JPS60151547A - Inspecting apparatus by means of nuclear magnetic resonance - Google Patents

Abstract

PURPOSE:To decrease bad influence of vibrating magnetic field and to improve an image quality by setting the interval of pulse sequences to a prescribed value corresponding to the frequency of an external vibrating magnetic field. CONSTITUTION:The output of a high frequency pulse generator 2 excites a coil 4 through an amplifier 3. The signal component received by the coil 4 is converted to the image by a signal processing device 7 through an amplifier 5 and a detector 6. The output of the generator 2 is made to the standard signal of a right angle phase detection by the detector 6. Gradient magnetic fields for Z direction and direction at right angle therewith are generated by coils 8-10, and said coils are driven by amplifiers 11-13. A coil 14 for generating static magnetic field is driven by an electric source 15. By setting the interval between 90 deg. and 180 deg. high frequency magnetic fields to the value near integer times of the period of the outer part vibrating magnetic field, the bad influence of the outer part vibrating magnetic field is decreased, and the image quality is improved. In this case, the frequency of the vibrating magnetic field is obtained by a Gauss meter 20 and a counter 21 having a sensor 19.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to nuclear magnetic resonance (N
The present invention relates to an apparatus for measuring MR) signals and determining the density of nuclei, relaxation time, or their respective spatial distributions, and particularly relates to a measurement number and suitable apparatus that are not adversely affected by external oscillating magnetic fields.

[Background of the invention]

2. Description of the Related Art Conventionally, X-ray CT and ultrasonic imaging devices have been widely used as devices for non-destructively inspecting internal structures such as the head and abdomen of a human body. In recent years, attempts to perform similar tests using nuclear magnetic resonance phenomena have been successful, and it has become clear that information that cannot be obtained with X-ray CT or ultrasound imaging devices can be obtained. In an inspection device that uses nuclear magnetic resonance phenomena, it is necessary to separate and identify signals from an inspection object in correspondence with each part of the object. This method includes methods such as projection reconstruction method (abbreviated as PR method) and Fourier transform method (abbreviated as FT method), which visualize the density and relaxation time distribution of this nucleus, and A method is known in which only signals are selectively detected and precise relaxation time measurements and chemical shift measurements are performed. In such an inspection device, a static magnetic field that is spatially uniform and temporally stable must be used. However, depending on the environment in which the device is installed, it may receive a disturbing magnetic field from the outside, resulting in significant image deterioration.

[Purpose of the invention]

The present invention has been made in view of these drawbacks, and is a nuclear magnetic resonance system that reduces the harmful effects of the oscillating magnetic field and significantly improves image quality by setting the pulse sequence interval according to the frequency of the external oscillating magnetic field. The object of the present invention is to provide an inspection device that uses the present invention.

[Summary of the invention]

Until now, there have been no reports on the influence of external oscillating magnetic fields on image quality. However, as a result of experiments conducted by the inventors, it was discovered that there is a close relationship between the frequency of the oscillating magnetic field and the degree of image quality deterioration. When we conducted an analysis to explain this experimental fact, the following became clear. That is,
Normally, the spin echo method is used for signal detection, which basically consists of 90" pulse - τ1 - 180 @ pulse - τ
This is a pulse sequence called 2-signal observation. τ1−τ
Although the condition of 2=τ is generally discouraged, here, for convenience of explanation, we will distinguish it using subscripts 1 and 2.

According to this spin echo method, the phase shift of the nuclear spin that occurs in the τ1 interval changes in the next τ2 interval in a direction that cancels out the previous shift due to the 180° pulse.

Therefore, when the following equation is satisfied, τ0. The signal attenuation in the τ2 interval is sufficiently small except for the attenuation due to T2, which is specific to the nuclear spin.

where bH (D is the magnetic field applied to the nuclear spins, H
(t)=Ho+G(t). Ho is a static magnetic field, and G(t) is a disturbance magnetic field.

If τ1=τ2=τkichi, equation (1) becomes becomes.

Now, in actual measurement, since the integral values in the sections before and after the 180° pulse are different, equation (2) does not necessarily hold true. Therefore, consider the following quantity Q.

Q= l f G(t)at −f G(t)dt I
(3) 0 τ This amount is the G (
It is the absolute value of the time integral of t), and is a quantity that ultimately represents the degree of disturbance to the nuclear spin. Now G(t) = gOcos
Suppose that a disturbance magnetic field is given in the form (ω = 1ψ). Here, ω is the angular frequency of the disturbance magnetic field, and ψ is a variable representing the phase difference that occurs because the 90° pulse and G(t) are not synchronized. This variable ψ changes randomly depending on signal addition or projection angle. Therefore, since Q changes in the same way, the expected value Q is taken as a quantity representing the degree of disturbance. From equation (2), the following can be obtained. Let this Q be an index representing the degree of influence of the disturbance G(t). FIG. 1 shows Q/Qmax obtained from equation (4) when τf is plotted on the horizontal axis. Here s Qm
ax is the maximum value of Q. As can be seen from Figure 1, τ
It can be seen that when f is an integer, it takes a value close to the minimum value.

