JPS58108482A - Positron ct apparatus - Google Patents

Abstract

PURPOSE:To reduce the missing of simultaneous counting otherwise causing non-linearity in the accidental simultaneous counting rate and the counting rate of a positron CT apparatus as probable noise component by decreasing a part of a dissipated gamma ray from the perimeter of a view having position information of a ray source with a member for absorbing gamma ray with a larger wavelength than it inserted between scintillators. CONSTITUTION:The chart shows changes in the simultaneous counting rate when the length of a tungsten plate is varied with a gamma ray incidence angle theta=25 deg. for example, employing tungsten for a gamma ray absorbing member. The simultaneous counting rate was reduced by about 10% when the tungsten plte was set at the reference length +4mm. in the length and 1mm. in the thickness and about 40% when the length of the tungsten plate exceeds the reference length +12mm. as compared with the case without the tungsten plate. The reference length +12mm. is found to be the optimum length of the tungsten plate. When the length of the tungsten plate was less than the reference length +4mm., the reduction rate in the simultaneous counting rate is small in the perimeter of the view. No substantial advantage was noted. The tungsten plate proved appropriate when it was 0.5-2mm. in the thickness and projected by more than 4mm. from the surface of scintillators in the length. Even in the case of a positron CT, the projection of the tungsten plate by more than 4mm. from the surface of the scintillators is expected to yield an effect of reducing the simultaneous counting rate in the perimeter of the view.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a positron CT apparatus, and particularly to a gamma ray detector for obtaining positional information of a positron radioisotope administered into a living body.

A positive CT device is a device that displays the concentration distribution of a ninth-order electron radioactive isotope (hereinafter referred to as a radiation source) administered into a living body on a plane perpendicular to the body axis. Here, the position information of the radiation source is obtained by simultaneous counting between a large number of gamma ray detectors surrounding the human body in a ring shape or a regular polygon shape. This method is described in detail in Japanese Unexamined Patent Publication No. 56-6175. ,
- A pair of annihilation gamma rays emitted from a radiation source distributed inside a living body undergoes gamma ray absorption in the human body through different routes depending on the location of the radiation source and the direction in which the annihilation gamma rays are emitted. IIi uniformly in the field of view! When the sources are distributed, the probability that a pair of annihilation gamma rays will reach the gamma ray detector without being absorbed is affected by the difference in gamma i-ray absorption rate due to the difference in the transmission paths. In FIG. 1K, a pair of annihilation gamma rays pass through the periphery of the visual field smoothly in the field of view. In FIG. 1, the horizontal axis t represents the distance from the visual field center 0 to the transmission path p, and the vertical axis represents the transmission probability. Note that the field of view radius was 15 m.

The configuration of the conventional positron CT gamma ray detector system was changed to a second one.
Fig. 3 shows an example of a coincidence projection obtained with a conventional detector system when a radiation source is uniformly distributed over the entire field of view, and its drawbacks are shown in Figs. I will explain about it. In the conventional detector system, a large number of cinch-length gamma ray absorbers 2 arranged in a ring shape are inserted (Fig. 2a), and positron CT is used to achieve high spatial resolution.
Device annihilation gamma! K only works by sacrificing the detection sensitivity for IK or preventing it from being detected by an adjacent detector due to scattering. Therefore, when the II source is uniformly distributed over the entire visual field 4, the coincidence rate is extremely high in the periphery of the visual field 4, where the probability of annihilation gamma ray transmission is high, as shown in Figure 3. . In addition, when the heavy metal plug 3 is attached, there is a problem that the aperture wi product of the scintillator 1 decreases and the coincidence rate decreases for annihilation gamma rays 5 that pass through the center of the field of view and enter the detector almost perpendicularly. There was a point. Furthermore, for the annihilation gamma Iw6 that passes through the periphery of the I/C field of view and has a large angle of incidence, the heavy metal glove 3 takes a trapezoidal or semicircular shape, so the shielding effect is small. As in the case, the coincidence projection data is extremely large at the periphery of the visual field.

The image quality of the source concentration distribution reproduced by a positron CT apparatus is inferior to the random coincidence count as a noise component, the statistical error of the coincidence count, and K. Coincidence is an event in which two annihilated cancer i-rays from different sources are counted simultaneously.
This probability is proportional to the square of the detector count rate. On the other hand, the statistical error is given by the square root of the coincidence count. Furthermore, when the counting rate of the detector becomes high, it is possible to
Due to dead time in the coincidence counting circuit and signal transmission system, the number of coincidence counts increases, and the linearity of the counting rate of the positron CT apparatus with respect to the source concentration is no longer guaranteed.

As mentioned above, in the conventional configuration, the coincidence rate of annihilation gamma rays passing through the center of the visual field is the lowest and the noise due to statistical errors is high, so the image quality of the reproduced image is determined by the statistical noise of the annihilation gamma rays passing through the center of the visual field. On the other hand, the high coincidence rate of the annihilated cancer i-rays passing through the periphery of the visual field contributes to improving the image quality in the periphery of the visual field due to small statistical noise, but does not contribute to improving the image quality in the central part of the visual field, which is of most interest. .

Rather, the increase in the counting rate of the detector causes an increase in coincidental coincidences and uncounted coincidences, and the positron C
This was a factor that deteriorated the maximum counting rate of the T device and the linearity of the counting rate with respect to the source concentration.

The present invention reduces a part of the annihilated gamma rays from the periphery of the field of view that have positional information of the source by inserting a longer gamma ray absorber in the scintillator, and reduces the chance coincidence rate that becomes a noise component. The purpose of this is to reduce the number of coincidence counts that cause non-linearity in the counting rate of positron CT devices.

