JP2006177799A - Positron ct system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positron CT system where a test subject will not be exposed, when data for absorption correction are being acquired. <P>SOLUTION: The positron CT system 1 acquires coincidence counting data, indicating on which line a photon pair has been generated through electron-positron annihilation in a measuring space 12 by a coincidence counting means 50. At the same time, by measuring the test subject 10 optically, to acquire outline data by a test subject shape measurement apparatus 30, on the basis of the coincidence counting data and the outline data thus acquired, and by performing absorption correction by a corrected projection data acquisition means 60, corrected projection data which are absorption-corrected projection are acquired data. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a positron CT apparatus.

  A positron emission computed tomography (PET) is applied to research or clinical examinations of living organisms and disease patients, and positron emitting nuclides (hereinafter referred to as “RI”) that are injected into living organisms and disease patients. It is an apparatus for imaging the distribution of the radiation source) and seeing biological functions. The distribution of the RI radiation source injected into the body of the subject is a photon detector in which a pair of photons emitted in directions opposite to each other by 180 ° due to the pair annihilation of the positron emitted from the RI radiation source and the electron in the normal substance are detected. And photon (γ-ray) pairs having energy of 511 keV from the detected photon pairs are obtained by energy discrimination and simultaneous counting.

  Since the photons detected by the photon detector are emitted from the RI radiation source in the subject, the photons may be absorbed in the subject before they come out of the body. Due to this absorption, the distribution of the RI source obtained by coincidence lacks accuracy. In order to solve such a problem, studies for obtaining transmission data (data representing the transmittance of photons) and performing absorption correction of coincidence count data have been conducted (Patent Document 1, Non-Patent Document 1).

In Patent Document 1, transmission data is obtained from data obtained by X-ray CT measurement, and absorption correction of coincidence count data is performed. On the other hand, in Non-Patent Document 1, transmission data is obtained by transmission measurement, and absorption correction of coincidence count data is performed. In the transmission measurement, as shown in FIG. 11, the subject 14 to which the RI source is not administered is placed at the same position as the emission measurement (measurement of the photon pair generated from the subject to which the RI source is administered). This means that the calibration RI radiation source 15 is rotated around the subject 14 to measure the photon (γ-ray) transmittance of the subject.
Japanese Patent Laid-Open No. 07-20245 M.Xu et al, “LOCAL THRESHOLD FOR SEGMENTED ATTENUATION CORRECTION OF PET IMAGING OF THE THORAX”, IEEE Trans. Nucl. Sci., NS-41, No. 4, pp. 1532-1537, 1994

  In the absorption correction method disclosed in Patent Document 1, transmission data for absorption correction is obtained from data obtained by X-ray CT measurement. Therefore, the subject is exposed when acquiring data for absorption correction. In the absorption correction method disclosed in Non-Patent Document 1, transmission data for absorption correction is obtained by transmission measurement. Therefore, the subject is also exposed when acquiring data for absorption correction.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a positron CT apparatus in which a subject is not exposed when acquiring data for absorption correction.

  In order to achieve such an object, the positron CT apparatus according to the present invention includes a plurality of photon detectors each outputting a photon detection signal corresponding to the energy of an incident photon, surrounding a measurement space around a predetermined axis. Input the arrayed ring and the photon detection signal, energy discriminate the photon pair generated by annihilation of electron / positron pair in the measurement space, and simultaneously count, and coincidence data indicating on which straight line the photon pair is generated The coincidence counting means for outputting, the subject shape measuring device for optically measuring the contour shape of the subject and outputting the contour shape data of the subject, and performing the absorption correction based on the coincidence counting data and the contour shape data And a corrected projection data acquisition means for generating corrected projection data that is projection data subjected to absorption correction.

  In this positron CT apparatus, the contour shape data of the subject can be obtained by the subject shape measuring device. Since the transmittance of the subject used for the absorption correction is obtained using the contour shape data, the positron CT apparatus can perform the absorption correction without performing transmission measurement. Further, the contour shape data used for the absorption correction is optically measured by the subject shape measuring apparatus. Therefore, the subject is not exposed when acquiring data for correction of absorption. This positron CT apparatus is particularly suitable for use in measuring small animals. This is because, in the case of a small animal, absorption of photon pairs is originally low, so that absorption correction based on the contour shape data of the subject is particularly effective.

