JPS62278479A - Measurement of radiation distribution - Google Patents

Abstract

PURPOSE:To reduce the total measuring time, by controlling the measuring time relying on the counting dosage rate of radiation at a desired position in a perimeter of an object to be measured. CONSTITUTION:A radiation detector 3 is moved with a driver 4 along the length of a person 1 to be inspected to detect a primary radiation in a body-heightwise. Here, at M equally-divided positions in the height of the person, the radiations inputted from respective input units 11 are measured for a time T0 (fixed). An output of the detector 3 is analyzed in the pulse height with a counter 7 and the nuclide of the radiation existing in the person 1 being inspected is identified with an arithmetic unit 9 based on the resulting pulse height distributions. Then, the body-heightwise primary counting rate is calculated with an arithmetic unit 10 from the photoelectric peak area of the pulse height distributions obtained at respective positions and the measuring time and the results are displayed on a display unit 12. Thus, the dosage rate counting efficiency of the radiation is measured efficiently thereby reducing the total measuring time.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to radioactivity distribution measurement, and particularly to a radioactivity distribution measurement method suitable for shortening the total measurement time.

[Conventional technology]

81 If the radioactivity intensity to be tested is weak,
As described in Japanese Unexamined Patent Publication No. 57-175272, an effective method is to obtain the radioactivity distribution inside the measurement object using the response function method. That is, the radiation count rate is measured around the object to be measured, and the radioactivity distribution inside the object to be measured is determined by analysis using a response function determined in advance based on the measurement data. However, no consideration was given to the radiation measurement time at any position around the object to be measured.

[The problem that the invention attempts to solve]

The above-mentioned conventional technology does not take into account the radiation measurement time at any position around the object to be measured, and when the radioactivity within the object is localized, Position, ie.

There is a risk that the radiation measurement time at a position with a low count rate and the measurement time at a position with a high count rate (close to localized radioactivity) will be the same, and the overall measurement time will become longer. there were.

In view of the above drawbacks, an object of the present invention is to provide a radioactivity distribution measurement method that can efficiently measure the radiation count rate and shorten the overall measurement time when the radioactivity within the measurement target is localized. The purpose is to provide the method.

[Means for solving problems]

The above object is achieved by controlling the measurement time depending on the radiation count rate at any position around the object to be measured. That is, the measurement time is shortened at a position where the counting rate is low, and conversely, the measurement time is lengthened at a position where the counting rate is high.

[Effect]

The radioactivity distribution of a single nuclide can be determined as Ai that minimizes the following equation using the least squares method theorem.

A;≧0...
(2) Here, C,: the count rate measurement value A at the j position
凰: Radioactivity intensity within i-region when the inside of the body to be measured is virtually divided.

RJI: Response function (a value defined by the counting rate of the detector at position j when there is radioactivity of unit intensity in region i, and can be determined in advance by calculation or preliminary experiment) SJ: CJ's Statistical error Therefore, when the radioactivity distribution is localized, the measured value CJ at a position j where RJl is small has a small contribution to the radioactivity distribution A to be determined, so the count rate measurement value at this position is No matter how high the accuracy is, it is meaningless in terms of analysis. Conversely.

Even if the accuracy of the measurement value at this position is low to some extent, R
If the count rate measurement value at the position where jt is large is determined with high precision, the radioactivity distribution can be determined with high precision.

Here, since Ta: is the measurement time at the j position, S, = -mi;r- (constant)
... (4) If the set value S (constant) is introduced, the measurement time TJ can be shortened at a position where Ci is small, and the measurement time TJ can be lengthened at a position where CJ is large. Therefore, the overall measurement time is shortened.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an apparatus for carrying out the present invention, and is an apparatus for determining the radioactivity distribution in the human body in the height direction. 1 is a test subject with radionuclides in the body, 2
3 is a bed that supports the subject 1; 3 is a radiation detector 5, for example, a NaI (Te) detector; 4 is a drive device for driving the radiation detector 3 in arbitrary steps in the height direction of the subject 1; 5; is the driving bag ffi! Rails supporting 4, 6 and 6
' is a rod that supports the rail 5. 7 is a counting device that counts electrical signals from the radiation detector, and 8 is a control device that controls the drive device 4. Reference numeral 9 denotes a calculation device that calculates the distribution of radionuclides in the height direction of the subject 1 from the output signal of the counting device 7 and the output signal of the control device 8. , 10 is an arithmetic device that determines the radiation measurement time at the height direction position of the subject 1, 11 is an input device that inputs the measurement time, etc., 12
is a display device that displays calculation results such as radioactivity distribution.

