JPH10206545A - Positron ct(computed tomograph) - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the reconstitution image of high resolution at a high S/N rate in a subject or a region of interest by increasing no volume of a memory accumulating projection data or reducing it while the projection data are accumulated for dispersion correction even in a simultaneous counting line not passing through the area which the subject occupies. SOLUTION: It is judged by an area judgement part 50 whether or not a simultaneous counting line mutually connecting a pair of photon detectors simultaneously detecting a photon pair generated with electron and position pair extinction out of the photon detectors constituting a ring 20 passes a subject 10. In a case where it is judged by the region judgement part 50 that the simultaneous counting line passes through the subject 10, a constant value is accumulated and added in the projection data accumulation area 60A of high sampling density. In another case where it is not judged, another constant value is accumulated and added in the projection data accumulation area 60B of low sampling density, and the projection data are accumulated in a t-θ memory 60.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures a substance distribution in a subject by detecting a photon pair emitted by the annihilation of an electron / positron pair generated by an RI source applied to the subject. The present invention relates to a positron CT apparatus.

[0002]

2. Description of the Related Art Positron Emissio
n Computed-Tomography; hereinafter "PET")
Is a device that is applied to the research or clinical examination of living bodies and diseases, and that images the distribution of positron-emitting nuclides (hereinafter, referred to as “RI radiation sources”) injected into the body to view 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 basic metabolism of a living body such as cerebral blood flow and oxygen consumption.

[0004] The detection section of such a PET comprises a large number of photon detectors (hereinafter, referred to as "rings") arranged in a ring shape, and an RI radiation source is injected or sucked into a measurement space in the ring. A subject such as a human body is placed.
Positrons emitted from the RI source in the subject are immediately combined with nearby electrons, and a pair of photons (gamma rays) each having an energy of 511 keV are emitted in opposite directions. Therefore, a pair of photons having an energy of 511 keV among the photons detected by the photon detectors constituting the ring are discriminated and coincidentally counted, so that the electron-positron pair annihilation can be determined by any straight line (hereinafter, referred to as the coincidence line ") Can be specified. PET
Accumulates such coincidence counting information (projection data) in a memory and performs image reconstruction processing to create a distribution image of an RI source.

[0005]

In such a PET, it is desired to increase the number of photon detectors forming a ring in order to obtain a high-resolution reconstructed image. In a two-dimensional PET (2D-PET), among the photon pairs emitted from the RI source and flying in all directions, only the photon pairs flying in the direction along the ring surface are detected. Since there is a problem that the probability of capturing the photon pairs emitted from the source is small, the statistical noise is large, and the detection sensitivity is low, a three-dimensional type PET (3D-PET) is used to improve the detection sensitivity. It is becoming.

As described above, the number of photon detectors constituting a ring tends to increase, and the number of combinations of photon detection pairs capable of detecting a photon pair also tends to increase. As the count increases,
The amount of projection data to be stored is further increased, and the time required to transfer the projection data stored in the memory to the image reconstruction unit is further increased.

Further, in order to maintain a normal physiological state without giving a pain or stress to a human body or other animal as a subject, free measurement is performed by allowing a certain amount of body movement without fixing the subject. Although the moving measurement is performed, the number of combinations of the photon detector pairs that can detect the photon pairs generated from the subject also increases by the free moving measurement, so the projection data to be stored increases.
The transfer time to the image reconstruction unit becomes longer. The larger the range in which the body movement of the subject is allowed, the larger the projection data to be stored.

By the way, the PET measurement involves a static measurement in which all the projection data during one measurement is stored in the same storage area, and the measurement time is divided into a plurality of frames and the projection data for each frame is divided into a plurality of frames. There is dynamic measurement (multi-frame measurement) stored in a corresponding storage area, but recently, dynamic measurement is often performed to measure temporal changes in biological functions, such as cerebral blood flow measurement. In this dynamic measurement, in order to observe the state of the temporal change of the biological function in more detail and reduce the calculation error, the frame time was shortened,
It is desired to measure at shorter time intervals. However, as described above, there is a large amount of projection data to be accumulated, and it takes a long time to transfer the large amount of projection data from the memory to the image reconstruction unit. Therefore, it is difficult to reduce the frame time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and it is possible to obtain a high-resolution reconstructed image without increasing or reducing the capacity of a memory for storing projection data. It is another object of the present invention to provide a positron CT apparatus capable of reducing a frame time in dynamic measurement.

[0010]

The positron CT apparatus according to the present invention comprises: (1) a plurality of photon detectors each of which outputs a photon detection signal corresponding to the energy of an incident photon are arranged around a measurement space; (2) Photon detection signal is input, photon pairs generated by annihilation of electron and positron pairs in the measurement space are subjected to energy discrimination, and photon detector pairs that detect each photon of the photon pair are detected. A coincidence circuit that outputs a signal, and (3) a coordinate conversion unit that outputs a coordinate value expressed in a polar coordinate system set in a measurement space with respect to a coincidence line connecting the photon detector pairs indicated by the detector identification signal, (4) area determination means for determining the passing area of the coincidence counting line in the measurement space; and (5) two or more areas having different densities corresponding to the address values corresponding to the coordinate values output from the coordinate conversion means. Has a projection data storage area of the constant, in any of the predetermined number of projection data storage area according to the determined passage region by region judging means,
A projection data storage unit that accumulates a fixed value to an address corresponding to the coordinate value output from the coordinate conversion unit and stores projection data, and (6) a predetermined number of projection data storage areas of the projection data storage unit. Image reconstruction means for calculating a spatial distribution of the frequency of occurrence of electron-positron annihilation in the measurement space based on the accumulated projection data and performing image reconstruction,
It is characterized by having.

According to this positron CT apparatus, when a photon enters one of a plurality of photon detectors constituting a ring, a photon detection signal corresponding to the energy of the incident photon is output from the photon detector. The output photon detection signal is input to the coincidence circuit. Based on the photon detection signal, the coincidence circuit discriminates the energy of the photon pair generated by the annihilation of the electron / positron pair in the measurement space, and identifies the photon detector pair that has detected each photon of the photon pair. A signal is output. Based on the detector identification signal, the coordinate conversion means outputs coordinate values expressed in a polar coordinate system set in the measurement space for a coincidence line connecting the photon detector pairs indicated by the detector identification signal. The passing area of the coincidence line in the measurement space is determined by the area determining means, and according to the determination result, any one of the two or more predetermined number of projection data accumulating areas of the projection data accumulating means is transferred from the coordinate converting means. A certain value is cumulatively added to an address corresponding to the output coordinate value, and projection data is accumulated. Then, based on the projection data accumulated in each of the predetermined number of projection data accumulation areas of the projection data accumulation means, the image reconstructing means calculates the spatial distribution of the occurrence frequency of electron-positron annihilation in the measurement space, and reconstructs the image. An image is obtained.