  Here, the subject shape measuring apparatus includes a light projecting unit that irradiates the subject with slit light, and a light detection unit that is disposed at a predetermined distance from the light projecting unit and detects reflected light from the subject. It is preferable to obtain the contour shape data of the subject by measuring the distance between the subject shape measuring apparatus and the surface of the subject by the triangulation method.

  The slit light irradiated from the light projecting unit hits the surface of the subject and is irregularly reflected, and a part thereof enters the light detecting unit. The incident angle or the incident position of the reflected light incident on the light detection unit varies depending on the irradiation position of the slit light on the subject. Therefore, the distance to the subject surface can be obtained by the triangulation method based on the incident angle or the incident position of the reflected light incident on the light detection unit. From the distance to the subject surface thus obtained, the subject shape measuring apparatus can obtain the contour shape data of the subject. In particular, the light projecting unit preferably scans the slit light at a predetermined angle in a plane perpendicular to the longitudinal direction of the slit light emitted from the light projecting unit. Thereby, position data for a plurality of different subject surface positions can be measured with one sensor.

  In addition, a driving unit that moves the support on which the subject is placed along a predetermined axis in the measurement space may be further provided, and the subject shape measuring apparatus may be provided around the predetermined axis. Thereby, in this positron CT apparatus, it is possible to acquire both coincidence count data and contour shape data while moving the subject. As a result, it is possible to obtain data for absorption correction in a short restraint time.

  Furthermore, it is preferable that the subject shape measuring apparatus is provided outside the measurement space in the predetermined axis direction. Thereby, it is possible to acquire coincidence count data without being obstructed by the subject shape measuring apparatus.

  According to the present invention, it is possible to provide a positron CT apparatus in which a subject is not exposed when acquiring data for absorption correction.

  Hereinafter, preferred embodiments of a positron CT apparatus according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

FIG. 1 is a system configuration diagram of a positron CT apparatus 1 according to the embodiment. The positron CT apparatus 1 uses a photon detector D _k (k = 1, 2, 3,...) To generate a photon pair generated by the combination of a positron emitted from an RI radiation source input to a subject 10 and nearby electrons. n), and the coincidence data for identifying on which straight line in the measurement space the photon pair is generated by energy-discriminating and simultaneously counting the photon pair having energy of 511 keV from the detected photon pair. obtain. Projection data storage means 62 stores the obtained coincidence data and generates projection data. Further, since photon pairs that are absorbed in the subject 10 are not reflected in the projection data, the positron CT apparatus 1 obtains the contour shape data of the subject 10 and calculates the transmittance of the subject 10 to obtain projection data. Correction correction data is obtained. In the positron CT apparatus 1, the ring 40 and the coincidence counting unit 50 are used to obtain coincidence count data, the subject shape measurement apparatus 30 is used to obtain contour shape data of the subject 10, and corrected projection data is used to perform absorption correction. Acquisition means 60 is provided. Further, in order to enable measurement at different positions of the subject 10, the positron CT apparatus 1 includes a bed 22 that is a support on which the subject 10 is placed, and the bed 22 in the direction of the central axis Ax of the ring 40. Driving means 24 to be moved. Each time the bed 22 moves by a distance corresponding to the slice pitch of the positron CT (distance between cross sections of the subject to be measured by the positron CT) by the driving means 24, coincidence count data and contour shape data are acquired. For example, a pulse motor is used as the driving unit 24.

