JPH0447789B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scintillation camera having a function of correcting "unevenness" in the energy response characteristics of a detector for incident gamma rays.

One of the main factors that determines the uniformity of scintillation cameras is the "unevenness" of the energy response characteristics of the detector for incident gamma rays. Since the energy of gamma rays is constant unless it is scattered, the magnitude of the energy signal of the scintillation camera due to gamma rays should also be constant, but in reality, the energy response characteristics of the detector are uneven. This causes slight differences depending on the position within the field of view of the scintillation camera.

Figure 1 somewhat emphasizes how the distribution of the magnitude of the energy signal of incident gamma rays (hereinafter referred to as energy spectrum), especially the energy signal value indicating its peak, changes depending on the position within the camera field of view. In the figure, positions within the camera field of view S and peaks a, b, and c of energy signal values of each energy spectrum are shown in correspondence with arrows. Note that the left side portion from approximately the center of each energy spectrum in the figure is due to scattered γ-rays.

In general, a scintillation camera is equipped with a wave height analysis circuit for removing scattered γ-rays, and the γ-ray of the energy signal value entering the window W is
Calculates the position only for the line and displays the image. Therefore, as shown in FIG. 1, when the peak of the energy signal value changes like a, b, c depending on the position within the camera field of view
The proportion of gamma rays in the energy signal value entering the image changes, resulting in a decrease in image uniformity.

Recently, a method for correcting the above problem using microcomputer control has been developed and is becoming popular. The principle will be explained with reference to FIG. That is, a scintillation camera having this type of correction function basically includes a two-stage wave height analysis circuit. Among these, the window W1 of the first-stage pulse height analysis circuit is set sufficiently wide with respect to the peaks a, b, and c that change depending on the position, and only the scattered γ-ray components are removed. The window W2 of the second stage wave height analysis circuit is a peak a of the energy signal value that changes depending on the position within the camera field of view S,
B and c are shifted as A, B, and C depending on the correction data. As a result, peaks a, b, c
Since the positions of the windows W2 relative to each other are constant, it is possible to effectively correct "unevenness" in the energy response characteristics. In this case, the correction data is obtained by previously measuring and processing the energy spectrum at each position within the camera field of view, in other words, the "unevenness" of the energy response characteristics. Another method is to multiply the energy signal by correction data and correct it so that its magnitude does not change depending on the position within the camera field of view.

By the way, in such a scintillation camera, the first stage wave height analysis circuit is only intended to remove the scattered gamma ray component, but since the position calculation process requires a certain amount of time, the counting rate is In order to improve the characteristics, it is necessary to narrow the range of the window W1 as much as possible to remove unnecessary γ-ray components.

However, with this type of conventional camera, it must be taken into consideration that the "unevenness" of the energy response characteristics worsens over time.
The range of the window W1 cannot be made very narrow, and as a result, there is a drawback that the count rate characteristics are significantly worse than those without a correction function.

The present invention was made in order to eliminate the above-mentioned drawbacks, and provides a scintillation camera that has a function of correcting the "unevenness" of the energy response characteristics of the incident gamma ray detector and has good count rate characteristics. The purpose is to

Embodiments of the present invention will be described below with reference to FIGS. 3 and 4. FIG. 3 is a block diagram showing an embodiment of a scintillation camera according to the present invention.
In the figure, 1 is a detector. This detector 1 includes a scintillator 2 and a plurality of photomultiplier tubes 3.
Here, the scintillator 2 converts the incident gamma rays into light, and the light enters the photomultiplier tube 3 while being diffused and is output as an electric signal 101. The adder circuit 4 adds all the electrical signals 101 to create an energy signal 102. The first stage pulse height analysis circuit (hereinafter referred to as the first PHA) 5 receives the energy signal 10.
2 and causes the position calculation circuit 6 to start calculation processing only for the γ-rays that have not been scattered. The position calculation circuit 6 calculates and processes the electric signal 101 from the photomultiplier tube 3 and sends the position signal 103 to an A/D conversion circuit (hereinafter referred to as ADC) 7 and a CRT.
Output to 8. The position signal 103 is converted into a digital signal by the ADC 7 and input to the memory circuit 9. The memory circuit 9 stores the correction data 104 of the γ-ray incident position in the second stage wave height analysis circuit (hereinafter referred to as the second PHA).
Send on 10. The second PHA 10 discriminates the magnitude of the energy signal 102 by shifting the window W2, for example, as shown in A, B, and C in FIG. The unblank signal 105 is output only for the γ-rays. CRT8 is position signal 103
A bright spot is displayed at a corresponding position on the tube surface by the unblank signal 105, and the bright spot is accumulated on a film (not shown) to form an image.

