JP4336596B2 - Positron CT system - Google Patents

Description

  The present invention relates to a positron CT apparatus that captures a distribution image of positron-emitting RI (radioisotope) distributed in a subject.

  The positron CT system collects data (emission data) by simultaneously counting two gamma rays emitted in the opposite direction of 180 ° when the positron emitted from the RI in the subject disappears, and calculates this data. Is used to obtain an RI distribution image. Since gamma rays are emitted outside through the subject, they are absorbed in the body. Therefore, it is necessary to correct this absorption. In order to obtain data for correction of absorption, a positron emitting radiation source is placed outside the subject, and gamma ray data (transmission) passing through the subject is collected.

Conventionally, transmission data is collected using a detector for collecting emission data, and these data are collected at different times and simultaneously. In the former, a radiation source is arranged around a subject before RI administration and transmission data is collected, and then RI is administered to the subject and emission data is collected in a state where the radiation source is removed. is there. In the latter, as shown in Patent Document 1 below, a radiation source is arranged around a subject to which RI is administered, and transmission data and emission data are collected simultaneously.
Japanese Patent Laid-Open No. 9-184885

  As the detector, a ring detector in which radiation detectors are arranged in a ring shape is used, and a multi-ring detector in which these are stacked in a multilayer in the central axis direction is also used. When a multi-ring type detector is used, the simultaneous count data between all the ring detectors facing each other is collected to capture a wide volume in the body axis direction of the subject, that is, a 3D image (3D positron CT imaging). However, in order to capture a larger volume in the body axis direction, the subject is moved relative to the detector. The amount of movement is the field width of the detector in the body axis direction or the width excluding the specified overlap width, and the bed or detector side is moved by the amount of movement, and data is collected for each position independently. The data is stored in the data matrix and the image is reconstructed for each position.

  However, there are problems with the conventional positron CT apparatus. When transmission data for absorption correction is collected at a time different from the collection of emission data, there arises a problem that the entire inspection time increases. If the transmission data collection and emission data collection are performed simultaneously, the inspection time can be shortened. However, since gamma rays are detected simultaneously by a common detector, both data are easily affected by each other and the external source intensity is increased. The transmission data cannot be collected at a high count rate. Furthermore, even if data is collected at the same time, absorption correction must be performed by post-processing, and cannot be corrected in real time.

  In addition, in the conventional 3D positron CT apparatus using a multi-ring detector, there is a problem that sensitivity unevenness occurs in the body axis direction when photographing a wide field of view in the body axis direction. In multi-ring detectors, the sensitivity distribution in the axial direction is higher at the center of the field of view and lower at the periphery. Therefore, simply aligning the reconstructed images for each position in the body axis direction Due to the sensitivity variation, image degradation is particularly noticeable in planar images parallel to the body axis, such as sagittal images and coronal images, resulting in a decrease in diagnostic ability. If the overlap width between the positions is increased in order to reduce this, useless data collection increases, and the memory utilization efficiency decreases. Further, if the range of data used for image reconstruction is limited to that from a ring-type detector having a small number of layers, the sensitivity unevenness is reduced, but the overall sensitivity is lowered, and the advantage of 3D acquisition cannot be utilized.

  In view of the above, an object of the present invention is to provide an improved positron CT apparatus.

  Another object of the present invention is to perform 3D positron CT imaging using a multi-ring type detector while moving the detector relative to the subject for a field of view larger than the field width in the body axis direction of the detector. It is an object of the present invention to provide a positron CT apparatus improved so as to eliminate sensitivity unevenness in the body axis direction.

  Still another object of the present invention is to be able to collect transmission data in a short time at a high count rate by using a strong external radiation source for collecting transmission data, and to perform absorption correction in real time. An object is to provide an improved positron CT apparatus.

In order to achieve the above object, in a positron CT apparatus according to the present invention, a multi-ring detector in which ring detectors are stacked in the axial direction in multiple layers, and an object to the multi-ring detector, A moving device that relatively moves the intervals of each ring of the ring detector, (1) a position where emission data is collected for each position of each ring detector, and (2) the emission data is simultaneously counted. i-th examples of the combination of the ring containing the same ring, and k-th, that the combination of the ring of the following positions (i + 1) -th, (k + 1) -th and then, combining the i-th of said ring, k-th coincidence data, when the ring combination (i + 1) -th in the mobile immediately and the k + 1-th coincidence data is data of the same position of the subject, the Li Combinations i th grayed, and k-th coincidence data, ring combination (i + 1) th in that immediately after the movement, by relatively moving the processing of adding those k + 1-th coincidence data for and the same direction at the same position However, it is characterized by being provided with a data collection device that collects data by repeating.