First, the ring 40 and the coincidence counting means 50, which are components for acquiring coincidence counting data, will be described. The ring 40 is installed in the opening of the gantry 20 so that the central axis Ax intersects the front surface 20 a of the gantry 20. The positron CT apparatus 1 is a two-dimensional type (2D-PET) composed of multiple layers of rings. In FIG. 2, one layer of the rings separated from each other by an inter-slice collimator is represented as a ring 40. The ring 40 includes a measurement space 12 in which the subject 10 is placed. In the ring 40, a plurality of photon detectors D _k (k = 1, 2, 3,..., N) that respectively output photon detection signals corresponding to the energy of incident photons are arranged around the central axis Ax. Yes. These photon detectors _Dk are directed toward the measurement space 12 and have their light receiving surfaces directed to detect photons generated in the measurement space 12. However, since the positron CT apparatus 1 is a two-dimensional type, only photon pairs that are emitted from the RI source and fly in all directions and fly in the direction along the ring surface are detected. The description will be continued with reference to FIG. 1 again. A signal line is provided between each of these photon detectors D _k (k = 1, 2, 3,..., N) and the coincidence counting unit 50 as shown in FIG. A photon detection signal corresponding to the energy of the detected photon is sent from D _k to the coincidence counting means 50.

The coincidence counting means 50 that receives the photon detection signals output from the photon detectors D _k (k = 1, 2, 3,..., N), respectively, has two photon detectors D _i and D _j in the ring 40. Simultaneously discriminates the simultaneous detection of photon (γ-ray) pairs having a predetermined energy (511 keV) generated along with the annihilation of electron-positron pairs. By simultaneously counting in this manner, the coincidence counting means 50 has a straight line (hereinafter referred to as “coincidence line”) L in which the photon pair generated by the annihilation of the electron / positron pair in the measurement space 12 connects the photon detectors D _i and D _j. Output coincidence data indicating that it occurred on _ij .

  Next, the subject shape measuring apparatus 30 that is a component for acquiring the contour shape data of the subject 10 will be described. An object shape measuring apparatus 30 that optically measures the contour shape of the object 10 includes sensors 31A, 31B, 31C, a bed position detecting means 32, a trigger signal generating means 33, a frame grabber 34, an object shape analyzing means 35, And a pulse drive circuit 36. The positron CT apparatus 1 includes three sensors 31A, 31B, and 31C for optically measuring the distance to the surface of the subject 10, and these are measured on the front surface 20a of the gantry 20 in the direction of the central axis Ax. 12 is installed so that it may be located outside. FIG. 3 schematically shows a cross section taken along the line II of the subject 10 and the sensors 31A, 31B, and 31C. As shown in FIG. 3, the three sensors 31 </ b> A, 31 </ b> B, and 31 </ b> C are arranged so as to surround the opening of the gantry 20, and the two sensors 31 </ b> A, 31 </ b> C are arranged at the same height as the subject 10. Since one sensor 31 </ b> B is disposed in the vicinity of the subject 10, it is possible to measure the entire periphery of the subject 10. In FIG. 3, a range in which each sensor can be measured is indicated by a dotted line indicating the range in which the slit light emitted from each sensor irradiates the subject 10. The shape of the subject 10 in contact with the bed 22 can be obtained from the shape of the bed 22. Therefore, the contour shape data of the subject 10 at the portion in contact with the bed 22 can be obtained without arranging a sensor that covers the portion in contact with the bed 22 whose shape is known.

  The sensors 31A, 31B, and 31C are reflective photoelectric sensors that perform distance detection by an optical triangulation method, and include, for example, a light projecting unit 131 and a light detecting unit 132 as shown in FIG. Although FIG. 4 shows the sensor 31A, the sensors 31B and 31C are configured in the same manner as the sensor 31A. Slit light for irradiating the subject 10 is emitted from the light projecting unit 131. The light detection unit 132 is arranged so as to be arranged at a predetermined distance from the light projection unit 131 in a direction perpendicular to the longitudinal direction of the slit light emitted from the light projection unit 131 and intersecting the light emission direction. Note that light having a wavelength included in the wavelength band of X-rays and γ-rays is not used as the slit light emitted from the sensors 31A, 31B, and 31C. It is preferable to use light having a wavelength from the visible region to the near infrared region (360 nm to 2500 nm) as the slit light.