Here, the predetermined number of projection data storage areas of the projection data storage means are different from each other in the correspondence of the addresses to the coordinate values output from the coordinate conversion means. The amount of projection data to be transferred from the projection data storage means to the image reconstruction means is smaller than in the case where the correspondence is uniformly dense. In the reconstructed image obtained by the image reconstructing means, a portion based on the projection data stored in the projection data storage area having a close correspondence is obtained with a high resolution.

Further, the image processing apparatus may further include a scatter correction means for performing scatter correction based on the spatial distribution image reconstructed by the image reconstruction means. In this case, the reconstructed image obtained by the image reconstructing means is subjected to the scattering correction by the
/ N ratio.

Further, the apparatus further comprises a contour detecting means for detecting a contour of a subject placed in the measurement space, and the area determining means determines a passing area based on the contour detected by the contour detecting means. It is good also as.
In this case, the contour of the subject placed in the measurement space is detected by the contour detection means, and the passing area of the coincidence line in the measurement space is determined by the area determination means based on the contour, and the projection is performed according to the result. A certain value is cumulatively added to any one of the predetermined number of projection data storage areas of the data storage means.

Further, the area determining means determines whether or not a coincidence line connecting the photon detector pairs indicated by the detector identification signal passes through a predetermined area in the measurement space. Of the predetermined number of projection data storage areas, the correspondence of the projection data storage area to which a certain value is cumulatively added when it is determined that the coincidence line passes through the predetermined area is the correspondence of the other projection data storage areas. It may be characterized as being denser. In this case, the area determination means determines whether or not the coincidence line connecting the photon detector pairs indicated by the detector identification signal passes through a predetermined area in the measurement space, and determines that the coincidence line passes through the predetermined area. If this is the case, a certain value is cumulatively added to the projection data storage area having a dense correspondence among the predetermined number of projection data storage areas of the projection data storage means. A certain value is cumulatively added to the area, and projection data is accumulated. Then, a high-resolution image is obtained for a predetermined area in the reconstructed image.

Further, the area determination means may be freely set with respect to the predetermined area. In this case, a predetermined area serving as a reference when judging whether or not the coincidence counting line passes is appropriately set by the area determining means in accordance with the subject and the attention area placed in the measurement space.

Further, the area determination means may be characterized in that either the area occupied by the object placed in the measurement space or the attention area in the object is set as the predetermined area. In this case, a high resolution image is obtained for the region occupied by the subject or the attention region in the subject in the reconstructed image.

Further, the area determining means sets a predetermined area to be an area obtained by adding a fixed width area around any one of an area occupied by an object placed in the measurement space or an attention area in the object. It is good also as. In this case, a high-resolution image can be obtained for the region occupied by the subject or the entire region of interest in the subject in the reconstructed image.

Further, the area determining means is characterized in that a spherical area including any one of an area occupied by the object placed in the measurement space and an attention area in the object is set as the predetermined area. Is also good. In this case, it is easy for the area determining means to determine whether or not the coincidence line passes through the predetermined area.

[0020]

Embodiments of the present invention will be described below in detail 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.

First, before describing the embodiment according to the present invention, the simultaneous scatter count and the scatter correction which are one factor of the S / N ratio deterioration of the reconstructed image will be described.

The scattering coincidence counting is a phenomenon in which a pair of photons (scattered rays) emitted by the electron-positron annihilation event and one or both of them are scattered are simultaneously detected by a pair of photon detectors. This phenomenon of coincidence of scattering gives wrong information about the annihilation position of the electron-positron pair.
It is necessary to remove it. In the case of 2D-PET,
Although the inter-slice collimator removed the detection of scattered radiation to some extent, in the case of 3D-PET, since the inter-slice collimator was removed, the detected scattered radiation was 2D-PE.
More than 10 times more than in the case of T, it is necessary to remove the scatter coincidence by another method.

As a method for removing the scatter coincidence without using the interslice collimator, utilizing the fact that the energy (511 keV) originally possessed by the photon is reduced by scattering, the photon has an energy of 511 keV without being scattered. It is conceivable to eliminate scattering coincidence by energy discrimination of photon pairs. However, this method can eliminate scatter coincidence to some extent, but cannot completely eliminate it.

A method of performing scattering correction is also conceivable.
FIG. 7 is an explanatory diagram of scatter coincidence counting. FIG. 7 (a)
FIG. 7B shows the distribution of projection data (emission data E (t, θ ′)) in a certain θ ′ direction in the polar coordinate system set in the measurement space 11 together with the subject 10 and the ring 20. , The distribution of projection data (emission data E (t, θ)) stored in the t-θ memory 60 is schematically shown. As shown in FIG. 7A, the area occupied by the subject 10 in the projection data accumulated by performing the emission measurement with the subject 10 to which the RI source is administered placed in the measurement space 11 in the ring 20 is shown. Projection data for passing coincidence lines (FIG. 7A)
In the projection data distribution range A), the scattering coincidence count is accumulated in addition to the true coincidence count, and the projection data for the coincidence line that does not pass through the area occupied by the subject 10 (the projection data in FIG. 7A). In the range B) of the data distribution, only the scattering coincidence count is accumulated.

That is, the projection data accumulation area of the t-θ memory 60 for accumulating projection data for each value of the azimuth θ and the distance t from the origin for all coincidence lines detected simultaneously is shown in FIG. As shown in FIG.
7 (b), and the projection data of the coincidence counting line that does not pass through the area occupied by the subject 10 (the area A of the projection data in FIG. 7B). (The area B of the projection data in FIG. 7B). Scatter correction, using the above,
By estimating the scattering data included in the entire projection data based on the projection data for the coincidence counting line that does not pass through the area occupied by the subject 10, and subtracting the estimated scattering data from the entire projection data, the true simultaneous This is for obtaining projection data relating to counting.

What is important in the scattering correction is to accurately estimate the scattering data included in the entire projection data based on the projection data on the coincidence line that does not pass through the area occupied by the subject 10. As the estimation method, there are an analysis method based on the laws of physics, an analysis method based on an actually measured response, a method based on simple linear approximation, and an eclectic method of these. Regardless of which estimation method is used, the success or failure of the scattering correction depends on the quantity and quality of projection data for coincidence lines that do not pass through the area occupied by the subject.

The area in the measurement space detected by the photon detector constituting the ring is limited to the area occupied by the subject and a limited area around the area, thereby reducing the capacity of the memory for storing projection data. Techniques are known (JP-A-58-6499, JP-A-2-87092). However, in this technique, it is impossible to estimate the scattering data included in the entire projection data based on the projection data for the coincidence line that does not pass through the area occupied by the subject, or the estimation accuracy is poor, and therefore, The scatter coincidence cannot be eliminated.

As described above, the projection data for the coincidence line that does not pass through the area occupied by the subject is indispensable for performing the scattering correction. Therefore, it is necessary to accumulate not only projection data for coincidence counting lines passing through the area occupied by the subject placed in the measurement space, but also projection data about coincidence counting lines not passing through the area occupied by the subject.

