JPH0287092A - Positron ct apparatus - Google Patents

Abstract

PURPOSE:To improve an S/N and the deterioration of a high counting rate characteristic by inputting only the output of detector groups in the region of measurement determined in accordance with the rotating position of a corrective radiation source into a simultaneous counting circuit. CONSTITUTION:The output of detectors in a group is converted into a binary address signal A in an address encoder circuit 29. If the signal A is in the range of the detectors l*-i*(or j*-k*) with a comparative circuit 19(or 20), an AND gate 22 is opened (or used) to output timing T23 respectively. Thereby the circuit 29 is provided every group and timing output T1-TN in each address encoder is input into a coincidence circuit 30 to perform the counting of a simultaneous counting event. An detector address in a pair of group concerned in simultaneous counting in the circuit 31 and address (binary) and strobe signals in a pair of detector groups concerned in the simultaneous counting in the circuit 30 are output. Accordingly, a scatter simultaneous counting event and an accidental simultaneous counting event which occur out of the region can be removed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a coincidence circuit for a positron CT apparatus, and in particular, to improvement of the S/N ratio of sensitivity correction data of a detector and absorption correction data of a subject. The present invention relates to a coincidence circuit suitable for improving high counting rate characteristics of the coincidence circuit.

[Conventional technology]

Conventionally, a positron CT apparatus (hereinafter abbreviated as rPCTJ) is used.

), the sensitivity correction data of the detector and the absorption correction data of the specimen are measured by using a rod-shaped or plate-shaped positron-emitting nuclide, such as 6&G, between the detector and the specimen.
This is done by placing a correction radiation source consisting of e-88G a, moving it along the tip of the detector, and counting coincidence events of annihilation radiation emitted from this radiation source. However, each of the obtained correction data contains false information such as scattered coincidence events and accidental coincidence events from outside the measurement area, and the S
This was a factor that deteriorated the /N ratio. Furthermore, since these false information and information outside the area are counted, it is also a factor that deteriorates the high counting rate characteristics of the coincidence circuit. Conventionally, to improve the S/N ratio of these correction data,
IEEE, Transaction On
Nuclear Science, 35. Nα1 (1988
) pages 735 to 739 (IEEE, Tran
s, Nucl, Sci,, Vofl.

35, No. 1. Feb, 1988. PP735-73
9) and elsewhere.

[Problem to be solved by the invention]

1. The conventional technology uses a coincidence circuit to measure 81 correction data and then removes unnecessary data outside the measurement area, so no consideration is given to the input counting rate of the coincidence circuit. , there was a problem of deterioration of high counting rate characteristics such as loss of count at high counting rates. The purpose of the present invention is to remove false data contained in sensitivity correction data and absorption correction data before inputting them to a coincidence circuit, improve the S/N ratio, and improve the deterioration of high count rate characteristics. be.

[Means to solve the problem]

FIG. 2 shows an example of a coincidence event when measuring PCT absorption correction data. In FIG. 2, 1 is the subject, 2 is a rod-shaped correction radiation source, and 3 is the position of the tip of a group of radiation detectors arranged closely circumferentially (hereinafter referred to as "
4 is the rotational trajectory of the correction radiation source, the solid arrow indicates annihilation radiation, 5, 8.9 is a true coincidence event, the dashed line 6 is a scattered coincidence event, and the dashed line 7 is an accidental one. Each shows a coincidence event. Normally, absorption correction data is obtained by moving a correction radiation source 2 placed between the detector and the subject 1 on a rotating orbit 4 around the subject 1 at a constant speed.
Subject 1 of annihilation radiation emitted from correction radiation source 2
Transmittance data can be obtained for the entire circumference.

As can be seen from FIG. 2, the annihilation radiation is isotropically emitted from the correction radiation source 2, leaving a large amount of coincidence components that do not contribute to the transmittance data. Although the measurement of the sensitivity correction data of the detector is performed by removing the subject 1, it is basically the same as the measurement of the absorption correction data, so only the measurement of the absorption correction data will be described.

FIG. 3 shows the M1 measurement area of annihilation radiation when the PCT correction radiation source 2 is located at an arbitrary position P on the rotating orbit 4. That is, in Fig. 3, annihilation radiation 10.1
The angle α formed by 0′ corresponds to the ideal measurement area when the correction radiation source 2 is located at the point P. 1+
j++Q indicates a detector corresponding to the boundary of an ideal measurement area among a group of detectors closely arranged in a circular ring. Therefore, when there is a correction radiation source 2 at point P, the detector for measuring annihilation radiation is set to Q
By limiting the range to ←1+J←, there is no need to measure unnecessary coincidence events. Therefore, the above object can be achieved by inputting only the outputs of the detector group within the ideal measurement area determined according to the rotational position of the correction radiation source 2 to the coincidence circuit.

[Effect]

FIG. 4 shows the concept of the ideal measurement area calculation method described in FIG. In FIG. 4, ro is the radius when the subject 1 is assumed to be a circle or the radius of the field of view, rl is the radius of the rotating orbit 4, and R is the radius of the detector ring 3.

If the position of the correction radiation source 2 (here represented by the rotation angle, Os) is θ80°, the angle of the detector l, Ol, is (1
) formula % formula % rz , O'≦θ, ≦360°. The angle θj of the detector j is expressed by the following equation using equation (1).

θ0 θa ” 1800+ Ot...(
2) (0°≦04≦3600) Similarly, the angle to the detector, θ, is expressed by the following equation.

O θh=1806 +--01...(3
) (0″≦θ,≦360°) The angle 0 plate of the detector Q is expressed by the following equation.

θ, = 360°−〇, ... (4) (
0'≦θ<360') In reality, there are a finite number of detectors, and they are arranged on the detector ring 3 at equal intervals. Therefore, if we arrange the detectors in symmetrical positions with respect to the vertical line passing through the center of the detector ring 3 and no detector is placed on the vertical line, the angle On made by the nth detector is It is expressed by the following formula.

0,,=(2n+1)80 ・= (4),
,'1 (0'≦On≦360°) Here, 80 is the negative angle of the angle between the detectors, n=0.1
,2.・It is. From equations (4) and (1), the △ first detector is expressed by the following equation.

Here, [ ] is the symbol for Gagara, and the reason for adding 1 to the right side is to secure the measurement area.

Similarly, for j, Qth, equation (1) and (2) ~
From equation (3), it is expressed as follows.

Therefore, when the correction radiation source 2 is at θs=o°, the measurement area should be limited to only the range of the i-th detector, the Q-th detector, and the range of the third detector. Bye. Actually, since the correction radiation source 2 rotates, it is necessary to consider the rotation angle θ3. Now, if this rotation angle is 03, the measurement area will be the range from the i*th to the Q ratio and the *th range from the jth. here,
For example, the measurement area can be limited using a ROM (read-only memory).

Here, only a rod-shaped radiation source was considered, but in the case of a plate-shaped radiation source, it is preferable to widen the measurement area by considering the width of the radiation source. In addition, a PC using multiple correction radiation sources
T can also be realized by overlapping a plurality of measurement areas and determining an area that includes all of them.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 5.6 and 7.1. FIG. 5 shows means for outputting the rotation angle of the correction radiation source 2. As shown in FIG. Clock output 2 of clock generator 24
5 is input to the stepping motor 26, and the radiation source 2 moves on the rotating orbit 4. On the other hand, the clock output 25 is input to the counter 27, and the counter 27 receives the rotation angle signal, O
Outputs 3. Figure 6 shows O3 detector], “yJ” + k
The ROMs 11 to 14 are shown to be converted into m and α books.

The contents of this data conversion are determined by equations (9) to (12). FIG. 7 shows the configuration of a circuit called an address encoder circuit. Normally, in PCT, in order to simplify the coincidence counting circuit, a method is adopted in which the detector group is divided into several groups and the coincidence counting between the groups is measured.

The address encoder circuit converts the outputs of the detectors in the group into 2
It converts into a decimal address signal and outputs a timing signal representative of the group. In the figure, 15 is the output of the detector group belonging to the group, 16 is the encoder, 1
7 is the ORed detector output, 18 is the encoded detector address signal A, and 19 is a comparison circuit (1). If the address signal A is in the range of Q* to i*, the AND gate 22 is opened. and outputs a timing signal T23. 20 is also a comparison circuit (2), and the address signal A is j$~*
If it is within the range, the AND gate 22 is used to output the timing signal T23. 21 is an OR gate. FIG. 1 shows the overall configuration of a coincidence circuit according to the present invention. In the figure, address encoder circuits 29 are provided for each group, and the timing outputs of each address encoder, T1 to TN, are input to a coincidence circuit 30, where coincidence events are counted. The address circuit 31 outputs the addresses of the detectors in the pair of groups involved in the coincidence count, and the coincidence circuit 30 outputs the addresses (binary) and strobe signals of the pair of detector groups involved in the coincidence count. .

According to this embodiment, there is an effect of preventing the detector output outside the H1 measurement area defined in FIG. 4 from being input to the coincidence circuit 30.

〔Effect of the invention〕

According to the present invention, only coincidence events due to annihilation radiation within the counting area where the correction radiation source 2 looks into the subject 1 and in a point-symmetric area with this area and the correction radiation source as the center can be calculated. , a scattering simultaneous H1 number event occurring outside the region,
This has the effect of eliminating accidental coincidence events and improving the SIN ratio of correction data. Furthermore, since true coincidence events that occur outside the area can also be removed, this has the effect of minimizing deterioration of the high counting rate characteristics of the coincidence circuit.

[Brief explanation of drawings]

Fig. 1 is a coincidence circuit diagram of an embodiment of the present invention, Fig. 2 is a diagram showing types of PCT coincidence events, Fig. 3 is a diagram specifying an ideal measurement area when measuring absorption correction data of PCT, Fourth
The figure is an angle display diagram of the measurement area shown in Figure 3, Figure 5 is a circuit diagram for detecting the rotation angle of the correction radiation source 2, Figure 6 is a configuration diagram of the ROM that specifies the measurement area, and Figure 7. is a circuit diagram of an address encoder. 1... Subject or field of view, 2... Radiation source for correction, 3... Detector ring, 4... Rotational trajectory of the radiation source for correction. Fukou Qin zu zu zu bong zu zu

Claims (1)

[Claims] 1. Positron emission radiation that moves on a rotating trajectory along the tip of the radiation detector group provided in the space between the radiation detector group arranged in an annular shape around the subject and the subject. In a positron CT apparatus having a positron emission radiation source, a region where the positron emission radiation source located at an arbitrary position on the rotating orbit looks into the subject, and an area which is point symmetrical with respect to the region with the positron emission radiation source as the center. A positron CT apparatus characterized by having means for inputting only incident annihilation radiation to a coincidence circuit.