When τf=0, that is, f=o, the external oscillating magnetic field corresponds to magnetic field inhomogeneity, which is naturally compensated by the 180° pulse and has no effect at all.

From the above results, when the disturbance magnetic field changes with a specific period, its influence can be significantly reduced by setting τ, that is, the interval τ between the 906 pulse and the 180' pulse, to a value close to an integral multiple of the period of the external oscillating magnetic field. I don't know what I can do.

Furthermore, according to Fig. 1, if the value of τf is increased, that is, if the interval τ between the 90″ pulse and the 1800 pulse is selected to be sufficiently long compared to the period (1/f ) of the external oscillating magnetic field, then the Q
It can be seen that the value of /Qmax becomes sufficiently small, and the influence of the external oscillating magnetic field can be reduced.

On the other hand, when there is subharmonic external magnetic field vibration, τf
The closer the value of Q/Qmax is to zero, the closer the value of Q/Qmax is to zero. That is, in this case, 90° pulse and 180° pulse
The influence of external oscillating magnetic fields can be reduced by making the pulse interval τ as short as possible.

[Embodiments of the Invention] Hereinafter, embodiments of the present invention will be described in detail based on the drawings. FIG. 2 shows the configuration of an inspection device that is an embodiment of the present invention.

The control device outputs various commands to each device at constant timing. The output of the high frequency pulse generator 2 is amplified by an amplifier 3 and excites a coil 4. The coil 4 also serves as a receiving coil, and the received signal component passes through an amplifier 5, is detected by a detector 6, and then converted into an image by a signal processing device 7. The output of the high frequency pulse generator 2 is used as a reference signal when quadrature phase detection is performed by the wave detector 6. The generation of magnetic gradient fields in the Z direction and in the direction perpendicular thereto takes place by coils 8, 9, 10, respectively, which are driven by amplifiers 11, 12, 13, respectively. The static magnetic field is generated by a coil 14, and the coil 14 is driven by a power source 15. The coil 10 has the same shape as the coil 9, is rotated 90 degrees around the Z axis, and generates gradient magnetic fields orthogonal to each other. A human body 16 to be examined is placed on a bed 17, and the bed 17 is moved on a support stand 18. FIG. 3 shows the envelope and signal of the high frequency magnetic field H1 from among the pulse sequences used in imaging. Although the pulse sequences differ between the PR method and the FT method, Hl is the same in both. In addition, FIG. 4 shows the inversion-recovery (In-version-Recovery)
This shows a case where signal measurement is performed by the method, and the number of 180° pulses for reversing magnetization is increased by one compared to the case of FIG. 90° pulse after td waiting time after reversal, 18
When 0° pulse deviation signal measurement is performed, a signal including the effect of T1 can be obtained. In FIGS. 3 and 4, a pulse having a rectangular envelope is used as the 180° pulse, but the shape is of course not limited to this, and a shape modulated by Gaussian or 5INC may be used. Conversely, the 90° pulse may have a rectangular shape.

Now, as mentioned above, when an oscillating magnetic field is applied from the outside,
The image quality deteriorates depending on the expected value Q of the integral value of the oscillating magnetic field in the interval τ shown in FIGS. 3 and 4.

Therefore, corresponding to the frequency f of the oscillating magnetic field, the value of τ is approximately n
/ f. However, n is a natural number. Since the pulse sequence is stored in the RAM in the control device 1, changing the τ value only requires rewriting the contents of the RAM.

The frequency of the oscillating magnetic field can be measured by detecting the oscillating magnetic field using a Gaussmeter 20 having a sensor 19 or a search coil type magnetic field measurement device, and counting the frequency using a frequency counter 21, as is well known.

〔Effect of the invention〕

According to the present invention, the influence of external oscillating magnetic fields can be significantly reduced even in locations where external oscillating magnetic fields exist, making it an effective method for obtaining high-quality images or spectra.

[Brief Description of the Drawings] Fig. 1 is a diagram showing the relationship between the frequency of an external oscillating magnetic field and an evaluation function Q representing signal deterioration, Fig. 2 is a configuration diagram of the device used in the present invention, Fig. 3, FIG. 4 is a diagram showing the sequence of the high frequency magnetic field H1, and FIG. 5 is a diagram showing a circuit for detecting the frequency of the oscillating magnetic field. 1...Control device 2...High frequency pulse generator 3.
...Amplifier 4...Coil 5...Amplifier 6...
・Detector 7...Signal processing device 8,9.10...
Coil 11゜12.13...Amplifier 14...Static magnetic field generating coil 15...Power supply 81

Claims (1)

[Claims] 1. Each magnetic field emitting means for a static magnetic field and a high-frequency magnetic field, and a signal detection means for detecting a nuclear magnetic resonance signal from an object to be examined;
In an inspection apparatus using nuclear magnetic resonance having a computer for calculating a detection signal of the signal detection means and a means for outputting the calculation result by the computer, an interval between a 90 degree high frequency magnetic field and a subsequent 180 degree high frequency magnetic field, An inspection device using nuclear magnetic resonance, characterized in that the period is selected to be approximately a natural number multiple of the period of the disturbance magnetic field, or sufficiently long compared to the period of the disturbance magnetic field.