JPS58140628A - Inspecting device using nuclear magnetic resonance - Google Patents

Abstract

PURPOSE:To correct the non-linearity of the ramp magnetic fields to be applied upon an inspecting object easily by providing means of rotating a coil which generates the ramp magnetic fields at every projection. CONSTITUTION:The output of a high frequency generator 9 is applied through an electric power amplifier 10 to a coil 11 for high frequency generating magnetic fields by the command of a control device 8. The signal detected with the coil 11 serving also as a receiver is converted to picture images with a signal processing unit 14 through an amplifier 12 and a detector 13 and the images are displayed. A coil 15 for generating the ramp magnetic fields in the Z direction and the direction perpendicular to the same are driven by electric power sources 16, 17. Static magnetic fields are generated by a coil 21 for generation of static magnetic fields driven by an electric power source 22. The coil 15 is rotated around the X axis by a motor 23 through a belt 24, and since the correction of the entire projected data is made possible with the correction data at one projection angle, the non-linearity of the ramp magnetic fields is easily corrected with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inspection apparatus using a nuclear magnetic resonance phenomenon, and more particularly to an inspection apparatus that facilitates correction of deterioration of a regenerating monk due to nonlinearity of a gradient magnetic field.

Conventionally, the X11iCTJP ultrasonic imaging device has been widely used as a method for non-destructively inspecting the internal structure of a human body or 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 X-ray CT or ultrasonic imaging devices can be obtained.

Inspection equipment using nuclear magnetic resonance (hereinafter simply referred to as "inspection equipment")
That's what it means. ) By non-destructively determining the nuclear spin density distribution, relaxation time distribution, etc. in the target object using nuclear magnetic resonance phenomena, the desired inspection area of the target object can be detected using a method similar to X-ray CT. This is for purchasing and outputting the cross section. In such an inspection apparatus, it is necessary to separate and identify signals from the object to be inspected in correspondence with each part of the nuclear object.

One method is to obtain position information by applying a gradient magnetic field to the object to be inspected, varying the static magnetic field placed on each part of the object, and thereby varying the resonance frequency of each part. FIG. 1 is a companion diagram for explaining the principle. When a gradient magnetic field G is applied to the object l to be inspected, the gradient magnetic field G,
A signal intensity distribution 2 is obtained by integrating the signals from the nuclear spins on a line perpendicular to , as a function of the static magnetic field H. In nuclear magnetic resonance, the relationship f=rH/(2g) holds, so it can be said that the signal intensity 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, by changing the direction of application of the gradient magnetic field and applying the gradient magnetic field to 0*', signal intensity distribution 3 is obtained. If a similar signal intensity distribution, that is, projection data, is obtained by varying the direction of application of the gradient magnetic field, the density distribution or relaxation time distribution of nuclear streaks in the object to be inspected can be obtained using an algorithm similar to that of X-ray CT. can be reconfigured.

By the way, the gradient magnetic field applied to obtain projection data is not spatially linear but includes a nonlinear region.

In this way, when projection data is obtained using a nonlinear gradient magnetic field, the reconstructed image is distorted, which is a major cause of deterioration of the image quality. This point will be explained in more detail using the drawings.

A solid line 4 in FIG. 2 indicates a spatially linear gradient magnetic field, and a solid line 5 represents projection data obtained from the target object 1'. On the other hand, if the projection data is spatially nonlinear as shown by 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 image, and the nonlinearity of the gradient magnetic field also causes significant image quality deterioration. However, if this nonlinearity is always constant regardless of the projection angle, it can be corrected by determining the spatial distribution of the gradient magnetic field in advance.

In that case, if the image consists of a 128×12B matrix, for example, correction data for at least 160,000 points is required. However, in the conventional method in which the gradient magnetic field is rotated by current flowing through two sets of coils, the nonlinearity changes for each projection, so the data required for correction is % 128 xl 28X (number of projections). Since the number of projections is usually about 128, there is a drawback that the memory and the time required for calculation are enormous.

Therefore, the present invention has been made in view of these points, and its purpose is to provide an inspection device 1 that eliminates the above-mentioned drawbacks of conventional inspection devices and facilitates correction of nonlinear bodies of gradient magnetic fields.
It is to serve.

In order to achieve this object, the present invention is characterized in that the direction of application of the gradient magnetic field is changed by rotating the gradient magnetic field generating coil for each projection.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 3 schematically shows the configuration of an inspection device that is an embodiment of the present invention. The control device 8 outputs various commands to each device at constant timing. The output of the high frequency pulse generator 9 is amplified by a power amplifier 10 and excites a high frequency magnetic field generating coil 11.

The coil 11 also serves as a receiving coil, and the signal component detected by the cough coil 11 passes through an amplifier 12, is detected by a detector 13, and is converted into an image by a signal processing device 14 and displayed. The other output from the high frequency pulse generator 9 is used as a reference signal when the detector 13 performs quadrature detection. Generation of gradient magnetic fields in two directions and in a direction perpendicular thereto is performed by a gradient magnetic field generation coil 15, which is driven by power sources 16 and 17. The power source 16 is for a gradient magnetic field in the Z direction, and the power source 17 is for a gradient magnetic field in a direction perpendicular to the Z direction. A human body 18 to be examined is placed on a bed 19,
Move on the support stand 20. Magnetic.

