JPH10206546A - 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: Coordinate values showing a simultaneous counting line by a polar coordinate are converted into a location at different sampling density according to whether or not the 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 constitution a ring 20 passes through a subject 10. The location is output from a t-θconversion part 40. In a case where the simultaneous counting line passes through the subject 10, a constant value is accumulated and added at high sampling density. In another case where it does pass through the subject 10, another constant values is accumulated and added at 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 coincidence line that connects the photon detector pairs indicated by the detector identification signal to each other according to a correspondence relationship in which addresses for the polar coordinate system set in the measurement space have a dense / dense distribution. Coordinate conversion means for outputting an address corresponding to a coordinate value expressed in a polar coordinate system with respect to (4)
Based on the projection data accumulation means for accumulating a constant value to the address output from the coordinate transformation means and accumulating projection data, and (5) projection data accumulated in the projection data accumulation means,
Image reconstruction means for calculating a spatial distribution of occurrence frequency of electron-positron pair annihilation in the measurement space and performing image reconstruction.

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, a coordinate conversion unit connects the photon detector pairs indicated by the detector identification signal to each other in accordance with a predetermined correspondence between addresses and coordinate values in a polar coordinate system set in the measurement space. The address corresponding to the coordinate value expressed in the polar coordinate system is output. Then, the projection data accumulating means accumulates and adds a fixed value to the address output from the coordinate transforming means, accumulates projection data, and based on the projection data accumulated in the projection data accumulating means, Then, a spatial distribution of the frequency of occurrence of electron-positron annihilation in the measurement space is calculated, and a reconstructed image is obtained.

Here, since the correspondence of the addresses to the coordinate values in the polar coordinate system set in the measurement space has a coarse / dense distribution in the coordinate conversion means, the correspondence of the addresses to all the coordinate values is uniform. The amount of projection data to be stored by the projection data storage means and transferred to the image reconstruction means is smaller than in the case where the density is high. 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 the subject placed in the measurement space, wherein the coordinate transforming means has a density distribution 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 coordinate conversion means uses the coincidence line based on this contour in accordance with the correspondence having a density distribution corresponding to the passing area in the measurement space. A certain value is cumulatively added to any one of the predetermined number of projection data storage areas of the projection data storage means.

Further, the coordinate conversion means may be freely set with respect to the coarse / fine distribution. In this case, the correspondence of the address to the coordinate value representing the coincidence line is appropriately set by the coordinate conversion means according to the subject and the attention area placed in the measurement space.

Further, the coordinate conversion means may determine whether the coincidence counting line connecting the photon detector pairs indicated by the detector identification signal passes through a predetermined area in the measurement space. The correspondence of the address to the coordinate value when passing through the area may be denser than the correspondence of the address to the coordinate value when the coincidence line does not pass through the predetermined area. In this case, depending on whether or not the coincidence line connecting the photon detector pairs indicated by the detector identification signal passes through a predetermined region in the measurement space, when the coincidence line passes through the predetermined region, the projection data is stored. A constant value is cumulatively added to a projection data storage area having a close correspondence among a predetermined number of projection data storage areas of the means, and otherwise, a constant value is cumulatively added to a projection data storage area having a rough correspondence. Then, the projection data is accumulated. Then, a high-resolution image is obtained for a predetermined area in the reconstructed image.

Furthermore, the coordinate conversion 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 coordinate conversion means is characterized in that a predetermined area is an area obtained by adding a fixed width area to either the area occupied by the object placed in the measurement space or the area of interest 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.

[0019] Further, the coordinate conversion means is characterized in that a spherical area including either an area occupied by the object placed in the measurement space or an attention area in the object is set as the predetermined area. Is also good. In this case, it is easy to convert the coordinate value into the address in the coordinate conversion means.

Further, the coordinate conversion means may be characterized in that the density of the correspondence changes stepwise at the boundary of the predetermined area, and the density of the correspondence changes from near the center of the predetermined area. It may be characterized by gradually changing over the periphery of the measurement space,
The density of the correspondence may gradually change in a fixed width area around the boundary of the predetermined area. In any of these cases, the coordinate conversion means preferably converts the coordinate values to addresses.

