JPS61275646A - Multiple tomographic picking up method for mri device - Google Patents

Abstract

PURPOSE:To distinguish tomographic surfaces and to obtain position resolution in a surface by applying a magnetic field perpendicular to tomographic surfaces, applying high frequency pulses corresponding to a position and exciting respective tomographic sections at the same time, and applying a magnetic field slanting from the perpendicular direction and imposing frequency modulation. CONSTITUTION:Tomographic surfaces #1 and #2 are parallel to an XY surface and are at positions Z1 and Z2. A magnetic field Gz is applied in a Z direction firstly and high frequency pulses of resonance frequencies f1+DELTAf and f1-DELTAf are applied to excite the tomographic surfaces #1 and #2 selectively at the same time. In this case, f1+DELTAf and f1-DELTAf are obtained by mixing f1 and DELTAf across a frequency band width corresponding to the thickness of a tomographic section. A magnetic field which slants by an angle theta determined by a magnetic field Gy, then a tomographic interval (b) and an X-directional visual field (a) and is generated by combining the Gx and Gy at the same time to receive a magnetic resonance video signal. This NMR signal is frequency-modulated in the slanting magnetic field, the quantity of modulation corresponds to the intensity of the magnetic field, and projections in the theta direction of each tomographic sections are obtained and never overlap with one another, so tomographic sections are distinguished from the quantity of modulation and the position in the X direction is known.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to an improvement in a multilayer imaging method in an MRI apparatus (magnetic resonance imaging apparatus).

BACKGROUND OF THE INVENTION MRI apparatuses have traditionally imaged a large number of tomographic planes simultaneously. As shown in FIG. 4, during the waiting time for longitudinal magnetization relaxation, that is, the pulse repetition time TI, a number of mutually parallel tomographic planes are first
, then tomographic plane #2°, etc. are selectively excited one after another, and NMR signals from each tomographic plane are received separately.

In this case, the number of slices N obtained is less than or equal to TR/Ts (
Ts is the pulse interval between each tomographic plane).

Problem to be Solved by the Invention Therefore, in the past, after applying a 90' pulse,
IR (InversionRe) to give a pulse
If the pulse sequence is complex, such as in the coma-ery method, the pulse interval between each tomographic plane becomes long, and the number of tomographic planes obtained becomes small.

Furthermore, when it is desired to perform high-speed imaging or to obtain T-enhanced images by shortening one repetition time, it is not possible to obtain a large number of tomographic images at the same time because the repetition time is short.

This invention is an MRI that can significantly increase the number of slices obtained within the same repetition time compared to conventional methods, and can simultaneously image multiple layers even in a short repetition time when conventional methods cannot obtain multiple layers. The purpose is to provide a multi-sectional imaging method for the device.

Means for Solving the Problems In the multi-sectional imaging method of the MRI apparatus of the present invention, while applying a gradient magnetic field in a direction perpendicular to the tomographic plane, an excitation high frequency having a plurality of frequency components corresponding to a plurality of tomographic plane positions is applied. A pulse is applied to simultaneously excite multiple tomographic planes, and when the signal is received, it is tilted from a direction perpendicular to the tomographic plane to a direction tilted in one direction within the tomographic plane, and the amount of modulation is A gradient magnetic field for one-frequency modulation that differs in direction and does not overlap among a plurality of tomographic planes is applied, and NMR signals from each tomographic plane are received simultaneously.

Effect: Since multiple tomographic planes are excited simultaneously, NMR signals are received from all of them. At the time of receiving this, conventional methods only apply a gradient magnetic field Gx that is tilted in the Since a gradient magnetic field with varying magnetic field strength is applied to the cross section, it is possible to distinguish between tomographic planes and to obtain positional resolution in the X direction.

Example As shown in FIG. 1, two tomographic planes $1. #2 shall be imaged at the same time. These two fault planes #1 and #2 are arranged in the Z direction and are parallel to each other. Each fault plane #1. The $2 plane is a plane parallel to the X-Y plane, and the direction perpendicular to this plane is the Z direction. Fault planes #l and #2 are located at positions Zl and Z2 in the Z direction, respectively.