However, even with PET in which the distribution of electron-positron annihilation occurrence in a subject is to be measured at a high resolution, compared with the sampling density of the projection data for the coincidence line passing through the area occupied by the subject, The sampling density of the projection data for the coincidence line that does not pass through the area occupied by the data need not be as dense as the same, but may be coarse. Because statistical noise is superimposed on the projection data of photons scattered in a complicated process, it is meaningless to sample this at a high density, and if there is no statistical noise, This is because the scattering data changes spatially smoothly. Furthermore, even within the area occupied by the subject,
Compared to the sampling density of projection data for coincidence lines passing through a region of interest (for example, a visual cortex in a brain activation experiment by visual stimulation) of the subject, projections on coincidence lines not passing through the region of interest are compared. The data sampling density does not need to be as dense as it is, and may be coarse.

The present invention has been made based on such considerations, and increases the memory capacity while accumulating projection data for scattering correction even for coincidence lines that do not pass through the area occupied by the subject. A positron CT capable of obtaining a high-resolution reconstructed image at a high S / N ratio for a subject or a region of interest without or with a reduced number
An apparatus is provided.

(First Embodiment) Next, a first embodiment will be described. FIG. 1 is a configuration diagram of a positron CT apparatus according to the first embodiment.

The PET according to the present embodiment is a 2D-PET. In this figure, one layer of the rings separated from each other by the inter-slice collimator is represented as a ring 20. The ring 20 includes a measurement space 11 in which the subject 10 is placed, and a number of photon detectors D _k (k = 1, 2, 3,..., N) for detecting incident photons are provided around a central axis. These photon detectors are arranged in a ring shape, and the photon detectors face the light receiving surface in the direction of the measurement space 11, and detect photons that fly and enter from the measurement space 11. A signal line is provided between each of the photon detectors D _k (k = 1, 2, 3,..., N) and the coincidence counting circuit 30. A photon detection signal corresponding to the detected photon energy is sent.

A coincidence circuit 3 for inputting photon detection signals output from the photon detectors D _k (k = 1, 2, 3,..., N)
0 represents two photon detectors D _i and D _j in ring 20
There recognized by energy discriminating that simultaneously detected the photon pair having a predetermined energy (511 keV) generated with the electron-positron pair annihilation, detecting respectively indicating two-photon detectors D _i and D _j at that time The device identification signals I and J are output.

The t-θ converter (coordinate conversion means) 40 which receives the detector identification signal pair (I, J) output from the coincidence counting circuit 30 outputs a photon indicated by the detector identification signal pair (I, J). for coincidence line connecting the two photon detectors D _i and D _j having detected pair with each other, coordinate values expressed in the set t-theta polar coordinates within the measurement space 11 (T, θ ') is converted to an output I do. Here, T is the coincidence counting line and t-
represents the distance from the origin of the θ polar coordinate system, and θ ′ represents the direction of the coincidence line.

The area determining unit 50 for inputting the coordinate values (T, θ ′) determines an area through which the coincidence line represented by the coordinate values (T, θ ′) passes in the measurement space 11. For example, it is determined whether or not the coincidence line passes the subject 10 placed in the measurement space 11. Alternatively, it may be determined whether or not the coincidence line passes through the attention area 10A of the subject 10 (for example, a visual cortex in a brain activation experiment using a visual stimulus with the human head as the subject 10). . The area determination unit 50 may determine the passing area based on the detector identification signal pair (I, J). When determining whether or not the coincidence counting line passes through the subject 10, the region determining unit 50 needs to detect and store the contour of the subject 10 in advance. The specific method will be described later. I do.

The t-θ memory (projection data storage means) 60 for storing projection data has two or more (two in the present embodiment) projection data storage areas 60A and 60B which are address spaces which do not overlap each other. The coordinate values (T, T) are stored in any of the projection data storage areas 60A and 60B.
A constant value is cumulatively added to the address corresponding to θ ′), and projection data for the photon pair generated in the measurement space 11 is accumulated. Here, in the projection data storage area 60A, the correspondence (sampling density) of the address to the coordinate value (T, θ ′) is higher than that in the projection data storage area 60B, that is,
The number of addresses per unit area on the t-θ plane is large. The difference between the density of the correspondence of the address to the coordinate value (t, θ) between the projection data storage areas 60A and 60B is t
May be provided for both coordinates and θ coordinates,
Any one of these may be provided.
In the t-θ memory 60 shown in FIG. 1, the distribution of the projection data on the t-θ plane is schematically shown for each of the projection data storage areas 60A and 60B.

The projection data storage area 60A is determined by the area determination section 50 when the coincidence line passes through a predetermined area in the measurement space 11 (an area occupied by the subject 10 or an attention area in the subject 10). Sometimes t-θ
Coordinate value output from the conversion unit 40 (T, θ ') to cumulatively adding a constant value to the address corresponding to accumulate projection data E _A. The other projection data storage area 60B stores the coordinate value (T, θ ′) output from the t-θ conversion section 40 when the area determination section 50 determines that the coincidence line does not pass through the predetermined area. cumulative addition to storing projection data E _B a constant value to a corresponding address. The projection data storage area 60B stores the coordinate value (T, θ ′) output from the t-θ conversion section 40 even when the area determination section 50 determines that the coincidence line passes through the predetermined area. A fixed value may be cumulatively added to the corresponding address.

[0038] Thus, the projection data storage area 60A, a true coincidence photon pairs generated in accordance with the electron-positron pair annihilation in the predetermined region and the projection data E _A relates scattered coincidence is high in the measurement space 11 sampling Accumulates in density. On the other hand, the projection data storage area 60B, the projection data E _B are accumulated at a lower sampling density for scattering coincidence.

[0039] The t-theta memory 60 projection data E _A and E _B are accumulated in the respective projection data storage area 60A and 60B of can be transferred image reconstruction unit (e.g., host computer) 70. Here, the t-θ memory 6
The projection data transferred from 0 to the image reconstruction unit 70 includes the projection data E _A stored in the projection data storage area 60A for the coincidence line passing through the predetermined area in the measurement space 11, and the projection data E _A in the measurement space 11. For the coincidence line that does not pass through the predetermined area, the projection data storage area 60B
It has been a projection data E _B accumulate. The former of projection data E _A, there is a high sampling density but is of a narrow region in the t-theta plane, the latter projection data E _B are those stored at low sampling density.

[0040] Then, the image reconstruction unit 70, t-theta based from the memory 60 to the transferred projection data E _A and E _B, to calculate the spatial distribution of the incidence of the electron-positron pair annihilation in the measurement space 11 Perform image reconstruction. That is,
Interpolating the projection data E _B to be the same sampling density and sampling density of the projection data E _A, the image reconstruction based on the interpolated projection data E _B and the projection data E _A. The image display unit 80 includes an image reconstruction unit 70
To display the reconstructed image.