[0021]

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 scattering coincidence counting and the scattering 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 radiation) emitted by one or both of them and scattered by an electron-positron annihilation event 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 of 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 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. 14 is an explanatory diagram of the scattering coincidence counting. FIG. 14 (a)
Is the measurement space 1 along with the subject 10 and the ring 20.
FIG. 14B also shows the distribution of projection data (emission data E (t, θ ′)) in a certain θ ′ direction in the polar coordinate system set to 1, and FIG. Projection data (emission data E (t,
θ)) is schematically shown. FIG. 14 (a)
As shown in the figure, the coincidence counting line passing through the area occupied by the subject 10 in the projection data accumulated by performing the emission measurement by placing the subject 10 to which the RI source is administered in the measurement space 11 in the ring 20. In the projection data (range A of the projection data distribution in FIG. 14A), the scattering coincidence count is accumulated in addition to the true coincidence count, and the subject 1
In the projection data for the coincidence counting line that does not pass through the area occupied by 0 (the range B of the projection data distribution in FIG. 14A), only the scatter coincidence is accumulated.

That is, the projection data accumulation area of the t-θ memory 60 for accumulating projection data for each value of the direction θ and the distance t from the origin for all coincidence lines detected simultaneously is shown in FIG. As shown in FIG.
The area where the projection data of the coincidence line passing through the area occupied by 0 is stored (area A of the projection data in FIG. 14B) and the projection of the coincidence line not passing through the area occupied by the subject 10 It can be divided into an area where data is stored (area B of projection data in FIG. 14B). Using the above, the scattering correction estimates 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 10, and calculates the estimated scattering data. By subtracting from the total projection data, projection data relating to true coincidence counting is obtained.

What is important in this 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 the 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 in 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 a ring separated from each other by an 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-.theta. Converter (coordinate conversion means) 40 for inputting the detector identification signal pair (I, J) output from the coincidence 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, to convert the coordinate values expressed in the set t-theta polar coordinates within the measurement space 11 (T, θ '), the Further, the coordinate value (T, θ ′) is converted into an address on a t-θ memory 60, which will be described later, according to a predetermined correspondence, and the address is output. Where T represents the distance between this coincidence line and the origin of the t-θ polar coordinate system,
θ ′ indicates the direction of the coincidence line.

The conversion from the coordinate values to the addresses in the t-θ converter 40 is performed based on the following determination. That is, the t-θ conversion unit 40 calculates the coordinate value (T,
The region through which the coincidence line represented by θ ′) passes in the measurement space 11 is determined. 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 a region of interest in 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). t-θ converter 40
May determine the passing area based on the detector identification signal pair (I, J). Note that the t-θ converter 40 needs to detect and store the contour of the subject 10 in advance when determining whether or not the coincidence line passes through the subject 10. Will be described later.

The t-θ converter 40 has a dense / dense distribution of the correspondence (sampling density) of the addresses to the coordinate values, and performs sampling when it is determined that the coincidence line passes through the area occupied by the subject 10. The density is higher than the sampling density in the other case, that is, the number of addresses per unit area on the t-θ plane is larger. The density difference in this sampling density is
It may be provided for both the t coordinate and the θ coordinate, or may be provided for any one of them.

The t-θ memory (projection data storage means) 60 for accumulating the projection data accumulates a constant value to the address output from the t-θ conversion section 40 and calculates the photon pairs generated in the measurement space 11. Is stored. The projection data storage area of the t-θ memory 60 is 2
The above (two in the present embodiment) projection data storage area 60
A and 60B. 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.

On the other hand, the projection data accumulation area 60A is used by the t-θ conversion section 40 for the coincidence counting line passing through a predetermined area (the area occupied by the subject 10 or the area of interest in the subject 10) in the measurement space 11. cumulatively adding a constant value to the outputted addresses to storing projection data E _a. The other projection data storage region 60B is for coincidence line which does not pass through the predetermined area, and cumulatively adding the constant value to the address output from the t-theta conversion unit 40 stores the projection data E _B.

