JPS62224335A - Nmr data collection method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to data acquisition methods for NMR (nuclear magnetic resonance) imaging, and in particular to improved pulse sequences that reduce data acquisition time.

Conventional technology In order to obtain an NMR tomographic image made up of NXN pixels, it is necessary to repeat the data acquisition sequence eight times using the commonly used two-dimensional Fourier transform method or two-dimensional projection restoration method. Furthermore, between each data collection, it is necessary to wait for the magnetization to recover according to the longitudinal relaxation time, so one sequence takes about 1 second. Therefore, it takes several minutes to obtain N=128 or 256 images.

Therefore, various proposals have been made to reduce the data collection time TI. The first proposal, as shown in the pulse sequence in Fig. 4, uses an RF pulse that tilts the magnetization by about 20' to 40°, instead of using an excitation RF pulse, which is usually a 90' pulse, to increase the gradient magnetic field Gx. An echo signal is generated by inversion. In this way the excitation RF
Because the pulse angle is small, recovery is quick, and data for image formation is collected in a few seconds by taking advantage of the fact that signal attenuation is small even if the sequence repetition time is shortened (Ma
gnetic Re5onance Imaging,
Vol, 3. pp.

297-299.1985).

The second proposal, as shown in Fig. 4, similarly uses an excitation RF pulse angle of 20' to 400, and after irradiation of this RF pulse, 1800 pulses (in the positive direction of the X axis) are applied to obtain an echo signal. This is a sequence in which the 180'th pulse (in the negative direction of the X axis) is irradiated to restore the longitudinal magnetization (Japanese Patent Application Laid-Open No. 6O-111647).

Problems to be Solved by the Invention However, in the first conventional proposal, since the echo signal is generated by reversing the gradient magnetic field GX, it is easily affected by the non-uniformity of the static magnetic field, and it is difficult to obtain a good result. It has been pointed out that there is a drawback that images cannot be obtained (rNMR).
Medicine J Vol, 4. pp. 18-33.1984).

Furthermore, in the second proposal, it is necessary to use two 180° pulses in one sequence in order to take advantage of the low angle, and the repetition time cannot be reduced much.

The present invention provides an NMR data acquisition method using a novel pulse sequence that can significantly shorten the repetition time and obtain a good #I image that is less affected by static magnetic field inhomogeneity. With the goal.

Means for Solving the Problems In the NMR data acquisition method of the present invention, the first RF is applied so that the magnetization created by placing the subject in a static magnetic field is tilted by an angle of uII from 90° to 180'. A second RF pulse that is excited with a pulse and then reversed 180 degrees is applied to generate an echo signal that is sampled for data collection.

Action The first RF pulse excites the spins and changes the magnetization from 9O0 to 1
intermediate angles up to 80°, e.g. 140' to 1600
The angle is small, so the recovery is quick and the sequence repeat time is shortened. can. Therefore, as much as the second 180° pulse is not required,
The repetition time can be reduced compared to the second proposal above.

Furthermore, since the magnetization is reversed at 1800 pulses to converge the dispersed magnetization and generate an echo signal, it is less susceptible to the non-uniformity of the static magnetic field. Therefore, a good image can be obtained.

Embodiment FIG. 1 shows a pulse sequence of an embodiment in which the present invention is applied to a two-dimensional Fourier transform method.The angle of the excitation RF pulse is determined by the longitudinal relaxation time of the target spin and the allowable repetition time Tr. The optimal angle can be assumed by It is assumed here that a 30' pulse is optimal. Therefore, first, as the first excitation RF pulse,
A pulse with an angle of °=150° is
Give in axial direction. This angle is determined by the strength and duration of the signal. Then, as shown in Figure 2 (1), the magnetization M formed by the static magnetic field in the direction of the static magnetic field (two directions) rotates the yz plane by 150 degrees and faces in the incoming direction, and then the magnetization changes in the direction of the magnetic field. Due to non-uniformity, the particles are dispersed over the circumference C shown in FIG. 2 (2).

