JPS58140630A - Inspecting device by using nuclear magnetic resonance - Google Patents

Abstract

PURPOSE:To correct the spacial non-linearity of ramp magnetic fields easily by measuring the distributions of at leat two sets of mutually orthogonally intersecting ramp magnetic fields beforehand and calculating the distribution of the ramp magnetic fields on an optional sectional surface. CONSTITUTION:Static magnetic fiels are generated by driving a coil for generation of static magnetic fields with an electric power source 18, and the output of a high frequency pulse generator 9 drives a coil 11 for generation of high frequencies with a control device 8 through an electric power amplifier 10. The signal detected with the coil 11 is converted to picture images with a signal processing unit 14 through an amplifier 12 and a detector 13 and the images are displayed. Ramp magnetic field coils 15, 15', 15'' are driven by electric power sources 16, 16', 16'', whereby the ramp magnetic fields in the directions x-z are generated. The samples 26 which generate the resonance signals movable in the directions x, y, z with movable arms 27-29 are disposed in the respective ramp magnetic fields, and are moved within the prescribed plane with the device 8. The magnetic field distributions are measured and the distributions of the respective ramp magnetic fields are stored in the memory in a computer 10, whereby the non-linearity is corrected easily.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inspection device that uses nuclear magnetic resonance phenomena to non-destructively determine the density distribution or relaxation time distribution of nuclear streaks in a target object.

2. Description of the Related Art Conventionally, X-ray CT and ultrasonic imaging devices have been widely used as methods for non-destructively inspecting internal structures of human bodies and the like. In recent years, attempts to conduct similar tests using nuclear magnetic resonance phenomena have been successful, and it has become clear that information that cannot be obtained with XMCT or ultrasound imaging devices can be obtained. Inspection equipment using nuclear magnetic resonance (hereinafter simply referred to as "inspection equipment") uses the nuclear magnetic resonance phenomenon to nondestructively determine the density distribution, relaxation time distribution, etc. of nuclear spin in a target object. This system constructs and outputs a cross-sectional image of a desired inspection region of a target object using a method similar to that of X-ray CT. In such an inspection apparatus, it is necessary to separate and identify signals from an object to be inspected in correspondence with each part of the object. One method is to obtain positional information by applying a gradient magnetic field to the object to be inspected, varying the static magnetic field placed in each part of the object, and thereby varying the resonance frequency of each part. FIG. 1 is a diagram for explaining the principle.

When a gradient magnetic field G is applied to the target object 1, the gradient magnetic field G
, a signal intensity distribution 2 is obtained by integrating the signals from the nuclear spins located perpendicular to , as a function of the static magnetic field H. In nuclear magnetic resonance, f=γH/(2π)...
Since the relationship (1) holds true, it can be said that the signal strength is a function of the resonance frequency f. Note that in the above equation, r is the nuclear gyromagnetic ratio, which is a value specific to nuclear spin. Next, the gradient magnetic field G is changed by changing the direction of application of the gradient magnetic field.
, a signal intensity distribution 3 is obtained when all the signals are applied. By varying the direction of application of the gradient magnetic field and obtaining similar signal intensity distributions, that is, projection data, it is possible to reconstruct the density distribution of nuclear spins in the object to be inspected using an algorithm similar to Xll11CT. can. However, the gradient magnetic field applied to obtain projection data is not spatially linear but includes a nonlinear region. Fruit #+1 in Figure 2
4 indicates a spatially linear gradient magnetic field, when the target object 1
The projection data obtained from ' is all solid line 5. On the other hand, if the projection data is spatially nonlinear like the broken line 6, the projection data becomes the broken line 7, and distorted projection data is obtained. Reconstruction of such distorted projection data results in a blurred fc image, and the nonlinearity of the tilted fiB field is a major factor in causing significant image deterioration. However, if the gradient magnetic field distribution is known in advance, it becomes possible to completely accurately associate each chest wave number component of the signal with the projection data, and it is possible to eliminate the adverse effects of the nonlinearity. However,
To perform such correction, it is necessary to measure the gradient magnetic field distribution for each projection and store the values. for example,
In order to store a two-dimensional gradient magnetic field distribution consisting of a 128x128 matrix for each part of 128 projections,
It requires a memory of at least 10' words, which has the disadvantage of resulting in a huge amount of memory.

