JPH05264736A - Positron ct apparatus - Google Patents

Abstract

PURPOSE:To improve sensitivity without dropping resolution by providing a slice collimator at a border of rings with a radiation detector placed at a border for each defined number of 1 or larger. CONSTITUTION:Four rings 1R to 4R are placed cylindrically along a body axis direction of a subject (long axis Z direction of a cylinder). A plurality of radiation (for example gamma-ray) detectors 1 are provided on each ring along the ring direction. A slice collimator 2B is provided only at a border for each defined number of 1 or more, that is only between rings 1R and 2R. gamma-rays incident to the rings 1R, 2R are identified as to which ring they have been incident. The same occurs with respect to gamma-ray incident to the rings 3R, 4R. Thus a slice collimator 2B provided only between the rings 2R, 3R suppresses an increase in scattered rays, while sensitivity increases between the rings 1R, 2R and the rings 3R, 4R as there is no slice collimator. On the other hand, resolution along a Z-direction may not drop.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positron CT apparatus (also referred to as PET) used for nuclear medicine diagnosis, and more particularly to a positron CT apparatus with an improved cylindrical radiation detecting section.

[0002]

2. Description of the Related Art In recent years, most radiation detectors of positron CT apparatuses (hereinafter referred to as PET) have a large number of radiation detectors (hereinafter simply referred to as detectors) arranged in a ring shape around a subject. It is based on a detector structure in which a large number of so-called detector rings are arranged in the body axis direction.

FIG. 2 shows the structure of the radiation detecting section. Figure 2
FIG. 2B is a layout view of FIG. 2A viewed from the side of the cross section of the subject 3.
Is a layout view seen from the aa cross section side. 4 in Fig. 2 (b)
The two rings 1R, 2R, 3R, 4R are arranged in a cylindrical shape along the body axis direction of the subject 3 (longitudinal axis Z direction of the cylinder). Each of the rings is shown in FIG.
As shown in (a), a plurality of radiation (eg, γ-ray) detectors 1 are arranged. A slice collimator 2 having a specified height is arranged at the boundary between adjacent rings,
A gamma ray removing collimator 2A is arranged at the outermost boundary. The slice collimator 2 has the role of determining the slice thickness when creating a positron image, and also has the purpose of removing scattered radiation and reducing random coincidence. The collimator 2A has a purpose of removing γ-rays emitted from the radiopharmaceuticals distributed in the subject 3 and distributed outside the visual field in the body axis direction. Collimator 2 and 2
A shows an example of the same height. The detector 1 generally consists of a scintillator, a photomultiplier tube, and an electronic circuit and is provided for each ring as shown in FIG. 2 (a). There is also a position detection circuit that can be used to find out which ring the γ-ray enters.

Generally, the ring width of each ring 1R-4R is 1.
The collimator 2A is made of lead having a thickness of about 30 mm, and the collimator 2 is made of lead or tungsten having a thickness of about 1 to 10 mm. 4, 4A, 5 and 5
A indicates a combination of coincidence counting between rings, 4 is coincidence counting in the first ring, 4A is coincidence counting in the second ring, 5 is coincidence counting between the first ring and the second ring 5,
A is the same as 5 but with the opposite combination of 5. The data of the first slice is created by 4, the data of the second slice is created by the sum of 5 and 5A, and the data of the third slice is created by 4A. In the example of FIG. 2, a total of 7 slices of data is obtained by the same procedure.

[0005]

In the prior art shown in FIG. 2, it is known that the detection efficiency of each slice data decreases in inverse proportion to the square of the slice thickness used. It is known that scattered radiation decreases in inverse proportion to slice thickness and collimator length. Z of slice data
It is known that the spatial resolution in the axial direction is 1/2 of the slice thickness at the center of the visual field. In PET, in order to create a high quality image, it is required to increase the resolution in the cross-sectional direction and the Z direction, increase the sensitivity, and reduce scattered radiation. Here, only the resolution in the Z direction will be considered. However, if the slice thickness is narrowed to increase the resolution in the Z direction, the sensitivity decreases, while if the length of the collimator is shortened or the slice thickness is increased to increase the sensitivity, the resolution deteriorates and scattered radiation also occurs. It can be seen that it is actually difficult to realize these two requirements such as increase.

It is an object of the present invention to provide a positron CT apparatus which can improve the sensitivity without lowering the resolution.