  As shown in FIG. 5, the light projecting unit 131 includes a light emitting element 131A and a light projecting lens 131B. The light emitting element 131A is, for example, a plurality of light emitting elements (for example, LED chips) arranged in a line in a direction perpendicular to the longitudinal direction of the slit light. Then, by controlling these light emitting elements to emit light sequentially, the light emitted from the light projecting unit 131 scans the subject 10 in the direction perpendicular to both the longitudinal direction of the slit light and the emission direction of the slit light. . The light projecting lens 131B is disposed in front of the light emitting element 131A, and collimates slit light emitted from the light emitting element 131A into parallel light.

  On the other hand, the light detection unit 132 includes a CCD 132A and a light detection lens 132B. The light detection lens 132B is disposed in front of the CCD 132A, and condenses the light emitted from the light projecting unit 131 reflected by the subject 10 on the surface of the CCD 132A. The pattern of the emitted light from the light projecting unit 131 reflected by the subject 10 thus collected (hereinafter referred to as a reflected light pattern) is imaged and output by the CCD 132A. The light emitting element 131A, the CCD 132A, the light projection lens 131B, and the light detection lens 132B are arranged so that their optical axes are parallel to each other.

Here, the measurement principle of the sensors 31A, 31B, and 31C will be briefly described with reference to FIG. The light emitted from the light emitting element 131A irradiates the subject 10 through the projection lens 131B, and is irregularly reflected on the surface of the subject 10. A part of the light irregularly reflected on the surface of the subject 10 is collected by the light detection lens 132B and enters the light detection position SP on the surface of the CCD 132A. At this time, the distance from the projection lens 131B to the subject 10 is L, the distance between the optical axis of the projection lens 131B and the optical axis of the light detection lens 132B is B, and the focal length of the light detection lens 132B, that is, the light detection lens. If the distance between 132B and the CCD 132A is f, and the distance from the optical axis of the light detection lens 132B to the light detection position SP is x, equation (1) is established due to the similarity of triangles.
L = Bf / x (1)

  Since x is obtained based on the reflected light pattern that is an output from the CCD 132A, the distance L from the light projecting lens 131B to the subject 10 is obtained from the equation (1) in the subject shape analyzing means 35 described later.

  In order to control the sensors 31A, 31B, and 31C and obtain contour shape data of the subject 10 based on the reflected light pattern that is output from the sensors 31A, 31B, and 31C, the subject shape measuring apparatus 30 detects the bed position. Means 32, trigger signal generation means 33, frame grabber 34, subject shape analysis means 35, and pulse drive circuit 36 are provided. That is, the bed position detection unit 32 that outputs the position information of the bed 22 on which the subject 10 is placed acquires a signal every time a pulse is generated from the pulse motor, for example, when the driving unit 24 is a pulse motor. The position information of the bed 22 is obtained by counting the number of generated pulses. Alternatively, the bed position detection means 32 may include an encoder to obtain the position information of the bed 22. The bed position detection means 32 outputs the position information of the bed 22 obtained in this way to the trigger signal generation means 33. The trigger signal generator 33 to which the position information of the bed 22 is input outputs a trigger signal every time the bed 22 moves a distance corresponding to the slice pitch of the positron CT. The pulse driving circuit 36 receives the trigger signal generated by the trigger signal generating means 33 as an input, drives the light projecting units 131 of the sensors 31A, 31B, 31C, and outputs slit light. The trigger signal generated by the trigger signal generation means 33 is sent to the frame grabber 34 as well as the pulse drive circuit 36. The frame grabber 34 that has received the trigger signal receives the reflected light pattern output from each of the sensors 31A, 31B, and 31C at the moment the trigger signal is received, and holds this. Thereby, the frame grabber 34 obtains the reflected light pattern at the moment when the pulse driving circuit 36 is driven. Based on this reflected light pattern, the subject shape analysis means 35 performs the above-described calculation based on the principle of triangulation, obtains the distances between the subject 10 and the sensors 31A, 31B, and 31C. Obtain contour shape data. The frame grabber 34 and the subject shape analysis means 35 may operate in the same computer. The accuracy of the contour shape data is preferably about the same as the resolution of the positron CT apparatus 1. Therefore, for example, when the resolution of the positron CT apparatus is 1.5 mm, it is preferable to obtain contour shape data with an accuracy of at least about 1 mm.