  The subject is moved relative to the multi-ring detector, and emission data is collected by the multi-ring detector, whereby coincidence count data between the rings is collected. The coincidence data between ring i and ring k is obtained at a certain position (first position), and the coincidence data between ring i + 1 and ring k + 1 is obtained at the next position (second position). Suppose. Since the distance between the first position and the second position is the interval between the rings of the multi-ring detector, these lines connecting the ring i and the ring k at the first position and the ring i + 1 at the second position And the ring k + 1 are the same lines for the subject. Therefore, data is collected by repeating the process of adding the coincidence count data between rings i and k and the coincidence count data between rings i + 1 and k + 1 while relatively moving. Thereby, uniform sensitivity is obtained over a wide visual field in the body axis direction, and unevenness in sensitivity in the body axis direction can be eliminated.

  In addition, the positron CT apparatus is flush with the first ring detector for collecting emission data for the subject, the second detector for collecting transmission data for the subject, and the second detector. The external radiation source arranged and the subject relative to the first and second detectors so that the subject moves from the second detector side to the first detector side. The moving device to be moved and the volume after the transmission data collection of the subject is moved, and while the emission data collection for the volume is being performed, the transmission data is processed to absorb absorption correction data on the coincidence line. And a data processing device that performs absorption correction immediately after the collection of the emission data for the volume is completed.

  In this case, since the subject is moved from the second detector side to the first detector side, transmission data is first collected by the second detector, and then the same volume of the subject is the first volume. Emission data is collected by the detector. In other words, when collecting emission data for a certain volume, transmission data collection for that volume has been completed, and during the emission data collection, transmission data for that volume is processed to obtain absorption correction data on the coincidence line. It becomes possible to calculate. As soon as the collection of the emission data for the volume is completed, the absorption correction for the emission data can be performed using the above-described absorption correction data, and the absorption correction in real time can be performed.

  Next, a positron CT apparatus embodying the present invention will be described with reference to the drawings.

  In FIG. 1, in a cylindrical gantry 10, ring detectors 11 in which radiation detectors are arranged in a ring shape are stacked in multiple layers in the central axis direction. Among the ring detectors 11, the multi-layer detector 11 excluding one layer (right end in the figure) closest to the entrance on the side where the subject 50 is inserted is a multi-ring detector 12. (See FIG. 3) and used for emission data collection. The ring-type detector 13 at the right end of the figure is for collecting transmission data. That is, a ring-type positron emitting external radiation source 14 is arranged on the plane of the ring-type detector 13, and is emitted from the radiation source 14 and positioned on the subject 50, that is, on the plane of the ring-type detector 13. The radiation passing through the slice 52 to be detected is detected by the ring detector 13. Since the slice ceptor 15 constituted by a lead shield is disposed so as to sandwich the external radiation source 14, the radiation emitted from the external radiation source 14 is a ring-type detector 13 for collecting transmission data at the right end. It is prevented from entering the other ring type detector 11, that is, the multi-ring type detector 12 for collecting emission data. The slice ceptor 15 also prevents radiation from a volume other than the slice 52 from entering the ring detector 13 for collecting transmission data at the right end. This external radiation source 14 may be a point radiation source instead of the ring type, and in this case, the body axis 51 is rotated around the subject 50 around the rotation center axis. When the point source is rotated in this way, it is possible to use a single photon emitting source instead of a positron emitting source. In this case, the ring detector 13 is used for counting single photons. Use a detector. When a detector for counting single photons is used, it is not necessary to use a ring type, and a partial ring type (a part of an arc) may be used. Rotate with rotation.

  Since the multi-ring detector 12 for collecting emission data and the ring detector 13 for collecting transmission data are placed at different positions and partitioned by the slice ceptor 15 as described above, Data can be collected separately, and both data can be prevented from affecting each other. Therefore, it is possible to efficiently collect transmission data in a short time with a high count rate by using an external radiation source 14 having a high radiation intensity.