The PET according to the present embodiment operates as follows. That is, when the subject 10 to which the RI source is administered is placed in the measurement space 11 in the ring 20, photon pairs are emitted inside the subject 10 as the electron-positron pairs disappear. Among the photon pairs, the photon pairs that have flown along the ring surface are a number of photon detectors D _k (k =
1, 2, 3,..., N), a photon detection signal indicating that a photon has been detected is _generated by the two photon detectors D _i and D _j. Output from each of _i and D _j and input to the coincidence circuit 30.

The coincidence counting circuit 30 for inputting these photon detection signals determines whether or not the photon pairs having a predetermined energy (511 keV) are detected at the same time. the, the two-photon detectors photon pairs detected D _i and D _j detector identification signal-indicating each (I, J) is output. Then, the detector identification signal pair (I, J) is input to t-θ.
The conversion unit 40 sets t in the measurement space 11 for the coincidence line indicated by the detector identification signal pair (I, J).
Converted into coordinate values (T, θ ′) expressed in a −θ polar coordinate system,
This coordinate value (T, θ ′) is output.

The coordinate values (T, θ ') are stored in the area determination section 50.
To enter. The coordinate values (T, θ ') based on whether the coincidence line connecting the two photon detectors D _i and D _j having detected a photon pair with each other passes through the subject 10,
The determination is made by the area determination unit 50. If it is determined that the coincidence line passes through the subject 10,
coordinate values of the projection data storage area 60A of the t-theta memory 60 (T, θ ') a constant value to the address corresponding to the projection data E _A is the cumulative addition is stored. Conversely, if it is determined that the coincidence line does not pass through the subject 10, t-θ
The coordinate values (T,
theta 'constant value to the address corresponding to) the projection data E _B are cumulatively added is accumulated.

[0044] The t-theta projection data E _A and E _B are accumulated in the respective projection data storage area 60A and 60B of the memory 60 is transferred to the image reconstruction unit 70, the transferred projection data E _A and E Based on _B , the image reconstruction unit 70 allows the
The spatial distribution of the positron pair annihilation frequency is calculated and the image is reconstructed. Then, the reconstructed image is displayed on the image display unit 80.
Is displayed.

Next, the scattering correction performed based on the image reconstructed by the image reconstruction unit 70 will be described.
The arithmetic processing in the scattering correction is, for example, processing performed by a host computer that also performs image reconstruction. FIG. 2 is a flowchart of the scattering correction.

First, in step S1, a point response function is obtained. This point response function is obtained as follows. That is, a container filled with water in a container having substantially the same shape as the subject 10 is placed at a position where the subject 10 is to be placed in the measurement space 11, and the RI source is placed at the center position of the ring 20. Measurement is performed in the same manner as when measuring the subject 10 and t
The projection data is stored in the -θ memory 60. At this time, the area determination unit 50 stores the projection data over the entire area on the t-θ plane of the projection data storage area 60A of the t-θ memory 60. Then, an image is reconstructed by the reconstructed image unit 70 based on the projection data. The reconstructed image obtained in this way is called a point response function.

In step S2 following step S1, a convolution of this point response function with the reconstructed image G0 obtained by the image reconstruction unit 70 is calculated.
In 3, calculates back projection data E1 on the basis of the convolution result obtained in step S2, in step S4, the projection data is interpolated by the actual projection data E0 (image reconstructing unit 70 E _B and the projection data E _A ) is subtracted from the projection data E1 obtained in step S3. What is obtained as a result of the subtraction in step S4 is obtained by adding an error to the true projection data not including the scattering data.

In step S5 following step S4, the reconstructed image G1 is calculated by reconstructing the image based on the projection data as the subtraction result in step S4. In step S6, the reconstructed image G1 obtained in step S5 is calculated. And the error between the reconstructed image G0 and the region other than the region occupied by the subject 10. Then, in a step S7, it is determined whether or not this error is smaller than the reference value ε. If it is determined that the error is smaller than the reference value ε, the scattering correction process ends, and if not, the process proceeds to step S8. In step S8, the reconstructed image G calculated in step S5
1 is newly set as a reconstructed image G0, and the process returns to step S2.

As described above, the loop processing consisting of steps S2 to S8 is repeated until the error becomes smaller than the reference value ε. However, in the second and subsequent processes in this loop process, the reconstructed image G0 referred to in each of steps S2 and S6 is the reconstructed image G1 calculated in step S5 in the preceding process. And
When the error is determined to be less than the reference value ε in step S7 and the scattering correction process is completed, the reconstructed image G
0 (or G1) is a result of image reconstruction based on true projection data from which scattering data has been removed. The reconstructed image subjected to the scattering correction is also displayed on the image display unit 80.

In addition to the above-mentioned scattering coincidence, the S / N ratio of the reconstructed image is degraded.
And the sensitivity among the many photon detectors constituting the ring 20 is not uniform. Therefore, it is also preferable to perform transmission measurement and blank measurement to perform absorption correction and sensitivity correction in addition to scattering correction.

Next, a method of detecting the contour of the subject 10 will be described. As a method of detecting the contour of the subject 10,
It is also preferable to use an optical 3D scanner. Also,
A method of detecting the contour of the subject 10 using transmission data obtained by transmission measurement is also suitable. Hereinafter, the latter will be described. FIG.
FIG. 4 is an explanatory diagram of a method of detecting a contour of a subject 10 using transmission data. FIG. 3A shows the subject 1
0, along with the calibration RI source 12 and the ring 20, projection data (transmission data T (t, t) in a certain θ ′ direction in the polar coordinate system set in the measurement space 11.
θ ′)), and FIG. 3B shows the distribution of t−θ.
3 schematically shows a distribution of projection data (transmission data T (t, θ)) stored in a memory 60.

Transmission measurement refers to placing the subject 10 to which the RI source has not been applied at the same position as the emission measurement (measurement of the photon pair generated from the subject 10 to which the RI source has been applied), The measurement is performed by rotating the calibration RI source 12 around the subject 10 about an axis. Further, the transmission data is a t-θ memory 6 based on the transmission measurement.
The projection data stored in 0 is data used for correcting photon absorption in the subject 10.

As shown in the figure, the transmission data is projection data (a range B in FIG. 3) for a coincidence counting line that does not pass through the area occupied by the subject 10. The value is large and substantially uniform as compared with the projection data (range A in FIG. 3) for the counting line. Therefore, the contour of the subject 10 can be detected using this fact. The area determination unit 50 determines the contours of the subject 10 obtained in this manner by using the functions t _s (θ) and t
_e (θ) and store it at the time of emission measurement.
-Θ coordinate values output from the conversion unit 40 (T, θ ') is t _s (θ' depending on whether they meet _{a) ≦ T ≦ t e (θ} ') ... (1) relational expression, the subject It is determined whether or not the coincidence line passes through the area occupied by 10.

