JPS6324696B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a radiation detector, and particularly to an improvement in a radiation detector that detects incident radiation in a scanning manner by arranging a plurality of unit radiation detectors on a plane, such as those used in computer tomography. Conventional radiation detectors of this type had rectangular detection characteristics, and unless the aperture width was made small, the phase of high frequency components of the signal would be reversed, frequency components would not be transmitted properly, and other problems would occur that would degrade image quality. It has the disadvantages of This invention improves the aperture transfer function of the detector system by placing absorbing objects corresponding to each unit detector to modify its rectangular detection characteristics.
-Function<ATF>) to solve these shortcomings. That is, the present invention provides a radiation detector in which a plurality of unit radiation detectors are arranged in a row on a plane and radiation that has passed through a subject is made to enter each unit radiation detector. The present invention relates to a radiation detector characterized in that, corresponding to each opening, a radiation absorber is interposed to cause the intensity of incident radiation to vary in a Gaussian distribution over each opening. FIG. 1 is a schematic diagram for explaining the theory of the present invention, in which 1 is an X-ray intensity distribution curve at the focal plane of the X-ray tube, 2 is a curve showing the detection characteristics at the aperture of the radiation detector, and 3 is a curve showing the detection characteristics at the radiation detector aperture. A curve illustrating the blur function resulting from the object moving during the data collection time is shown in conjunction. Let f(x) be the intensity distribution 1 of the X-ray tube focal point, d(x) be the detection characteristic 2 of the detector, and t(x) be the blur function resulting from movement during data acquisition time, then the aperture of the X-ray beam is The transfer function ATF is the product of one-dimensional Fourier transform of each response function and can be expressed as follows. ATF=F(f)×F(d)×F(t) [F is one-dimensional Fourier transform] Generally, the above f(x) is a Gaussian function, d(x), t
(x) is a rectangular function. That is, f(x)=exp[−(x/x ₀ ) ² ],

[Formula]. Aperture transfer function ATF of the detector system at the position of the object located midway between the X-ray tube focal point and the detector aperture
can be expressed by the following formula. ATF=exp [-π ² f ² x ² / ₀ (M-1) ² /M ² ] × sin π fa / M / π fa / M × sin π ft ₀ / π ft ₀ Now, let the aperture width a of the radiation detector be 3 mm, and The ratio of the distance D between the ray tube focal point and the radiation detector opening position to the distance L between the X-ray tube focal point and the subject center position, that is, the magnification M=D/L, is set to 4/3, and the radius of the X-ray tube focal point ( Figure 2 shows each ATF when these standard values are entered, assuming that X ₀ ) is 0.75 mm and the image movement distance t ₀ within the subject data collection period is 0.5 mm.
In Figure 2, the vertical axis shows the ATF%, the horizontal axis shows the frequency cycle, the curve shows the ATF curve as a function of the radius of the X-ray tube focus, and the curve shows the moving distance of the subject during the data collection period. The curve shows the ATF curve of the radiation detector aperture. The finer the sampling interval is to improve image accuracy, that is, the higher the Nyquist frequency, the more the ATF characteristic (curve) of the detector aperture will be affected. In the figure, it is 4.4 when data is taken at 0.5 mm intervals.
The phase of the frequency components of ~8.8 cycles is reversed, and there are also frequency components that are not detected at all, degrading the image. FIG. 3 is a schematic diagram showing an absorber according to an embodiment of the present invention. Reference numeral 2' denotes a detector opening, which is arranged in a row on a plurality of planes as described above, one of which is shown in the figure. 4 is a radiation absorber whose thickness is centered on the axis passing through the center of the opening;
It has a shape that is thin at that part and increases in thickness as a quadratic function as it goes outward. In the figure, I ₀ is the radiation intensity at any time, x
is the distance between the position and the center, and y is the thickness. The shape of this absorber allows the ATF characteristics to drop gently without taking zero or negative values as shown in Figure 2. There are various functions that gradually decrease, but for example, a Gaussian function is shown in Fig. 3. Since the Fourier inverse transform of a Gaussian function is a Gaussian function, the ATF characteristics can be made into a Gaussian function. In other words, if an object whose thickness changes quadratically as shown in the figure is placed in front of a detector with uniform detection characteristics, the detection characteristics will be I ₁ (x) = I ₀ e ^- 〓 ^x2 , which is a Gaussian function. Therefore, typical ATF characteristics can be obtained. This X-ray absorber can be placed in front of the opening of the detector, and the gist remains the same even if it is placed just before the opening or between the subject and the focal point of the X-ray tube. Figure 4 shows their positional relationship. In the figure, 1' is the X-ray tube focal point,
Reference numeral 5 denotes an object, and in this invention, the absorber is usually placed at the position in front of the detector opening shown by the solid line, but it can also be placed at the position shown at 4'.
In this case, there is also the effect of reducing the radiation exposure dose of the subject. One X-ray absorber is provided for each radiation detector aperture. Any material that absorbs radiation can be used as the material for the X-ray absorber. As shown in FIG. 5, each detector has a plurality of scintillators 6 cut into small strips arranged in a row on a flat surface, which converts the visible wavelength light, and then a photomultiplier tube 7. When using a detection method in which the scintillator elements 6 are placed in the same direction, it is possible to place an optical filter 8 having a transmission characteristic that is quadratically different as described above at the rear of each scintillator element 6, and this is within the scope of the present invention. Since the present invention is configured and operates as described above, it is possible to make the sampling interval finer in a scanning type composite radiation detector system such as a computer tomography machine, and to increase the number of scanning lines to improve image resolution. The beam width can be easily corrected, and when capturing the same number of photons as a conventional radiation detector with rectangular detection characteristics, the spatial resolution can be improved compared to the conventional one. It is effective.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram for explaining the theory of this invention, Figure 2 is an aperture transfer characteristic (ATF) curve diagram, Figure 3 is an explanatory diagram of a radiation absorber according to an embodiment of this invention, and Figure 4 is FIG. 5 is an explanatory plan view showing the positional relationship between the absorber of the present invention, the X-ray tube focal point, the subject, and the radiation detector, and FIG. 5 is a diagram showing a modified embodiment. 1'...X-ray tube focal point, 2'...detector aperture, 3...
...Scanning pulse including movement of the subject during data collection, 4...Radiation absorber, 5...Subject, 6...
Scintillator, 7...photomultiplier tube, 8...optical filter.

Claims (1)

[Claims] 1. In a radiation detector in which a plurality of unit radiation detectors are arranged side by side in a row on a plane, and the radiation that has passed through the subject is made to enter each unit radiation detector, each of the unit radiation detectors has a Correspondingly, a radiation detector characterized in that a radiation absorber is interposed to cause the intensity of incident radiation to vary in a Gaussian distribution over each of its openings.