  The subject 50 is placed on the bed 20, and the bed 20 is moved in the arrow direction (left direction in the figure) by the bed moving device 21, whereby the hollow portion of the cylindrical gantry 10 including the ring type detector 11. Are inserted from right to left. The body axis 51 of the subject 50 is aligned with the central axis of the ring detector 11.

  FIG. 2 shows a signal system of this embodiment, and a detection signal from the emission data multi-ring detector 12 is inputted to the coincidence counting data collection device 31. The emission data multi-ring detector 12 is composed of the ring detectors 11 stacked in multiple layers as described above, and the simultaneous counting within each ring and the simultaneous counting between the rings are performed.

  For example, when the multi-ring detector 12 is constituted by five layers (first to fifth layers) of the ring detector 11 as shown in FIG. 3, the first to fifth layers. Simultaneous counting is performed between the detectors arranged in the circumferential direction within each layer and between the detectors between each layer. In the case of simultaneous counting by the detector of the first ring 1 and the detector of ring 1, the case of simultaneous counting by ring 1 and ring 2, the case of simultaneous counting by ring 1 and ring 3, etc. Simultaneous counting is performed for each combination as indicated by a line connecting the three detectors. That is, since the coincidence data is collected not only within the ring but also between detectors across the ring, the coincidence line exists for each combination of rings. Therefore, the counting at each coincidence line is performed in each of the matrices corresponding to the ring combinations as shown in FIG.

  In each of these matrices, position data in a plane crossing the body axis 51 is collected as a sinogram. The sinogram is represented by the distance from the central axis and the inclination angle in the circumferential direction of a line (simultaneous counting line) connecting two detectors that simultaneously detect radiation. As described above, in the case of 5 rings, sinogram data is collected in each of 5 × 5 matrices.

  Considering the state where the bed 20 is stationary, the body axis direction sensitivity distribution within the entire body axis direction field width D (= 4d) of the multi-ring detector 12 is as shown in FIG. It has a triangular shape that is higher at the center of the field of view and lower at the periphery. As shown in FIG. 3, this is intuitively understood from the fact that the number of lines passing through the subject 50 and connecting each ring is as large as the center and only one at the end.

  The bed 20 is sequentially moved by the bed moving device 21 at intervals d of the ring detector 11. If the respective stationary positions at that time are the first position P1, the second position P2, the third position P3,..., For example, the position where the line connecting the rings 2 and 4 at the first position P1 passes through the subject 50 is The line connecting the rings 3 and 5 at the next second position P2 is the same as the position passing through the subject 50. The same applies to other ring combinations. Therefore, as shown in FIG. 6, the matrix collected at P2 can be shifted by 1 in the vertical and horizontal directions and added to the matrix collected at P1.

In general, the combination i- th and k- th coincidence data of the ring at a certain position Pj and the ring combination i + 1- th and k + 1- th coincidence data of the next position Pj + 1 are determined by the subject 50. Since these are the same part of the data, they can be added. Therefore, as shown in FIG. 6, the data obtained every time the position is moved are sequentially added while shifting the matrix, and this is repeated for m positions. Thus, when the number of rings is n (n is 5 in the above), (2n-1)
X (m-1) + n * n sinogram data can be collected.

This is based on the assumption that the ring type detector 11 having (m−1) + n number of rings covering the entire travel distance of (m−1) × d is provided.
This is the same as the data collected when limited to only one ring. The sensitivity distribution in the body axis direction is a trapezoid that is flat in the body axis direction as shown in FIG. 7, and the sensitivity is uniform except for both ends. On the other hand, if an image is reconstructed using only data collected at a certain position as in the prior art, an image having a sensitivity distribution as shown in FIG. 5 can be obtained for each position. Thus, a non-uniform sensitivity distribution in which a peak appears for each position is obtained. If the position interval is narrowed and the overlap width is widened, the sensitivity approaches uniform, but the data collected by overlapping and overlapping is wasted, collecting time is wasted, and memory utilization efficiency is reduced.

