gradient coil power supply 5, the transmission unit 7, and the reception unit 9 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 a user's designation.

The first pulse sequence shown in FIG.
In this example, gradient moment nulling (GMN) is applied to D high-speed spin echo imaging. As is well known, in the fast spin echo method, after a magnetization is once excited by a high-frequency pulse for excitation (90 °), a high-frequency pulse for refocusing (180 °) is continuously applied at a constant interval to obtain a plurality of pulses. Is used to generate a single image. In this way, a large number of data required to create one image are collected by one excitation. This realizes shortening of the photographing time. In order to convert such a high-speed spin echo method to 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.
D is used for the purpose of taking a general image of, for example, cervical sagittal in the high-speed spin echo method. On the other hand, in recent years, it has been found that 3D conversion of the high-speed spin echo method can be used as so-called MR angiography for obtaining a blood vessel morphological image.

Therefore, GMN is applied to 3D imaging by the high-speed spin echo method. Various pulse waveforms can be taken in the GMN method. Among them, a typical pulse sequence is, as shown in FIG. Then, a gradient magnetic field pulse (shaded portion) of the opposite polarity is applied. As a result, the magnetization spin of the blood flow causing a phase shift in the echo time becomes
It is rephased before the next application of the gradient magnetic field pulse for refocusing. As a result, the signal from the blood flow is maintained at a high level over a plurality of echoes, and a flow-enhanced image (blood flow emphasized image) is obtained.

Of course, in order to complete the collection of 3D data necessary for blood flow delineation in a short imaging time, two-dimensional imaging using a single shot half Fourier method is also required.
Dimensionally extended techniques are useful. Here, the half Fourier method is a method of creating data necessary for imaging from slightly more than half of collected data by utilizing the symmetry of MRI data to increase the speed. 3 using this half Fourier method
It is also possible to apply the GMN method to the D fast spin echo method.

The second pulse sequence shown in FIG.
The GMN method and the spoiler method are used in combination for D high-speed spin echo imaging. Although there are a plurality of types of GMN waveforms, in the waveform shown in FIG. 2, an FID signal generated from a refocus pulse (flop pulse) forms a signal when collecting an echo signal necessary for a video. . Therefore, the feature of the second pulse sequence shown in FIG. 3 is to apply a spoiler gradient magnetic field pulse for eliminating this signal. This second pulse sequence is shown in FIG.
In 3D high-speed spin-echo imaging using GMN shown in (1), flow enhancement is performed by GMN in a specific direction, and the FID signal is rendered non-signal by spoiler pulses in other directions.

In particular, as a method of applying a spoiler, RF
A constant flop spoiler (CSF) indicated by oblique lines in FIG. 3, which applies a fixed amount of gradient magnetic field after pulse application, is employed (Japanese Patent Application No. 11-353168). In the CSF method, since a spoil effect can be obtained very efficiently, the GM which is likely to cause an artifact due to the FID signal is generated.
The compatibility with the N method is very good. 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-described method, it is inevitable that dephase occurs in the flow in the slice direction.
Moreover, application of a spoiler is essential. So further,
The VIPS method is applied to the gradient magnetic field pulse for the spoiler. This VIPS method will be described in detail with reference to FIG. FIG. 4A is equivalent to the slice selection gradient magnetic field pulse train combined with CFS of FIG.

The slice selection gradient magnetic field is shown in FIG.
Slice encoding (2) shown in (b) and FIG.
The three functions of slice selection (1) and spoiler (3) shown in FIG. Among these, the VIPS method is used for the slice selection (1) and the spoiler gradient magnetic field (3) which are not variable portions for phase encoding (slice encoding) in the slice direction. FIG. 4 shows the VIPS method.
As shown by the arrow (c ′), the initial gradient moment (G
M), for example, to make the primary GM up to the second echo half of the GM between subsequent echoes. Therefore, the waveform of the slice selection gradient magnetic field finally formed is obtained by synthesizing the waveform of FIG. 4 (c ′) with the waveform of the slice encoding of FIG. 4 (b), as shown in FIG. The waveform shown in a ') is obtained.

As a result, the dephasing in the slice direction used as a spoiler can be suppressed to a minimum, and artifacts can be suppressed.

Next, the fourth pulse sequence will be described. This pulse sequence is applied to the fast spin echo method using the GMN method in combination with the missing gradient (Mis
singGradient (MG) method.