The object of the present invention is to provide two or three mutually orthogonal
By measuring the #4 oblique magnetic field distribution of each of the inclined hot water generation coils in advance and calculating the entire gradient magnetic field distribution on an arbitrary cross section based on the gradient magnetic field distribution, it is possible to correct the spatial nonlinearity of the gradient magnetic field. We provide an inspection device' (F-) that facilitates the inspection.

Hereinafter, practical examples of the present invention will be explained in detail based on the drawings. FIG. 3 shows a schematic configuration of an inspection device that is an embodiment of the present invention. The control device 8 outputs obscene commands to each device at a constant timing. The output of the frequency pulse generator 9 is amplified by the cage amplifier 10, and excites the high frequency magnetic field generating coil 11t. The coil 11 also serves as a receiving coil, and the signal component detected by the coil passes through an amplifier 12, is detected by a detector 13, and then sent to a signal processing device 14.
Convert to 1 and display 9. The other output from the high frequency pulse generator 9 is used as a reference signal when performing quadrature phase detection in the wave detector 13. Generation of gradient magnetic fields in the Z direction and directions perpendicular to the Z direction is performed using gradient magnetic field generation coils 15.15.
' , 15" and power supply 16.1 to these coils.
6' and 16". Also, the static magnetic field is generated by static @
This is done using a field generating coil 17, which is connected to a power source 18.
is driven by. The coil 15 for generating hot water at an angle in the X direction is a coil for generating a gradient magnetic field in the X direction 15'1r, which is rotated 90 degrees around the Z axis, and all the hot water is generated at an angle at right angles to each other. The object 19i1r lies on the bed 2°, and the bed 20 moves entirely on the platform 20'. FIG. 4 shows an example of the coil 15'' for generating a gradient magnetic field in the X direction, and the arrow indicates the direction of the t flow. FIG. 5 shows an example of the coil 15 for generating a gradient magnetic field in the Z direction.

As mentioned earlier, iK, which applies a gradient magnetic field to a target object and obtains the density distribution of nuclear spins in the target object with high precision from the projection data, requires a gradient magnetic field with excellent linearity. be. FIG. 6(a) is a contour diagram showing an example of the gradient magnetic field generated by the coil shown in FIG.
The deviation from linearity increases as the distance from the center of the coil increases.

The projection 21 when using such a gradient magnetic field is shown in Figure 6 (
As shown in the figure, the signal component of frequency f is the integral value of the signal from the nuclear spin in the target object 23 corresponding to the contour line 22. When the integral path is curved in this way, it is possible to reconstruct the original nuclear spin density distribution from projection data if the path is known in advance. for example,
When reproducing data using the back projection method, it is sufficient to distribute the data along a curved path instead of the normally done straight path. Now, in order to know all such integral paths, it is necessary that the gradient magnetic field distribution is known. The magnetic field distribution data required to correct one projection data is 128×1 which is almost equal to the picture element of the reproduced image.
It is about 28 points. However, as is conventional practice, when two orthogonal gradient magnetic field distributions are combined to generate a rotating gradient magnetic field, the magnetic field distribution changes for each projection angle, so the data required for correction is 128×12
It is the projection times 8. This is a two-dimensional case, but in order to obtain an arbitrary cross section, it is necessary to multiply the three-dimensional magnetic field distribution data by projection, resulting in a huge amount of data. The present invention measures in advance the magnetic field distribution generated by each of the gradient magnetic field generating coils that generate mutually orthogonal gradient magnetic fields, calculates and corrects the magnetic field distribution of an arbitrary cross section using these magnetic field distributions. This is used as data.

For example, when inspecting the xy cross section 24 as shown in FIG. 7, the gradient magnetic field is rotated around the Z axis to capture projection data. The rotation of the gradient magnetic field is x, the gradient magnetic field G in the X direction,
This is done by synthesizing G. Now, let x be a unit vector 'kin j in the
It becomes a magnetic field whose magnitude is constant and whose direction changes with θ. Therefore, the magnetic field distribution H, (x
, y), H a (x, y) If we calculate the humidity of the meeting, %
Combined magnetic field strength H−y(X,y) at P(x,y)
can be calculated using the following formula.

HIIF (Xs y)=Hx(xe y)ωSθ+H
y (xs y) sin θ ・・・・・・・・・ (4
) The gradient magnetic field at that time is given by (5). Therefore, H, (x, y) and H a < x, y
The gradient magnetic field for any projection angle can be calculated using only two sets of magnetic field distributions.

