JP3075746B2 - 3D MRI method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a three-dimensional MRI method, and more particularly, to a three-dimensional MRI method for performing three-dimensional encoding using an RF signal.

[0002]

2. Description of the Related Art FIGS. 6 and 7 show a conventional three-dimensional MRI.
2 illustrates a pulse sequence in the method. These pulse sequences insert a three-dimensional encoding gradient into a slice gradient GSLICE in a direction perpendicular to the tomographic plane in order to perform three-dimensional encoding. That is, in the pulse sequence of FIG. 6, the three-dimensional encoding gradient E is inserted after the rephase gradient R of the slice gradient GSLICE. Further, in the pulse sequence of FIG. 7, the gradient E for three-dimensional encoding is superimposed on the rephase gradient R.

[0003]

In the conventional pulse sequence shown in FIG. 6, the echo time (TE) becomes longer by the time of the gradient E for three-dimensional encoding.
There is a problem that E becomes difficult. On the other hand, the conventional pulse sequence shown in FIG. 7 has a problem that the load on the gradient amplifier increases because the maximum amplitude of the slice gradient GSLICE increases. Accordingly, an object of the present invention is to provide a three-dimensional MRI method capable of performing three-dimensional encoding without increasing the echo time (TE) or increasing the load on the gradient amplifier. is there.

[0004]

The three-dimensional MRI of the present invention
The method is to collect an NMR signal from the tomographic plane by adding an RF signal having a frequency band-limited waveform while applying a magnetic field in which a static magnetic field and a gradient magnetic field in a direction perpendicular to the tomographic plane are superimposed, Repeat while shifting the center of the waveform,
It is characterized in that a three-dimensionally encoded NMR signal group is obtained. In the above configuration, examples of the waveform whose frequency band is limited include a sinc waveform and a Gaussian waveform.

[0005]

The waveform of the RF signal whose frequency band is limited is represented by g (t).
When the Fourier transform is represented by G (f), a waveform g (t−τ) obtained by shifting the center of the waveform g (t) by τ
Is G (f) .exp [-j2πτf]. In other words, the RF signal having a waveform shifted from the center of the waveform whose frequency band is limited has a phase change in the thickness direction of the tomographic plane. Since this does not deviate from three-dimensional encoding, it is not necessary to insert a three-dimensional encoding gradient into a gradient in a direction perpendicular to the tomographic plane. Therefore, it is possible to avoid an increase in the echo time (TE) and an increase in the load on the gradient amplifier.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the embodiments shown in the drawings. It should be noted that the present invention is not limited by this. FIG. 2 is a block diagram showing an MRI apparatus 1 for implementing the three-dimensional MRI method of the present invention. The computer 2 controls the entire operation based on an instruction from the console 13.

A sequence controller 3 activates a magnetic field drive circuit (including a gradient amplifier) 4 based on the stored sequence, and a static magnetic field and a gradient magnetic field of a static magnetic field coil and a gradient magnetic field coil of a magnet assembly 5. Generate. Further, the modulation circuit 7 is controlled, and the RF oscillation circuit 6 is controlled.
Is modulated into a predetermined waveform and applied from the RF power amplifier 8 to the transmission coil of the magnet assembly 5.

The NMR signal obtained by the receiving coil of the magnet assembly 5 is input to a phase detector 10 via a preamplifier 9 and further input to a computer 2 via an AD converter 11. The computer 2 reconstructs the image based on the NMR signal data obtained from the AD converter 11 and displays the image on the display device 12.

The three-dimensional MRI method according to the present invention
And the procedure stored in the sequence controller 3. FIG. 3 is a flowchart of one embodiment of the three-dimensional MRI method of the present invention.

In step S1, a shift amount τn is set based on the number of slices N, slice thickness S, slice gradient GS, and RF pulse width T. That is, n = N / 2,..., 0,..., −N / 2 + 1 τn = n / γ · GS · S (However, N / γ · GS · S ≦ T must be satisfied.) In step S2, , N / 2 into n.

In step S3, the center of the RF signal is set to τn
Shift only. This will be described with reference to FIGS. FIG. 4 shows a normal sinc waveform and its frequency band. In this case, there is no phase change in the slice thickness direction. FIG. 5 shows that the center of the sinc waveform is τ.
The waveform is shifted by n. Due to the shift, the portion outside the pulse width T is turned back to the opposite side. In the sinc waveform whose center is shifted, the frequency band is the same as the sinc waveform in FIG. 4, but the phase changes in the slice thickness direction. That is, phase encoding is performed.

In step S4, two-dimensional data is obtained by adding an RF signal having a waveform whose center is shifted by τn. FIG.
Is 2D by adding an RF signal whose center is shifted by τn.
4 shows a pulse sequence for obtaining data. R is the rephase gradient.

According to steps S5 and S6,
Steps S3 and S4 are repeated while changing the shift amount in order until n = −N / 2 + 1. This makes it possible to obtain a 3D-encoded data group.

In step S7, a 3D Fourier transform is performed on the obtained data group. Thereby, a three-dimensional image can be generated. In step S8, the generated three-dimensional image is output to the display device 12.

According to the three-dimensional MRI method of the above embodiment,
Since there is no need to insert a three-dimensional encoding gradient into the slice gradient GSLICE, the echo time (TE) does not increase, and a short TE is possible. Also, the load on the gradient amplifier does not increase.

If the amount of phase change in the slice thickness direction caused by the shift of the center of the RF signal is made equal to the amount of rephase gradient (the area of the rephase gradient), the rephase gradient becomes unnecessary and a shorter short TE Becomes possible. Normally, the rephase gradient amount is about half the area of the slice gradient GSLICE. In this case, the shift amount τn (n = N / 2) of the center of the RF signal is T / 2.
Becomes However, since the actual rephase gradient amount is larger than half the area of the slice gradient, a rephase gradient is necessary for a portion larger than the half, and the rephase gradient cannot be completely eliminated. However, since the rephase gradient amount can be reduced, a shorter short TE can be achieved.

[0017]

According to the three-dimensional MRI method of the present invention,
Since there is no need to insert a three-dimensional encoding gradient into the slice gradient, the echo time (TE) does not increase, and a short TE becomes possible. In addition, the load of the gradient emblem does not increase.

[Brief description of the drawings]

FIG. 1 is a pulse sequence diagram of one embodiment of a three-dimensional MRI method of the present invention.

FIG. 2 is an MR illustrating a three-dimensional MRI method according to the present invention;
FIG. 2 is a block diagram illustrating an example of an I device.

FIG. 3 is a flowchart of one embodiment of a three-dimensional MRI method of the present invention.

FIG. 4 is an explanatory diagram of a normal sinc waveform.

FIG. 5 is an explanatory diagram of a sinc waveform whose center is shifted.

FIG. 6 is an explanatory view of a pulse sequence of an example of a conventional three-dimensional MRI method.

FIG. 7 is a pulse sequence diagram of another example of the conventional three-dimensional MRI method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 MRI apparatus 2 Computer 3 Sequence controller 4 Magnetic field drive circuit 5 Magnet assembly 6 RF oscillation circuit 7 Modulation circuit 8 RF power amplifier

Claims (1)

(57) [Claims] An NMR signal from a tomographic plane is obtained by applying an RF signal having a frequency band-limited waveform while applying a magnetic field in which a static magnetic field and a gradient magnetic field in a direction perpendicular to the tomographic plane are superimposed. Is repeated while shifting the center of the waveform to obtain a three-dimensionally encoded NMR signal group.