JPS61235741A - Nuclear magnetic resonance device - Google Patents

Abstract

PURPOSE:To eliminate the distortion of a resonance image by detecting the ramp of the flat top part of a ramp magnetic field pulse from the frequency spectral shape of a nuclear magnetic resonace signal and controlling the output current waveform of a power source for a ramp magnetic field coil thereby controlling the ramp in the flat top part to a prescribed value or below. CONSTITUTION:A measuring body is disposed in a static magnetic field. A high-frequency magnetic pulse (a) is passed to a high-frequency coil and the ramp magnetic field pulse (b) is passed to the ramp magnetic field coil. The intensity of the ramp magnetic field pulse is adequately selected as shown by (c) to excite the entire spin of the measuring body. A spin echo (d) is observed. The ramp in the flat top part of the ramp magnetic field pulse intensity is detected and the output electric power waveform of the power source for the ramp magnetic field coil for generating the ramp magnetic field is so controlled that the ramp thereof attains the prescribed value or below in accordance with the result of the detection. Since the ramp in the flat top part of the ramp magnetic field is decreased in the above-mentioned manner, the distortion and blur of the unclear magnetic resonance image is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a nuclear magnetic resonance apparatus, particularly a nuclear magnetic resonance tomography apparatus. .

[Conventional technology]

FIG. 6 is a block diagram showing a conventional nuclear magnetic resonance apparatus. F.America
Journal of Radiology: /
This is a simplified version of the one described in Volume 3 and p sob (tqtr2). In the figure, (1) is a magnet, (2) is an object to be measured, such as a human body, lying in the static magnetic field of this magnet (1), (3) is a high-frequency coil wound around the human body (2), (The transceiver transmits electromagnetic waves to the high-frequency coil (3) and receives the electromagnetic waves from the human body (2). (5) is located between the magnet (1) and the high-frequency coil (3) and consists of multiple pairs. A gradient magnetic field coil, (6) a power source for this gradient magnetic field coil (S), (7) a control circuit that controls the gradient magnetic field coil power source (6) and the transmitter/receiver,
(za) is a computer connected to this control circuit (7), (t)
is connected to a computer via an image display.

Next, the operation will be explained. The human body (
A uniform static magnetic field is applied to the human body (2), and an electromagnetic wave corresponding to the Zeeman energy of the specific atomic nucleus in the human body (2) is irradiated from the transmitter/receiver (2) through the high-frequency coil (3). A specific atomic nucleus in (2) causes a resonant transition from the ground state to an excited state.After this, when the irradiation of electromagnetic waves is stopped, electromagnetic waves are emitted from the atomic nuclei in one human body (2).

This electromagnetic wave is passed through a high-frequency coil (3) and detected by the transmitter/receiver (receiver). At this time, a gradient magnetic field coil (Y) creates a gradient in the static magnetic field to determine which position on the human body (2) the signal is coming from. In addition, the computer (za) controls the gradient magnetic field coil power supply (6) for supplying current to the gradient magnetic field coil (5) and the transmitter/receiver (ri) via the control circuit (ri). The obtained image is displayed on an image display (te).

[Problems to be solved by the invention] Since the conventional nuclear magnetic resonance apparatus is configured as described above, especially when a superconducting magnet is used as a magnet, a box for cooling and supporting the superconducting magnet when applying gradient magnetic field pulses is required. Eddy currents flow through the body, distorting the gradient magnetic field pulse waveform, resulting in problems such as distortion and blurring of nuclear magnetic resonance images.

FIGS. 7(a) and 7(b) show examples of output current waveforms of the power source for gradient magnetic field coils and gradient magnetic field waveforms generated within the same time period. This example shows that the portion of the gradient magnetic field waveform (ll) corresponding to the constant current portion (10) has a slope, and furthermore, a residual magnetic field (12) is generated.

This invention was made to solve the above-mentioned problems, and an object of the present invention is to obtain a nuclear magnetic resonance apparatus that can remove distortion, blur, etc. from nuclear magnetic resonance images.

[Means for solving problems]

The nuclear magnetic resonance apparatus according to the present invention includes means for detecting the inclination of the flat top portion of the gradient magnetic field pulse and controlling the output current waveform of the power source for the gradient magnetic field coil so that the absolute value thereof is equal to or less than a predetermined value. It is something that

[Effect]

The nuclear magnetic resonance apparatus of the present invention reduces distortion and blurring of nuclear magnetic co-images by controlling the gradient magnetic field pulse waveform to an ideal shape.

〔Example〕

Hereinafter, an embodiment in which the present invention is applied to a nuclear magnetic resonance tomography apparatus will be described with reference to the drawings. The configuration of this embodiment is the same as the configuration shown in FIG.

The only difference is that the program in the computer (ff) is different and that a predetermined object to be measured is used instead of a human body (2) when optimizing the output current waveform of the gradient coil power source (6).

That is, in addition to the same functions as conventional ones, the computer in this invention measures the frequency spectrum of a nuclear magnetic resonance signal obtained from a predetermined object to be measured under a predetermined imaging sequence, and calculates the gradient magnetic field pulse. A function to detect the inclination of the flat top part.

The control circuit (7) is controlled based on this detection result, and a feedback signal is sent from the control circuit (ri) to the power source (6) for the gradient magnetic field coil so that the absolute value of the slope of the flat top portion is equal to or less than a predetermined value. It has a function to output.

