JP3409506B2 - Positron CT system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positron CT apparatus, and more particularly to a positron CT apparatus in which a multi-layer detector ring is moved with respect to a subject so as to have a range equal to or larger than the visual field width of the multi-layer detector ring. The present invention relates to a positron CT apparatus for imaging an image. 2. Description of the Related Art A positron CT apparatus uses positron-emitting radionuclides, detects annihilation gamma rays, and takes an image of the nuclide distribution. For example, when a drug labeled with a positron-emitting radionuclide is administered to the human body, it accumulates in specific organs. At that time, gamma rays emitted outside the human body are detected by a detector placed outside the human body, data is collected, and the data is processed by a predetermined algorithm, so that the concentration distribution of nuclides in a predetermined cross section is obtained. Reconstruct the image. This reconstructed image is used for diagnosis of a specific organ. [0003] A scintillation detector or the like is used as a detector for detecting gamma rays outside the subject, and these are arranged in a number of rings. Since gamma rays emitted from the nuclides located in the plane of arrangement of the detector ring and emitted in parallel with the above plane enter one of the ring-arranged detectors and are detected, The data of the specimen on this plane (slice plane) will be collected, and the reconstructed image will be the nuclide concentration distribution image on this slice plane. Usually, the detector ring is formed in a plurality of layers so that data acquisition and image reconstruction can be performed not only on one slice plane but also on a plurality of slice planes stacked in a certain range. [0004] If the data acquisition range in the direction (layer direction) perpendicular to the slice plane in the multilayer detector ring is called a field width, if this field width is moved with respect to the subject (multilayer detector By moving the ring or the subject), data can be collected over a wide range beyond the field of view. Thereby, for example, by moving the detector ring from the head to the toe of the human body, data collection and image reconstruction of the whole body, that is, a so-called whole body scan can be performed. [0005] When data is collected over a wider range than the field of view, such as a whole body scan, using such a multilayer detector ring, conventionally, the subject is moved for each field of view, and then stopped. Simultaneous counting data is collected during the stationary period. Using the data collected in this way, a tomographic image in each slice is reconstructed, arranged in the slice thickness direction (layer direction), and a three-dimensional image is obtained in a range wider than the field width in the layer direction. Alternatively, an image in a layer direction such as a sagittal image or a coronal image can be directly reconstructed from the collected data. However, as in the prior art, when the whole body scan or the like is performed by moving the subject in steps while making the moving distance of the subject coincide with the field-of-view width in the layer direction of the multilayer detector ring. However, there is a problem that an artifact such as a discontinuous stripe pattern in the layer direction occurs in the reconstructed image. [0007] This is because the sensitivity distribution of the multilayer detector ring is not uniform in the layer direction. Actually, the sensitivity distribution in the layer direction of the multilayer detector ring is as shown in FIGS. In FIG. 3, a ring-shaped collimator 14 is arranged in front of (inside) the multilayer detector ring 13 to perform collimation between the layers, and only the radiation in the layer enters the detector rings of each layer of the multilayer detector ring 13. Thus, two-dimensional data collection is performed by the detector ring of each layer. In this case, the sensitivity distribution in the layer direction is as shown by the broken line 15 within the layer-direction viewing width W, and the sensitivity at the center is substantially constant, but the sensitivity gradually decreases at the ends. [0008] Such a so-called two-dimensional data collection type has been used for a long time.
A collimator 14 for collimating between the layers as shown in FIG.
Without using the method, it is also performed to detect all the incident radiation obliquely across the layers. In this case, since the coincidence data also includes position information in the layer direction,
This is called three-dimensional data collection. Here, the sensitivity distribution in the layer direction within the layer direction viewing width W is as indicated by a broken line 16, and is highest at the center in the layer direction, becomes lower toward the end, and the difference is about 10 times. As described above, since the sensitivity distribution of the multilayer detector ring is non-uniform in the layer direction and becomes lower toward the end in the layer direction, the statistical noise of the collected data increases in the portion where the sensitivity is low. Therefore, when a sagittal image or a coronal image continuous in the layer direction is reconstructed, a portion having a small amount of statistical noise and a portion having a large amount of statistical noise appear in the layer direction, which becomes a stripe pattern and deteriorates image quality. In view of the above, the present invention does not cause artifacts such as stripes in a reconstructed image when data is collected in a wide range that is equal to or larger than the field-of-view width of a multilayer detector ring, such as a whole body scan. It is an object of the present invention to provide a positron CT apparatus improved as described above. In order to achieve the above object, a positron CT apparatus according to the present invention comprises a multilayer detector ring in which detector rings in which radiation detectors are arranged in a ring shape are stacked in multiple layers. The object is inserted into the hollow portion in the multilayer detector ring, and the object is set in a predetermined direction such that the field of view of the multilayer detector ring overlaps with the multilayer detector ring only near the end. It is characterized by being provided with a moving device that performs relative movement in a stepwise manner for each distance, and an image reconstruction device that performs an image reconstruction process using coincidence data collected during a stationary period of the stepwise movement. I have. The subject moves stepwise with respect to the multilayer detector ring by a predetermined distance in the layer direction so that the field width of the multilayer detector ring in the layer direction partially overlaps near the end. Let me do. Then, data is collected during the stationary period of the stepwise movement. Therefore, coincidence data is collected so that the field width of the multilayer detector ring in the layer direction partially overlaps near the end. Data is not lost in the layer direction without using the data of the overlapped portion. The overlapped portion is an area where the sensitivity is low, which corresponds to the end of the field-of-view width in the layer direction of the multilayer detector ring, so if this was not used, the image could be reconstructed without using the data with reduced sensitivity. That is, artifacts of the reconstructed image can be reduced. Even if the data of the overlapped portion is added to each other to increase the data amount of the portion, it is possible to compensate for the sensitivity decrease near the end of the field width of the multilayer detector ring in the layer direction. A preferred embodiment of the present invention will be described below in detail with reference to the drawings. In FIG. 1, a gantry 11 is provided with a tunnel section 12, and a multilayer detector ring 13 is arranged so as to surround the tunnel section 12. A bed 21 is arranged beside the gantry 11, and a top plate 22 is inserted into the tunnel section 12 of the gantry 11. The bed top plate 22 is configured to move in a direction penetrating the tunnel section 12, and the subject 20 lies on the bed top plate 22. Signals from the respective detectors of the multilayer detector ring 13 are input to a coincidence counting circuit 31, where it is detected that these two signals have entered simultaneously, and for each set of detectors that have issued signals at the same time. A count is made in the data collection memory 32. The bed 21 is controlled by the control device 34 to move the top 22. Information on the position of the top plate 22 is also stored in the data collection memory 32.