As described above, in the PET according to the present embodiment, projection data is stored at a low sampling density for coincidence lines that do not pass through the area occupied by the subject 10. The amount of projection data to be transferred to the image reconstruction unit 70 is small,
Therefore, the transfer time is short. For example, the diameter of the subject 10 is 1/3 of the diameter of the measurement space 11, the sampling density of the projection data storage area 60A is equivalent to the conventional one, and the sampling density of the projection data storage area 60B is Assuming that the sampling density is 1/16 with respect to the sampling density of the region 60A, P
The amount of the projection data to be transferred in the ET and the transfer time are 1/3 + 1/16. (1-1 / 3) = 3/8... (2) as compared with the case of the conventional PET. . Further, the ring configuration is multi-layer and the subject 1
When 0 is small (for example, when the subject 10 is a small animal such as a rat), not only the diameter of the subject 10 becomes smaller than the diameter of the measurement space 11 but also the projection data accumulation becomes larger at the end layer of the ring. Since the area 60A can reduce the amount of projection data to be stored at a high sampling density, the amount of projection data to be transferred is further reduced,
Transfer time is reduced.

In the PET according to the present embodiment, projection data is also accumulated for coincidence lines that do not pass through the area occupied by the subject 10, so that scattering correction can be performed with high accuracy. Therefore, also in the PET according to the present embodiment, a high-resolution reconstructed image equivalent to the reconstructed image obtained when accumulating projection data at a high sampling density on all t-θ planes as in the conventional PET is obtained. can get.

(Second Embodiment) Next, a second embodiment will be described. FIG. 4 is a configuration diagram of a positron CT apparatus according to the second embodiment. PE according to the present embodiment
T is different from PET according to the first embodiment in that a region determining unit 51 and a t-θ memory 61 are provided instead of the region determining unit 50 and the t-θ memory 60, respectively. .

The area determination section 51 receives the coordinate values (T, θ ′) output from the t-θ conversion section 40, and inputs the coordinate values (T, θ ′), similarly to the area determination section 50 in the first embodiment. The region through which the coincidence line represented by (T, θ ′) passes in the measurement space 11 is determined. However, the area determination unit 51 in the present embodiment determines that the coordinate value (T, θ ′)
It is determined whether or not the T value satisfies the relational expression of t _min ≦ T ≦ t _max (3) based on only the value.

Here, the t _min value and the t _max value are both constant values that do not depend on the θ value, and are output from the t-θ conversion unit 40 for the coincidence line passing through the area occupied by the subject 10. The T value is a value determined so as to always satisfy the expression (3). That is, based on the contour of the subject 10 on the t-θ plane, the t _min value and the t _max value are respectively the minimum (or less) and maximum value (or less) of the t coordinate value of the contour. Or more). This means that subject 1
This means that the area determination unit 51 determines whether or not the coincidence line passes through a spherical area including the area occupied by 0. In addition, the area determination unit 51 may determine the passing area based on the detector identification signal pair (I, J).

The t-θ memory (projection data storage means) 61 for storing projection data includes two or more (two in this embodiment) projection data storage areas 61A and 61B which are address spaces which do not overlap each other. And a constant value is cumulatively added to an address corresponding to the coordinate value (T, θ ′) to one of the projection data storage areas 61A and 61B,
The projection data on the photon pairs generated in the measurement space 11 is stored. Here, the projection data storage area 61A has a denser correspondence (sampling density) of addresses to coordinate values (T, θ ′) than the projection data storage area 61B, that is, per unit area on the t-θ plane. There are many addresses. The coarse / dense difference in the correspondence of the address to the coordinate value (t, θ) between the projection data storage areas 61A and 61B may be provided for both the t coordinate and the θ coordinate, or any one of them. May be provided. Note that, in the t-θ memory 61 shown in FIG.
4 schematically shows a distribution of projection data on a −θ plane.

One projection data storage area 61A has a coordinate value (T, θ) output from the t-θ conversion section 40 when the area determination section 51 determines that the coincidence line satisfies the expression (3). ') to cumulatively adding a constant value to the address corresponding to accumulate projection data E _a. The other projection data accumulation area 61B has the coordinate value (T, θ ′) output from the t-θ conversion section 40 when the area determination section 51 determines that the coincidence line does not satisfy Expression (3). cumulative addition to storing projection data E _B a constant value to the address corresponding to the. The projection data accumulation area 61B has the coordinate values (T, T) output from the t-θ conversion section 40 even when the area determination section 51 determines that the coincidence line satisfies the expression (3).
A constant value may be cumulatively added to the address corresponding to θ ′).

Therefore, even in the PET according to the present embodiment, the true coincidence count and the scatter coincidence count of the photon pairs generated due to the annihilation of the electron / positron pair in the predetermined area in the measurement space 11 are also stored in the projection data accumulation area 61A. projection data E _A relates are accumulated at a higher sampling density. On the other hand, the projection data storage area 61B, the projection data E _B are accumulated at a lower sampling density for scattering coincidence.

The operation of the PET according to this embodiment is substantially the same as that of the first embodiment. However, in the first embodiment, the coordinate values (T,
Whether or not the coincidence line represented by θ ′) passes through the subject 10 is determined by the area determination unit 50, and which of the two projection data storage areas 60A and 60B of the t-θ memory 60 is determined according to the result. While a certain value is cumulatively added to the crab to accumulate projection data, in the present embodiment,
The region determination unit 51 determines whether the T value of the coordinate values (T, θ ′) output from the t-θ conversion unit 40 satisfies Expression (3).
And a certain value is cumulatively added to one of the two projection data storage areas 61A and 61B of the t-θ memory 61 in accordance with the result, and the projection data is stored.

Therefore, in this embodiment, the conventional PE
Compared with T, the amount of projection data to be transferred from the t-θ memory 61 to the image reconstruction unit 70 is smaller, and the transfer time is shorter.
Also, highly accurate scattering correction can be performed. Compared with the first embodiment, in the present embodiment, the content of the determination in the area determination unit 51 is simple, but on the other hand, the projection data accumulation area 61A which is a high sampling density area.
Is large in projection data amount. However, ring 20
Is a substantially circular cross section when the subject 10 is cut in parallel to the slice plane of the subject 10.
When the subject 10 is placed such that the center of the circle is located on the center axis of the ring 20, the difference in effect between the present embodiment and the first embodiment is small.

(Third Embodiment) Next, a third embodiment will be described. FIG. 5 is a configuration diagram of a positron CT apparatus according to the third embodiment.

