JPH03211486A - Scattered ray removing and correcting method for scintillation camera - Google Patents

Abstract

PURPOSE:To facilitate data processing and to shorten the processing time by sectioning the crest value of the output signal of the scintillation camera into two areas and gathering data, and making scattered ray removal corrections at respective points of an image. CONSTITUTION:The output signal 4 of a gamma-ray detection part 3 is sent to a position calculating circuit 5 to calculate the XY coordinates of a gamma- ray incidence position. Two energy windows are set for the data gathering and the smoothing processes (F1 and F2) are performed by convolution arithmetic in this case. Then N1 and N2 which are found previously by the smoothing processes are used to calculate g11 and A1 and the g11A1N1 and N1 are used to calculate g11A1. In this case, statistical variation in g11A1 is controlled with only the N1 since the statistical variation of the N1 and N2 is small, so an increase in error is suppressed.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to the functions of collecting image data and processing the data in a scintillation camera that is widely applied in nuclear medicine diagnosis. This invention relates to a function that makes it possible to improve the contrast of an image by removing the contribution of scattered gamma rays contained as components.

[Conventional technology]

A scintillation camera is a device that measures gamma rays emitted by a radiopharmaceutical administered to a subject via intravenous injection, etc. outside the subject, and depicts the distribution of the radiopharmaceutical in the body as an image. Although the information provided is useful for medical diagnosis, a few problems remain. One of them is that the number of scattered gamma rays included in the image data degrades the contrast of the radiopharmaceutical distribution. For this reason, various methods have been proposed to remove this scattered radiation component. For information on this scattered radiation removal correction method, please see the video information magazine.

It is introduced in the October 1986 issue (7), pages 1034 to 1040, as part of the current status of research on improving the quantitative properties of rsPEGT. 5PECT is a single photon transverse tomographic imaging method (Single PhotonEmiss
ion Computed Toi+ography)
As of now.

It is also common to have a scintillation camera with a detection unit that rotates around the subject.

Although the scattered radiation correction method is often discussed in connection with 5PECT as shown in this document, it is fundamentally a problem related to the collection and processing of image data from a scintillation camera.

As for the scattered radiation removal correction method, it was mentioned earlier that various methods have been proposed, but all of them have advantages and disadvantages.
There is no definitive method.

This means that the distribution of scattered radiation components included in the image data of the scintillation camera is based on the size of the object.

This is thought to be due to the fact that it changes in an extremely complex manner depending on the shape and distribution of the radiopharmaceutical in the body.

Now, the scattered radiation removal correction method utilizes the following two facts. One is due to the fact that when a single point of radiopharmaceutical exists, the obtained image data will be spread around the single point due to the contribution of scattered radiation components. The spatial spread of image data due to scattered rays is measured in advance using a phantom, and this can be used to perform scattered ray removal correction. Although this is a powerful method, it has the drawback that it cannot perform correct correction when a large amount of radiopharmaceuticals are distributed near the boundaries of the subject.

Another fact that is exploited for anti-scatter correction is that scattered gamma rays have lower energy than non-scattered gamma rays.
The output signal of a scintillation camera has a low peak value. There is a method in which image data is collected corresponding to a plurality of peak values, and this image data is combined to create image data from which scattered radiation components are removed. Although this method has the disadvantage that the amount of image data to be collected is large, it has the advantage that it does not depend on the shape and size of the subject or the distribution of radiopharmaceuticals. Recently, a scattered radiation removal correction method that simultaneously utilizes the above two facts has been proposed.

When looking at the scattering #i removal correction method from the perspective of practicality, the following 5 points are considered.
points must be taken into consideration.

(1) Is it necessary to add special hardware to a normal scintillation camera?

(2) Whether it is necessary to collect other data in addition to the image data that should be collected.

(3) Is the data processing complicated and does it take a long time to process the data?

(4) Is it possible to perform scattered radiation removal correction that does not depend on the shape and size of the subject or the distribution of radiopharmaceuticals?