  Next, the corrected projection data acquisition unit 60 that obtains corrected projection data by performing absorption correction based on the coincidence count data and the contour shape data will be described. The corrected projection data acquisition unit 60 includes a projection data storage unit 62 and an absorption correction unit 64. The projection data storage unit 62 stores the coincidence data output from the coincidence unit 50 to generate projection data. The absorption correction unit 64 obtains corrected projection data by performing absorption correction of the projection data generated by the projection data storage unit 62 using the contour shape data transferred from the subject shape analysis unit 35. In order to perform absorption correction, the absorption correction means 64 first obtains the transmittance of photon pairs (γ rays) in the subject 10 from the contour shape data. The corrected projection data is obtained by dividing the projection data by the transmittance thus obtained.

With reference to FIG. 6, the principle of obtaining the transmittance of the subject 10 from the contour shape data will be briefly described. If the length of the _coincidence line L _ij crossing the subject 10 is a, the length of the bed 22 is b, the absorption coefficient of the subject 10 is μ1, and the absorption coefficient of the bed 22 is μ2, the coincidence line L _ij The transmittance T _ij of each subject 10 is expressed by the following equation (2).
T _ij = exp {− (μ1 · a + μ2 · b)} (2)

The length b of length a and coincidence line L _ij of the coincidence line L _ij crosses the object 10 crosses the bed 22 can be determined from the contour data of the object 10. Further, the absorption coefficient μ2 of the bed 22 can be easily obtained by actual measurement or the like. For the absorption coefficient μ1 of the subject 10, for example, the absorption coefficient of water is used. Therefore, the transmittance T _ij of the subject 10 as transmission data can be obtained from the equation (2).

  Based on the corrected projection data obtained by the corrected projection data acquisition unit 60, the image reconstruction unit 90 calculates a spatial distribution of the occurrence frequency of electron / positron pair annihilation in the measurement space 12 and performs a re-image configuration. The image display unit 92 displays the image reconstructed by the image reconstruction unit 90.

  Next, a measurement method using the positron CT apparatus 1 will be described. FIG. 7 is a flowchart of measurement in the positron CT apparatus 1.

  First, in step S1, the positron CT apparatus 1 acquires contour shape data of the subject 10. That is, the moving distance of the bed 22 is output to the trigger signal generating means 33 by the bed position detecting means 32. The trigger signal generating means 33 determines whether or not the moving distance of the bed 22 is a distance corresponding to the slice pitch of the positron CT, and if it is determined that the distance corresponds to the slice pitch, the trigger signal generating means 33 outputs a trigger signal. Is output to the pulse drive circuit 36 and the frame grabber 34. The pulse driving circuit 36 receives the trigger signal, drives the light projecting portions 131 of the sensors 31A, 31B, and 31C to irradiate the subject 10 with slit light. On the other hand, the frame grabber 34 holds a reflected light pattern that is an output from the CCD 132A at the moment when the trigger signal is given. The frame grabber 34 outputs the reflected light pattern thus held to the subject shape analysis means 35. The subject shape analyzing means 35 obtains contour shape data of the subject 10 based on the principle of triangulation using the reflected light pattern held by the frame grabber 34.

Subsequently, in step S2, the positron CT apparatus 1 acquires coincidence count data. That is, among the photon pairs emitted along with the annihilation of the electron / positron pair in the subject 10 located at the measurement position in the measurement space 12, the photon pair flying along the ring surface detects the photon that forms the ring 40. When detected by any two photon detectors D _i and D _j of the vessel D _k, photon detection signal indicating that the detected photons is output from each of the two-photon detectors D _i and D _j To the coincidence counting means 50. When the coincidence counting means 50 that receives these photon detection signals as input, it is determined whether or not photon pairs having a predetermined energy (511 keV) are detected at the same time. Outputs the coincidence count data specifying the straight line connecting the two photon detectors D _i and D _j that detect the photon pair.

  The coincidence count data obtained in step S2 is stored in the projection data storage means 62 in step S3. By repeating step S2 and step S3 for a fixed time, projection data is generated.