The correction data 104 contains the energy spectrum at each position within the field of view of the camera (detector 1), in other words, the "unevenness" of the energy response characteristics, and is suitable for normal image data collection (inspection imaging). In advance, a position signal 10 is generated by a correction data creation circuit 12 having a built-in microcomputer.
3 and energy signal 102 and is obtained by measuring and processing the energy signal 102 and stored in the memory circuit 9.

The window W2 of the second PHA 10 is set by the window signal 106 from the window setting circuit 11,
Further, the window W1 of the first PHA 5 is set by the window signal 107.

This window signal 107 is created as follows. That is, the correction data creation circuit 12
As described above, the correction data 104 is obtained and stored in the memory circuit 9. When the correction data 104 is stored, its maximum and minimum values are detected and sent to the window control circuit 1 as the window correction signal 108.
Output to 3. This window control circuit 13 includes
A window signal 106 having a window range W2 similar to that given to the second PHA 10 is also input from the window setting circuit 11. Window control circuit 13
The window correction signal 108 widens the window range of the window signal 106 from W2 to the range W1 shown in FIG. A new window signal 107 in this window range W1 is input to the first PHA 5, and the minimum γ required to form an image is input to the first PHA 5.
The position calculation circuit 6 is caused to perform position calculation processing only for lines.

Here, the correction data 104 will be explained in more detail. The correction data 104 is obtained by uniformly irradiating the entire field of view of the detector 1 with gamma rays, and measuring and processing the position signal 103 and energy signal 102 by the correction data generation circuit 12. That is, the correction data creation circuit 12 measures the magnitude distribution (energy spectrum) of the energy signal 102 at each position within the field of view of the detector 1 using the energy signal 102 and the position signal 103. Then, energy signal values indicating the peaks of the energy spectrum at each position within the field of view of the detector 1 (positions on the horizontal axis in FIG. 4) are detected and stored in the memory circuit 9 as correction data 104.

When collecting image data, each time one gamma ray is incident, the incident position (position signal 103) is determined.
Corrected data 104 is output from the memory circuit 9 in accordance with the correction data 104, and the center position of the window W2 of the second PHA 10 is shifted to the position indicated by the corrected data 104 (position on the horizontal axis in FIG. 4) or by the amount of the corrected data 104. and set. As a result, the energy signal value (position on the horizontal axis in Figure 4) indicating the peak of the energy spectrum according to the incident position of the γ-ray is changed to
The center positions of the windows W2 of the 2PHA 10 are matched.

Further, the maximum value and minimum value of the correction data 104 are detected from the correction data 104 stored in the memory circuit 9 as described above, but this is based on the energy spectrum at each position within the field of view of the detector 1. This corresponds to the maximum and minimum values of the energy signal value indicating the peak (position on the horizontal axis in FIG. 4). That is,
The maximum and minimum shift position (or shift amount) of the window W2 of the second PHA 10, and the width of the window W1 of the first PHA 5 are the width of the window W2 of the second PHA 10.
, plus the width between the maximum and minimum values of this shift position (or shift amount).

This will be explained with reference to FIG. In Figure 5, if the energy signal values indicating the peaks of the energy spectrum for each position a, b, c (see Figure 1) in the field of view are Ea, Eb, Ec, the maximum value is Eb, and the minimum value is is Ec. Now, the appropriate windows for the peaks a, b, and c of the energy spectrum are W2a, W2b, and W2c, and the absolute value of their range (width) is W2, which is equal to each other, and each window W2a, W2b, and W2c is 1/ of range
It is assumed that the peaks a, b, and c are set to be located at position 2.