The PET according to this embodiment is a 3D-PET, and the ring 22 includes a measurement space 11 in which the subject 10 is placed, and a number of photon detectors for detecting incident photons are provided around a central axis. Are arranged in a ring shape and in a multilayer shape in the direction of the central axis, and the inter-slice collimator is removed. In these photon detectors, the light receiving surface is directed in the direction of the measurement space 11, and detects photons that fly and enter from the measurement space 11. A signal line is provided between each of the photon detectors and the coincidence counting circuit 32, and a photon detection signal corresponding to the energy of the detected photon is sent from the photon detector that has detected the photon to the coincidence counting circuit 32. Sent.

[0066] coincidence circuit 32 for inputting a photon detection signals output from the photon detector is of a predetermined two-photon detectors D _i and D _j in the ring 22 is generated due to electron-positron pair annihilation energy recognized by energy discriminating that simultaneously detected the photon pair having (511 keV), the two-photon detectors D _i and D _j detector identification signals I and J indicate the respective time thereof, as well as two photons that 2, each detector D _i and D _j belongs
A difference signal RD (Ring Difference) between the two single-layer rings is output.

An xy-θ-φ converter (coordinate conversion means) 42 for inputting the detector identification signal pair (I, J) and the ring difference signal RD output from the coincidence circuit 32 outputs a detector identification signal. signal-to (I, J) for coincidence line connecting together two photon detectors D _i and D _j having detected a photon pair indicated, x-y-θ-φ which is set within the measurement space 11
It is converted into coordinate values (x, y, θ, φ) expressed in a polar coordinate system and output. Here, θ and φ represent the azimuth of the coincidence line (broken arrow in the figure), and x and y represent the position in the orthogonal coordinate system on the projection plane perpendicular to the coincidence line. Represent.

The area determining unit 52 for inputting the coordinate values (x, y, θ, φ) determines the area through which the coincidence line represented by the coordinate values (x, y, θ, φ) passes in the measurement space 11. judge. For example, the subject 1 placed in the measurement space 11
It is determined whether or not the coincidence counting line passes 0.
Alternatively, it may be determined whether or not the coincidence line passes through the attention area of the subject 10. The area determination unit 52 may determine the passing area based on the detector identification signal pair (I, J) and the ring difference signal RD.

In this embodiment, when determining whether or not the coincidence line passes through the subject 10, the contour of the subject 10 is detected in the same manner as in the first embodiment, and The determination unit 52 stores the contour of the subject 10 as a curve on the xy plane for each (θ, φ) direction, and then stores the contour in the xy-θ-φ conversion unit 42.
It is determined whether or not the coincidence line passes through the area occupied by the subject 10 depending on whether or not the point indicated by the coordinate value (x, y, θ, φ) output from is inside the curve. .

The xy-θ-φ memory (projection data storage means) 62 for storing projection data has two or more (two in this embodiment) projection data storage areas 62A which are address spaces which do not overlap each other. And a constant value cumulatively added to an address corresponding to the coordinate value (x, y, θ, φ) to one of the projection data storage areas 62A and 62B, and the photon generated in the measurement space 11 is obtained. Accumulate projection data for pairs. Here, the projection data storage area 62A
Are coordinate values (x,
The correspondence (sampling density) of addresses to (y, θ, φ) is dense, that is, the number of addresses per unit volume in the xy-θ-φ space is large. This projection data storage area 62
Coordinate values (x, y, θ, φ) between A and 62B
The density difference of the correspondence of the address to
It may be provided for all of the θ coordinates and φ coordinates, or may be provided for some of them.
In the xy-θ-φ memory 62 shown in FIG. 5, the projection data storage areas 62A and 62B
Although the distribution of projection data on the xy plane in a certain (θ, φ) direction is schematically shown, actually, each (θ, φ) direction is projected on the xy plane. Data is accumulated.

One of the projection data accumulation areas 62A is determined by the area determination section 52 when the coincidence line passes through a predetermined area in the measurement space 11 (an area occupied by the subject 10 or an attention area in the subject 10). Sometimes xy
The coordinate values (x, y,
theta, and cumulatively adding the constant value to the address corresponding to phi) for storing projection data E _A. The other projection data storage area 62B
Is the coordinate value (x, y, θ, φ) output from the xy-θ-φ conversion unit 42 when the area determination unit 52 determines that the coincidence line does not pass through the predetermined area. cumulative addition to storing projection data E _B a constant value to a corresponding address. The projection data accumulation area 62B stores the coordinate values (x, x, y) output from the xy-θ-φ conversion section 42 even when the area determination section 52 determines that the coincidence line passes through the predetermined area. (y, θ, φ) may be cumulatively added to a fixed value.

[0072] Thus, the projection data storage area 62A, a true coincidence photon pairs generated in accordance with the electron-positron pair annihilation in the predetermined region and the projection data E _A relates scattered coincidence is high in the measurement space 11 sampling Accumulates in density. On the other hand, the projection data storage area 62B, the projection data E _B are accumulated at a lower sampling density for scattering coincidence.

[0073] The x-y-θ-φ projection data E _A and E _B are accumulated in the respective projection data storage area 62A and 62B of the memory 62 is transferred image reconstruction unit (e.g., host computer) 72 . Where xy
The projection data transferred from the −θ-φ memory 62 to the image reconstruction unit 72 includes projection data E _A stored in the projection data storage area 62A for coincidence lines passing through a predetermined area in the measurement space 11, and a projection data E _B stored in the projection data storage area 62B for coincidence lines not passing through the predetermined region in the measuring space 11. The former of projection data E _A, there is a high sampling density but x
-Y-theta-phi are those of a narrow region in space, the latter projection data E _B are those stored at low sampling density.

Then, the image reconstructing section 72 calculates the xy-θ
The projection data E _A and E transferred from the memory 62
Based on _B , the spatial distribution of the occurrence frequency of electron-positron annihilation in the measurement space 11 is calculated, and image reconstruction is performed. That is, by interpolating the projection data E _B to be the same sampling density and sampling density of the projection data E _A,
This is an image reconstruction based on the interpolated projection data E _B and the projection data E _A. The image display unit 82 displays the image reconstructed by the image reconstruction unit 72.

The PET according to the present embodiment operates as follows. That is, when the subject 10 to which the RI source is administered is placed in the measurement space 11 in the ring 22, photon pairs are emitted inside the subject 10 as the electron-positron pairs disappear. When the photon pair is detected by either of two photon detectors D _i and D _j of the multiple photon detectors constituting the ring 22, the photon detection signal indicating that detected photons are two that respectively output from the photon detector D _i and D _j, is input to the coincidence circuit 32.

The coincidence counting circuit 32 for inputting these photon detection signals determines whether or not the photon pairs having a predetermined energy (511 keV) are detected at the same time. the, the two-photon detectors that detected the photon pair D _i and D _j detector identification signal-indicating each (I, J) and the inter-ring difference signal RD is output. Then, this detector identification signal pair (I, J) and the inter-ring difference signal RD are input as xy.
The -θ-φ conversion unit 42 detects the detector identification signal pair (I,
J) and the coincidence line indicated by the ring-to-ring difference signal RD are converted into coordinate values (x, y, θ, φ) expressed in the xy-θ-φ polar coordinate system set in the measurement space 11. The coordinate values (x, y, θ, φ) are output.

