JPH0511980B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an MR imaging device that obtains in-vivo image information using nuclear magnetic resonance (NMR).

[Conventional technology]

In the field echo method, as shown in Figure 4, a gradient magnetic field for slice plane selection (referred to as Gz) is applied at the same time as the excitation high-frequency signal is applied, and then a gradient magnetic field for phase encoding (referred to as Gy) is applied.
Then, a sequence is performed in which data is collected by sampling the NMR signal generated while applying a gradient magnetic field for frequency encoding (referred to as Gx). This sequence is repeated for a repetition time TR while changing the magnitude of the phase encoding gradient magnetic field. A high-speed imaging method is known in which data is collected at high speed by shortening the repetition time TR of this field echo method, but when an image is reconstructed from the data obtained in this way, the center of the reconstructed image 51 as shown in FIG. A linear artifact 52 in the frequency modulation direction (in this case, the X direction) occurs in the area. This artifact 52 is caused by the shortening of the repetition time TR, which generates a steady state signal (ADRIAN P.CRAWLEY et al.
al., “Elimination of Transverse Coherences
in FLASH MRI”, MAGNETIC
RESONANCE IN MEDICINE, 8, P248−
260, 1988). Therefore, as shown in FIG. 6, it has been conventionally proposed to randomly change the magnitude of the gradient magnetic field Gz at the end of the repetition time TR to add an extra gradient magnetic field Gz'. (JENS FRAHM et al.,
“Transverse Coherences in Rapid FLASH
NMR Imaging”, Journal of Magnetics
Resonance, 72, 307-314, 1987). This is based on the fact that Gz' disturbs the phase of the NMR signal generated each time and suppresses the generation of steady state signals.

[Problem to be solved by the invention]

However, when the phase is disturbed using the gradient magnetic field Gz in this way, there are the following problems. First, as shown in Figure 7, the strength of the gradient magnetic field Gz in the Z direction is tilted in the Z direction without changing much near the center O, so the effect of disturbing the phase depends on where the slice plane S is set. It turns out.
Therefore, when the selective excitation slice plane S is set at a position away from the center O in the Z direction, the intensity of Gz' becomes too large, causing a problem that the image quality deteriorates due to the influence of the eddy current. This invention provides a high-speed imaging method using a field echo method that shortens the repetition time without causing the problem of image quality deterioration due to eddy current.
An object of the present invention is to provide an MR imaging device that can suppress the generation of steady-state signals and remove artifacts.

[Means to solve the problem]

In order to achieve the above object, according to the present invention,
An MR imaging device that repeats a field echo sequence with a short repetition time and, at the end of each repetition time, applies pulses of randomly varying intensity of a gradient magnetic field whose magnetic field strength is gradient in the thickness direction of the selected excitation slice. , a means for determining the distance of the set slice position from the center in the slice thickness direction, and a change range of the gradient magnetic field pulse whose intensity changes randomly according to the distance from the center. The invention is characterized in that it has a means.

[Effect]

