JPH0452421B2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to a scintillation camera.

A scintillation camera includes a flat scintillator and a number of photoelectric converters optically coupled to the back surface of the scintillator via a light guide.
The principle is to output a pulse from each photoelectric converter in response to the scintillation light generated when radiation enters a scintillator, and calculate the radiation incident position based on the magnitude of the peak value of the output pulse of each photoelectric converter. It is something to do. Therefore, if two or more radiations are incident at the same time, the output pulses will overlap, making accurate position calculation impossible. Therefore, conventionally, a pileup removal circuit has been provided which detects when two or more radiation incidences overlap in time, outputs a pileup signal, and removes the position calculation result using this pileup signal. Therefore, there is a problem that the counting rate cannot be increased. Specifically,
The output pulse occurs for about 1 μsec, so for example, the first
When radiation is incident on two points 12 and 13 within the field of view 11 in the figure (approximately corresponding to the planar shape of the flat scintillator) at an interval of 500 nsec, the output pulses 21 and 22 overlap as shown in Figure 2. Therefore, the calculation of both positions is removed by the pileup signal. Therefore, the maximum counting rate is usually
It is about 200 kilocounts/sec.

In view of the above, the present invention is configured such that even if a plurality of radiations are incident successively in time, and the incident positions are far apart, position calculations are performed independently by a plurality of position calculation circuits. An object of the present invention is to provide a scintillation camera that improves counting rate characteristics by counting things that were conventionally omitted.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 3, each output pulse of a photomultiplier tube 31, which is a large number of photoelectric converters, is sent to a switching circuit 33 via a preamplifier 32, and in this switching circuit 33, each output pulse is divided into two In each of the position calculation circuits 34 and 35,
It is designed to be distributed sequentially for each incident radiation. Individual radiation incidences are detected as follows. Each output pulse from each of the preamplifiers 32 is sent through each resistor 41 to an adder 42, where they are added together to obtain a summation signal of the output pulses. This summation signal consists of a capacitor and a resistor. After being differentiated by the differentiating circuit 43, it is compared with the reference voltage V2 by the comparator 44, and the comparator 44 outputs a pulse corresponding to each incident radiation.
This pulse is sent to a flip-flop 45, which repeats inversion every time radiation is incident.

The switching circuit 33 includes a delay circuit 51, analog switches 52 and 53, a comparator 54, AND circuits 55 and 56, and monostable circuits 57 and 58. A comparator 54 compares each output pulse of the photomultiplier tube 31 that has passed through the preamplifier 32 with a reference voltage V1, and when the peak value of each output pulse is greater than or equal to the reference voltage V1, the output is output to AND circuits 55 and 56. send to
At this time, one of the two AND circuits 55 and 56 must receive a "1" from the flip-flop 45.
is input, and this "1" is the input side.
An output is generated from the AND circuit, and the monostable circuit 57,5
One of the analog switches 52 and 53 is closed through one of the switches 8 and 8. Next, when radiation is incident, the flip-flop 45 is inverted and the other AND
The other of analog switches 52, 53 is closed so that an output is produced from the circuit.

In this way, each output pulse is sequentially distributed to each of the two position calculation circuits 34 and 35 each time radiation is incident.

Here, the monostable circuits 57 and 58 determine the time when the analog switches 52 and 53 are closed.
This time is usually set to 1 μsec or less. The delay circuit 51 delays the output pulse in accordance with the timing delay when the analog switches 52 and 53 are closed.

Due to the function of the comparator 54, only output pulses having a predetermined value or more are used for position calculation. Therefore, for example, when radiation is incident on point 12 in FIG. 4, a large output pulse from the photomultiplier tube 31 located in the area surrounded by the dotted line 14 in the vicinity is sent to the position calculation circuit 34, for example. If radiation is subsequently incident on a distant point 13, then the output pulse from the photomultiplier tube 31 within the dotted line 15 in the vicinity is sent to the other position calculation circuit 35.
Therefore, as shown in FIG. 5, these output pulses 21 and 22 are sent independently to the respective position calculation circuits and do not overlap with each other. As a result, even if these two radiation incidences overlap in time, they are counted at their respective positions, improving the count rate characteristics.

Note that a sample-and-hold circuit is used to create one position signal from two position signals output from the two position counting circuits 34 and 35. 2
Two position signals may be sent to one display device and combined on one display screen.

By the way, in the above example, if radiation is incident on the third point 12 again, the output pulses sent to the same position calculation circuit may overlap, and in such a case, the output pulses sent to the same position calculation circuit may overlap. Accurate position signals cannot be obtained. Therefore, a pileup removal circuit similar to the conventional one is provided inside each of the position calculation circuits 34 and 35.

Although two position calculation circuits are used in the above embodiment, it is of course possible to use three or more.

As described above with respect to the embodiments, according to the present invention, it is now possible to calculate the position and count even radiation incident that was not counted due to pile-up in the past, so the count rate characteristics are better than in the past. can be improved. Furthermore, since only the large output pulses of the photoelectric converter are used for position calculations, small output pulses that contain more noise can be removed from position calculations, and the spatial resolution of images can be improved at the same time. I can do it.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a radiation incident point for explaining a conventional example, FIG. 2 is a signal waveform diagram for explaining a conventional example, and FIG. 3 is a block diagram showing an embodiment of the present invention. FIG. 4 is a schematic diagram showing a radiation incident point for explaining the operation of the same embodiment, and FIG. 5 is a signal waveform diagram for explaining the operation of the same embodiment. 11... Field of view, 12, 13... Radiation incidence point,
31...Photomultiplier tube, 32...Preamplifier, 3
3...Switching circuit, 34, 35...Position calculation circuit,
42... Adder, 43... Differential circuit, 44, 54
... Comparator, 45 ... Flip-flop, 51 ...
...Delay circuit, 53, 53...Analog switch,
55, 56...AND circuit, 57, 58...monostable circuit.

Claims (1)

[Claims] 1 A scintillator and a large number of photoelectric converters optically coupled to the back surface of the scintillator, each photoelectric converter outputs a pulse in response to scintillation light generated when radiation enters the scintillator, and each photoelectric converter outputs a pulse. In a scintillation camera that calculates a radiation incident position based on the magnitude of each peak value of an output pulse of a scintillation camera, a sum signal is obtained by adding all the output pulses of at least two position calculation circuits and each of the photoelectric converters. adder,
It includes a differentiating circuit that differentiates this summation signal and a comparator that compares the differential signal output from this differentiating circuit with a reference voltage, and when the differential signal is larger than the reference signal, the signal output from the comparator is applied to the radiation input. a radiation incidence detection circuit for generating a detection signal; and controlled by the radiation incidence detection signal, the output pulses of each of the photoelectric converters are sequentially distributed to each of the at least two position calculation circuits for each radiation incidence. a switching circuit that includes a comparator that detects that the output pulse of each photoelectric converter is greater than a predetermined value, and transmits only the output pulse that is greater than or equal to the predetermined value. A scintillation camera characterized by: