JPH0990043A - Positron ct device and its picture reconstructing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct the body movement of a subject even when the subject moves during measurement and, at the same time, to obtain an accurate reconstructed picture by performing sensitivity correction with accuracy. SOLUTION: A position CT detects photon pairs generated from a ray source 110 for correction by means of a ring 121 and acquires projection data stored in an x-y-θ-ψ memory 150 as reference sensitivity correcting data. Then the CT device measures the relative position of a subject 100 to the ring 121 and the azimuth to the subject 100 from the ring 121 by means of a body movement measuring section 123, generates the existence density distribution of the subject 100 by means of an existence density distribution generating section 190, and acquires measurement data by compensating the projection data with respect to the subject 100 for body movement. Thereafter, the CT device generate sensitivity correcting data by compensating the body movement of the subject 100, accurately correct the measurement data for sensitivity from the correcting data, and obtains an accurate reconstructed picture by means of a picture reconstructing device 160.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention detects a photon pair emitted upon annihilation of an electron-positron pair generated by an RI radiation source placed in an object to be measured to detect a substance in the object to be measured. The present invention relates to a positron CT apparatus for measuring distribution and an image reconstruction method thereof.

[0002]

2. Description of the Related Art Positron Emissio
n Computed-Tomography; hereinafter referred to as PET) has been applied to the study of living organisms and diseases, clinical examinations, etc., and has been introduced into the body as a positron-emitting nuclide (hereinafter referred to as RI source).
Is a device for visualizing the distribution of the, and observing biological functions.

[0003] RI-ray source, FDG related to glucose metabolism in dopamine and the body that are involved in neurotransmission ⁽¹⁸ F-
It is used by being partially added to an in-vivo substance such as fluorodeoxyglucose) or a drug under development, for example. PET makes it possible to observe the distribution, consumption, or temporal change of such a substance in a living body. PET can also measure basal metabolism of the living body such as cerebral blood flow and oxygen consumption.

The detector of such a PET is composed of a large number of photon detectors arranged in a ring shape (this is called a ring), and a human body in which an RI radiation source is injected or inhaled into the ring. An object to be measured is placed. Positrons emitted from the RI radiation source in the object to be measured are immediately combined with nearby electrons and each has an energy of 511 keV.
Paired photons (gamma rays) are emitted in opposite directions.
By simultaneously counting the pair of photons by the ring, it is possible to specify on which straight line the electron-positron pair annihilation occurs. PET accumulates such coincidence count information and performs image reconstruction processing to create a distribution image of the RI radiation source.

The time required for measurement by such a PET depends on the half-life of the RI radiation source and may extend to several tens of minutes to several hours. Conventionally, the object to be measured is positioned and positioned within this measurement time. Both directions had to be fixed. In this way, when the measurement time is long, it is very painful for the human body and other animals as the measurement object, and the position and orientation of the measurement object may change due to movement within the measurement time. is there. If the measurement object moves even a little during measurement, accurate measurement cannot be performed, and the image obtained by image reconstruction has an artifact due to the movement of the measurement object, and an accurate reconstructed image is obtained. I couldn't.

Even if the object to be measured could be fixed, since the object to be measured often measures its physiological state in the awake state rather than the anesthesia state, stress caused by long-term fixation is Therefore, the physiological state changes. This change adversely affects the measurement, and again an accurate measurement cannot be performed.

Therefore, without fixing the measuring object,
That is, the movement related to the position and orientation of the measurement target (hereinafter referred to as body movement) is allowed to be measured while maintaining the physiological state, and the body movement is measured during the measurement, and the body movement information is obtained. There is known a technique for correcting coincidence counting information by using the technique (for example, Japanese Patent Laid-Open No. 2-209133).
Japanese Patent Laid-Open No. 4-128679).

[0008] In order to improve the spatial measurement resolution of the photon detector of the ring, the ring is rotated or wobbled, but the measurement resolution may also be increased by moving the object to be measured. Is improved. The body movement correction technique is also effective in this respect.

[0009]

By adopting the above-mentioned body movement correction technique, it becomes possible to obtain a more accurate reconstructed image than before even when the object to be measured moves.

On the other hand, in general, since there is unevenness in sensitivity among a large number of photon detectors forming a ring, the coincidence counting information (emission data) as it is, which is accumulated by detecting the photon pairs generated from the measurement object as described above. If the image reconstruction processing is performed based on, the accurate RI source distribution image cannot be obtained. Therefore, the sensitivity correction is performed by acquiring the sensitivity correction data (blank data) using the calibration radiation source and dividing the emission data by the blank data.

However, when the object to be measured moves, there is a problem in simply performing the sensitivity correction in this way. That is, even for a photon pair that occurs at the same position for the measurement object and flies in the same direction, if the position and orientation of the measurement object are different, the photon pair will have different photon detector pairs with different detection sensitivities. Is received by. Nevertheless, in the conventional sensitivity correction, since the emission data is sensitivity-corrected with the same blank data, accurate sensitivity correction cannot be performed.
Therefore, in order to accurately measure a moving measurement target, a body movement correction technique and a sensitivity correction technique suitable for this case have been demanded.

The present invention has been made to solve the above-mentioned problems, and even when the object to be measured moves during the measurement, the body motion is corrected and the sensitivity is corrected with high accuracy. It is an object of the present invention to provide a positron CT apparatus and an image reconstruction method therefor capable of obtaining a highly accurate reconstructed image.

[0013]

In a positron CT apparatus according to a first aspect of the present invention, an electron-positron pair annihilation in a measurement space is performed by a ring composed of a plurality of photon detectors which surround the measurement space and are arranged around a predetermined axis. The photon pairs generated in accordance with the above are detected, and a predetermined number is cumulatively added to the address corresponding to the coordinate value expressed in polar coordinates set in the measurement space for the straight line connecting the two photon detectors that detected each photon of the photon pair. A positron CT device for measuring the spatial distribution of the occurrence frequency of electron-positron pair annihilation based on the projection data accumulated by (1) a calibration line for generating photon pairs accompanying electron-positron pair annihilation Calibration source rotating means for rotating the source relative to the ring around a predetermined axis in the measurement space, and (2) Directing the rotation movement of the calibration source to the calibration source rotating means. And calibrate In a state in which the source is rotating motion,
Reference sensitivity correction data generation means for detecting the photon pairs generated from the calibration radiation source and accumulating projection data as reference sensitivity correction data, and (3) Relative to the ring of the measurement object placed in the measurement space. Position and direction measuring means for outputting position and direction data according to the specific position and direction, and (4) detecting the photon pair generated from the measurement object in the state where there is no calibration source in the measurement space, and measuring the position and direction data. Measuring means for compensating relative position and azimuth changes based on the above, and acquiring projection data accumulated during measurement time as measurement data; and (5) inputting position and azimuth data during measurement time An abundance density distribution generation unit that maps the relative position and azimuth represented by the azimuth data to the coordinates on the polar coordinates and generates the abundance density distribution on the polar coordinates of the measurement target during the measurement time; and (6) Reference sensitivity Correction data Data, the sensitivity correction data generation means for generating sensitivity correction data based on the result of the weighted average calculation of the existence density distribution, and (7) the sensitivity correction means for correcting the sensitivity of the measurement data based on the sensitivity correction data, 8) Image reconstructing means for performing image reconstruction by calculating a spatial distribution of occurrence frequency of electron-positron pair annihilation in the measurement object based on the measurement data whose sensitivity is corrected by the sensitivity correcting means. And

The operation of this device is as follows. The reference sensitivity correction data generating means acquires the projection data accumulated by detecting and accumulating the photon pair generated from the calibration radiation source rotated by the calibration radiation source rotating means by the ring as the reference sensitivity correction data. The position and orientation measuring means measures the relative position and orientation of the measuring object placed in the measurement space with respect to the ring, and outputs the position and orientation data. The measuring means projects projection data about the measuring object into the measuring object. Is acquired as measurement data by compensating the movement of the above, and the existence density distribution representing the movement distribution of the measurement object is generated by the existence density distribution generation unit. Then, the sensitivity correction data generation means generates sensitivity correction data in which the movement of the measurement object is compensated, the sensitivity correction means corrects the sensitivity of the measurement data, and the image reconstruction means obtains an accurate reconstructed image.

In the positron CT apparatus according to claim 2,
The abundance density distribution generation means accumulates the coordinate values at each time during the measurement time, and generates the abundance density distribution after the elapse of the measurement time based on the accumulated coordinate values. In the positron CT apparatus according to the third aspect, the abundance density distribution generation means sequentially generates the abundance density distribution based on the coordinate values at each time during the measurement time.

According to a fourth aspect of the image reconstruction method of the positron CT apparatus, the electron-positron pair annihilation in the measurement space is performed by a ring composed of a plurality of photon detectors arranged around a predetermined axis so as to surround the measurement space. The photon pair generated with it is detected, and a predetermined number is cumulatively added to the address corresponding to the coordinate value expressed in polar coordinates set in the measurement space for the straight line connecting the two photon detectors that detected each photon of the photon pair. An image reconstruction method of a positron CT device that measures the spatial distribution of the occurrence frequency of electron-positron pair annihilation based on the projection data accumulated by (1) generation of photon pairs due to electron-positron pair annihilation In the state where the calibration radiation source is rotated relative to the ring about the predetermined axis in the measurement space, the pair of photons generated from the calibration radiation source is detected and based on the accumulated projection data. The first step of acquiring as quasi-sensitivity correction data, and (2) measuring the relative position and orientation of the measurement object placed in the measurement space with respect to the ring in the absence of a calibration radiation source, The projection data accumulated during the measurement time is acquired as the measurement data by detecting the photon pair generated from the measurement object and compensating for the change in the relative position and direction, and the relative position and The second step of mapping the azimuth to the coordinate values on the polar coordinates and generating the existence density distribution of the measurement object on the polar coordinates during the measurement time, and (3) Weighted average calculation of the reference sensitivity correction data with the existence density distribution. A third step of generating sensitivity correction data based on the result, and (4) a fourth step of sensitivity correction of the measurement data based on the sensitivity correction data,
(5) Fifth step of performing image reconstruction by calculating a spatial distribution of occurrence frequency of electron-positron pair annihilation in the measurement object based on the measurement data whose sensitivity is corrected in the fourth step Is characterized by.

The operation of this method is as follows. First
In the step, the projection data accumulated by detecting the photon pairs generated from the calibration radiation source rotated by the calibration radiation source rotating means by the ring are acquired as the reference sensitivity correction data. In the second step, the position and orientation data corresponding to the relative position and orientation of the measurement target placed in the measurement space with respect to the ring are acquired, and the existence density distribution representing the motion distribution of the measurement target is generated. The projection data of the measuring object is compensated for the movement of the measuring object and acquired as the measuring data. And
Sensitivity correction data that compensates for the movement of the measurement object is generated in the third step, the sensitivity of the measurement data is accurately corrected in the fourth step, and an accurate reconstructed image is obtained in the fifth step.

In the image reconstruction method of the positron CT apparatus according to claim 5, the second step is to store the coordinate values at each time during the measurement time, and to exist after the measurement time has elapsed based on the stored coordinate values. Generate a density distribution. Claim 6
In the image reconstruction method of the positron CT apparatus according to
In the step (1), the existence density distribution is sequentially generated based on the coordinate values at each time during the measurement time.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

Prior to the description of the essential parts of the embodiment, the configuration, operation, image reconstruction method, sensitivity correction method and the like of the PET to which the embodiment is applied will be described.

First, the structure and operation of 2D-PET will be described. FIG. 8 is a system configuration diagram of 2D-PET.

A ring in which n photon detectors D _k (k = 1,2,3, ..., n) are arranged in a ring shape around a central axis surrounding a measurement object in the measurement space. Is the photon detector D _k (k = 1,
2, 3, ..., N) and the coincidence counting circuit 30 are connected by signal lines. When any photon detector of the ring 20 detects a photon, a signal corresponding to the photon energy is sent to the coincidence counting circuit 30 through the signal line, and the coincidence counting circuit 3
0 is a predetermined photon energy (51) emitted from the RI source 10 by two photon detectors in the ring 20 based on the signals arriving from the respective photon detectors in the ring 20.
It recognizes that a photon pair having 1 keV) is detected simultaneously, and outputs a detector identification signal (I, J) indicating each of these two photon detectors at that time.

These detector identification signals (I, J) are t-
Two variables (t, t used to represent the straight line L1 that is input to the θ conversion unit 40 and connects two photon detectors that have detected photon pairs in polar coordinates in the measurement space within the ring 20)
θ) is converted into a mapping position (T, θ ′) on the t-θ plane with each coordinate axis. Here, T is the distance between this straight line and the coordinate origin, and θ ′ is the angle of the perpendicular line drawn from this coordinate straight line to the coordinate system main axis. The t-θ memory 50 cumulatively adds 1 to the projection data stored in the address corresponding to this (T, θ ′).
In this way, the coincidence counting information for the pair of photons generated in the RI radiation source 10 is stored in the t-θ memory 50.

The coincidence count information of a large number of photon pairs generated by the RI radiation source 10 and detected by the ring 20 in this manner is t
It is stored in the θ memory 50 as projection data. FIG.
FIG. 6 is a diagram for explaining photon pair detection and image reconstruction in a ring. The image showing the distribution of the RI radiation source 10 viewed from the θ ′ direction (direction of the straight line L1 in FIG. 9) of the polar coordinate system set in the measurement space in the ring 20 is the polar coordinate system in the accumulated projection data. Then, the image reconstruction unit 60 reconstructs from the projection data corresponding to each T point on θ = θ ′, and the reconstructed image is displayed on the image display unit 70.

However, not all photon pairs generated in the measurement space are detected. FIG. 10 is an explanatory diagram of photon pair detection in the ring. The photon pairs from the RI source 10 are emitted from every position where the RI source 10 exists and fly in all directions, in which the photon pairs are generated on the surface of the ring 20 and the direction along the surface of the ring 20. Only photon pairs that flew to can be detected at ring 20. For example, as shown in FIG. 9, even with a photon pair generated from the RI radiation source 10 existing in the ring 20, a photon pair flying in directions L2 to L4 other than the direction along the plane of the ring 20 is not detected. . The photon pair flying in the direction L1 along the plane of the ring 20 is detected by one of the photon detectors I and J,
It is converted to (T, θ ′) by the t-θ conversion unit 40, and 1 is added to the projection data of the corresponding address of the t-θ memory 50.

The photon detector has a block structure because it is easy to manufacture and inexpensive. In this type of ring, as shown in FIG.
_In each _p , a large number of photon detectors D _pq are arranged in parallel, and these blocks B _p are arranged in a ring shape (p = 1,2,3, ..., q = 1,2,3). , ...). This block type PE
T is different only in the arrangement of photon detectors,
The operation principle, accumulation of projection data, image reconstruction, and the like are the same as those described above.

In the above-mentioned PET, since only the photon pairs which are emitted from the RI radiation source and fly in all directions and which fly in the direction along the ring surface are detected, the photons emitted from the RI radiation source are detected. There is a problem that the probability of capturing a pair is low, the detection sensitivity is low, and the statistical noise is large. It is possible to inject a large amount of RI radiation source into the measurement target in order to improve the detection sensitivity, but there is a limit when the measurement target is a living body.

Therefore, in such a case, a three-dimensional type PET (3D-PET) is used. Figure 12
FIG. 13 is a system configuration diagram of 3D-PET, and FIG.
FIG. 6 is a configuration diagram of a PET ring. The 3D-PET multilayer ring 21 is a _single- layer ring R ₁ , R ₂ , ..., R _m of the photon detector similar to that shown in FIG. 9 or FIG.
The photon pairs emitted from the RI radiation source 10 and flying in the direction of the straight line L1 are generated by two photon detectors belonging to different monolayer rings R _p and R _q (p ≠ q). Can also be counted simultaneously. When any two photon detectors in the multilayer ring 21 simultaneously detect a photon pair of energy 511 keV emitted from the RI source 10, the coincidence counting circuit 31 identifies the two photon detectors respectively. Signal (I, J) and the difference signal R between the two single layer rings to which the two photon detectors belong respectively
D (Ring Difference) is output. The detector identification signal (I, J) and the ring difference signal RD are xy-
Four variables (x, y, θ) that are input to the θ-ψ conversion unit 41 and are used to represent the straight line L1 that connects two photon detectors that have detected a photon pair in polar coordinates in the measurement space in the multilayer ring 21. , Ψ) as coordinate axes, and xy-
It is converted into a mapping position (x, y, θ, ψ) on the θ−ψ space. Here, θ and ψ represent the direction of the straight line L1, and x and y
Represents the position in a rectangular coordinate system on a projection plane (Projection Plane) perpendicular to the straight line L1. The xy- [theta]-[psi] memory 51 cumulatively adds 1 to the projection data stored in the address corresponding to this (x, y, [theta], [psi]). In this way, the coincidence count information about the pair of photons generated by the RI radiation source 10 is accumulated in the xy-θ-ψ memory 51 as projection data, and the image reconstruction unit 6 is created from this projection data.
1 and the reconstructed image is reconstructed by the image display unit 7
Displayed as 1.

In any of the above types of PET,
Since the detection sensitivities of the photon detectors are not constant and have many irregularities, the sensitivity of the photon detectors is corrected as follows. That is, as shown in FIG. 14, the measurement object 10a to which the RI radiation source is administered is placed in the ring for measurement, and the coincidence counting information is accumulated via the coincidence counting circuit 32 and the t-θ conversion unit 42. This is called emission measurement, and the data stored in the t-θ memory 52 by this is called emission data and is represented by E (t, θ). Separately from this, as shown in FIG. 15, in the state where there is no object to be measured in the ring, an R for calibration is provided around the center axis of the ring in the ring.
The I-ray source 10b is rotated and simulated parallel light is made incident on each photon detector to perform measurement (blank measurement). The data (blank data B (t, θ)) stored in the t-θ memory 52 in this way represents the detection sensitivity variation of the photon detector pair. And the emission data E
The sensitivity correction is performed by dividing (t, θ) by the blank data B (t, θ).

Further, the photon absorption of the measurement object is corrected as follows. As shown in FIG. 16, the measurement object 10c to which the RI radiation source has not been administered is placed in the ring at the same position as during emission measurement, and the RI radiation source 10b for calibration is rotated in the ring as in the blank measurement. Data (transmission measurement), and the data accumulated in the t-θ memory 52 (transmission data T
(T, θ)) is acquired. If this transmission data T (t, θ) is divided by the blank data B (t, θ), the absorption coefficient on the coincidence counting line is obtained, and the emission data E (t, θ) is divided by this absorption coefficient to obtain the absorption. Can be corrected. In the transmission measurement, the weak RI radiation source is further absorbed by the measurement object, and the number of detected photon pairs is reduced, so that the statistical accuracy is significantly deteriorated. As a countermeasure, T (t, θ) / B (t,
θ) is smoothed with a filter or the like.

The true projection data P (t, θ) after the sensitivity correction and the absorption correction have been performed in this way are emission data E (t, θ), blank data B (t, θ) and transmission data T. Using (t, θ), P = (E / B) / <T / B> (1) Here, the symbol <> means smoothing. The same applies to the case of 3D-PET.

The present invention is to obtain a correct reconstructed image by correcting body movements and correcting sensitivity of emission data with high accuracy even for a moving measurement object.

Next, the main part of this embodiment will be described. This embodiment is a block type 3 in which the ring is stationary.
It is D-PET. FIG. 1 is a system configuration diagram of the PET according to the present embodiment.

The PET according to the present embodiment is (1) a ring 121 consisting of a large number of photon detectors for detecting photons, and a body movement measuring section 123 for measuring the position / direction of the measuring object 100 and outputting position / direction data. , And (2) it is judged whether the photon detected by the ring 121 is a photon pair generated due to the annihilation of the electron-positron pair. In addition, a straight line connecting the coincidence counting circuit 130 for identifying the photon detector pair detecting the photon pair generated by the annihilation of the electron-positron pair and (3) the photon detector pair identified by the coincidence counting circuit 130 is polar coordinated. Xy converted to the coordinate value (x, y, θ, ψ) represented by
-Θ-ψ conversion unit 140, and (4) coordinate values (x, y, θ,
ψ), an xy-θ-ψ memory 150 for cumulatively adding a predetermined value to the projection data stored in the address corresponding to (5),
An abundance density distribution generation unit 190 that inputs the position and orientation data output from the body movement measurement unit 123 and generates an abundance density distribution on the polar coordinates of the measurement object 100, and (6) xy-θ-ψ
Image reconstruction device 1 for generating reconstructed image data based on projection data accumulated in the memory 150 and subjected to sensitivity correction, etc.
60, (7) the image display unit 170 that displays the reconstructed image based on the reconstructed image data, and (8) the above-mentioned units are controlled, and the projections stored in the xy-θ-ψ memory 150 are controlled. A computer 200 for reading data and performing a predetermined process, and (9) a storage device 180 for storing data handled by the computer 200 are provided.

Detection unit 1 including measurement space 122 therein
A large number of photon detectors are arranged in a ring shape on the ring 121 of 20.
The light receiving surface is oriented in the direction of the measurement space 122 where 00 is placed to receive the photons generated in the measurement space 122. The detection unit 120 further includes a body movement measurement unit (position / direction measurement unit) 12
3 to measure the position and orientation of the measuring object 100 placed in the measurement space 122, and output position and orientation data. Further, the detection unit 120 is provided with a calibration radiation source 110 used for blank measurement and transmission measurement and a rotation mechanism for rotating the calibration radiation source 110. This rotating mechanism is, for example, a calibration radiation source 110.
The computer 200 is controlled by a support mechanism that supports and rotates about a central axis, a belt that transmits rotation to the support mechanism, and a motor that generates the rotation. During use (during blank measurement and during transmission measurement), the calibration radiation source 110 rotates on the ring 121 surface around the center axis of the ring 121, whereby the photon pair generated from the calibration radiation source 110 is generated. Those flying on the surface of the ring 121 are detected by the photon detector pair. On the other hand, when it is not used (at the time of emission measurement), the photon pair generated from the calibration radiation source 110 is retracted to a position where it does not reach any photon detector. Alternatively, the calibration source 110 may be removed from the device.

When one of the photon detectors in the ring 121 receives a photon, a signal corresponding to the photon energy is transmitted through the signal line between each photon detector and the coincidence counting circuit 130. Sent to. Ring 1
The coincidence counting circuit 130, which has received the signals arriving from the respective photon detectors 21 of the photons, has a photon having a predetermined energy (511 keV) generated by the annihilation of electron-positron pairs by two photon detectors of the ring 121. Recognizing that the pair has been detected simultaneously, it outputs a detector identification signal (I, J) and an inter-ring difference signal RD indicating each of these two photon detectors at that time.

The detector identification signal (I, J) and the inter-ring difference signal RD are input to the xy-θ-ψ conversion section 140, and the photon pair indicated by the detector identification signal (I, J) is detected. The four variables (x, y, θ, ψ) used when expressing the straight line connecting the two photon detectors described above in polar coordinates set in the measurement space 122 in the ring 121 are xy- It is mapped to a position on the θ−ψ space. This x
The -y-θ-ψ conversion unit 140 also receives the position and orientation data output from the body movement measurement unit 123, and measures
, And outputs the (x, y, θ, ψ) value.

The xy-.theta .-. Psi. Memory 150 for accumulating projection data receives the (x, y, .theta., .Psi.) Value output from the xy-.theta .-. Psi. , Y, θ,
the projection data stored in the address corresponding to the (ψ) value,
A predetermined value (for example, "1") is cumulatively added. In this way, a large number of coincidence information about the photon pairs generated from the measurement object 100 is accumulated in the xy-θ-ψ memory 150 and becomes projection data.

The position and orientation data output from the body movement measuring unit 123 is also input to the existence density distribution generating unit 190, and the position and orientation of the measuring object 100 in the measurement space 122 are converted into x-y-θ-ψ. X-
It is mapped to a position on the y-θ-ψ space. The position and orientation data of the measuring object 100 within a fixed time are successively mapped to the points on the xy-θ-ψ space in this way, and the xy-θ of the measuring object 100 within the fixed time. -Abundance density distribution in ψ space is generated.

The image reconstructing device 160 for reconstructing an image of the distribution density of the RI radiation source in the measuring object 100 is x-
From the projection data (emission data) accumulated in the y-θ-ψ memory 150 or the emission data after correction processing described later, the measurement target viewed from the predetermined θψ direction of the polar coordinate system set in the measurement space 122 in the ring 121. Thing 1
Reconstructed image data representing the photon pair generation distribution at 00 is generated, and the image display unit 170 inputs the reconstructed image data and displays the reconstructed image.

The above-mentioned respective parts are the computer 200
Are integrated via the bus line 210 and controlled. That is, the computer 200 controls the rotation and retreat of the calibration radiation source 110. Also, the abundance density distribution function generated by the abundance density distribution generation unit 190 is acquired. Further, all contents of each address of the xy-θ-ψ memory 150 are zero-cleared before the measurement, and the projection data accumulated at each address is acquired after the measurement. Also, xy
Based on the projection data (emission data, blank data, transmission data) acquired from the-?-? Memory 150, a correction process (sensitivity correction, absorption correction) described later is performed. Further, emission data is sent to the image reconstructing device 160, and reconstructed image data generated by the image reconstructing device 160 is acquired. Further, the reconstructed image data is sent to the image display unit 170 to display the reconstructed image.

Each data handled in each process of the computer 200 is stored in the storage device 180 and read from the storage device 180 when necessary. That is,
The storage device 180 stores xy-θ- after emission measurement.
The emission data read from the ψ memory 150 is stored, and the abundance density distribution function read from the abundance density distribution generation unit 190 is stored. In addition, the storage device 180 stores blank data obtained by blank measurement,
The transmission data obtained by the transmission measurement is also stored. When calculating the sensitivity correction and the absorption correction, the storage device 180 stores the emission data,
Blank data, transmission data, etc. are read, and intermediate data of the calculation and emission data after sensitivity correction are stored. Further, in the image reconstruction process, the storage device 180 reads the emission data after the sensitivity correction and stores the reconstructed image data generated by the image reconstruction device. The storage device 180
Has a storage area divided into a plurality of areas, and stores each of the above data in different areas as necessary. The storage device 180 may be a magnetic disk, a magnetic tape, or the like, or may be a main storage device including a semiconductor memory in the computer 200.

The respective units described above do not operate independently of each other, but according to the instruction of the computer 200,
Work in coordination with each other. That is, the computer 200 acquires (1) a reference sensitivity correction data generation module 201 that rotates the calibration radiation source 110 to obtain blank data (reference sensitivity correction data), and (2) obtains emission data and an existing density distribution function. Measurement module 2
02, and (3) a sensitivity correction data generation module 20 for generating sensitivity correction data in which the body movement of the measurement object 100 is compensated.
3 and (4) a sensitivity correction module 204 for sensitivity correction of emission data, and (5) an image reconstruction module 205 for performing image reconstruction processing and image display. Each module has a corresponding processing. In the step
The operation of each part is integratedly controlled.

Reference sensitivity correction data generation module 201
Indicates the rotation of the calibration radiation source 110 and zero-clears the data in the xy-θ-ψ memory 150 prior to the blank measurement. Then, after the blank measurement is completed, xy
The reference sensitivity correction data accumulated in the −θ−ψ memory 150 is acquired, and the reference sensitivity correction data is stored in the storage device 180.

Further, the measurement module 202 for acquiring emission data issues an instruction to stop and save the calibration radiation source 110 and zero-clears the data in the xy-θ-ψ memory 150 prior to emission measurement. After the emission measurement is completed, the emission data accumulated in the xy-θ-ψ memory 150 and the abundance density distribution function generated by the abundance density distribution generation unit 190 are acquired, and the emission data and the abundance density distribution are acquired. Function and storage 18
0 is stored.

Further, the sensitivity correction data generation module 20
3 reads the reference sensitivity correction data and the abundance density distribution function stored in the storage device 180, generates sensitivity correction data based on these data, and stores the sensitivity correction data in the storage device 180.

Further, the sensitivity correction module 204 reads out the emission data and the sensitivity correction data stored in the storage device 180, and corrects the sensitivity of the emission data based on these data. Furthermore, absorption correction may be performed. Then, the corrected emission data is stored in the storage device 180.

The image reconstructing module 205 also reads the sensitivity-corrected emission data stored in the storage device 180, and uses that data as the image reconstructing device 1.
60, the reconstructed image data obtained by being processed by the image reconstructing device 160 is acquired, and the data is sent to the image display unit 170 for image display.

Next, the detector of the PET according to this embodiment will be described in detail. FIG. 2 shows the PE according to the present embodiment.
It is sectional drawing of the detection part of T. The detection unit 120 includes a ring 121 including a large number of photon detectors D and a measurement target 100.
The body movement measuring unit 123 and the like for measuring the position and azimuth of

Ring 12 having measurement space 122 inside
1 is a multi-layer block type single layer ring (5 in this figure).
Layers), each layer has a plurality of blocks each composed of a plurality of photon detectors D arranged in a ring shape, and each photon detector D of each block is an object to be measured. Measurement space 12 in which 100 is placed
The light receiving surface is oriented in the direction of 2. Shield shield 12
4 is for preventing the photon pairs from leaking out of the measurement space 122, and does not separate the single-layer rings from each other. Therefore, it is possible to detect photon pairs between different monolayer rings.

The body movement measuring unit 123 for measuring the position and azimuth of the measuring object 100 is based on, for example, optical distance measuring sensors 123a to 123c each including a light emitting element and a light receiving element, and outputs from these sensors. And a body movement data processing unit 123d for outputting position and orientation data. Each of the distance measuring sensors 123a to 123c has a fixed relative positional relationship with the ring 121, and is located at a predetermined position of the measuring object 100 (in this figure, the measuring object 1
Each of the markers 101a to 101c provided on the human head (00) is irradiated with a light beam to receive the reflected light, and the distances to the markers 101a to 101c are measured. The distance data obtained from each of the three distance measuring sensors 123a to 123c is input to the body movement data processing unit 123d, and the position and azimuth of the measuring object 100 are obtained.

Alternatively, an image pickup camera may be used as the body movement measuring unit 123, and the obtained image may be analyzed to obtain the position and orientation of the measurement object 100. Alternatively, the body movement measuring unit 12
As 3, the acceleration sensor provided at a predetermined position of the measuring object 100 may be used, and in this case, the position and orientation of the measuring object 100 can be obtained based on the output from the acceleration sensor.

Next, the measurement of the position and orientation of the measuring object 100 and the generation of the existing density distribution by the body movement measuring section 123 will be described in more detail. FIG. 3 is an explanatory diagram of body movement measurement in PET according to this embodiment.

The position of the measuring object 100 is determined by the ring 121.
Is represented by coordinate values (X, Y, Z) of a predetermined point (for example, the center point of the measurement object 100) in the measurement object 100 in the XYZ orthogonal coordinate system set in the measurement space 122 inside. It In addition, the azimuth of the measuring object 100 is XYZ.
Orientation (Θ, Ψ) of measurement object 100 in Cartesian coordinate system
It is represented by Here, Θ is the angle of the azimuth projected on the XY plane from the X-axis, and Ψ is the elevation angle from the XY plane. The position / orientation data (X, Y, Z, Θ, Ψ) representing the position and orientation of the measuring object 100 is based on the distance data output from the distance measuring sensors 123a to 123c in the body movement data processing unit 123d. Desired.

The body movement measuring section 123 is used for measuring the object 1 to be measured.
For the position and orientation of 00, the displacement amount from the reference value (for example, the position and orientation of the measuring object 100 at the start of measurement) may be measured and output. This body movement measuring unit 12
3 measures the displacement amount of the position and orientation of the measuring object 100 at fixed time intervals (for example, several milliseconds to several tens of milliseconds) from the measurement start time to the measurement end time of the emission measurement, and each time t _k. Position and orientation data (ΔX _k , ΔY
_k , ΔZ _k , ΔΘ _k , ΔΨ _k ) is obtained by the body movement measuring unit 123 (k = 0,1, ..., n, ...). The position / orientation data representing the position and orientation of the measurement object 100 thus measured is
It is input to the xy- [theta]-[psi] conversion unit 140 and also to the existence density distribution generation unit 190.

Position / azimuth data (ΔX) at each time t _k
k , ΔY _k , ΔZ _k , ΔΘ _k , ΔΨ _k ), the existence density distribution generation unit 190 receives the x-y-θ-ψ conversion unit 140.
In the same manner as the above, each position / azimuth data is mapped to a position (Δx _k , Δy _k , Δθ _k , Δψ _k ) in the xy-θ-ψ space (k = 0,1, ..., n, ...). FIG. 4 shows P according to the present embodiment.
6 is a chart of position and orientation data in ET and a position on an xy-θ-ψ space. In this way, by obtaining the distribution of mapping positions on the xy-θ-ψ space within a fixed time, the existence density distribution of the measurement object 100 on the xy-θ-ψ space is represented. Density distribution function Cθψ (Δ
x, Δy, Δθ, Δψ) is generated.

This existence density distribution function Cθψ (Δx, Δ
y, Δθ, Δψ), photon pairs generated at the same point on the measurement object 100 and flying in the same direction are detected by the ring 121, and coincidence count information is stored in the xy-θ-ψ memory 150. At this time, the measuring object 100 has a reference position and a reference azimuth (for example, a position at the start of emission measurement,
When it is in the azimuth direction, the coincidence count information indicates the coordinate value (x,
y, θ, ψ) is stored in the address, but the measurement object 100 is displaced (Δx, Δy, Δθ, Δ) during the measurement.
ψ), the coincidence count information indicates coordinate values (x + Δx, y + Δy, θ + Δθ, ψ + Δψ).
Is stored in the address corresponding to, and the ratio is stored in the value Cθψ (Δ
x, Δy, Δθ, Δψ).

The existing density distribution function Cθψ generally differs depending on the θ value and the ψ value, that is, the projection direction. FIG. 5 is an explanatory diagram of the existence density distribution at each position and each direction of the measurement target in the PET according to this embodiment. This figure shows a measurement target (human head) 100.
FIG. 4 is a diagram viewed from the top of FIG. 1, in which the existing density distribution functions Cθψ in the projection directions of La, Lb, and Lc are represented as Ca, Cb, and Cc, respectively.
The existence density distribution function is actually a function of four variables, but is simplified in this figure. As shown in this figure, when the body movement of the measuring object 100 is biased in a certain direction or a certain rotation direction, the existing density distribution function Cθψ becomes θ.
And ψ are different functions.

Next, a method for compensating the body movement of the object measured by the body movement measuring unit 123 to generate sensitivity correction data and further correcting the sensitivity will be described. Note that the sensitivity correction data generation module 203 of the computer 200 performs the sensitivity correction data generation and the sensitivity correction described below, respectively.
And sensitivity correction module 204 respectively. FIG. 6 is a data flow diagram of sensitivity correction data generation and sensitivity correction in the PET according to this embodiment.

The abundance density distribution function Cθψ (Δx, Δ) generated by the abundance density distribution generation unit 190 during emission measurement.
y, Δθ, Δψ) and the reference sensitivity correction data B (x, y,
θ, ψ) and the sensitivity correction data B1 in which the body movement of the measuring object 100 is compensated after the emission measurement is completed.
(X, y, θ, ψ)

[0061]

[Equation 1]

It is calculated by the following relational expression. Here, the quadruple summation operation Σ is calculated over the range of body movement of the measuring object 100. This calculation may detect the same photon pair even if the same photon pair from the measurement object 100 is detected by different photon detector pairs according to the body movement of the measurement object 100. Existence density distribution C for photon detector pairs
This means calculating the averaged detection sensitivity in consideration of the weight represented by θψ.

From the above, the sensitivity correction data B1 (x, y, obtained by compensating the body movement of the measurement object 100 is the emission data E (x, y, θ, ψ) obtained by the emission measurement.
By performing the sensitivity correction with θ, ψ), the emission data E1 (x, y, θ, ψ) after the sensitivity correction is obtained by E1 = E / B1 (3) This emission data E1 is not only the body movement of the measurement object 100 is corrected for emission measurement, but also the sensitivity is corrected with the blank data B1 that compensates for the body movement of the measurement object 100, so that accurate sensitivity correction is performed. If an image is reconstructed based on this, an accurate reconstructed image can be obtained.

If absorption correction is further performed, transmission measurement is performed in the same manner as emission measurement, and body movement-compensated transmission data T and the existing density distribution function during transmission measurement are obtained. The sensitivity correction data B2 is calculated based on the equation (2). Then, the true line sum P1 is obtained by the relational expression P1 = (E / B1) / <T / B2> (4). Here, the symbol <> means smoothing.

Next, the operation of PET and the image reconstruction method according to this embodiment will be described. FIG. 7 is a flowchart of measurement in PET according to this embodiment.

First, in step S1, the computer 2
Blank measurement is performed according to an instruction from the reference sensitivity correction data generation module 201 of 00. That is, ring 1
The radiation source 1 for calibration is placed without placing the measuring object 100 inside 21.
10 is rotated on the surface of the ring 121 about the central axis to generate simulated parallel light. In that state, the photon pairs generated from the calibration radiation source 110 are detected by the multiple photon detectors forming the ring 121, and the coincidence counting circuit 1
The energy is discriminated at 30, and the xy-θ-ψ conversion unit 140
Is converted into the (x, y, θ, ψ) value corresponding to the straight line connecting the two photon detectors that detected the photon pair,
y, θ, ψ) value corresponding to xy-θ-ψ memory 150
Is stored as projection data at the address. Reference sensitivity correction data B accumulated as projection data in this way
(X, y, θ, ψ) is read by the reference sensitivity correction data generation module 201 and stored in the storage device 180.

Subsequently, in step S2, emission measurement is performed according to an instruction from the measurement module 202 of the computer 200. That is, the calibration radiation source 110 is retracted, the measurement object 100 into which the RI radiation source is injected is placed in the measurement space 122 in the ring 121, and in this state, photon pairs generated from the measurement object 100 are detected. Then, the body movement is corrected based on the position / orientation data indicating the position and orientation of the measurement object 100 output from the body movement measuring unit 123, and the projection data is stored in the xy-θ-ψ memory 150. At the same time, the position and orientation data is stored in the existence density distribution generation unit 1.
90, and the existence density distribution function Cθψ (Δx, Δ
y, Δθ, Δψ) is generated. Emission data E (x, y,
θ, ψ) and the existing density distribution function Cθψ (Δx, Δy,
Δθ, Δψ) is read by the measurement module 202 and stored in the storage device 180.

If absorption correction is also performed,
Perform transmission measurement. That is, the measurement object 100 into which the RI radiation source is not injected is placed in the measurement space 122, the calibration radiation source 110 is rotated, and in this state, the measurement is performed in the same manner as the emission measurement, and the transmission data T and the existence density distribution are measured. Function and obtain the memory device 18
Store to 0.

The blank measurement (step S1), the emission measurement (step S2) and the transmission measurement may be performed in any order.

Upon completion of each of the above measurements, subsequently, in step S3, sensitivity correction data is generated based on the data obtained by these measurements. When generating the sensitivity correction data, the sensitivity correction data generation module 203 of the computer 200 reads the existing density distribution function Cθψ and the reference sensitivity correction data B (x, y, θ, ψ) at the time of emission measurement from the storage device 180. , Sensitivity correction data B1 (x, y, θ, ψ) is calculated based on the equation (2).

Subsequently, sensitivity correction is performed in step S4.
The sensitivity correction module 204 of the computer 200 uses the emission data E read from the storage device 180.
(X, y, θ, ψ) is divided by the sensitivity correction data B1 (x, y, θ, ψ) obtained in step S3 to correct the sensitivity unevenness between the photon detectors forming the ring 121. Emission data E1 (x, y,
θ, ψ) is obtained.

If absorption correction is also performed,
The transmission data T is corrected based on the existing density distribution function at the time of transmission measurement in the same manner as in step S3, and the true projection data P1 is calculated based on the equation (4).

Then, based on the emission data E1 after the sensitivity correction (or the true projection data P1) obtained in step S4, image reconstruction is performed in step S5.
That is, the image reconstruction module 205 of the computer 200 uses the emission data E1 after the sensitivity correction.
(Or the true projection data P1) to the image reconstruction device 16
0, the reconstructed image data generated in the image reconstructing device 160 is acquired, and the reconstructed image data is sent to the image display unit 170 to display the reconstructed image. By doing so, a more accurate spatial distribution of the occurrence frequency of electron-positron pair annihilation can be measured.

The present invention is not limited to the 3D-PET described above, but can be applied to other types of PET.

For example, the present invention can be applied to 2D-PET having a multi-layered ring and 2D-PET having a single-layered ring. The 2D-PET including the multi-layered rings can be considered to be substantially the same as a plurality of 2D-PETs including the single-layered ring, and thus the latter will be described. Also in this case, the movement of the measurement object is equivalent to the movement of the ring, and in particular, the movement of the measurement object in the central axis direction of the ring is equivalent to the movement of the ring in the central axis direction. Therefore, when the measurement object is fixed, only one cross section can be measured, whereas when the measurement object moves, within the range of its body movement,
It is possible to measure many cross sections. That is, a simulated 3
It can also be said to be D-PET. However, since the ring actually has one layer, the detection sensitivity is lower than that of 3D-PET. Therefore, even in the case of 2D-PET, as in the above embodiment, the existence density distribution function Cθψ of the measurement object is generated by the existence density distribution generation unit during the emission measurement, and the emission is compensated for the body movement. Data E is x-
The sensitivity correction data B1 which is stored in the y-θ-ψ memory and compensates for the body movement is calculated by the equation (2), and the emission data E1 after the sensitivity correction is obtained by the equation (3) to perform image reconstruction. be able to.

Further, even when the ring makes a motion such as a rotary motion or a wobbling motion, it is similarly applicable. In this case, the body movement measuring unit fixed to the ring measures the position / orientation of the measurement target, or both the rotational position of the ring and the position / orientation of the measurement target,
Based on the relative movement of the ring and the measurement object obtained by this, the true line sum subjected to the body movement compensation and the sensitivity correction can be obtained in the same manner as the above embodiment.

Further, in the above embodiment, the abundance density distribution generator accumulates / generates the abundance density distribution during emission measurement, but during emission measurement, the coordinate values (Δx, Δy, Δθ, Δψ) at each time. Is stored as time-series data, and after the emission measurement is completed, the existing density distribution function Cθψ
May be calculated.

[0078]

As described above in detail, in the positron CT apparatus according to the present invention, the photons generated from the calibration radiation source rotated by the calibration radiation source rotating means by the reference sensitivity correction data generating means. The projection data accumulated by detecting the pair by the ring is acquired as the reference sensitivity correction data. The position and orientation measuring means measures the relative position and orientation of the measurement object placed in the measurement space with respect to the ring and outputs position and orientation data, and the measurement means outputs projection data for the measurement object of the measurement object. The abundance density distribution function of the object to be measured is generated by the abundance density distribution generation means while the motion is compensated and acquired as measurement data. Then, the sensitivity correction data generation unit generates sensitivity correction data in which the movement of the measurement object is compensated based on the existing density distribution function, the sensitivity correction unit corrects the measurement data, and the image reconstructing unit reconstructs the reconstructed image. obtain. With the above configuration, not only the measurement data (emission data) in which the body movement of the measurement object is compensated can be obtained, but also the sensitivity correction data in which the body movement is compensated can be obtained, and Even when moving without being fixed, it is possible to perform sensitivity correction with accurate sensitivity correction data, and thus it is possible to obtain an accurate reconstructed image without the occurrence of artifacts.

[Brief description of drawings]

FIG. 1 is a system configuration diagram of a PET according to an embodiment.

FIG. 2 is a cross-sectional view of a detection unit of PET according to the embodiment.

FIG. 3 is an explanatory diagram of body movement measurement in PET according to the embodiment.

FIG. 4 is a chart of position and orientation data and a position on an xy-θ-ψ space in PET according to the embodiment.

FIG. 5 is an explanatory diagram of an existing density distribution at each position and each direction of the measurement target in the PET according to the embodiment.

FIG. 6 is a data flow diagram of sensitivity correction data generation and sensitivity correction in the PET according to the embodiment.

FIG. 7 is a flowchart of measurement in PET according to the embodiment.

FIG. 8 is a system configuration diagram of 2D-PET.

FIG. 9 is an explanatory diagram of photon pair detection and image reconstruction in a 2D-PET ring.

FIG. 10 is an explanatory diagram of photon pair detection in a 2D-PET ring.

FIG. 11 is a block diagram of a block PET ring.

FIG. 12 is a system configuration diagram of 3D-PET.

FIG. 13 is a configuration diagram of a ring of 3D-PET.

FIG. 14 is an explanatory diagram of emission measurement.

FIG. 15 is an explanatory diagram of blank measurement.

FIG. 16 is an explanatory diagram of transmission measurement.

[Explanation of symbols]

100 ... Object to be measured, 110 ... Calibration source, 120 ... Detecting section, 121 ... Ring, 122 ... Measuring space, 123 ... Body motion measuring section, 124 ... Shielding shield, 130 ... Simultaneous counting circuit, 140 ... xy -Θ-ψ conversion unit, 150 ... xy-
θ-ψ memory, 160 ... Image reconstruction device, 170 ... Image display unit, 180 ... Storage device, 190 ... Presence density distribution generation unit, 200 ... Computer.

Claims (6)

[Claims] 1. A photon pair generated by annihilation of an electron-positron pair in the measurement space is detected by a ring composed of a plurality of photon detectors which surround the measurement space and are arranged around a predetermined axis. Based on the projection data accumulated by cumulatively adding a predetermined number to the address corresponding to the coordinate value represented by the polar coordinates set in the measurement space with respect to the straight line connecting the two photon detectors that have detected each photon of the pair, A positron CT device for measuring a spatial distribution of occurrence frequency of electron-positron pair annihilation, wherein a calibration radiation source for generating photon pairs accompanying electron-positron pair annihilation is centered on the predetermined axis in the measurement space. A calibration radiation source rotating means that rotates relative to the ring; and an instruction to rotate the calibration radiation source to the calibration radiation source rotation means such that the calibration radiation source rotates. In this state, a reference sensitivity correction data generation unit that detects the photon pairs generated from the calibration radiation source and acquires the accumulated projection data as reference sensitivity correction data, and a measurement object placed in the measurement space A position and orientation measuring means for outputting position and orientation data according to a relative position and orientation with respect to the ring, and detecting a photon pair generated from the measurement object in a state where the calibration radiation source is not present in the measurement space. A measuring means for compensating for changes in the relative position and orientation based on the position and orientation data and acquiring the projection data accumulated during the measurement time as measurement data; and the position and orientation during the measurement time. Data is input to map the relative position and azimuth represented by the position and azimuth data to coordinate values on the polar coordinates, and the measurement object is measured during the measurement time. An abundance density distribution generating means for generating an abundance density distribution on coordinates, based on the result of weighted average calculation of the reference sensitivity correction data in the abundance density distribution, a sensitivity correction data generating means for generating sensitivity correction data, Sensitivity correction means for correcting the sensitivity of the measurement data based on the sensitivity correction data; and a space of occurrence frequency of electron-positron pair annihilation in the measurement object based on the measurement data whose sensitivity is corrected by the sensitivity correction means. An positron CT apparatus comprising: an image reconstructing unit that calculates a distribution and reconstructs an image. 2. The abundance density distribution generation means accumulates the coordinate values at each time during the measurement time, and generates the abundance density distribution after the measurement time elapses based on the accumulated coordinate values. The positron CT apparatus according to claim 1, wherein: 3. The positron CT apparatus according to claim 1, wherein the existing density distribution generating means sequentially generates the existing density distribution based on the coordinate values at each time in the measurement time. 4. A photon pair generated by annihilation of electron-positron pairs in the measurement space is detected by a ring composed of a plurality of photon detectors arranged around a predetermined axis so as to surround the measurement space. Based on the projection data accumulated by cumulatively adding a predetermined number to the address corresponding to the coordinate value represented by the polar coordinates set in the measurement space for the straight line connecting the two photon detectors that have detected each photon of the pair, An image reconstruction method of a positron CT apparatus for measuring a spatial distribution of occurrence frequency of electron-positron pair annihilation, comprising a calibration radiation source for generating photon pairs accompanying electron-positron pair annihilation in the measurement space. In the state of rotating relative to the ring around the center, the projection data accumulated by detecting the photon pairs generated from the calibration radiation source is used as reference sensitivity correction data. The first step of obtaining, and measuring the relative position and orientation of the measurement object placed in the measurement space with respect to the ring in the absence of the calibration source in the measurement space, The projection data accumulated during the measurement time is obtained as measurement data by compensating for the change in the relative position and orientation generated by detecting the photon pair generated from the relative position and the relative position during the measurement time. A second step of mapping the azimuth and azimuth to coordinate values on the polar coordinates, and generating a density distribution of the measurement target on the polar coordinates during the measurement time; A third step of generating sensitivity correction data based on the result of the weighted average calculation in step 4, and a fourth step of correcting the sensitivity of the measurement data based on the sensitivity correction data. A fifth step of performing image reconstruction by calculating a spatial distribution of occurrence frequency of electron-positron pair annihilation in the measurement object based on the measurement data whose sensitivity is corrected in the fourth step. An image reconstructing method for a positron CT apparatus, comprising: 5. The second step accumulates the coordinate values at respective times during the measurement time, and generates the existence density distribution after the measurement time elapses based on the accumulated coordinate values. An image reconstructing method for a positron CT apparatus according to claim 4, wherein 6. The image of the positron CT apparatus according to claim 4, wherein in the second step, the existing density distribution is sequentially generated based on the coordinate values at each time in the measurement time. Reconstruction method.