Next, a first embodiment will be described with reference to FIGS. FIG. 1 is a flowchart showing a procedure for correcting a gradient magnetic field pulse waveform, FIG. 1 is a time chart showing a pulse sequence for gradient magnetic field correction, and FIG.
The figure is a power spectrum diagram of the obtained spin echo signal, and Fig. D is a perspective view of a rectangular parallelepiped sample as a predetermined object to be measured.

As shown in Fig. 9, first, a rectangular parallelepiped sample (JA), for example, a container containing water, is placed so that each side of the sample (JA) coincides with the direction of the gradient magnetic field Gx r Gy * Gz (first Step S/) in the figure.Next, the gradient magnetic field correction function is started (Step S2).

There are three types of gradient magnetic fields, and they are adjusted one by one. That is, the high-frequency magnetic field pulse shown in Fig. 1 (a) is passed through the high-frequency coil (3), and the gradient magnetic field current pulse Gα (α is any one of X r Y + Z) shown in Fig. 2 (b) is applied. The current is applied to the gradient magnetic field coil (j). At this time, the high-frequency magnetic field pulse and the gradient magnetic field pulse intensity shown in FIG. 2(C) are appropriately selected so that all the spins in the rectangular parallelepiped sample (core A) are excited. As a result.

The spin echo signal shown in FIG. 3(d) is observed. At this time, the gradient magnetic field pulse intensity during spin echo signal observation is not constant as shown in the figure due to the influence of eddy currents. Calculate the power spectrum of this spin echo signal (step 8
．． 7). Figure 3 shows (a) the case where the gradient magnetic field pulse strength is ideally constant; (b) (C) the slope ha of the spin echo signal when the gradient β of the gradient magnetic field pulse strength gradually increases; The power spectrum with hb and howo is shown. It can be seen that the absolute value of the slope h in FIG. 3 decreases as the slope β of the gradient magnetic field pulse intensity increases. therefore.

By maximizing this slope h, correction can be made to minimize the slope β of the gradient magnetic field pulse intensity waveform. Specifically, the control circuit (
7) Send a feedback signal to the gradient coil power source (6) to change the output current waveform of the gradient coil power source (6) (steps Be and S3);
If the absolute value of becomes larger than the predetermined value (2), the same process is performed for the gradient coil power supplies of the three channels one after another.

After that, proceed to the imaging sequence (step S
a).

Next, the fourth embodiment will be described. The fourth embodiment is the same as the first embodiment except that the time chart shown in FIG. 3 is used as the gradient magnetic field correction pulse sequence. Figure S shows a pulse sequence that is effective when applying gradient magnetic field pulses frequently, such as in simultaneous multilayer tomography.
A plurality of spin echo signals are measured using the G sequence, and the frequency spectrum, such as the power spectrum, of each spin echo signal is evaluated.

In the above embodiments, the shape of the high-frequency magnetic field pulse is a Gaussian function in the first embodiment, and a rectangular waveform in the fourth embodiment, but other function shapes may be used. It is also conceivable to make the rectangular parallelepiped sample small and insert it into the imaging space at all times.

Although the power spectrum was simply measured using a program in Calculation, the speed may be increased by using dedicated hardware such as an FFT analyzer.

〔Effect of the invention〕

As described above, according to the present invention, there is provided a means for detecting the inclination of the flat top portion of the gradient magnetic field coil, and an output current waveform of the power source for the gradient magnetic field coil such that the absolute value of the inclination is equal to or less than a predetermined value. Since the means for controlling this is installed in the computer, it has the effect of eliminating distortion and blurring of nuclear magnetic resonance images.

[Brief explanation of drawings]

FIG. 1 is a flowchart showing the gradient magnetic field pulse waveform correction procedure according to an embodiment of the present invention, FIG. 2 is a time chart showing the gradient magnetic field correction pulse sequence, and FIG. 3 is the power of the obtained spin echo signal. Spectrum diagram, Figure 9 is a diagram showing a rectangular parallelepiped sample and the direction of a gradient magnetic field, Figure 3 is a time chart diagram showing a pulse sequence for gradient magnetic field correction in another embodiment, and Figure 6 is a schematic configuration of a nuclear magnetic resonance apparatus. FIG. 7 is a diagram showing the output current waveform of the gradient magnetic field coil power supply and the actual gradient magnetic field waveform. (1) is a magnet, (2) is an object to be measured, (2 people) is a rectangular parallelepiped sample, (3) is a high frequency coil, (5) is a gradient magnetic field coil,
(6) is a power supply for gradient magnetic field coils, (7) is a control circuit, (
t) is a calculator. In each figure, the same reference numerals indicate the same or corresponding parts. Leather Figure 1 Figure 2 Figure 4 ^ 2A: 11! τ body sample leather 3 figure 5

Claims (2)

[Claims] (1) In a nuclear magnetic resonance apparatus that applies a high-frequency magnetic field and a gradient magnetic field to an object to be measured placed in a static magnetic field and obtains a nuclear magnetic resonance signal from the object to be measured, a predetermined object to be measured is used to Pulse characteristic detection means detects the slope of the flat top portion of the gradient magnetic field pulse based on the frequency spectrum shape of the magnetic resonance signal, and the absolute value of the slope of the flat top portion is determined to be a predetermined value based on the detection result of the pulse characteristic detection means. A nuclear magnetic resonance apparatus comprising: a control means for controlling an output current waveform of a power source for a gradient magnetic field coil that generates the gradient magnetic field so that the waveform of the output current is equal to or less than a value of the gradient magnetic field. (2) The nuclear magnetic resonance apparatus according to claim 1, wherein the predetermined object to be measured is a rectangular parallelepiped sample.