(5) Whether or not the statistical fluctuation of one image data increases when the scattered radiation removal correction is applied to the collected image data.

[Problem to be solved by the invention]

The objects of the present invention are as follows: 1. Data can be collected using only the functions of a normal scintillation camera, data processing is relatively simple, and the shape and size of the object can be measured.

It is an object of the present invention to provide a scattered radiation removal correction method that allows for correction of scattered radiation that does not depend on the distribution of radiopharmaceuticals, and has the characteristics of suppressing an increase in statistical fluctuations in image data caused by the scattered radiation removal correction.

[Means to solve the problem]

In order to achieve the above object, the present invention adopts a method in which data is collected by dividing the peak value of the output signal of a scintillation camera into two regions, and scattering radiation removal correction is performed at each point in the image. . Scintillation cameras currently on the market collect data by setting the peak value of the output signal within a certain range, and this is called an energy window. Energy Wind is configured to be multi-configurable and does not require additional hardware with new functionality. Furthermore, in the method adopted in the present invention, the error (statistical variation) in the calculated value may become large because the scattered radiation removal correction performs subtraction between two quantities with relatively small count values. We will adopt a method to keep this low by using data processing techniques.

[Effect]

The basics of the present invention will be explained below.

Hereinafter, the gamma rays emitted from the radiopharmaceutical in the subject will be referred to as primary rays, and the gamma rays generated when the -order rays are scattered within the subject will be referred to as secondary rays.

FIG. 3 shows an example of the wave height distribution of the output signal of the scintillation camera, where 31 and 32 represent the wave height distributions of the primary line and the secondary line, respectively, and 33 represents the sum of both. Now, it is assumed that the wave height distribution shown on the horizontal axis is divided into two regions for measurement. The region of the wave height distribution is called the energy window. In FIG. 3, numeral 35 indicates an energy window 1 that includes the photoelectric absorption peak of the primary line, and this energy window is primarily intended for counting the primary line.

36 in FIG. 3 indicates an energy window 2 set to a lower peak value than 35, and is mainly intended for counting secondary lines.

Now, let AI and AZ be the amounts of primary and secondary lines (which can also be called the number of gamma rays) at a position corresponding to one point in the image data of the scintillation camera.

When these are measured, the counts obtained in an energy window of 1°2 are expressed as Nt and N2. A1. Az and Nl
, NZ has the following relationship.

Here, gJk represents the probability that the k-th line is counted in energy window j. In the case of a scintillation camera, gJk can be considered to be constant except for the periphery of the field of view, and these can be measured in advance. . If gJb is found, Ax,
Az can be calculated. If the inverse matrix [h Jh] of the matrix [gJk] is obtained, AI and A2 can be calculated using the following equations.

Considering that Nl and NZ follow the Poisson distribution, N
+ variance σN? =Nl. Using this, A
The variance σ^l of I is as follows.

σL=hlx” σL+hsxz” Jz=htx”s
t+hzz2Nz Here, since the h-th value is almost 1, the error in A1 is larger than the error in Nl. Therefore, in order to reduce the error caused by the count value Nz, the following fact is used.

The amount A2 of scattered radiation usually does not change rapidly spatially. Therefore, the smoothed version of A2 obtained for each point is close to the actual value. A spatially smoothed version of A2 is represented by A2. AX is obtained by weighting and averaging the values of the point currently being considered and the points surrounding it. This can be performed as a smoothing process using a spatial filter. Examples of smoothing spatial filters are shown in FIGS. 4(a) and 4(b).

A2 is expressed by the following formula.

Az=hzx+Ns+hzz' NZNs and Nz are obtained by subjecting Nt and Nz to spatial filter processing, respectively.

AI may be determined using this Az. Here, if gllAx is determined as a kite corresponding to N1 instead of A1, the following equation is obtained.

g1 Emperor Al: N1-g+zAz=N+-N1+gtx(
httNs+h 52N2)" In the introduction of the two equations, g l+ h tt+ g 1zh zl= 1 +g
The following relationship is used: ++ h 12+ g tzh zz=0.