  The addition of data in which the positions of the matrix are shifted is performed in the data collection memory 32 based on the movement information from the bed moving device 21. On the other hand, the detection signal from the transmission data ring type detector 13 is input to the coincidence counting data collecting device 33, and the coincidence counting is performed and the data collecting memory 34 collects the data. This data is transmission data for the slice 52 in the planar subject 50 where the ring detector 13 is located. Since this ring type detector 13 is on the insertion side of the subject 50 with respect to the multi-ring type detector 12 for collecting emission data, the transmission data is first collected and then its position is moved. Emission data is collected. In other words, when collecting emission data of a certain volume, transmission data for that volume has already been collected in the data collection memory 34, so it is sent to the processor 35 during the emission data collection and all the corresponding coincidence counts are sent. Absorption correction data for the line can be calculated, and this absorption correction data can be stored in the table memory 36 as an absorption correction table. That is, transmission data for the slice 52 stored in the memory 34 is read out, and an image is reconstructed to obtain an absorption coefficient map (absorption coefficient distribution image) for the slice 52. By performing this for a large number of slices based on the position information from the bed moving device 21, an absorption coefficient map (a three-dimensional distribution image of the absorption coefficient) in the volume is obtained, and further, this three-dimensional image is forward projected. Then, the absorption correction data for all coincidence lines in the volume is obtained. In this way, absorption correction data for all coincidence lines in the volume can be sequentially stored in the table memory 36 as an absorption correction table.

  As a result, the addition of the data with the matrix positions shifted is performed in the data collection memory 32 as described above, and the corresponding absorption correction table is read from the memory 36 for the matrix for which the addition has been completed, and all the coincidence lines are read. The processor 35 can perform absorption correction of emission data. Such absorption correction is sequentially performed in real time for each matrix for which addition has been completed. The processor 35 performs a three-dimensional image reconstruction operation from the corrected emission data, that is, sinogram data of all matrices, so that the volume larger in the body axis direction than the entire body axis direction field width D of the multi-ring detector 12 is obtained. 3D positron CT images can be reconstructed.

  In this example, a multi-ring detector and a transmission data ring detector positioned in front of the multi-ring detector are combined, but one emission data ring detector and transmission data ring detector Or a configuration that collects data by adding the matrix positions shifted as described above using a multi-ring detector, and a configuration that does not use a ring detector for transmission data. It is. In addition, the specific configuration and the like can be variously changed without departing from the spirit of the present invention.

  According to the present invention, when performing 3D positron CT imaging using a multi-ring type detector while moving the detector relative to the subject for a field of view larger than the field width in the body axis direction of the detector. A positron CT apparatus improved so as to eliminate sensitivity unevenness in the body axis direction can be realized. In addition, it is possible to realize a positron CT apparatus capable of collecting transmission data in a short time at a high count rate by using a strong external radiation source for collecting transmission data and capable of performing absorption correction in real time.

1 is a schematic side view showing an embodiment of the present invention. The block diagram which shows the signal system | strain of the embodiment. Explanatory drawing explaining the positional relationship of the to-be-examined subject and a multi-ring type | mold detector. The figure which shows the matrix which collects sinograms in one position. A graph showing the sensitivity distribution in the body axis direction of data collected at one position. Explanatory drawing for demonstrating adding the data collected in each of many positions, if a matrix is shifted. The graph which shows the body axis direction sensitivity distribution of the data collected by adding if the matrix is shifted. The graph which shows the body axis direction sensitivity distribution at the time of reconstructing each image by the data collected for every conventional position.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Gantry 11 Ring type detector 12 Multi ring type detector 13 Transmission type ring detector 14 Ring type external radiation source 15 Slice ceptor 20 Bed 21 Bed moving device 31 Emission coincidence data collecting device 32 Emission data collecting memory 33 Transmission Count Data Collection Device 34 Transmission Data Collection Memory 35 Processor 36 Table Memory 50 Subject 51 Body Axis 52 Slice

Claims (1)

Multi-ring detector in which ring-type detectors are stacked in multiple layers in the axial direction, and a moving device that moves a subject relative to the multi-ring detector relative to each ring interval of the multi-ring detector (1) Emission data is collected for each position of each ring detector, and (2) Examples of combinations of rings including the same ring at a certain position for simultaneous counting in the emission data are i-th and k-th. , the combination of the ring of the following positions (i + 1) -th, (k + 1) -th and then, combining the i-th of said ring, and k-th coincidence data, the rings in the immediately following movement combination i + 1-th and k + 1 th and coincidence data, when the data of the same position of the subject, the combination i-th of said ring, and k-th coincidence data, put on the immediately following movement The combination (i + 1) th ring, k + for the first coincidence data is repeated while the processing is relatively moved to add those same portion and the same direction, and characterized in that it comprises a data collection device to collect data Positron CT device.

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