After setting the position of the slice plane for selective excitation,
The distance of that position from the center in the slice thickness direction is determined. Then, the variation range of the gradient magnetic field pulse whose intensity changes randomly is determined according to this distance. Therefore, even if the slice plane is set far from the center, the maximum amplitude of the gradient magnetic field pulse, whose intensity changes randomly, is suppressed accordingly, which prevents excessive phase disturbance and the effects of eddy current. This can prevent image quality from deteriorating. Therefore, no matter where the slice plane is set, the same effect of disturbing the phase can be obtained.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. According to an embodiment of this invention
In the MR imaging device, a pulse sequence as shown in FIG. 1 is performed. This pulse sequence is basically based on a conventional high-speed imaging method based on a field echo method with a short repetition time TR. In other words, when applying a high-frequency signal for excitation, a gradient magnetic field for slice plane selection is simultaneously applied.
Gz is applied, then a gradient magnetic field Gy for phase encoding is applied, and a gradient magnetic field Gx for frequency encoding is applied while sampling the generated NMR signal and collecting data. This pulse sequence is repeated with a short repetition time TR while changing the magnitude of the phase encoding gradient magnetic field.
And according to this invention, each repetition time TR
At the end of , the strength of the gradient magnetic field Gz is repeated for the time TR
An extra gradient magnetic field pulse Gz′ is generated by randomly changing the gradient magnetic field pulse Gz′. The variation range M of Gz' is determined according to the position of the set slice plane. Figure 2 shows an MR that performs the above pulse sequence.
This shows the system configuration of the imaging device.
1 is laid on the bed 12 and the main magnet 21
is sent into a static magnetic field formed by Inside this main magnet 21, there are three orthogonal axes (X, Y, Z).
A gradient magnetic field coil 22 for generating three gradient magnetic fields whose magnetic field strengths are gradient in directions, and an RF coil 2
3 are arranged. The gradient magnetic field coil 22 is connected to a gradient magnetic field power supply 31 and is supplied with a current for generating a gradient magnetic field. The RF coil 23 is connected to the RF transmitting/receiving device 32, and irradiates the subject 11 with an RF signal to excite it, and collects NMR generated within the subject 11.
Receive a signal. The transmission and reception of these RF signals and the generation of gradient magnetic fields and correction magnetic fields are controlled by the measurement control device 34, and the data obtained from the received NMR signals is sent to the image reconstruction device 35, where it undergoes two-dimensional Fourier transformation, etc. After undergoing image reconstruction processing, the reconstructed image is sent to the image display device 36 and displayed. A computer 37 performs overall control. The position of the slice plane to be selectively excited is set by the setting device 33. When the setting device 33 sets the position of the slice plane to be selectively excited, the position information is input to the computer 37. Then, the computer 37 determines the distance L of the slice position from the center O in the Z direction (see FIG. 7). Then, within the computer 37, the range of variation M of the randomly varying gradient magnetic field Gz' is determined by the procedure shown in FIG. That is, first, it is determined whether the set slice position is not more than a certain distance Lo from the center O. This can be done by determining whether |L|>Lo holds true. this
Lo corresponds to a selective excitation slice position where the slice position is not far from the center O and the image quality does not deteriorate. If this formula does not hold true, the set slice position will be set at a position not far from the center O in the Z direction, so the change range M will be left as is and the slice position will be randomly set at the end of the repetition time TR. Change
A command is issued to the measurement tracking device 34 to add the Gz' pulse. If the above equation holds true, the set slice position is set far away from the center O, so if the change range of the Gz' pulse is left unchanged as M, the maximum amplitude of the Gz' pulse will be large. If it becomes too large, excessive phase disturbance will occur and the image quality will deteriorate due to the influence of the eddy current. Therefore, it is necessary to reduce the range of change M. Here, the value of M×(Lo/|L|) is determined, and this is set as the variation range M. When the change range M has been changed in this way, a command is issued from the computer 37 to the measurement control device 34, and Gz', which changes randomly in the range M, is added at the last part of the repetition time TR. Therefore, when applying a Gz pulse whose magnitude changes randomly within a predetermined range as a gradient magnetic field pulse Gz′ for disturbing the phase in the last part of the repetition time TR, that range is set at the slice setting position. Since it can be suppressed accordingly, it is possible to prevent excessive phase disturbance and image quality deterioration due to eddy current.

【Effect of the invention】

According to the MR imaging device of the present invention, in a high-speed imaging method using a field echo method with a short repetition time, the generation of steady-state signals is suppressed and artifacts are removed.
Since the change range of the random magnetic field that disturbs the phase of the NMR signal is determined according to the slice position, the NMR signal will be the same no matter where the slice plane is set.
It is possible to disturb the phase of the signal and prevent deterioration of image quality due to eddy current.

[Brief explanation of drawings]

Fig. 1 is a time chart showing a pulse sequence according to an embodiment of the present invention, Fig. 2 is a block diagram showing the system configuration of the same embodiment, and Fig. 3 is a procedure for determining the variation range of random Gz' pulses. Fig. 4 is a time chart showing the pulse sequence of the field echo method;
The figure shows artifacts that occur in high-speed imaging using the field echo method with a shortened repetition time. Figure 6 is a time chart showing the conventional pulse sequence for removing artifacts. Figure 7 is the strength of the gradient magnetic field Gz. It is a figure showing Z direction distribution of. 11... Subject, 12... Bed, 21... Main magnet, 22... Gradient magnetic field coil, 23...
RF coil, 31... gradient magnetic field power supply, 32... RF
Transmitting/receiving device, 33... Setting device, 34... Measurement control device, 35... Image reconstruction device, 36... Image display device, 37... Computer, 51... Reconstructed image, 52... Artifact, S...Sliced surface.

Claims (1)

[Claims] 1. MR imaging in which a field echo sequence with a short repetition time is repeated, and at the end of each repetition time, pulses of randomly varying intensity of a gradient magnetic field whose magnetic field strength is gradient in the thickness direction of the selected excitation slice are applied. In the apparatus, means for determining the distance of the set slice position from the center in the slice thickness direction, and a change range of the gradient magnetic field pulse whose intensity randomly changes according to the distance from the center. An MR imaging device characterized by having means for determining.