[0007]

The present invention comprises a plurality of rings arranged along the major axis of a cylinder, each ring having a plurality of radiation detectors arranged in the ring direction. Imaging using a circular radiation detection unit, a ring-shaped slice collimator provided only on the boundary between rings and at a prescribed number of 1 or more, and detection signals simultaneously counted by the cylindrical radiation detection unit. Imaging means for
(Claim 1).

Further, the present invention comprises a plurality of rings arranged along the major axis of the cylinder, each ring comprising a plurality of radiation detectors arranged in the ring direction, and a cylindrical radiation detector. , A ring-shaped slice collimator having a specified length provided on each boundary between the rings and at least one specified number of boundaries, and the above-mentioned definition provided on an inner boundary of the adjacent boundaries where the collimator is provided. It comprises a ring-shaped collimator having a length shorter than the length and an imaging means for imaging by using the detection signals simultaneously counted by the cylindrical radiation detecting section (claim 2).

[0009]

According to the present invention, the slice collimator is provided only at the boundary for each specified number of 1 or more, and the sensitivity can be increased without substantially deteriorating the spatial resolution in the Z direction (claim 1).

Further, according to the present invention, a slice collimator having a prescribed height is provided at each one or more prescribed boundaries, and further, the inside of the adjacent border where the adjacent slice collimators are provided has the above-mentioned structure. A slice collimator lower than the height of the slice collimator is provided, whereby the sensitivity can be increased without substantially deteriorating the spatial resolution in the Z direction, and at the same time the scattered ray component can be reduced (claim 2). ..

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an embodiment of the cylindrical radiation detector of the present invention, and is a sectional view in the body axis direction (Z direction). In this embodiment, no slice collimator is installed at the boundary between the first ring 1R and the second ring 2R and at the boundary between the third ring and the fourth ring, and at the boundary between the second ring 2R and the third ring 3R. Only the slice collimator 2B was installed. The height of this slice collimator 2B was the same as that of the collimator 2A. Therefore, in comparison with FIG. 1, it is equivalent to removing the slice collimator at the boundary between 1R and 2R and the slice collimator at the boundary between 3R and 4R.

The operation and effect of this embodiment will be described in comparison with the conventional example. 3A, 3B, 3C, and 3D are detection efficiencies for each slice for comparison between the conventional example of FIG. 2 and the present embodiment of FIG. 1 (here, they are treated as sensitivity for convenience. FIG.

In FIGS. 3A and 3B, 6 and 7 are shown in FIG.
2 shows a response function when a point source is scanned in the horizontal direction along the Z-axis in the conventional example. These response functions represent the sensitivity of each slice. Here, the odd-numbered slices are called the in-plane 7 and the even-numbered slices are called the cross-plane 6. The sensitivity of the cross plane becomes higher than the sensitivity of the in-plane as the height of the collimator 2 becomes shorter, and becomes four times as high as the collimator 2 does not have. Now, the sensitivity S _{T of} all slices is given by the following equation.

[Equation 1] S _T = 4S _IN + 3S _CR = 4S _IN + 3 × 1.5S _IN = 8.5S _IN Here, the in-plane sensitivity S _IN corresponds to the entire area of the response function 6, and the cross-plane sensitivity S _CR Is assumed to be 1.5 times S _IN for convenience.

Next, the slice sensitivity according to the present embodiment of FIG. 1 (all slices when the slice collimator 2 between the first R and the second R and the slice collimator 2 between the third R and the fourth R are removed Sensitivity) Consider S _T1 . The response functions 8 and 9 in FIG. 3C give the in-plane and cross-plane sensitivities S _IN1 and S _CR1 in this case, respectively. The calculation of S _T1 is as follows.

[Equation 2] S _T1 = 2S _IN1 + S _CR1 = 2S _IN1 + 1.5S _IN1 = 3.5S _IN1 = 3.5 × 4S _IN = 14S _IN

Next, the total slice sensitivity S _T2 when all the slice collimators 2 are removed will be considered. Figure 3
The response function 10 of (d) shows the sensitivity S _{IN2 in} this case.
S _T2 is