Here, when a 180' pulse is applied again in the X-axis direction after time Δ, the magnetization is reversed by 180° around the X-axis and moves up the circumference C°, as shown in Fig. 2 (3). Further, after a time Δ, the light converges at point A', and an echo signal is generated. Therefore, this echo signal is sampled and input into a computer. In this case, the magnetization is the circumference C at an angle of 30 degrees from the two axes.
Since it is on the top, recovery is quick and the repetition time Tr can be shortened.

In FIG. 1, the gradient magnetic field Gz is used to selectively irradiate a certain portion in the Z direction, thereby selecting one tomographic plane perpendicular to the Z axis. The gradient magnetic field Gx is a frequency encoding gradient magnetic field for adding position information in the X direction within the tomographic plane. The #Ji oblique magnetic field ay is a phase encoding gradient magnetic field for adding positional information in the Y direction within the tomographic plane, and is gradually increased each time the sequence is repeated every time Tr. These gradient magnetic fields Gx
, Gy, and Gz are the same as the normal two-dimensional Fourier transform method.

In this way, the sequence is repeated a number of times corresponding to the number of pixels on one side of the image matrix, data collection is completed, a fast two-dimensional Fourier transform is performed by a computer, and NMR
The parameter distribution image is reconstructed as a tomographic image. In this case, since the echo signal is obtained by converging the magnetization with the 180' pulse, a good image that is not affected by the non-uniformity of the static magnetic field can be obtained.

Note that the 180° pulse is in the negative direction of the X axis, that is, 180°
The same result is obtained with the -X pulse. It is also possible to use a pulse in the positive direction of the y-axis, that is, a tao'y pulse, or a pulse in the negative direction of the y-axis, that is, a 180°-y pulse. In this case, as shown in FIG.
The point where magnetization gathers after rotating 180' around ilib is A.
'', the effect is the same. Generally, the 180' pulse can be applied in any direction on the xy plane.

Although the embodiments applied to the two-dimensional Fourier transform method have been described above, the present invention can be applied not only to the two-dimensional post-projection method, but also to the three-dimensional Fourier transform method, the three-dimensional post-projection method, and the like.

In short, it can be applied to any sequence in which the repetition time can be shortened by reducing the rotation angle (clip angle) of magnetization.

Furthermore, it is also possible to create a multi-221 sequence by adding an even number of 180° pulses after the 1800 pulses in the 1 sequence.

Effects of the Invention According to the present invention, in a conventionally used sequence such as a two-dimensional Fourier transform method.

You only need to adjust the initial RFX pulse clip angle from 90° to 180' instead of 90'.
It is extremely easy to shorten the repetition time and enable high-speed data collection. Furthermore, since the echo signal is generated using a 180° pulse, the image is less susceptible to deterioration due to non-uniformity of the static magnetic field.

[Brief explanation of drawings]

Fig. 1 is a time chart showing the pulse sequence of one embodiment of this invention, Fig. 2 is a schematic diagram showing the direction of magnetization in this embodiment, and Fig. 3 is a schematic diagram showing the direction of magnetization in another embodiment. 4 and 5 are time charts showing conventional pulse sequences, respectively. Note 2nd C1 nos (g) Jun 3rd play qth toe - WS Tr ---) 5th circle

Claims (2)

[Claims] (1) The first RF pulse excites the magnetization created by placing the subject in a static magnetic field by an angle intermediate between 90° and 180°, and then the second RF pulse reverses it by 180°. An NMR data collection method characterized in that an echo signal is generated by giving a signal, and data is collected by sampling the echo signal. (2) The first RF pulse changes the magnetization from 140° to 160°
2. The NMR data collection method according to claim 1, wherein the NMR data collection method is performed by tilting the NMR data only by tilting the NMR data.