[0040] 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.

[0041] 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.

[0042] 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 circuit 30 that inputs these photon detection signals allows
Photon pair having a predetermined energy (511 keV) is determined whether or not the detected simultaneously, when it is determined that coincidence is two photon detectors D _i which detected the photon pair and D _j detector identification signal-indicating each (I, J) is output.

Then, this detector identification signal pair (I, J)
Are detected by the t-θ conversion unit 40 by the detector identification signal pair (I,
The coincidence counting line indicated by J) is converted into a coordinate value (T, θ ′) expressed in the t-θ polar coordinate system set in the measurement space 11 and further corresponds to this coordinate value (T, θ ′). The address is converted to an address, and this address is output from the t-θ converter 40. Upon conversion to address from the coordinate values, the address coincidence line connecting the two photon detectors D _i and D _j having detected a photon pair each other corresponding with high sampling density in the case is to pass through the subject 10 Is converted to
Otherwise, it is converted to the corresponding address at a low sampling density.

The address output from the t-θ conversion unit 40 is input to the t-θ memory 60, and a certain value is cumulatively added to the address in the t-θ memory 60. Here, if, when the coincidence lines are those that pass through an object 10, t-theta projection data E _A constant value is cumulatively added to the address of the projection data storage area 60A of the memory 60 is accumulated You. Conversely, the coincidence lines when those that do not pass through the subject 10 is a constant value to the address of the projection data storage area 60B of the t-theta memory 60 is projection data E _B are cumulatively added is accumulated.

[0046] 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, t
The -θ conversion unit 40 accumulates projection data at a constant sampling density over the entire area 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, the 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, an image is reconstructed based on the projection data as a result of the subtraction in step S4 to calculate a reconstructed image G1, and 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.

The factors that degrade the S / N ratio of the reconstructed image are, in addition to the above-described scattering coincidence, the subject 10
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 for 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 time of emission measurement (measurement of a pair of photons 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 this figure, the transmission data is the projection data for the coincidence counting line that does not pass through the area occupied by the subject 10 (range B in FIG. 3). 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. t-theta conversion unit 40 stores the thus function as a variable contour theta values of the subject 10 determined in the t _s (θ) and t _e (θ), further, the coordinates on the basis of the function Prepare a conversion table from values to addresses.

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 and the transfer time of the projection data to be transferred in the ET are 1/3 + 1/16. (1-1 / 3) = 3/8 (1) 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.

Further, in the PET according to the present embodiment, since the projection data is also stored for the coincidence line that does not pass through the area occupied by the subject 10, 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.

Next, a preferred example of the correspondence (sampling density) of the address to the coordinate value in the t-θ converter 40 will be described with reference to FIGS. Each of these figures shows a cut surface of the ring 20 and the slice surface of the subject 10 and sampling density distributions in two directions θ forming 90 degrees with each other, but the same applies to each θ direction. is there.

The sampling density distribution shown in FIG.
For values respectively, t values in the case of passing through a region occupied by the object 10 is the coincidence lines _{(t s (θ) <t} <
t _e (θ)), otherwise coarse. The t-θ conversion unit 40 prepares a correspondence between addresses and coordinate values based on the contour of the subject 10.

The sampling density distribution shown in FIG.
For each a value, t value when passing through the area obtained by adding the areas of constant width Delta] t in the region occupied by the object 10 is the coincidence lines _{(t s (θ) -Δt <} t <t e (θ) + Δt)
Is coarse, otherwise coarse. t
The -θ conversion unit 40 prepares a correspondence relationship between the coordinate values and the addresses based on an area obtained by expanding the contour of the subject 10 by a constant width Δt.

The sampling density distribution shown in FIG.
For values respectively, t values in the case of passing through a region occupied by the object 10 is the coincidence lines _{(t s (θ) <t} <
t _e (θ)) relative to a dense, t values in the case where a region obtained by adding a region of constant width Δt in the region occupied by the object 10 is the coincidence lines do not pass through _{(t <t s (θ)} -Δt , T _e
(θ) + Δt <t), and the t value (t) when the coincidence line passes through the other area is coarse.
_{s (θ) -Δt <t <} t s (θ), t e (θ) <t <t e (θ) + Δ
In t), it gradually decreases / increases. t-θ converter 40
Prepares a correspondence between addresses and coordinate values based on the contour of the subject 10 and an area obtained by expanding the contour.