Next, the most important arithmetic device in the present invention 9.10
The calculation procedure will be explained in detail.

The driving device 4 moves the radiation detector 3 in the height direction to detect radiation in the height direction. At this time, input devices at the positions where the height is divided into M equal parts! To constant waiting time inputted from 11) radiation is measured. The output of the radiation detector 3 is subjected to pulse height analysis in the counting device 7, and based on this pulse height distribution, the arithmetic device 9 identifies the radioactive nuclide present in the subject 1. Next, for each nuclide, the counting rate in the height direction-dimension can be calculated from the photoelectric peak area of the wave height distribution at each position and the measurement time. Hereinafter, the explanation will be limited to the case where a single nuclide exists in the subject 1. This is because counting rates can be calculated independently for multiple nuclides.
This is because it is sufficient to repeat the single-nuclide method multiple times.

The radioactivity distribution can be determined as Ai (single nuclide) that minimizes the following equation using the principle of least squares method.

Here, the subscript O means the first rough measurement, and the meaning of each variable is the same as in equation (1). However, M≧N.

It is. The response function RJ 1 is calculated in advance by preliminary measurement as the counting rate given to the detector at the j position when the subject 1 is virtually divided into N equal parts in the height direction and the unit radioactivity intensity is present in the i region. Or it can be calculated. That is, it can be determined by multiplying the counting efficiency of the detector, the geometric efficiency determined by the solid angle of looking into the detector at the j position from the i region, and the radiation transmittance within the subject.

Therefore, by rough measurement, the total amount X of radioactivity present in the subject 1 and the error 7 due to the counting rate statistical error at this time can be determined by the following equation.

Here, HIJ is a constant determined from the least squares method of equation (3), and is a function of RJI.

Letting the statistical error of the count rate measurement value at each measurement position be S (constant), the following is obtained from equations (6) and (7). ;7
/τ is the accuracy for determining the radioactivity distribution, and the input bag ffi! 1
If you input from 1, you can determine the number from the above formula.

Next, based on this, the measurement time TJ at each position is
Find it as follows.

Based on this measurement time, precision measurement is performed again, that is, CJ 1 is determined, and the radioactivity distribution Ai is determined in the same manner as above.

FIG. 2 schematically represents the measurement time at each measurement position, assuming that there is a point source within the subject 1. The measurement time is longer nearer to the source and shorter as it is farther away from the source.

In the above embodiment, the measurement time for each position was calculated by rough measurement, but the value of equation (10) may be inputted in advance from the input device 11. In this case, rough measurement becomes unnecessary and the measurement time can be further shortened.

Further, in the above embodiments, only -dimensional distribution measurement was targeted, but the present invention can also be applied to three-dimensional distribution measurement in which the radiation detector is movable in the circumferential direction of the human body. In this case, the subject 1 may be divided three-dimensionally.

Furthermore, although the above embodiment has a configuration in which the detector is moved, the measurement of the radioactivity distribution is the same even if the configuration is such that the subject 1 is moved.

〔Effect of the invention〕

According to the present invention, the overall measurement time can be shortened without significantly deteriorating the accuracy of radioactivity distribution measurement values. For example, when the radioactivity is localized, this method has a particularly large effect, and the total measurement time can be reduced to 1/2 or less compared to conventional methods.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a diagram showing the distribution of measurement times according to the present invention. DESCRIPTION OF SYMBOLS 1... Subject, 2... Bed, 3... Radiation detector, 4... Drive device, 7...-Counter, 8... Drive control device, '9... Radioactivity distribution computing device, i
o...Measurement time calculation device/

Claims (1)

[Claims] 1. In a device that measures the count rate distribution around the object to be measured using a radiation detector and calculates the radioactivity distribution inside the object based on the measured value, statistics on the measured value of the count rate at any position. A radioactivity distribution measuring method characterized by measuring the count rate distribution around the measurement object so that the error is less than or equal to a set value.