  After generating the projection data in this way, in step S4, the projection data is subjected to absorption correction using the contour shape data, and corrected projection data is obtained.

  In step S5, the image reconstruction means 90 calculates the spatial distribution of the occurrence frequency of electron-positron pair annihilation in the measurement space 12 based on the corrected projection data, and performs the image reconstruction. The reconstructed image is displayed by the image display means 92. Thereafter, the bed 22 is moved by the slice pitch of the positron CT by the driving means 24, and steps S1 to S4 are repeated.

Here, step S4 will be described in more detail with reference to the flowchart shown in FIG. First, in step S41, the contour shape data of the subject 10 is arranged in a positron CT image area on a memory such as a computer that operates the absorption correction means 64. Next, in step S42, the absorption correction means 64 substitutes absorption coefficients μ1 and μ2 for the area corresponding to the subject 10 and the bed 22 in the positron CT image area, respectively. Next, in step S43, the cutting lengths a and b of the subject 10 and the bed 22 on the _coincidence line L _ij are calculated, and the transmittance T _ij in the _coincidence line L _ij is calculated based on the equation (2). To do. Thereafter, in step S44, the absorption correction means 64 writes data obtained by dividing a portion corresponding to the measurement on the coincidence line L _ij in the projection data by the transmittance T _ij on the memory. Steps S43 and S44 are repeated for the number of samples of the acquired coincidence data, and corrected projection data is obtained to complete the absorption correction.

  The effects of the positron CT apparatus 1 according to the above embodiment will be described.

  Transmission data used for the absorption correction, that is, the transmittance of the subject 10 can be obtained from the contour shape data as shown in the equation (2). In the positron CT apparatus 1, the contour shape data of the subject 10 is obtained by the subject shape measuring device 30. Therefore, the positron CT apparatus 1 can perform projection data absorption correction without performing transmission measurement.

  The positron CT apparatus 1 is particularly preferably used for measurement of small animals. In the case of small animals, the absorption of photon pairs is originally low. Therefore, in order to obtain the transmittance from the contour shape data of the subject, there is almost no need to vary the absorption coefficient μ depending on the region, and the transmittance is obtained from the equation (2) with the absorption coefficient μ inside the subject being a constant value. Even a very accurate transmittance can be obtained. Alternatively, the positron CT apparatus 1 is also effective for measuring a part such as the head that can be regarded as having an almost uniform absorption coefficient. This is because if the absorption coefficient is substantially uniform, even if the absorption coefficient μ inside the subject is set to a constant value and the transmittance is obtained from the equation (2), a very accurate transmittance can still be obtained. . For example, when measuring the head, it is preferable to use the water absorption coefficient as the absorption coefficient μ inside the subject.

  The contour shape data used for the absorption correction is optically measured by the subject shape measuring apparatus 30. That is, the CCD 132A of each sensor outputs a reflected light pattern of the slit light emitted from the light emitting elements 131A of the sensors 31A, 31B, and 31C on the subject 10. Based on this reflected light pattern, the object shape analyzing means 35 calculates the distance between the object 10 and each of the sensors 31A, 31B, 31C based on the triangulation method (equation (1)), and the contour shape data of the object 10 is obtained. Desired. Therefore, the subject 10 is not exposed when acquiring data for absorption correction.

  In particular, the light projecting unit 131 preferably scans the slit light at a predetermined angle within a plane perpendicular to the longitudinal direction of the slit light. This is because position data for a plurality of different subject 10 surface positions is measured by one sensor.

  Further, in the positron CT apparatus 1, data for absorption correction is obtained by being measured by the sensors 31A, 31B, 31C and the like. Therefore, it is cheaper than the case of using X-ray CT measurement, and means for acquiring data for absorption correction can be provided.

  In the positron CT apparatus 1, as described above, contour shape data that is data for absorption correction is optically measured and obtained. Therefore, unlike the transmission measurement, the positron CT apparatus 1 can also acquire contour shape data when acquiring coincidence count data. As a result, it is possible to obtain data for absorption correction in a short restraint time.