The sampling density distribution shown in FIG.
For values respectively, t values in the case of passing through a region occupied by the object 10 is the coincidence lines _{(t s (θ) <t} <
t _e (θ)), and gradually becomes coarser as the distance from this region increases. The t-θ conversion unit 40 calculates the distance based on the contour of the subject 10 and the distance from the contour.
The correspondence of the address to the coordinate value is prepared.

When accumulating projection data in accordance with the sampling densities shown in FIGS. 5 to 7, a high-resolution reconstructed image can be obtained over the entire subject. That is,
The high-resolution area and the low-resolution area in the reconstructed image do not change abruptly at the boundary of the sampling density of the coincidence line, but change gradually over a fixed width area on both sides of the boundary. Therefore, the range in which the projection data is stored at a high sampling density is assumed to be expanded by a certain width around the area occupied by the subject in addition to the area occupied by the subject. If the image is reconstructed based on the data, a high-resolution reconstructed image can be obtained over the entire subject.

FIGS. 4 to 7 described above show the object 10
Although a preferred example of the sampling density distribution is shown by focusing on, the same applies to the sampling density distribution in the case of focusing on a region of interest (for example, visual cortex) which is a partial region in the subject (for example, head) 10. is there. For example, the sampling density distribution shown in FIG. 8 corresponds to that shown in FIG. 4, and for each θ value, the t value when the coincidence line passes through the attention area 10A in the subject 10 is shown. Is dense, otherwise it is coarse. However, there are other suitable examples of the sampling density distribution when attention is paid to the attention area in the subject 10 as shown in FIGS.

The sampling density distribution shown in FIG.
For each value, the t value (t _s1 (θ) <t <t) when the _coincidence line passes through the attention area 10A
_e1 (θ)), the t value (t <t) when the coincidence line does not pass through the area occupied by the subject 10.
t _s2 (θ) and t _e2 (θ) <t) are rough, and the t value ( _ts2 (θ) <t) when the _coincidence line does not pass through the region of interest 10A but passes through the subject 10 <T _s1 (θ),
For t _e1 (θ) <t <t _e2 (θ)), the density is intermediate.
In addition, the sampling density distribution shown in FIG. 10 indicates that, for each θ value, the t value (t) when the coincidence line does not pass through the region of interest 10A but passes through the subject 10.
_{For s2} (θ) <t < _ts1 (θ) and _te1 (θ) <t < _te2 (θ), the values gradually increase and decrease. The functions t _s1 (θ) and t _e1 (θ) represent the contour of the attention area 10A, and the functions t _s2 (θ) and t _e2 (θ) represent the contour of the subject 10, respectively.

In both cases of FIGS. 9 and 10,
The t-θ converter 40 includes the subject 10 and the attention area 10A.
Based on each contour, a correspondence between addresses and coordinate values is prepared. Note that the attention area 10A exists inside the subject 10 and its outline cannot be detected. However, for example, when a brain activation experiment is performed as a test using PET, the subject (head) There is no problem because it is known in advance which region in 10 is to be activated.

In any of the above sampling density distributions, projection data is accumulated at a high sampling density for the coincidence line passing through the subject 10 (or the area of interest 10A). The image has a high resolution for the subject 10 (or the attention area 10A). However, FIG. 9 or FIG.
The reconstructed image obtained based on the projection data accumulated according to the sampling density distribution shown in FIG.
0A has a high resolution, and the area of the subject 10 other than the attention area 10A has a medium resolution.
Regions other than 0 have low resolution. On the other hand, since the projection data is accumulated at a low sampling density for the coincidence line that does not pass through the area occupied by the subject 10, the projection data amount is smaller and t-θ The capacity of the memory 60 is small,
Also, the time required to transfer the projection data from the t-θ memory 60 to the image reconstruction unit 70 is short. Furthermore, a reconstructed image having a high S / N ratio can be obtained by performing scattering correction based on projection data accumulated for coincidence lines that do not pass through the area occupied by the subject 10 (or the area of interest 10A).