FIG. 8 shows a more detailed structure of the magnetic field distribution measuring device 25 shown in FIG. The magnetic field distribution can be measured using equation (1) from the frequency at which the resonance signal occurs. A sample 26 that generates a resonance signal is placed at the tip of an arm 27, and the arm 27 moves on a rail on an arm 28. The arm 28 moves entirely on the rail on the arm 29, and the arm 29 moves on the rail on the stand 30. Therefore, the sample 26 can move in any direction of xe)'s''.

For example, as shown in FIG. 9, the arm is moved by a pulley 32 attached to the shaft of a pulse motor 31, and an arm 34 fixed to the belt 33 is moved.
This can be easily achieved by moving the . Such a mechanism is used for the arms 27, 28 and 29. The measurement of the magnetic field distribution is carried out prior to the inspection of the target object 19, by placing the sample 26 of the measuring device 25 in each gradient magnetic field generating coil, and driving the pulse motor 31tl by a command from the control device 8. By intermittently moving the sample 26 within a predetermined plane, detecting the resonance signal from the sample 26 at each position, and storing it in the memory in the signal processing device 14,
Measure the @outside cloth on the above predetermined plane.

For example, the magnetic field distribution H, (x, y) can be obtained by driving the gradient magnetic field generating coil 15' in the X direction and moving the sample 251 within the rxy plane under the gradient magnetic field G0 in the X direction. Ru.

FIG. 10 shows the main part of another embodiment of the present invention, in which a plurality of devices are used to measure the magnetic field distribution, as shown in FIG. 10(a).
For example, this is the case where sample 35 is used, in which 128 samples are arranged in one row. Assume that a gradient magnetic field G0 is applied in the fy@ direction. In this case, the coordinate y is
If the resonance signal 36 from the sample 35 placed in
Since t/γ is determined, the magnetic field distribution at 128 points can be known at one time. By moving these samples 35 in the X direction, the fB magnetic field distribution on the XY plane can be changed to yl1 in a short time.
It will be monkey at 1 fixed.

As is clear from the above explanation, not only on the XY plane but also on any plane, the gradient magnetic field G, , G a, If G and i are given f, @J projection data can be obtained by generating an arbitrary rotating magnetic field. Here, α and β are the angle that the projection of the gradient a field vector onto the xy plane makes with the X axis, and the angle that the gradient magnetic field vector makes with the two axes. 'E fc,' s J *kux, y, z
is the unit vector of the axis. In this case, the gradient magnetic field G,
, Ga, and G1 may be generated individually and the magnetic field distribution at the other end may be measured in advance.

As explained above, according to the present invention, by measuring in advance the magnetic field distribution of each coil that generates all the gradient magnetic fields in the x, y, and z directions, the nonlinearity of the gradient magnetic field at any projection angle can be investigated. It can be easily corrected and the It! of the reproduced image can be easily corrected.
It has a remarkable effect on improving 1ft.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the principle for obtaining projection data, Fig. 2 is a diagram to explain the nonlinear influence of gradient magnetic fields, Fig. 3 is a block diagram showing an embodiment of the present invention, and Fig. 4 , Fig. 5 is a diagram showing the structure of the gradient magnetic field generation coil, Fig. 6(a)
, (b) is a diagram showing the relationship between the isotonicity of the gradient @ magnetic field and the projection data, Figure 7 is a diagram for explaining the method of synthesizing the gradient magnetic field, and Figures 8 and 9 are the diagrams of the present invention. Figures 10(a) and 10(b) are diagrams illustrating an example of a magnetic field distribution measuring device used for the hot water distribution measuring device. , 1lPi61ffi(b)
) PiV Figure Pigeon δ Eye 6 'vJ'? Vito Diagram: [

Claims (1)

[Scope of Claims] 1. In an inspection device using nuclear magnetic resonance, which applies each magnetic field of a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to an inspection object and detects a nuclear magnetic resonance signal from the inspection object. , a measuring means for measuring the magnetic field distribution of a coil that generates the gradient magnetic field, and a gradient magnetic field distribution on a desired cross section of the inspection object from at least two sets of mutually orthogonal gradient magnetic field distributions measured in advance by the measuring means. An inspection device using nuclear magnetic resonance, characterized by comprising means for calculating. 2. The measuring means includes a sample and a moving means for moving the entire sample within a predetermined plane, and detects a nuclear magnetic resonance signal from the sample located within the plane to determine the gradient magnetic field distribution. 2. An inspection device according to claim 1, characterized in that the inspection device performs measurement. 3. The inspection device according to claim 2, wherein the sample is composed of a plurality of samples arranged in a row.