First, a gradient magnetic field Gz in the Z direction is applied to create fault planes #1 and #1.
2 are selectively excited at the same time. Therefore, the frequency components l+Δf corresponding to the positions Zl and Z2.

Give a high frequency pulse with fl-Δf. Such a high frequency signal having two frequency components is, for example, a second
As shown in the figure, it can be made with a simple circuit in which a signal with a frequency fl and a signal with a frequency Δf are mixed by a mixer MIX. This frequency f1+Δf, fl−Δ
f is a resonance frequency corresponding to the positions Zl and Z2, respectively, and actually has a frequency bandwidth corresponding to the thickness of the tomographic planes #1 and #2.

Next, in the two-dimensional Fourier transform method, a Y-direction gradient magnetic field Gy is applied to perform phase modulation in the Y-direction.

Next, when the field of view in the X direction is a, and the interval between fault planes #1 and #2 is b, a gradient magnetic field is applied that is tilted in a direction tilted from the Z direction by tanθ = b/a (see Figure 3). do. Such a gradient magnetic field can be easily obtained by combining and simultaneously generating the X-direction gradient magnetic field Gx and the Z-direction gradient magnetic field GV.

Then, while applying this gradient magnetic field, an NMR signal is received.

This NMR signal is frequency modulated by the gradient magnetic field, and the amount of modulation corresponds to the strength of the magnetic field. and,
Since the strength of the magnetic field is changing in the direction of angle 0 as described above, the amount of modulation is the projection of the X-direction position of each tomographic plane #1 and #2 in the direction of angle 0. In other words, in the case of the 3rd FgJ, since the modulation amounts at each position on the tomographic planes #1 and #2 do not overlap with each other, it is possible to determine which of the tomographic planes #1 and #2 is located based on the modulation amount and their respective values. The position in the X direction can be known. Note that if the angle θ is smaller than the above, the resolution of the X-direction position will be poor, and if it is larger, the modulation amount will be lower than that of the tomographic plane #1. Since there is an overlapping part in #2, the fault plane #1. #
It becomes impossible to distinguish between the two.

The two fault planes #l and #2 were explained above, but
It is easy to extend to a larger number of fault planes. As a result, a large number (N
”), and if it is repeated N times as in the conventional method within one repetition time T, then N'X
N multi-sectional imaging becomes possible.

Note that, although the two-dimensional Fourier transform method has been described above, it can be similarly applied to other methods such as a projection restoration method.

Effects of the Invention According to the multi-tomographic imaging method of the MRI apparatus of the present invention, the number of tomographic planes obtained within a limited repetition time can be greatly increased.
This makes it possible to eliminate the limitation on the number of tomographic images simultaneously taken due to the pulse sequence. Moreover, since multiple tomographic imaging is possible even in short-time imaging, ie, short-time imaging, particularly high-speed screening is possible.

[Brief explanation of drawings]

Figures 1, 2, and 3 are related to one embodiment of the present invention. Figure 1 is a schematic diagram explaining the positional relationship of a plurality of tomographic planes, and Figure 2 has two frequency components. FIG. 3 is a block diagram showing a configuration for obtaining a high-frequency signal, FIG. 3 is a schematic diagram for explaining the gradient direction of a gradient magnetic field, and FIG. 4 is a time chart showing a pulse sequence in a conventional multi-tomography method. #1, #2...on the fault plane! X...Mixer

Claims (1)

[Claims] (1) While applying a gradient magnetic field in a direction perpendicular to the tomographic plane, excitation high-frequency pulses having multiple frequency components corresponding to the positions of multiple tomographic planes are given to simultaneously excite multiple tomographic planes, and when receiving signals, , the frequency modulation is inclined from a direction perpendicular to the fault plane to a direction tilted in one direction within the fault plane, and the amount of modulation is different in the above-mentioned direction within the fault plane and does not overlap between multiple fault planes. A multi-sectional imaging method using an MRI apparatus, characterized in that NMR signals from each tomographic plane are simultaneously received by applying a gradient magnetic field for .