(Second Embodiment) Next, a second embodiment will be described. FIG. 11 is a configuration diagram of a positron CT apparatus according to the second embodiment. P according to the present embodiment
ET is t- compared to PET according to the first embodiment.
The difference is that a t-θ conversion unit 41 and a t-θ memory 61 are provided instead of the θ conversion unit 40 and the t-θ memory 60, respectively.

The t-θ converter 41 is, like the t-θ converter 40 in the first embodiment, a coincidence counting circuit 30.
Detector identification signal pair (I, J) output from
Regarding the coincidence line connecting the two photon detectors that have detected the photon pair indicated by the detector identification signal pair (I, J), the coordinate value represented by the t-θ polar coordinate system set in the measurement space 11 ( T, θ ′), and further converts the coordinate values (T, θ ′) into a t-θ memory 6 according to a predetermined correspondence relationship.
The address is converted to the upper address, and the address is output. However, in the present embodiment, the correspondence (sampling density) of the address to the coordinate value in the t-θ converter 41 is such that the T value of the coordinate value (T, θ ′) is t _min ≦ T ≦ t _max . (2) When the relational expression is satisfied, the density is higher, that is, the number of addresses per unit area on the t-θ plane is larger than when the relational expression is not satisfied. The density difference in the sampling density may be provided for both the t coordinate and the θ coordinate, or may be provided for any one of them.

Here, the t _min value and the t _max value are both constant values independent of the θ value, and are obtained by the t-θ converter 41 for the coincidence line passing through the area occupied by the subject 10. The value is a value determined so as to always satisfy the expression (2). 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 sampling density differs depending on whether or not the coincidence line passes through a spherical area including the area occupied by 0.

The t-θ memory (projection data storage means) 61 for accumulating the projection data accumulates a constant value to the address output from the t-θ conversion unit 41, and adds the address to the measurement space 11.
And accumulates projection data for the photon pairs generated in step (1). The projection data storage area of the t-θ memory 61 can be divided into two or more (two in this embodiment) projection data storage areas 61A and 61B. FIG.
The t-θ memory 61 shown in FIG.
The distribution of projection data on the t-θ plane for each of 1A and 61B is schematically shown.

One projection data storage area 61A is
(2) Simultaneous counting line satisfying formula, and cumulatively adding the constant value to the address output from the t-theta conversion unit 40 stores the projection data E _A. The other projection data storage area 61
B is a coincidence line that does not satisfy the expression (2).
t-theta a constant value outputted from address converter 40 cumulative addition to storing projection data E _B.

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 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 sampling density according to whether or not the coincidence counting line represented by the detector identification signal pair (I, J) output from the coincidence counting circuit 30 passes through the subject 10, t
While a fixed value is cumulatively added to one of the two projection data storage areas 60A and 60B of the -θ memory 60 to store projection data, in the present embodiment, the coincidence line is defined in the t-θ polar coordinate system. One of the two projection data storage areas 61A and 61B of the t-θ memory 61 at a sampling density according to whether or not the T value of the represented coordinate values (T, θ ′) satisfies the expression (2). Is that a constant value is cumulatively added to the data to accumulate projection data.

Therefore, also in this embodiment, the conventional PE
As compared with T, the amount of projection data stored in the t-θ memory 61 is small, the capacity of the t-θ memory 61 is small, and the projection data is transferred from the t-θ memory 61 to the image reconstruction unit 70. The transfer amount is small and the transfer time is short. Also, highly accurate scattering correction can be performed. The first
Compared to the embodiment, the present embodiment is simpler in the content of the conversion process to the address in the t-θ converter 41,
On the other hand, the amount of projection data stored in the projection data storage area 61A, which is a high sampling density area, is large. However, when the subject 10 is cut in parallel with the slice plane of the ring 20, the cross-sectional shape is substantially circular, and the subject 10 is positioned such that the center of the subject 10 is located on the center axis of the ring 20. , The difference in effect between the present embodiment and the first embodiment is slight.