In the imaging of the GMN high-speed spin echo system, in a typical method of the GMN as shown in FIG. 2, the area of the readout gradient magnetic field between the periodically applied refocusing pulses (180 °) is reduced. It is set to be zero. In the fast spin echo method, in the most common imaging method, the area of the gradient magnetic field for phase encoding is generally set to be zero. Therefore, some of these gradient magnetic field pulses can be omitted.

In particular, MR having a long effective echo time (TE)
In imaging such as CP, data near the center of k-space necessary for half reconstruction (half-Fourier method) should be left, and an image must be created without necessarily acquiring echoes up to a point other than near the center of k-space. Can be.

As shown by a dotted line in FIG. 5, a method in which a section in which a gradient magnetic field is not applied is partially provided in one or both of a direction using GMN and a direction in which phase encoding is applied, that is, a missing gradient method (Generally, the MIGHT method (Missing Gradient Half-Four
ier fast spin echo technique).

For example, assuming that the echo interval is 6 ms and the effective echo time is 240 ms, the 40th echo is usually the 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 generated, and acquire data centering on the 40th echo for the first time from the 35th echo. In this case, the S / N ratio decreases as much as no data is acquired, but the blood flow rendering ability is improved.

Although the gradient magnetic field in the slice direction is not shown, any of the methods described so far may be used, such as a simple spoiler combination method, a CFS combination method, or CFS + VI.
It is possible to use a PS combined method.

The MIGHT method is not limited to 3D. 2D can also be realized by using a simple spoiler technique in the slice direction.

The present invention is not limited to the embodiments described above,
Various modifications are possible.

[0027]

As described above, according to the present invention, 3
By applying the gradient moment nulling method to the D or 2D high-speed spin echo method, the occurrence of ghost artifacts can be suppressed, and the occurrence of signal loss due to dephase can be suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

FIG. 2 shows a first pulse sequence (3) according to the present embodiment.
The figure which shows GMN in D and uses CFS (constant flop spoiler) in a slice direction.

FIG. 3 shows a second pulse sequence (3) according to the present embodiment.
FIG.

FIG. 4 shows a third pulse sequence (3D
FIG. 5 is a diagram showing a waveform of a read gradient magnetic field of (a GMN is used and VIPS is used in a slice direction).

FIG. 5 is a diagram showing a fourth pulse sequence (using a missing gradient method) according to the embodiment;

FIG. 6 is a diagram showing a pulse sequence (3D does not use GMN) in a conventional example.

FIG. 7 is a diagram showing a conventional pulse sequence (using GMN in 2D).

[Explanation of symbols]

 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 ... Transmitting part, 9 ... Receiving part, 10 ... Sequencer, 11 ... Data collection unit, 12: Computer system, 13: Console, 14: Display.

Claims (6)

[Claims] 1. A magnetic resonance imaging apparatus for acquiring data necessary for imaging by a pulse sequence according to a three-dimensional fast spin echo method, wherein the pulse sequence includes a gradient magnetic field pulse according to a gradient moment nulling method. A magnetic resonance imaging apparatus characterized by being added. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence corresponds to a half Fourier method. 3. A magnetic resonance imaging apparatus for acquiring data necessary for imaging by a pulse sequence according to a three-dimensional fast spin echo method, wherein the pulse sequence includes a gradient magnetic field pulse according to a gradient moment nulling method. A magnetic resonance imaging apparatus wherein a gradient magnetic field pulse for a spoiler is added in a specific direction and a spoiler gradient magnetic field pulse is added in another direction. 4. The magnetic resonance imaging apparatus according to claim 3, wherein the gradient magnetic field pulse for the spoiler is based on a constant flop spoiler method. 5. The magnetic resonance imaging apparatus according to claim 3, wherein an application timing of the spoiler gradient magnetic field pulse corresponds to a VIPS method. 6. A magnetic resonance imaging apparatus for acquiring data required for imaging by a pulse sequence according to a three-dimensional fast spin echo method, wherein the pulse sequence includes a gradient magnetic field pulse according to a gradient moment nulling method. A magnetic resonance imaging apparatus, wherein the application of at least one of the gradient magnetic field pulse and the phase encoding gradient magnetic field pulse is stopped in a partial section.