Compared to the following equation that uses A2 instead of A2, NINm has a smaller variance than Nll Nz.
1 error is small.

g LIA1= Nt −N1+ g tt(h xt
Nt+ h tzNz) [Example] Hereinafter, an example of the present invention will be described with reference to FIGS. 1 and 2.

FIG. 2 is a diagram explaining the outline of a scintillation camera. 1 is the subject, 2 is the gamma ray emitted from the radiopharmaceutical administered to the subject, and 3 is the scintillation radiation.
The gamma ray detection section of the camera 10 is composed of a collimator, a scintillator, a photomultiplier tube, etc. The output signal 4 of the gamma ray detection section 3 is sent to a position calculation circuit 5, which calculates the XY coordinates of the gamma ray incident position. On the other hand, the output signal 4 of the gamma ray detection section 3 is also sent to the energy analysis circuit 6, and it is determined which energy window the output signal corresponds to. The value of the energy window and the XY coordinates determined in this way become an address in the storage device (memory) of the data processing device 20, and a count is added to the memory corresponding to this address.

Two energy windows are set for data collection according to the present invention. One is mainly for counting primary lines, and the other is mainly for counting secondary lines. The collected data is transferred to the data processing device 20, where data processing as described below is performed.

FIG. 1 shows a flow of data processing based on the present invention. This processing flow is based on the method described earlier.

The image data smoothing process (Fl, F2) is called a convolution operation and is based on a first-order calculation formula.

Nt(x+y)=ΣΣF(x-x' l Y-y')
'Nt(x' + y') router, examples of which are shown in FIGS. 4(a) and 4(b). This type of arithmetic processing is often applied to image data processing for scintillation cameras, and calculation programs are prepared for data processing equipment used in nuclear medicine.

Next, the -water dose is calculated for each point of the image data (F3, F4). Nl obtained earlier by the smoothing process
, N2 to calculate gsIAx, and then g t s
gtxAi using A 11 N s v N s
Calculate. In this case, the statistical fluctuation of gztAt finally determined is dominated only by N1 since the statistical fluctuations of Nt and N2 are small, and an increase in error (statistical fluctuation) is suppressed.

〔Effect of the invention〕

The scattered radiation removal correction method according to the present invention has the following features.

(1) No special equipment is required other than the usual scintillation camera and data processing equipment.

(2) Data processing is relatively simple and processing time is short.

(3) Scattered radiation removal correction can be performed regardless of the shape and size of the subject or the distribution of radiopharmaceuticals.

(4) Increase in errors (statistical fluctuations) in image data caused by scattered radiation removal correction can be suppressed.

Although the drawback is that it is necessary to collect twice as much image data because the energy window is set twice, it is not a particular burden on a normal scintillation camera. Therefore, when considered together with the above characteristics, this is an extremely practical scattered radiation removal correction method.

[Brief explanation of drawings]

FIG. 1 shows the calculation procedure for the scattered radiation removal correction based on the present invention. FIG. 2 shows an overview of a scintillation camera and a data processing device, and is a diagram for explaining the present invention in detail. FIG. 3 is a diagram showing an example of the wave height distribution of the output signal of the scintillation camera and explaining the energy window. FIG. 4 shows an example of a spatial filter used for smoothing processing of image data.

Claims (1)

[Claims] 1. In a scintillation camera that can set multiple energy windows, image data N_1 and image data N_ were collected by setting the first energy window to a range of peak values that includes the photoelectric absorption peak of non-scattered gamma rays.
Image data obtained by applying spatial smoothing processing to 1 @
N_1@ and image data @ obtained by performing spatial smoothing processing on image data collected by setting the second energy window to a range of peak values lower than the range of peak values of the first energy window. N_2@ and a constant K_1 that depends on the characteristics of the detection part of the scintillation camera.
, K_2, calculation formula N=N1-@N_1@+K_
1. Scintillation to calculate image data N from which scattered radiation components are removed by @N_1@+K_2 and @N_2@
Camera scatter removal correction method.