(Third Embodiment) Next, a third embodiment will be described. FIG. 12 is a configuration diagram of a positron CT apparatus according to the third embodiment.

The PET according to the present 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.

[0078] 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
Converted to coordinate values (x, y, θ, φ) expressed in polar coordinate system,
Further, the coordinate value (x, y, θ, φ) is converted into an address on the xy-θ-φ memory 62 according to a predetermined correspondence, and the address is output. Here, θ and φ represent the azimuth of the coincidence line (broken arrow in the figure), and x and y represent the projection plane (Proj
Section Plane) on a rectangular coordinate system.

The conversion from the coordinate values to the addresses in the xy-θ-φ converter 42 is performed based on the following determination. That is, the xy-θ-φ conversion unit 42 determines an area through which the coincidence line represented by the coordinate values (x, y, θ, φ) 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, subject 10
It may be determined whether or not the coincidence line passes through the attention area. The xy-θ-φ converter 42 may determine the passage area based on the detector identification signal pair (I, J) and the ring difference signal RD.

In the present embodiment, when converting to an address corresponding to the passing area of the coincidence line, the first
In the same manner as in the embodiment, the contour of the subject 10 is detected in advance, and the xy-θ-φ conversion unit 42 converts the contour of the subject 10 on the xy plane for each (θ, φ) direction. Of the coordinate values (x, y, θ,
Depending on whether the point indicated by φ) is inside the curve,
It is determined whether or not the coincidence line passes through the area occupied by the subject 10.

The xy-θ-φ converter 42 determines that the coincidence line passes through the area occupied by the subject 10 because the xy-θ-φ converter 42 has a coarse / dense distribution of the correspondence of the address to the coordinate value (sampling density). In this case, the sampling density is higher than the sampling density in the other cases, that is, the number of addresses per unit volume in the xy-θ-φ space is larger. The coordinate values (x, y, θ, θ) between the projection data storage areas 62A and 62B
The coarse / dense difference in the correspondence of the address to φ) may be provided for all of the x coordinate, y coordinate, θ coordinate, and φ coordinate, or may be provided for some of them.

It should be noted that x of 3D-PET according to this embodiment is
The sampling density distribution in the -y-θ-φ converter 42 is also preferably the same as that in the 2D-PET t-θ converter 40 (FIGS. 4 to 10) according to the first embodiment. However, the sampling density distribution in the case of 2D-PET was expressed as a function of the variable t for each θ value, whereas the sampling density distribution in the case of 3D-PET was a variable x for each (θ, φ) direction. And as a function of y.

An xy-θ-φ memory (projection data storage means) 62 for accumulating projection data accumulates a fixed value to the address output from the xy-θ-φ conversion section 42 and performs measurement. This is for accumulating projection data on photon pairs generated in the space 11. The projection data storage area of the xy-θ-φ memory 62 can be divided into two or more (two in this embodiment) projection data storage areas 62A and 62B. In the xy-θ-φ memory 62 shown in FIG. 12, two (θ, φ) directions and (θ ′, φ) are set for each of the projection data storage areas 62A and 62B.
Although the distribution of projection data on the xy plane in each of the (φ ′) directions is schematically illustrated, in practice, each (θ,
φ) Projection data on the xy plane is accumulated for each direction.

On the other hand, the projection data storage area 62A is provided for the coincidence counting line passing through a predetermined area (the area occupied by the subject 10 or the area of interest in the subject 10) in the measurement space 11 for xy-θ-φ. cumulatively adding a constant value to the address output from the conversion unit 42 to accumulate the projection data E _a. The other projection data storage area 62B has xy-θ-φ for coincidence lines that do not pass through the predetermined area.
Cumulatively adding a constant value to the address output from the conversion unit 42 to accumulate the projection data E _B.

[0086] 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.