According to this, the peak a of the second PHA 10,
The windows corresponding to b and c are W2a, W2b,
It becomes W2c. The window W1 of the first PHA 5 has a range of (Eb - Ec) + W2, and the energy signal value is {(Ec - (1/2) x W2) ~ (Eb + (1/2)
2)×W2)}, as shown by W1 in FIG.

This makes it possible to measure the peaks of the energy spectrum at each position within the field of view of the detector.

When the energy signal 102 enters the window W1 of the first PHA 5 as described above, the position calculation circuit 6 outputs a position signal 103 corresponding to the incident position of the γ-ray. The position signal 103 becomes a digital signal by the ADC 7 and enters the memory circuit 9. The memory circuit 9 is γ
The correction data 104 corresponding to the line incidence position is
Give to 2PHA10. The second PHA 10 shifts the window W2 to the position indicated by the correction data 104 (or by an amount corresponding to the correction data 104),
For example, when position c within the field of view in FIG. 1 is calculated as the gamma ray incident position 103, W2 is shifted to position C in FIG. 4, and the energy signal 102 is analyzed for wave height.

Here, the window control circuit 13
The window W1 is set within the range described above. This window W1 setting operation is not performed for each gamma ray, but is performed from the correction data creation circuit 12 for each set of correction data obtained before the start of image data collection regarding the target gamma ray energy (nuclide). The maximum value and minimum value of the correction data 104 are inputted into the window control circuit 13 to be set. That is, the operations of the window control circuit 13 and the correction data creation circuit 12 are performed before the start of image data collection (each time the γ-ray energy (nuclide) is different, and when it is desired to correct changes over time even if the γ-ray energy (nuclide) is the same). ) is executed. Note that different γ
The maximum and minimum values of the correction data 104 are obtained in advance for each linear energy (nuclide), and the correction data memory 9
It may be stored in .

In the above embodiment, the present invention is applied to a scintillation camera that corrects "unevenness" in the energy response characteristic by shifting the window W2, but the energy signal 102 is multiplied by the correction data 104 to The first step is also used in scintillation cameras that correct for “unevenness” in response characteristics.
Since the action of 1PHA5 is the same, it can be applied in the same way.

Further, in the above embodiment, when obtaining correction data,
The window range control means for detecting the maximum and minimum values of the correction data and controlling the window range of the first PHA 5 using the detected values is configured using the correction data creation circuit 12, the window control circuit 13, etc. Not limited to only.

As described above, the present invention obtains correction data in a camera equipped with a pulse height analysis circuit for removing scattered gamma ray components and correcting "unevenness" in the energy response characteristics of a detector. Occasionally,
Since the maximum and minimum values of the correction data are detected and the window range of the pulse height analysis circuit is controlled based on these values, the window range is determined each time the maximum and minimum values of the correction data are detected. Become. Therefore, the window range does not need to be set wide in advance in consideration of secular changes in the "unevenness" of the energy response characteristics, unlike conventional cameras.
It has the effect of being able to maintain the necessary minimum width and improving the count rate characteristics.

[Brief explanation of the drawing]

Figure 1 shows how the distribution (energy spectrum) of the energy signal of incident gamma rays changes depending on the position within the camera field of view, and Figure 2 shows the energy response characteristics of the scintillation camera detector. FIG. 3 is a block diagram showing an embodiment of the scintillation camera according to the present invention, and FIGS. 4 and 5 are diagrams for explaining the operation of the camera. It is a diagram. 1...Detector, 4...Addition circuit, 5...1st PHA, 6
…Position calculation circuit, 7…ADC, 8…CRT, 9…Memory circuit, 10…Second PHA, 11…Window setting circuit, 12…Correction data creation circuit, 102…Energy signal, 103…Position signal, 14…Correction data , 106, 107...window signal, 108...window correction signal, W1...window of first PHA, W
2...Second PHA window.

Claims (1)

[Claims] 1 Equipped with a wave height analysis circuit to remove scattered gamma ray components, calculates the position only for the gamma rays that enter the window, and measures the distribution of the size of the energy signal according to the position in advance. In a scintillation camera that uses the obtained correction data to correct "unevenness" in the energy response of the detector, when obtaining the correction data, the maximum and minimum values of the correction data are detected, and the A scintillation camera characterized by comprising window range control means for controlling the window range of the wave height analysis circuit according to a value.