The coordinate values (x, y, θ, φ) are input to the area determination section 52. The coordinate values (x, y, θ, φ ) based on whether the coincidence line connecting the two photon detectors D _i and D _j having detected a photon pair with each other passes through the subject 10, this region The determination is performed by the determination unit 52. If it is determined that the coincidence line passes through the subject 10, the xy-θ-φ memory 62 stores the address corresponding to the coordinate value (x, y, θ, φ) of the projection data storage area 62A. projection data E _A is stored constant value is cumulatively added. Conversely, if it is determined that the coincidence line does not pass through the subject 10, it corresponds to the coordinate value (x, y, θ, φ) of the projection data storage area 62B of the xy-θ-φ memory 62. projection data E _B is stored constant value at the address is cumulatively added.

[0078] The x-y-θ-φ projection data E _A and E _B are accumulated in the respective projection data storage area 62A and 62B of the memory 62 is transferred to the image reconstruction unit 72, the transferred projection data Based on E _A and E _B , the image reconstruction unit 72 calculates the spatial distribution of the occurrence frequency of the annihilation of the electron-positron pair in the measurement space 11 and reconstructs the image. Then, the reconstructed image is displayed by the image display unit 82.

As described above, in the PET according to the present embodiment, similarly to the case of the first embodiment, the subject 10
Since the projection data is stored at a low sampling density for the coincidence line that does not pass through the area occupied by, the amount of projection data to be transferred from the xy-θ-φ memory 62 to the image reconstruction unit 72 is Less, therefore
Transfer time is short. In particular, in the present embodiment, 3D-PET is used, and the projection data to be stored is much larger than in 2D-PET. Therefore, the effect of reducing the amount of projection data and shortening the transfer time of projection data is great.

That is, as in the case where the subject 10 is a small animal such as a rat, the size of the subject 10 in the ring axis direction is smaller than the thickness of the ring, and the diameter of the subject 10 is smaller than the diameter of the measurement space 11. Is smaller than
Projection data to be accumulated at a high sampling density is accumulated only in a certain central area on the xy plane for each (θ, φ) direction, as schematically shown in the projection data accumulation area 62A of FIG. Is done. Therefore, compared to the case of the first embodiment, the ratio of the amount of projection data to be stored at a high sampling density in the projection data storage area 62A is small, and the reduction of the projection data amount and the reduction of the projection data transfer time are achieved. The effect is great. Generally, this effect is even greater because the axial thickness of the 3D-PET ring is greater than the thickness of the 2D-PET ring.

In the PET according to the present embodiment, projection data is also accumulated for coincidence lines that do not pass through the area occupied by the subject 10, so that scattering correction can be performed with high accuracy. Therefore, even in the PET according to the present embodiment, high resolution equivalent to a reconstructed image obtained when projection data is accumulated at a high sampling density on all xy-θ-φ spaces like conventional PET is obtained. A reconstructed image is obtained.

(Fourth Embodiment) Next, a fourth embodiment will be described. FIG. 6 is a configuration diagram of a ring in the positron CT apparatus according to the fourth embodiment.

The overall structure of the PET according to this embodiment is the same as that of the third embodiment. However, in the present embodiment, the subject 10 placed in the measurement space 11 in the PET ring 22 is the head of a subject (a person or another animal),
Attention area 10A to be observed by this PET
Is the visual cortex in the back of the head. That is,
In the present embodiment, a brain activation experiment is performed for a region of interest (visual area) 10A.

For example, while showing a simple figure such as や or ×, a scenery, a pattern, etc. to the subject (subject) 10, the area of interest (visual area) 10 A is measured by PET, and projection data is accumulated. Obtain a reconstructed image based on the data.
On the other hand, without showing anything to the subject (subject) 10, P
The ET measures the area of interest (visual area) 10A, accumulates projection data, and obtains a reconstructed image based on the projection data. Then, subtraction is performed between these two reconstructed images, and a subject (subject) is determined based on the subtraction result.
The regions in the 10 brains that are activated by the visual stimulus are specified. Such an experiment or test is called a brain activation experiment.

In this brain activation experiment, the subject (subject) 1
0 (the visual cortex) 10A depending on the content of the visual stimulus given to the subject 0 or the subject (subject) 10
In many cases, the time during which the metal is strongly activated is as short as several tens of milliseconds to several seconds. In the case of repeatedly measuring, the subject (subject) 10 may become accustomed to the visual stimulus, and the brain activation may be weakened. Therefore, shortening of the measurement time by PET is strongly desired.

On the other hand, in order to reconstruct an image based on the projection data, it is necessary to accumulate projection data not only for the region of interest (visual area) 10A but also for coincidence counting lines passing through the subject (subject) 10. In order to perform the scattering correction, it is necessary to accumulate projection data even for coincidence lines that do not pass through the area occupied by the subject (subject) 10.

Therefore, when it is known in advance which region in the brain is to be activated, as in the case where a brain activation experiment is performed as a test, the region judging section 52 determines that the region of interest (visual area) 10A is It is determined whether or not the coincidence line passes through the area occupied, and the projection data accumulation area 62A accumulates projection data of the coincidence line passing through the area occupied by the attention area (visual area) 10A at a high sampling density. , The projection data storage area 62B is a region of interest (visual area).
Projection data may be accumulated at a low sampling density for coincidence lines that do not pass through the area occupied by 10A. If the image is reconstructed based on the projection data accumulated in this way, the total amount of projection data can be reduced. On the other hand, the obtained reconstructed image has a high S / N ratio after the scattering correction is performed. And attention area (visual area) 1
For 0A, the resolution is high.

The present invention is not limited to the above embodiment, and various modifications are possible. For example, in each of the first and third embodiments, the region determination unit determines whether the coincidence line passes through the region occupied by the subject or the region of interest in the subject, but is not limited to this. . The high-resolution area and the low-resolution area in the reconstructed image do not suddenly change at the boundary when the area determination unit determines the passing area of the coincidence line,
It gradually changes over a constant width area on both sides of the boundary. Therefore, it is assumed that the boundary at the time of judging the passing area of the coincidence line by the area judging unit is extended by a fixed width from the contour of the subject or the area of interest, and the coincidence counting that passes through the internal area of the extended boundary is performed. By accumulating projection data at a high sampling density only for lines, accumulating projection data at a low sampling density for coincidence counting lines that do not pass through the inner region of the expanded boundary, and reconstructing an image based on this projection data Thus, a high-resolution reconstructed image can be obtained over the entire subject or attention area.

Also, in the case of 3D-PET described in the third and fourth embodiments, as in the case of the second embodiment, the region determining section determines the region occupied by the subject or the region of interest. Not whether or not the coincidence line passes,
It may be determined whether or not the coincidence line passes through a spherical area including the area occupied by the subject or the area of interest.