[0087] 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 that inputs these photon detection signals determines whether or not the photon pairs having the predetermined energy (511 keV) are detected at the same time. the two-photon detectors photon pairs detected 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)
The xy-θ-φ converter 42 that inputs the inter-ring difference signal RD sets the coincidence counting line indicated by the detector identification signal pair (I, J) and the inter-ring difference signal RD in the measurement space 11. Is converted into a coordinate value (x, y, θ, φ) expressed in the xy-θ-φ polar coordinate system, and further converted to an address corresponding to the coordinate value (x, y, θ, φ). This address is output from the xy-θ-φ converter 42. Upon conversion to address from the coordinate values, the address coincidence line connecting the two photon detectors D _i and D _j having detected a photon pair each other corresponding with high sampling density in the case is to pass through the subject 10 To the corresponding address at a low sampling density otherwise.

The address output from the xy-θ-φ conversion unit 42 is input to an xy-θ-φ memory 62,
A fixed value is cumulatively added to the address in the y-θ-φ memory 62. Here, if the coincidence line passes through the subject 10, the xy-θ-φ memory 62
A constant value to the address of the projection data storage area 62A is the projection data E _A is the cumulative addition is stored in. Conversely, if the coincidence line does not pass through the subject 10, the projection data storage area 6 in the xy-θ-φ memory 62
A constant value is cumulatively added to that address of 2B to obtain projection data E
_B accumulates.

The projection data E _A and E _B stored in the projection data storage areas 62A and 62B of the xy-θ-φ memory 62 are transferred to the image reconstructing section 72, and 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, as in 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
The projection data to be stored at a high sampling density is shown in FIG.
As schematically shown in the projection data storage area 62A of
Each (θ, φ) direction is accumulated only in a certain central area on the xy plane. 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, 3D-PET
This effect is even greater because the axial thickness of the ring is thicker 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 accurately. 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. 13 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, a target area (visual area) 10A is measured by PET while a simple figure such as や or × or a scenery / pattern is shown to the subject (subject) 10, 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, such as when a brain activation experiment is performed as an examination, the xy-θ-φ conversion unit 42 (Visual area) Coordinate values (x, y, θ, φ) representing the coincidence line are converted into addresses at different sampling densities according to whether or not the coincidence line passes through the area occupied by 10A, and the projection data is obtained. The accumulation area 62A accumulates projection data of coincidence lines passing through the area occupied by the attention area (visual area) 10A at a high sampling density, and the projection data accumulation area 62B accumulates the attention area (visual area) 10A.
The projection data may be accumulated at a low sampling density even for coincidence lines that do not pass through the area occupied by. 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, and a high resolution is obtained for the attention area (visual area) 10A.

The present invention is not limited to the above embodiment, but can be variously modified. For example, even in the case of 3D-PET described in the third and fourth embodiments, as in the case of the second embodiment, the xy-θ-φ conversion unit outputs the area occupied by the subject or Depending on whether or not the coincidence line passes through the region of interest or whether the coincidence line passes through a spherical region including the region of interest or the region occupied by the object, Coordinate values (x, y, θ,
φ) may be converted to an address.

The t-θ memory and the xy-θ-φ
It is preferable that the memory can freely set the distribution of the sampling density when converting the coordinate value representing the coincidence counting line into the address. 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.

[0104]

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, coincidence counting that connects the pair of photon detectors to each other is performed by coordinate conversion means (t-θ conversion unit and xy-θ-φ conversion unit). The coordinate value of the line expressed in the polar coordinate system is converted into an address according to a predetermined correspondence, and the address is output. Projection data storage means (t-θ memory, xy-θ-
A certain value is cumulatively added to the address output from the coordinate conversion means in any one of two or more predetermined numbers of projection data storage areas of the φ memory) to store projection data. 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, since the correspondence of the addresses to the coordinate values indicating the position and orientation of the coincidence line has a coarse / dense distribution in the coordinate conversion means, the correspondence of the addresses to all the coordinate values is uniform. 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 in which the density is high. 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 an explanatory diagram of a first preferred example of a correspondence relationship between a coordinate value and an address in a t-θ conversion unit.

