JP3018439B2 - Scintillation camera - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scintillation camera used in nuclear medicine, and more particularly, to an improvement in a counting correction.

[Prior art]

The scintillation camera emits scintillation light generated by gamma rays incident on the scintillator using a photomultiplier, a photoelectric converter, as an electrical signal (pulse signal).
The scintillator is combined with a plurality of photomultipliers, the emission position, that is, the gamma ray incidence position is calculated from the magnitude of each output signal, and the number of gamma ray incidence is counted for each incidence position. The gamma ray is used to capture an image. In this scintillation camera, the width of the pulse signal generated in response to the incidence of the gamma ray is due to the response speed of the scintillator. Therefore, when two or more gamma rays enter in close proximity in time, signal pile-up occurs.
Also, this pulse signal is integrated and prepared for later processing,
It takes time for this integration. For this reason, the counting time (the number of gamma rays incident per unit time) increases, mainly due to the time required for pile-up and integration of such signals, and counting is lost. In view of this, conventionally, a part of performing a countdown correction process by obtaining a correction coefficient for the countdown correction according to the counting rate of the scintillation camera has been performed.

[Problems to be solved by the invention]

However, in the conventional countdown correction processing, a correction coefficient is obtained based on the count rate of gamma ray incidence detected by a scintillation camera, and there is a problem that accurate countdown correction cannot always be performed. That is, a scintillation camera generally detects that the energy signal wave height is within a predetermined energy window and generates an unblank signal, and only when this unblank signal occurs, the gamma ray incidence is determined for each incident position. Conventionally, the counting rate is measured using the unblank signal, and a correction coefficient is obtained according to the measured counting rate. However, the output count rate of such a scintillation camera is far away from the ideal characteristic (dotted line) corresponding to the incident count rate, as shown by the solid line in FIG. However, as the incident count rate increases, the incident count rate decreases. Therefore, when the output count rate is equal to or higher than the incident count rate at which the output count rate becomes the maximum value, an accurate correction coefficient cannot be obtained. In the example of FIG. 4, when the output count rate is Ka, the incident count rate is Kb. Therefore, when the count value of the scintillation camera is multiplied by a correction coefficient of (Kb / Ka), the count-down correction is performed. However, when the output count rate is Ka, the actual incident count rate is Kb when the output count rate is Ka as shown in the solid line in FIG.
Or Kc, it is difficult to determine whether the correction count is (Kc / Ka) or not (Kb / Ka), and the output count rate is equal to or greater than the incident count rate at which the output count rate reaches the maximum value. In this case, the counting correction cannot be performed accurately. SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a scintillation camera which has been improved so that accurate counting correction can be performed.

[Means for Solving the Problems]

In order to achieve the above object, in a scintillation camera according to the present invention, a scintillator, a plurality of photoelectric converters coupled to the scintillator, and energy discrimination and position calculation to which respective outputs of the plurality of photoelectric converters are input. Circuit, a counting circuit that counts each position signal from the energy discrimination and position calculation circuit in accordance with the unblank signal obtained by the energy discrimination and position calculation circuit, and outputs each of the plurality of photoelectric converters. An adder for adding, a discrimination circuit for discriminating an output of the adder from a noise level, a circuit for measuring a count rate from an output of the discrimination circuit, and a circuit for obtaining a correction coefficient based on the measured count rate. ,
A circuit for correcting the count of the counting circuit according to the correction coefficient.

[Action]

Each time an unblank signal occurs, a count is made for each position signal. That is, each time radiation enters the scintillator, a count is made for each incident position. This forms an image due to the radiation. On the other hand, the respective outputs of the plurality of photoelectric converters coupled to the scintillator are added, and the added output is discriminated from the noise level. Thus, no matter what kind of energy radiation is incident, the incident radiation is detected. Then, the count rate is measured from the discrimination output. The count rate measured in this way is close to the count rate of incident radiation (the number of actually incident radiation per unit time) and monotonically increases with the incident count rate, and corresponds to the incident count rate on a one-to-one basis. ing. Therefore, the incident count rate can be accurately obtained from the measured count rate, and a correct correction coefficient for the count rate of the unblank signal corresponding to the incident count rate can be obtained. In accordance with this correction coefficient, the count for each incident position is calculated as 1.01 when one unblank signal is generated, for example.
For example, when one hundred unblank signals are generated, an extra unblank signal is randomly generated at a rate of one and counted and corrected. The extra count at this time is determined by the incident count rate obtained based on the above measured count rate, and further by the correction coefficient relating to the unblank signal obtained from the incident count rate. Can also perform accurate countdown correction.