It is preferable that the area judging section can freely set an area in the measurement space, which is a criterion for judging whether or not the coincidence line passes. In this case, when the shape and size are different depending on the subject, or when the same subject is placed at a different position or attention area, the projection data is stored at a high sampling density accordingly. The area to be set can be set appropriately.

The t-θ memory (or xy-θ)
-Φ memory) is provided with three projection data storage areas having different densities from each other, and the area determination unit determines whether the coincidence line passes through the attention area and whether it passes through the subject area. The projection data for the coincidence lines passing through is stored in the highest-density projection data storage area, and the projection data for the coincidence lines that do not pass through the attention area but pass through the subject area is stored in the medium-density projection data storage area. However, it is also preferable to store the projection data for the coincidence line that does not pass through the subject area in the projection data accumulation area having the lowest density. The reconstructed image obtained in this case has a high S / N ratio after the scattering correction has been performed, and has a high resolution for the region of interest, a medium resolution for the region of the subject other than the region of interest, and a non-subject region. Becomes low resolution.

[0092]

As described above in detail, according to the present invention, any one of a large number of photon detectors constituting a ring makes it possible to generate electrons and electrons in a measurement space in the ring. When a photon pair generated due to the annihilation of the positron pair is detected, a coordinate value expressed in a polar coordinate system with respect to a coincidence line connecting the pair of photon detectors is represented by coordinate conversion means (t).
-Θ converter, xy-θ-φ converter), and the area through which the coincidence line passes in the measurement space in the ring is determined by the area determination means. In accordance with this determination result, the projection data storage means (t-θ memory, x
-Y-θ-φ memory), a predetermined value is cumulatively added to an address corresponding to the coordinate value output from the coordinate conversion means in any one of two or more predetermined numbers of projection data storage areas, and the projection data is stored. Is done. Then, based on the projection data accumulated in each of the predetermined number of projection data accumulation areas of the projection data accumulation means, the image reconstructing means calculates the spatial distribution of the occurrence frequency of electron-positron annihilation in the measurement space, and reconstructs the image. An image is obtained.

Here, the predetermined number of projection data storage areas of the projection data storage means are different from each other in the correspondence of the addresses to the coordinate values indicating the position and orientation of the coincidence line, so that all the coordinate values are different. The amount of projection data to be transferred from the projection data storage unit to the image reconstructing unit is small, and the time required for the transfer is short, as compared with the conventional case where the correspondence of addresses to the image data is uniformly dense. . Therefore, the frame time can be reduced in the dynamic measurement. In addition, despite the fact that the amount of projection data is reduced as compared with the conventional case, among the reconstructed images obtained by the image reconstructing means, the correspondence is based on the projection data accumulated in the dense projection data accumulation area. A high resolution image can be obtained for the portion (subject image or attention area), and a reconstructed image having a high S / N ratio comparable to that of the related art can be obtained by performing scattering correction. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a positron CT apparatus according to a first embodiment.

FIG. 2 is a flowchart of a scattering correction.

FIG. 3 is an explanatory diagram of a method of detecting a contour of a subject.

FIG. 4 is a configuration diagram of a positron CT apparatus according to a second embodiment.

FIG. 5 is a configuration diagram of a positron CT apparatus according to a third embodiment.

FIG. 6 is a configuration diagram of a ring in a positron CT apparatus according to a fourth embodiment.

FIG. 7 is an explanatory diagram of scattering coincidence counting.

[Explanation of symbols]

10 subject, 10A attention area, 11 measurement space, 1
2 ... RI source for calibration, 20,22 ... Ring, 30,32
... Simultaneous counting circuit, 40 ... t-.theta. Converter, 42 ... xy-
.theta .-. phi. converter, 50, 51, 52... area determination unit, 60.
t-θ memory, 60A, 60B ... projection data storage area,
61: t-θ memory, 61A, 61B: projection data storage area, 62: xy-θ-φ memory, 62A, 62B ...
Projection data storage area, 70, 72... Image reconstruction unit, 8
0, 82: Image display unit.

Claims (8)

[Claims] 1. A ring in which a plurality of photon detectors each outputting a photon detection signal corresponding to the energy of an incident photon are arranged so as to surround a measurement space; A coincidence circuit that energy-discriminates photon pairs generated by annihilation of electron / positron pairs and outputs a detector identification signal indicating a photon detector pair that has detected each photon of the photon pair; Coordinate conversion means for outputting a coordinate value expressed in a polar coordinate system set in the measurement space for a coincidence line connecting the photon detector pairs to each other; and an area determination for judging a passing area of the coincidence line in the measurement space. Means, and two or more predetermined number of projection data storage areas having different densities of correspondence of addresses to coordinate values output from the coordinate conversion means. Then, in any one of the predetermined number of projection data accumulation areas according to the pass area determined by the area determination means, a fixed value is assigned to an address corresponding to the coordinate value output from the coordinate conversion means. Projection data accumulation means for accumulating and adding projection data, based on the projection data accumulated in each of the predetermined number of projection data accumulation areas of the projection data accumulation means,
A positron CT apparatus, comprising: image reconstruction means for calculating a spatial distribution of the frequency of occurrence of electron-positron pair annihilation in the measurement space and performing image reconstruction. 2. The positron CT apparatus according to claim 1, further comprising a scatter correction unit that performs scatter correction based on the image of the spatial distribution reconstructed by the image reconstruction unit. 3. The image processing apparatus according to claim 1, further comprising: a contour detecting unit configured to detect a contour of the subject placed in the measurement space, wherein the area determining unit determines the passing area based on the contour detected by the contour detecting unit. The positron CT apparatus according to claim 1, wherein: 4. The area determination means determines whether a coincidence counting line connecting the photon detector pairs indicated by the detector identification signal passes through a predetermined area in the measurement space. The means may include, among the predetermined number of projection data accumulation areas, the correspondence of the projection data accumulation areas to which the fixed value is cumulatively added when it is determined that the coincidence line passes through the predetermined area, 2. The positron CT apparatus according to claim 1, wherein the density is higher than the correspondence of the projection data storage areas. 3. 5. The apparatus according to claim 4, wherein the area determining means can freely set the predetermined area.
The positron CT apparatus according to the above. 6. The apparatus according to claim 4, wherein the area determination unit sets any one of an area occupied by the subject placed in the measurement space or a region of interest in the subject as the predetermined area. Positron CT device. 7. The area determining unit determines a predetermined area as an area obtained by adding a fixed width area to an area occupied by an object placed in the measurement space or a periphery of an attention area in the object. 5. The positron CT apparatus according to claim 4, wherein: 8. The method according to claim 1, wherein the predetermined area is a spherical area including any one of an area occupied by a subject placed in the measurement space and an area of interest in the subject. The positron CT apparatus according to claim 4, wherein