FIG. 5 is an explanatory diagram of a second preferred example of a correspondence relationship between addresses and coordinate values in a t-θ conversion unit.

FIG. 6 is an explanatory diagram of a third preferred example of a correspondence relationship between addresses and coordinate values in a t-θ conversion unit.

FIG. 7 is an explanatory diagram of a fourth preferred example of a correspondence relationship between addresses and coordinate values in a t-θ conversion unit.

FIG. 8 is an explanatory diagram of a fifth preferred example of a correspondence relationship between addresses and coordinate values in a t-θ conversion unit.

FIG. 9 is an explanatory diagram of a sixth preferred example of a correspondence relationship between addresses and coordinate values in a t-θ conversion unit.

FIG. 10 is an explanatory diagram of a seventh preferred example of a correspondence relationship between a coordinate value and an address in a t-θ conversion unit.

FIG. 11 is a configuration diagram of a positron CT apparatus according to a second embodiment.

FIG. 12 is a configuration diagram of a positron CT apparatus according to a third embodiment.

FIG. 13 is a configuration diagram of a ring in the positron CT apparatus according to the fourth embodiment.

FIG. 14 is an explanatory diagram of scattering coincidence counting.

[Description of Signs] 10 subject, 10A attention area, 11 measurement space, 1
2 ... RI source for calibration, 20,22 ... Ring, 30,32
... Simultaneous counting circuit, 40, 41 ... t-.theta. Converter, 42 ... x
-Y-θ-φ converter, 60... T-θ memory, 60A, 6
0B: Projection data storage area, 61: t-θ memory, 61
A, 61B ... projection data accumulation area, 62 ... xy-θ-
φ memory, 62A, 62B ... projection data storage area, 7
0, 72: image reconstructing unit, 80, 82: image display unit.

Claims (11)

[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 pairs; and a coincidence counting circuit set in the measurement space. According to a correspondence relationship in which the address with respect to the coordinate value in the polar coordinate system has a coarse-dense distribution, a coordinate for outputting an address corresponding to the coordinate value expressed in the polar coordinate system for a coincidence line connecting the photon detector pairs indicated by the detector identification signal. Conversion means; projection data accumulation means for accumulating projection data by accumulatively adding a fixed value to the address output from the coordinate conversion means; Image reconstruction means for calculating a spatial distribution of the frequency of occurrence of electron-positron annihilation in the measurement space based on the projection data stored in the data storage means and performing image reconstruction. Positron CT device. 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 further comprises: contour detection means for detecting a contour of a subject placed in the measurement space, wherein the coordinate conversion means has the coarse / dense distribution based on the contour detected by the contour detection means. The positron CT apparatus according to claim 1, wherein: 4. The apparatus according to claim 1, wherein said coordinate conversion means is capable of freely setting the density distribution.
The positron CT apparatus according to the above. 5. The coincidence counting line according to 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. Wherein the correspondence of addresses to coordinate values when passing through the predetermined area is denser than the correspondence of addresses to coordinate values when the coincidence line does not pass through the predetermined area. Item 7. A positron CT apparatus according to Item 1. 6. The coordinate conversion unit according to claim 5, wherein the predetermined area is any one of an area occupied by the subject placed in the measurement space and a region of interest in the subject. Positron CT device. 7. The coordinate conversion means sets 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, as the predetermined area. The positron CT apparatus according to claim 5, wherein: 8. The method according to claim 1, wherein the coordinate conversion unit sets a spherical area including any one of an area occupied by the subject placed in the measurement space and an attention area in the subject as the predetermined area. The positron CT apparatus according to claim 5, wherein 9. The positron CT apparatus according to claim 5, wherein said coordinate transformation means changes the density of said correspondence stepwise at a boundary of said predetermined area. 10. The positron CT according to claim 5, wherein said coordinate conversion means changes the density of said correspondence gradually from near a center of said predetermined area to a periphery of said measurement space. apparatus. 11. The positron CT apparatus according to claim 5, wherein said coordinate transformation means changes the density of said correspondence gradually in a fixed width area around a boundary of said predetermined area.