  The positron CT apparatus 1 includes driving means 24 that moves the bed 22 along the center axis Ax of the ring 40, and sensors 31A, 31B, and 31C are provided around the center axis Ax. As a result, the positron CT apparatus 1 can acquire both coincidence count data and contour shape data for each slice pitch of the positron CT while moving the subject 10. As a result, data for absorption correction can be obtained in a short restraint time.

  In the positron CT apparatus 1, the sensors 31A, 31B, and 31C are all provided outside the measurement space 12 in the direction of the central axis Ax. Therefore, the coincidence count data can be acquired without being disturbed by the sensors 31A, 31B, and 31C.

  The positron CT apparatus according to the present invention is not limited to the above-described embodiments and configuration examples, and various modifications are possible. For example, instead of obtaining corrected projection data by performing absorption correction on the projection data that is the accumulation of coincidence count data as in the above embodiment, the corrected projection data is stored by storing the data subjected to absorption correction. It is good also as a structure to obtain. FIG. 10 shows a system configuration diagram of the positron CT apparatus in this case. In this case, the corrected projection data acquisition unit 160 includes an absorption correction unit 164 and a corrected projection data storage unit 166. The absorption correction means 164 outputs data obtained by dividing a constant value by the transmittance corresponding to each coincidence data. The corrected projection data storage unit 166 stores the data output from the absorption correction unit 164 at the address corresponding to the coincidence data used to obtain the data, and obtains corrected projection data.

Further, the light detection unit 132 of the sensor is not composed of a CCD, but may be, for example, a PSD (semiconductor position detection element) or a two-part semiconductor position detector. FIG. 9 shows a case where the light detection unit 132 of the sensor includes a PSD 132C and a light projection lens 132B. The PSD 132C has a light detection surface 132d made of a resistance layer, and has electrodes 132e and 132f provided on the upper and lower ends with the light detection surface 132d interposed therebetween. When light is incident on the light detection surface 132d, the light detection unit 132 generates a photocurrent at the light detection position, and the photocurrent is divided into electrodes 132e and 132f and flows. At that time, the respective photocurrents flowing through the electrodes 132e and 132f are divided according to the resistance value between the light detection position and the electrodes 132e and 132f. This resistance value is proportional to the distance between the electrode and the light detection position. Therefore, the ratio of the currents flowing through the electrodes 132e and 132f changes according to the light detection position of the incident light in the light detection unit, and the detection position of the incident light is obtained by measuring each current value. The measurement principle of this sensor will be briefly described with reference to FIG. The light emitted from the light emitting element 131A is diffusely reflected on the surface of the subject 10 through the light projection lens 131B, a part of the light is collected by the light detection lens 132B, and enters the light receiving position SP of the light detection surface 132d of the PSD 132C. . At this time, the distance from the projection lens 131B to the subject 10 is L, the distance of the optical axis center between the projection lens 131B and the light detection lens 132B is B, and the focal length of the light detection lens 132B, that is, the light detection lens 132B. When the distance between the PSDs 132C is f, the distance between the electrodes 132e and 132f is C, and the distance from the optical axis center of the light detection lens 132B to the light detection position SP is x1, Equation (3) is established.
x1 = Bf / L (3)

Further, when the sum of the output currents IA and IB from the electrodes 132e and 132f is I0, and the distance between the light detection position SP and the electrode 132f is x, the output currents IA and IB are respectively calculated between the light detection position SP and the electrode. Since it is inversely proportional to the distance, equation (4) holds.
IA = I0 · X / C
IB = I0 · (C−X) / C (4)

Furthermore, equation (5) is established by equations (3) and (4).
L = Bf / C · (1 + IB / IA) (5)

  As can be understood from the equation (5), the distance L between the sensor and the subject 10 can be obtained from the ratio of each current.

  Further, the subject shape measuring apparatus is not constituted by the sensor, the bed position detecting means, the trigger signal generating means, the pulse driving circuit, the frame grabber, and the subject shape analyzing means, and may not include all of them. Particularly when the subject is fixed and measured without moving the bed by the driving means, the bed position detecting means, the trigger signal generating means, the pulse driving device, and the frame grabber may not be provided.