[Equation 3] S _T2 = S _IN2 = 4S _IN1 = 4 · 4S _IN = 16S _IN

As described above, when the slice collimator is provided for every two rings, the sensitivity is 1.6 times that of the prior art,
When all of them are removed, it becomes about 1.9 times. Figure 4 shows the calculation formula of E. Tanaka et al “Analytical stady of the
performance of a multilayerpositron computed tomo
graphy scanner ”J.Comput.Assist.Tomogr., vol.6, pp.3
50-364,1982.) Shows the in-plane and cross-plane sensitivities, the ratios of random and scattered radiation to the slice thickness, which were evaluated by using the method. As can be seen from FIG. 4, the proportion of scattered radiation is about 20% when the slice thickness doubles,
When it becomes four times, it will increase to 40%. Therefore, in order to improve the image quality of PET, it is necessary to appropriately increase the sensitivity without unnecessarily increasing scattered radiation or random coincidence and further degrading the Z-direction resolution.

FIG. 5 shows the response function 11 added in the slice direction in the case of the prior art, the response function 12 obtained in the same manner in this embodiment, and the response function 13 obtained when all the slice collimators 2 are removed. Is shown. Comparing these, it can be seen that the distribution of the response function becomes flat in the order of 13, 12, and 11, and becomes uniform over all slices. Therefore, installing the slice collimator 2 rather than removing all the slice collimators 2 makes it possible to make the sensitivity distribution in the visual field in the Z direction uniform.

Therefore, in this embodiment, the slice collimator 2 is provided only between the second R and the third R. For the γ-rays incident on the first R and the second R, which ring the γ-rays are incident on is identified. 3rd and 4th
The same applies to γ rays incident on R. Thus, the slice collimator provided only between the second R and the third R suppresses the increase of the scattered radiation component, while eliminating the slice collimator 2 between the first R and the second R and between the third R and the fourth R. Increased sensitivity can be expected. According to the present embodiment, the spatial resolution in the Z direction is maintained at almost the same value as in the prior art,
The sensitivity can be suppressed to about 1.5 times, and the increase of scattered radiation can be suppressed to about 2 times.

FIG. 6 shows another embodiment. In FIG.
A slice collimator 13 having a length shorter than that of the slice collimator 2 is installed between the first R and the second R.
The same applies to between the third R and the fourth R. In this case, due to the effect of the slice collimator 13, the in-plane response function 14 and the cross-plane response function 15 are slightly deformed. In particular, although the sensitivity of the cross plane is lower than that of the embodiment shown in FIG. 5, the scattered ray component can be reduced.

In this embodiment, the imaging from the detection signals by the simultaneous detection from the cylindrical radiation detecting section was not described, but if the coordinate conversion of the coincidence counting data is performed and the corresponding slice data is created. Needless to say, it can be realized by the conventional procedure.

[0021]

According to the embodiments of the present invention, the increase in scattered radiation can be suppressed to about twice and the sensitivity can be increased by about 1.5 times with almost no deterioration in the spatial resolution in the Z direction. As a result, an improvement in the S / N ratio of the PET image can be expected.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a cylindrical detector of the present invention.

FIG. 2 is a diagram showing a conventional cylindrical detector.

FIG. 3 is a comparison diagram of the present embodiment and a conventional example.

FIG. 4 is a characteristic diagram of slice thickness and sensitivity.

FIG. 5 is a comparison diagram of the present embodiment and a conventional example.

FIG. 6 is a diagram showing another embodiment of the cylindrical detector of the present invention.

[Explanation of symbols]

 1 Detector 1R-4R Ring 2, 2B Slice Collimator 2C Collimator for γ-ray removal

Claims (2)

[Claims] 1. A cylindrical radiation detector comprising a plurality of rings arranged along the long axis direction of a cylinder, each ring comprising a plurality of radiation detectors arranged in the ring direction, and a ring. It comprises a ring-shaped slice collimator provided only on the boundary of the rings at a predetermined number of 1 or more, and imaging means for imaging using the detection signals simultaneously counted by the cylindrical radiation detecting section. Positron CT system. 2. A cylindrical radiation detecting section comprising a plurality of rings arranged along the long axis direction of a cylinder, each ring comprising a plurality of radiation detectors arranged in the ring direction, and a ring. A ring-shaped slice collimator having a specified length provided at each boundary of the rings at a specified number of 1 or more, and a length longer than the specified length provided at an inner boundary of adjacent boundaries provided with the collimator. A positron CT apparatus comprising a ring-shaped collimator having a short length and an imaging means for imaging using detection signals simultaneously counted by the cylindrical radiation detecting section.