JP2699474B2 - PMT gain adjustment method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of adjusting a gain of a PMT (photomultiplier tube), and particularly to a ring type ECT device employing an Anger type positioning method. A method for adjusting the gain of a number of PMTs.

[Conventional technology]

In a ring-type ECT device employing an anchor-type positioning method, one or a plurality of integrated scintillators are formed in a ring shape, and a plurality of PMTs are arranged for one scintillator outside thereof. The radiation incident position in one scintillator is obtained by the Anger method, that is, the method of weighting and adding each PMT output according to the position.

[Problems to be Solved by the Invention] By the way, in the ring type ECT device of the anchor type,
If the gains of a large number of PMTs are not uniform, the positioning accuracy will be poor, and there will be a problem that large distortion will occur in the reconstructed image. However, in reality, the gain of many PMTs fluctuates even when the same high voltage is applied, and changes over time, so it is necessary to adjust the gains of the PMTs.

The applicant has already proposed a method of adjusting the gain of each PMT by inserting a ring-shaped radiation shielding plate provided with a radiation transmission hole corresponding to the center position of each PMT inside the ring scintillator. According to this adjusting method, the gain of each PMT can be adjusted accurately and the gain of each PMT can be made uniform, but a radiation shielding plate provided with radiation transmitting holes is indispensable. ， Trouble adjusting.

In view of the above, an object of the present invention is to provide a PMT gain adjustment method capable of simply and accurately adjusting a PMT gain without using a radiation shielding plate.

[Means for solving the problem]

In the PMT gain adjustment method according to the present invention, first, a source is placed inside a ring-type scintillator. And
After eliminating one gain adjustment circuit provided for each PMT and setting the gain to zero, a signal obtained by adding a plurality of PMT outputs arranged for one scintillator is guided to, for example, a single channel analyzer, and the gain adjustment circuit is provided. The number of times of entering a predetermined window is counted every time while increasing the gain of a small number sequentially.

Then, by taking the count into the CPU and determining the rate of change in the count, the gain of the PMT can be calculated from the calculated rate, and the gain of the gain control circuit that is not set to zero in the calculated gain is adjusted. This operation is repeated for each of the adjustment circuits.

[Action]

If a gain adjustment circuit is connected to each of a large number of PMTs and the gain is zero except for one of the PMTs, the signal obtained by adding the multiple PMT outputs in one scintillator is connected to that one gain adjustment circuit Only one PMT output is supported.

In this case, the plurality of scintillators are one PMT
If optically coupled, the output is an output corresponding to the radiation incident on the entire surface of the plurality of scintillators.

Then, the above-mentioned added signal of the PMT output is led to, for example, a single channel analyzer, and the number of times the signal enters a predetermined window is counted while changing the gain of the gain adjustment circuit, and the PMT gain is calculated from the change rate of the count.
If the gain of one gain adjustment circuit that is not zero is adjusted to the calculated gain, the gain can be adjusted.

If this operation is repeated for all the gain adjustment circuits, the gain is uniform for all of a large number of PMTs, the positioning accuracy of the Anger method is increased, and the distortion of the reconstructed image can be removed.

〔Example〕

In FIG. 1, a ring-type ECT device that performs position calculation using an anger method includes a scintillator (1) integrally formed in a ring shape and a cylindrical light guide (2) outside the scintillator.
And a number of PMTs (3) arranged via the
The scintillator (1) is made of, for example, a NaI crystal.

As shown in FIG. 2, the output of each PMT (3) is sent to an anchor-type position calculation circuit via a gain adjustment circuit (7) and an amplifier (8), and to an addition circuit (9). Then, the outputs of all PMTs (3) are added to obtain an energy signal α. This energy signal α is subjected to wave height classification by a window comparator (single channel analyzer) (10) to determine the energy of the incident radiation, and is input to the CPU (11), whereby position calculation and counting are performed only for radiation of a specific energy. Is performed. The energy signal α is sent to the position calculation circuit and used for normalizing the position signal.

The upper level of the window comparator (10)
The UL and the lower level LL correspond to the energy of the photopeak when the radiation emitted from the reference source placed inside the ring-type scintillator during gain adjustment is incident near the center of the PMT photocathode. It is set by the window voltage generation circuit (12) controlled by the window width adjustment signal from (11).

When adjusting the gain, a reference radiation source (4) is placed inside the ring-type scintillator (1). Radiation emitted from the radiation source (4) is incident on the entire surface of the scintillator (1) and produces a photoelectric effect.