  In the above embodiment, the number of sensors is three, but the number of sensors is not limited to three as long as the shape can be measured while covering the entire periphery of the subject. However, it is clear that at least two sensors are necessary to accurately measure the shape of the subject, and three or more sensors are preferably arranged in order to reliably measure the entire surrounding shape. In addition, the contour shape data may be acquired separately from the acquisition of the coincidence count data without installing the sensor in the gantry. Further, when anatomical information is already obtained by MRI measurement, the transmittance for each tissue part of the subject can be obtained by fitting the anatomical information and the contour shape data. . In this case, more accurate transmission data (transmittance) is obtained, and as a result, an image representing the distribution of the RI source is also more accurate.

  In addition, this invention is not limited to 2D-PET which consists of a multilayer ring mentioned above, It can apply also to another type of positron CT apparatus. For example, the present invention can also be applied to 2D-PET composed of a single layer ring and 3D-type 3D-PET.

1 is a system configuration diagram of a positron CT apparatus according to an embodiment. It is a figure for demonstrating the principle which acquires coincidence count data. It is a figure for demonstrating the measurement of the outline shape of the subject by a sensor. It is a schematic perspective view which shows the structure of a sensor. It is a figure for demonstrating the measurement principle of a sensor. It is a figure for demonstrating the principle which calculates the transmittance | permeability of a subject from outline shape data. It is a flowchart of the measurement in a positron CT apparatus. It is a flowchart of absorption correction. It is a figure for demonstrating the measurement principle at the time of using PSD for the light detection part of a sensor. It is a system block diagram of the positron CT apparatus which concerns on a modification. It is a figure for demonstrating the conventional transmission measurement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Positron CT apparatus 10, 14 ... Subject, 12 ... Measurement space, 15 ... RI source for calibration, 20 ... Gantry, 20a ... Front surface of gantry, 22 ... Bed, 24 ... Drive means, 30 ... Shape of subject Measuring device, 31A, 31B, 31C ... sensor, 32 ... bed position detection means, 33 ... trigger signal generation means, 34 ... frame grabber, 35 ... subject shape analysis means, 36 ... pulse drive circuit, 40 ... ring, _Dk ... Photon detector, L _ij ... Simultaneous counting line, 50 ... Simultaneous counting means, 60, 160 ... Corrected projection data acquisition means, 62 ... Projection data storage means, 166 ... Corrected projection data storage means, 64, 164 ... Absorption correction means , 90 ... Image reconstruction means, 92 ... Display means, 131 ... Projection unit, 131 A ... Light emitting element, 131 B ... Projection lens, 132 ... Light detection unit, 132 A ... CCD, 132B ... light detection lens, 132C ... PSD, 132d ... light detection surface, 132e, 132f ... electrode

Claims (4)

A ring in which a plurality of photon detectors each outputting a photon detection signal corresponding to the energy of the incident photon are arranged around a predetermined axis surrounding the measurement space;
Simultaneously inputting the photon detection signal, energy-discriminating photon pairs generated by electron-positron pair annihilation in the measurement space and simultaneously counting, and outputting coincidence data indicating on which straight line the photon pair is generated Counting means;
A subject shape measuring apparatus for optically measuring the contour shape of the subject and outputting the contour shape data of the subject;
Corrected projection data acquisition means for generating corrected projection data which is projection data subjected to absorption correction by performing absorption correction based on the coincidence count data and the contour shape data;
A positron CT apparatus comprising:   The subject shape measurement apparatus includes a light projecting unit that irradiates the subject with slit light, and a light detection unit that is disposed at a predetermined distance from the light projecting unit and detects reflected light from the subject. The positron CT apparatus according to claim 1, wherein the contour shape data of the subject is obtained by measuring a distance between the subject shape measuring device and the surface of the subject by a triangulation method. Drive means for moving the support on which the subject is placed along the predetermined axis in the measurement space;
The positron CT apparatus according to claim 1, wherein the object shape measuring apparatus is provided around the predetermined axis. 4. The positron CT apparatus according to claim 1, wherein the object shape measuring apparatus is provided outside the measurement space in the predetermined axis direction. 5.

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