The image reconstruction device 33 performs image reconstruction using the data collected in the data collection memory 32. A so-called full-body scan is usually performed by moving the bed top plate 22 from the maximum position as shown in FIG. 1 in the retraction direction as indicated by the arrow. At this time, the movement is performed stepwise, and the movement amount D in one step is shorter than the field-of-view width W of the multilayer detector ring 13 in the layer direction. Then, at each position, as shown in (a), (b), (c), and (d) of FIG. 2, data is collected in the region W in the layer direction, that is, in the body axis direction of the subject 20. However, the range W overlaps at the end. Therefore, when reconstructing an image using the collected data at the four positions, data in the overlapping range is not used. Then, the data of the low sensitivity portion at both ends of the layer direction viewing width W is not used, and only the data of the central high sensitivity portion is used. Therefore, the sensitivity distribution in the body axis direction of the subject 20 is as shown in FIG. 2E, and there is no part with extremely low sensitivity. As a result, when reconstructing a sagittal image or coronal image in the body axis direction, striped artifacts do not occur. Alternatively, if data of the overlapped portion is added and used without discarding, the subject 20
The sensitivity distribution in the body axis direction is as shown in FIG. 2F, and the uniformity of the sensitivity distribution in the body axis direction is improved. In this case, it is desirable to perform weighted addition in consideration of the sensitivity of each position (each slice) in the layer direction within the layer direction viewing width W. As a result, theoretically, the sensitivity is almost the same at the center except for both ends of the scan in the body axis direction, and an image having almost the same S / N ratio is obtained. In the example shown in FIG. 2, so-called three-dimensional data collection is performed. However, the present invention can be applied to so-called two-dimensional data collection. The stepwise movement distance D can be set arbitrarily as long as a certain degree of overlap of the layer-direction viewing width W can be obtained. When applied at the time of two-dimensional data collection, striped artifacts may be removed only by determining the step movement distance D so as to overlap only one or two slices at the end and collecting data. In the above description, the gantry 11 containing the multilayer detector ring 13 is fixed, the bed top plate 22 is moved, and the subject 20 is moved with respect to the multilayer detector ring 13. Bowl ring 13
Since the movement of the subject 20 with respect to may be relative,
Needless to say, the subject 20 can be fixed and the gantry 11 in which the multilayer detector ring 13 is accommodated can be moved. As described above, according to the positron CT apparatus of the present invention, the multi-layer detector ring is used, and the field width of the multi-layer detector ring, such as a whole body scan, in the layer direction is larger than that of the multi-layer detector ring. When data is collected over a wide range, the S / S of data in the layer direction (scan direction)
It is possible to suppress a change in the N ratio and reduce artifacts of a reconstructed image due to data of a low-sensitivity portion within the layer-direction viewing width.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a positron CT apparatus according to one embodiment of the present invention. FIG. 2 is a diagram showing a sensitivity distribution at each position of a subject. FIG. 3 is a diagram showing a layer direction sensitivity distribution of a multilayer detector ring at the time of two-dimensional data collection. FIG. 4 is a diagram showing a layer direction sensitivity distribution of a multilayer detector ring at the time of three-dimensional data collection. [Description of Signs] 11 Gantry 12 Tunnel 13 Multi-layer detector ring 14 Collimator 20 Subject 21 Bed 22 Bed top plate 31 Simultaneous counting circuit 32 Data acquisition memory 33 Image reconstruction device 34 Control device

Claims (1)

(57) [Claims 1] A multilayer detector ring in which a plurality of detector rings in which radiation detectors are arranged in a ring shape are stacked in multiple layers, and a subject is placed in a hollow portion in the multilayer detector ring. Insert and
A moving device for moving the subject relative to the multilayer detector ring in a stepwise manner at predetermined intervals so that the layered field width of the multilayer detector ring overlaps only near the end; A positron CT apparatus, comprising: an image reconstruction apparatus that performs an image reconstruction process using coincidence counting data collected during the static period.