The field is generated by a static magnetic field generating coil 21, which is driven by a power source 22. The gradient magnetic field generation coil 15
is rotated about zmt- by the motor 23 through the belt 24. Here, the shape of the gradient magnetic field generating coil 15 will be explained in detail. FIG. 4 shows an example of a gradient magnetic field generating coil in a direction perpendicular to the Z direction, and it is assumed that a static magnetic field is applied parallel to the hz axis. In addition, Z
The directional gradient magnetic field coil is shown separately in FIG. 5 because the diagram is complicated. The supply of current to the coil is carried out by means of slip rings 26, 27 wound on the base 25 of the coil and brushes 28, 29 in contact with them. In this way, it is easy to supply current even if the coil 15 is rotated for each projection. The arrow in the figure represents the current direction t-, and when the coil comes to the position shown in FIG. 4, a gradient magnetic field in the Y direction is generated. Figure 5 shows the coil 1 shown in Figure 4.
It is the same as 5. In order to make the figure easier to read, the coils that generate the gradient magnetic field in the Z direction are shown.

Note that the belt 24 and motor 23 are the same as in FIG. 4, so they are omitted. The mechanism for supplying current by the brushes 28', 29' contacting the slip rings 26', 27' is also similar to that in FIG.

FIG. 6 shows the timing of the rotation of the coil and the acquisition of projection data. A pulse motor is used as the motor 23, and the pulse (a) from the control device 8 generates 180/N per projection.
Rotate degrees. This is the number of Nfl projections. When the coil has completely stopped after one hour of rotation, the control device 8
Signal detection is started by pulse (b) from .

Here, the reason for rotating the coil 15 will be described in a little more detail. As mentioned earlier, in order to apply a gradient magnetic field to a target object and obtain the density distribution of nuclear streaks in the target object with high precision from the projection data, a gradient magnetic field with excellent linearity is required. be.

Fig. 7 (14 is a contour map showing an example of the gradient magnetic field generated by the coil shown in Fig. 4, and the deviation from the linear shape increases as the distance from the center of the coil increases. The projection 32 in this case is shown in Fig. 7(b).
, the signal component of frequency f1 is the integral value of the signal from the nuclear spin in the projectile object 31 corresponding to the contour line 30. When the integral path is curved in this way, it is possible to reproduce 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, data may be distributed along a curved path instead of the normally used straight path. Let us now assume that the xy plane is divided into 128x128 parts for my own reproduction.

The required gradient a-field distribution data is approximately 16,000 points. These data are correction data necessary for one projection, but if we tried to electrically rotate the gradient magnetic field as in the past, these correction data would be required for each projection, which would increase memory and calculation time. It will be huge. In order to electrically rotate the gradient magnetic field, in addition to the coil shown in Figure 4, a coil rotated 90 degrees around the Z-axis is used, and when the current value of both is tL-It, the following formula is used. All you have to do is satisfy.

l8 = IocoSθ I = I6sinθ Here, θ is the rotation angle of the gradient magnetic field. Assuming that the gradient magnetic field generated by each coil is tGt -Gt and the unit vector eis j in the x and y directions, the combined magnetic field G of both is G '' Gt J + G@1 = K (I I
J + I t 1) = KI6 (cosi9 j + s
in#i). Here, Ki is a proportionality constant, and is a value determined by the shape and number of turns of the coil. Therefore, I.G.I.
=KI.

Thus, we obtain a magnetic field in which the magnitude of the gradient is always constant and only its direction changes. However, as shown in FIG. 7(a), since the gradient magnetic field generated by each coil is nonlinear, if θ changes, the contour lines of the composite magnetic field will also change. In order to use a curved path as correction data when performing compensation reproduction, data for each projection is required for the reasons mentioned above. However, if the gradient magnetic field generation coil is rotated in correspondence with the projection angle as in the present invention, the correction data only needs to be for one projection, and two
The advantage is that it is reduced by an order of magnitude. This makes it possible to perform corrections with higher precision than with electrical discharge rotation, resulting in high-quality reproduction scratches.

As explained above, according to the present invention, it is possible to correct the deterioration of the reconstructed image due to the spatial nonlinearity of the gradient magnetic field using only the correction data at one projection angle, and the reproduction image is significantly improved. It has the effect of improving wo. Furthermore, since the number of coils is 1 when electrically rotating the gradient i-field, there is also the advantage that the coil can be manufactured with high precision.

Although not shown in the embodiments, it is of course possible to transmit force to the coil bobbin using gears as a mechanism for rotating the coil. In either case, it is necessary to use a non-magnetic tube for the transmission mechanism.

Further, in the embodiment shown in FIG. 3, the coil that generates gradient magnetic fields in two directions is also rotated, but this coil is used to determine slices in two directions l), and therefore does not necessarily need to be rotated.

[Brief explanation of the drawing]

Fig. 1 is a diagram for explaining the principle of obtaining projection data, Fig. 2 is a diagram showing the relationship between the nonlinearity of a gradient magnetic field and projection data, and Fig. 3 is a block diagram showing an embodiment of the present invention. Figure 4. Figure 6 shows the structure of the gradient magnetic field generating coil.
The figure is a time chart showing the relationship between the rotation of the coil and signal detection, and Figures 7 (a) and (b) are the relationship between the contour melting of the gradient magnetic field and the projection data. A3 口z5 ZjFigure 6

Claims (1)

[Claims] 1. In an inspection device using nuclear magnetic resonance, which applies a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the inspection target and detects a nuclear magnetic resonance signal from the inspection target, the gradient magnetic field is generated. An inspection device using nuclear magnetic resonance, characterized in that it is provided with means for rotating a coil for each projection.