【Example】

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, the human body which is the subject
It is assumed that a drug containing RI (radioisotope) has been previously administered to 11, RI has accumulated in a specific organ 12, and this RI emits gamma rays. The gamma rays enter the scintillator 22 through the collimator 21 and cause scintillation light emission. This light is guided by the light guide 23 to each of the many photomultipliers 24. Each photomultiplier 24 generates an electric signal corresponding to the light incident thereon. This electric signal becomes pulsed in response to scintillation light emission due to the incidence of gamma rays,
In addition, the magnitude of the signal increases as the position is closer to the light emission position. The output of each photomultiplier 24 is sent to the energy discrimination and position calculation circuit 31. Here, all the outputs are added to form an energy signal, and a discrimination is made as to whether or not the wave height of the energy signal is within a predetermined energy window. Further, since the magnitude of each output of the photomultiplier 24 depends on the gamma ray incident position as described above, the incident position is calculated from the magnitude of each output, and the position signals X and Y are obtained. The position signals X and Y are sent to the memory 32 and used as address signals. When the unblank signal and the position signals X and Y occur at the same time, the unblank signal passed through the multiplier 33 is added to the value read from that address of the memory 32 in the adder 34 and stored again at that address. You. Thus, the number of gamma rays that have entered is counted at each address of the memory 32 for each incident position. When counting for a certain time is completed, an image based on gamma rays is reproduced in the memory 32, and the image is taken into the CPU. On the other hand, the outputs of a number of photomultipliers 24 are sent to an adder 41 to be added, and the added output is sent to a discriminator 42. As shown in FIG. 2, the detection level L of the discriminator 42 is set close to the limit so that the signal can be distinguished from the noise level. That is, since the added signal has a peak value corresponding to the energy of the incident radiation, the energy discrimination and position calculation circuit
At 31, the discrimination as to whether or not the radiation is in the predetermined energy window W is made.However, it is determined by the adder 41 and the discriminator 42 that radiation of any energy is incident separately. The detection is performed at an early timing, and the output of the discriminator 42 is sent as a timing signal to the energy discrimination and position calculation circuit 31 to start the processing of the energy discrimination and the position calculation. As described above, since the output of the discriminator 42 is the basis of the operation timing of the circuit, it needs to be generated as early as possible when radiation is incident. Therefore, the detection level L of the discriminator 42 is As described above, the signal is set to a level as low as possible so that the signal can be distinguished from the noise level. In this embodiment, the output of the discriminator 42 is sent to the counting rate measuring circuit 43, and the counting rate is measured. That is, the count rate is measured as the number of times the detection level L has been broken per unit time. Therefore, the characteristic of the output count rate (measurement count rate) thus obtained is close to the ideal output count rate characteristic (shown by a dotted line) completely corresponding to the incident count rate, as shown by the solid line in FIG. It increases monotonously with the incidence count rate. This is because, since the detection level L is lowered to the limit, the radiation incidence can be detected at an extremely early stage of the radiation pulse signal, and the influence of the pile-up of the signal and the integration time is less. . The correction coefficient calculation circuit 44 calculates a correction coefficient based on the count rate thus measured. For example, when the measured counting rate is Kd as shown in FIG. 3, the counting rate Ke of the actually incident radiation is obtained from the solid line in FIG. 3. From this, the correction coefficient (Kb / Ka) is calculated based on the solid line in FIG. , Or Kc / Ka). The correction coefficient calculating circuit 44 can be composed of a storage circuit for storing the plots of the solid line in FIG. 3 and the solid line in FIG. 4, a circuit for interpolating and calculating values between the plots, and the like. That is, the unblank signal obtained in the energy discrimination and position calculation circuit 31 is obtained when the wave height of the added signal of the outputs of all the photomultipliers 24 enters the predetermined energy window W, as described above. Therefore, the generation timing is late, the signal pile-up is affected, and the integration time is affected. As a result, the output count rate of the unblank signal is as shown by the solid line in FIG. Therefore, based on the incident count rate obtained from FIG. 3, a correction count for correcting the output count rate of the unblank signal is obtained from the solid line in FIG. 4, and this correction count is multiplied by the multiplier 33 to the unblank signal. By adding the calculated values, counting without counting down can be performed, and the counting down correction can be accurately performed. This correction coefficient is a value including a decimal part such as 1.01, for example. When the count-down correction is not performed, the value is added one by one according to one radiation incidence. In addition, instead of performing the countdown correction using the multiplier 33 as in this embodiment, the correction coefficient is set to 1.01 as described above.
, The probability according to the correction coefficient, that is, 10
Other counting-down correction circuits, such as a circuit that randomly generates two unblank signals from one unblank signal with a probability of once per 0 count, can also be used. Further, similar counting-down correction according to the correction coefficient can be performed by software. Further, the counting rate of the unblank signal may be separately measured, and the correction coefficient may be obtained using these two measured counting rates. In the above embodiment, the counting rate is measured from the output of the discriminator 42 for generating the timing signal. However, an adder and a level discriminating circuit (comparing circuit) may be separately provided for measuring the counting rate. Good (that is, in the above embodiment, the adder 41 for generating the timing signal and the discriminator 42 are also used for counting rate measurement).

【The invention's effect】

ADVANTAGE OF THE INVENTION According to the scintillation camera of this invention, the precision of a countdown correction can be improved and the quantitative nature in nuclear medicine measurement can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, FIG. 2 is a signal waveform diagram, and FIGS. 3 and 4 are graphs showing an output count rate with respect to an incident count rate. 11 ... human body, 12 ... organ, 21 ... collimator, 22 ... scintillator, 23 ... light guide, 24 ... photomultiplier, 31 ... energy discrimination and position calculation circuit, 32
…… Memory, 33 …… Multiplier, 34 …… Adder, 35 …… CP
U, 41 ... Adder, 42 ... Discriminator, 43 ...
Count rate measurement circuit, 44... Correction coefficient calculation circuit.

──────────────────────────────────────────────────続 き Continued on the front page (56) References JP-A-60-174974 (JP, A) JP-A-63-37283 (JP, A) JP-A-59-137874 (JP, A) JP-A-2- 52278 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01T 1/164 G01T 1/17

Claims (1)

(57) [Claims] 1. A scintillator, a plurality of photoelectric converters coupled to the scintillator, an energy discriminating and position calculating circuit to which respective outputs of the plurality of photoelectric converters are input, and an energy discriminating and position calculating circuit. A counting circuit for counting each position signal from the energy discrimination and position calculation circuit in accordance with the obtained unblank signal; an adder for adding each output of the plurality of photoelectric converters; and an output of the adder A circuit for measuring a counting rate from an output of the discriminating circuit, a circuit for obtaining a correction coefficient based on the measured counting rate, and a circuit for calculating the correction coefficient based on the correction coefficient. A circuit for correcting the count.