If the gain of the first PMT (3) is adjusted first, other gain adjustment circuits (7) other than the gain adjustment circuit (7) connected to the first PMT (3) are used. In 7), the gain is controlled to zero. Then, only the output of the first PMT (3) is added to the adder (9).
From the energy signal α.
This corresponds to the output of the PMT (3). However, since radiation is incident on the entire surface of the first PMT (3), the output of the first PMT (3) is equal to the output of the first PMT (3).
When the radiation is incident on the position corresponding to the center of the wave, the wave height becomes the highest, and the wave height becomes lower in other portions. However, the peak is constant. First, the energy signal α is sent to the window comparator (10) to detect whether or not the peak of the waveform is between the upper level UL and the lower level LL of the window as shown in FIG. If the number is counted. The upper level UL and the lower level LL of this window are set corresponding to the energy of the photopeak when the radiation emitted from the radiation source (4) is incident on the scintillator near the center of the PMT photocathode. The gain of the gain adjustment circuit 5 that is not set to zero is sequentially changed, and for example, the gain setting values are g ₁ , g ₂ ,... G _i .
… G _n , the count value in each unit time
Assuming that c ₁ , c ₂ ,..., c _i, ... c _n , a graph shown by a solid line in FIG. 4 is obtained. Then, a dashed graph close to a Gaussian distribution with the vertex P is obtained. That is, the point at which C _k −C _{k + 1} ≦ A (A is a threshold) is first detected in the solid line graph of FIG. 4 is defined as p, and the gain setting value c _gi closest to ｐ of p is found. By adjusting the adjustment circuit (7), the adjustment of the gain for capturing the photoelectric peak is completed.

When the gain adjustment for the first PMT (3) is completed in this way, the gain adjustment for the second PMT (3) is performed by the same operation, and further, the next PMT (3), and so on. The gain adjustment operation is sequentially performed for (3), and when all the PMTs (3) are completed, the gain has been adjusted to the same value for all the PMTs (3).

After the gain adjustment is completed, if clinical measurement is to be performed on an actual subject, the source (4) is removed, and a collimator is attached inside the scintillator (1) instead. insert. each
Since the gain of PMT3 has been adjusted, a reconstructed image with good uniformity and linearity can be obtained.

In addition, in the above description, the scintillator (1) is constituted by one piece integrally formed in a ring shape, but may be divided into several parts and joined to form a ring shape as a whole. In this case, it is particularly important that the gain be the same within the group of PMTs optically coupled to the scintillator of each part, since it is important to determine the positioning accuracy within the scintillator. The above-described gain adjustment may be performed only within a group.

In the above description, the adjustment of the gain of many PMTs in the ring-type ECT device has been described. However, this PMT gain adjustment method uses the PMT gains of all other radiation position detection devices that perform positioning calculation by the Anger method. Available for adjusting.

〔The invention's effect〕

According to the PMT gain adjustment method of the present invention, the gain can be accurately adjusted for all of a large number of PMTs with a simple operation, and as a result, the positioning accuracy of the Anger method is improved,
The distortion of the reconstructed image can be removed.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of one embodiment of the present invention, FIG. 2 is a circuit diagram of the embodiment, FIG. 3 is a waveform diagram of an energy signal, and FIG. 4 is a characteristic showing a relationship between a gain setting value and a count. FIG. DESCRIPTION OF SYMBOLS 1 ... Scintillator 2, 2 ... Light guide 3 ... PMT 4, Source 7 ... Gain adjustment circuit 8, Amplifier 9 ... Addition circuit 10 ... Window comparator 11 ... CPU 12 ... Window voltage Generator circuit

Claims (1)

(57) [Claims] 1. One or a plurality of integrated scintillators are formed in a ring shape, and a plurality of PMTs are arranged for each scintillator outside the ring-type scintillator to form one scintillator. In a ring-type ECT device in which the radiation incident position in the inside is determined by the Anger method, with the radiation source placed inside the above-mentioned ring-type scintillator, the gain is reduced to zero by removing only one gain adjustment circuit provided for each PMT. Then, the number of times that a signal obtained by adding a plurality of PMT outputs in one scintillator enters a predetermined window is counted while changing the gain of the gain adjustment circuit, and the PMT gain is calculated from the change rate of the count number. The operation of calculating the calculated gain and adjusting the gain of a gain adjustment circuit for which the gain is not set to zero is repeated for each of the gain adjustment circuits. And PMT
Gain adjustment method.