A method of inserting a longer radiation absorber between the radiation detection parts is known in XJIICT (for example, Japanese Patent Laid-Open No. 53
However, in X-ray CT, the Xs source exists outside the body, and the positional relationship with the detector is uniquely determined, unlike in positron CT equipment. and Xll1 determined by the detector aperture! The purpose is to remove scattered X-rays that enter from sources other than the passage. In other words, the purpose of using the radiation absorber in X#CTK is to remove scattered X-rays that become noise components of reproduced images.

Therefore, the radiation absorber does not affect the projection data in any way.Furthermore, since the absorption rate of X#l is low for becomes. In contrast, the gamma ray absorber of the positron CT apparatus according to the present invention reduces the number of annihilated gamma rays that have positional information and can be used for image reproduction, and smoothes the coincidence projection data, thereby reducing accidental coincidences and missed counts. , aimed at improving the count rate characteristics of positron CT equipment.
The purpose and effects are completely different from X-ray CT.

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 4 is a diagram showing the main parts of an embodiment of the positron CT apparatus according to the present invention, and shows the configuration of the gamma ray detector system.
In the figure, lO is the E-class Gunn i-ray absorber according to the present invention. The gamma ray absorber 10 does not interfere with the gamma rays 5 that pass through the center of the field of view and directly enter the scintillator IKfi.On the other hand, the gamma rays 6 that pass through the periphery of the field of view and have a large angle of incidence are blocked by the gamma ray absorber 10. Since the scintillator IK is reached after some absorption, the coincidence rate decreases. Rounding and uniform absorber to quantitatively evaluate the effect of this gamma ray absorber 10 (gamma ray absorption coefficient μ)
We have derived a calculation formula for determining the true coincidence rate for a pair of detectors when the KM source is distributed.

C*(')"//8#o (X e 7)"-""
' ” ” ” (' e ” If(”s F )
'''F... (1) In the above equation, C1 (#) is the true coincidence rate of gamma rays incident on the detector pair at an incident angle of -.8 is the slice width, pe (x a y ) is the source concentration distribution % '(' I
”) is the detection efficiency of gamma rays incident at an incident angle of -, f
Fy) is the geometric coincidence efficiency % when the source is located at the locus 1m (1, 7). t(a IX) means the length of the gamma ray transmission path. When the radiation source is uniformly distributed in the whole 1c of Uno 4 (radius r), the above equation can be rewritten as the following equation.

C・(Lee/"F(##X)dX However, 71≦R, IX-ω1/ω However,)'l>RIIX-ω1/ω Here, B,=Bcos#*z(lsixθ+bcos#)/ 2Y*=(
” (X + x l) Cao) V x 1 = 1sis
#-1Camθa: scintillator length (see Figure 4) b: scintillator width (see Figure 4); detector ring radius. - As an example, the gamma ray absorber 10 is made of tungsten, and the scintillator 2 is made of 24X24X12 (8x a
X b ) am” IDk'xwxjer i Nate (
BGO) scintillator, field of view 40 radius r 12.5m,
The radius B of the gamma ray detector ring 7 was set to 25 cIR, and coincidence projection data was calculated when the radiation source was uniformly distributed over the entire field of view. However, #(##X) is an incident angle of −(”*
We were able to calculate the absorption of gamma rays by the scintillator through the scintillator.

The results are shown in Figures 5a-if. In the figure, the horizontal axis is the gamma ray incident angle #, and the g axis is [true coincidence rate]/(
p・5i"74g), and the -(rammeter) of the curve is determined by the thickness of the tungsten plate.

The fifth composition is a case where a gamma ray absorber, which is 4 cm shorter than the scintillator, is inserted between the scintillators.

As the thickness of the tungsten plate increases, the distance between it and the adjacent BGO increases, so the G/M ray shielding effect of the adjacent BGO decreases.
The coincidence rate near the X side of the visual field increases. In this way, by inserting a tungsten plate shorter than %BGO between 800 and 800, it is impossible to reduce the coincidence rate in the peripheral part of the visual field. Figures 5b to '5f show 0wm+4 cod from the scintillator, respectively.

This is a case where an 8 mm, 512 m, 16-long H gun i-ray absorber is inserted between scintillators, and a part of the absorber protrudes from the surface of the scintillator by the above length. As is clear from these figures, when a part of the tungsten plate with a %BGO surface is made to protrude, as the length and thickness of the tungsten plate are increased, the coincidence rate of annihilation gamma rays passing through the peripheral part of the visual field increases. Decrease. However, since an increase in the thickness of the tungsten plate causes a decrease in the mounting density of the 0BGO in the gamma ray detector ring 6, there is a limit to the thickness, which is preferably 4- or less, and the most preferable range is 0,5. ~2 ws, and K for smoothing the projection data can be adjusted by adjusting the length of the tungsten plate. Figure 6 shows the gamma ray incident angle θ-2
B' is a diagram showing the change in the coincidence rate when the length of the tungsten plate is changed. Tungsten plate length +4-1
At a thickness of 1-1, the coincidence rate decreases by a factor of about 10 compared to a case without a tungsten plate, and at a length of +12 m or more, isn't it? '! 40
S decreases. From this, the optimal tungsten plate length is +12
- and) The umbrella reduction rate is small, which is a big advantage compared to the conventional configuration. 6 From the above explanation, 1. In the above configuration, the thickness, 10.5 to 2-1. Tungsten plate is suitable. Even in a positron CT having a configuration other than the above, if 4 or more tungsten plates are made to protrude from the surface of the scintillator, an effect of reducing the coincidence rate in the peripheral area of the visual field can be expected.

(1) When the scintillator size is the same and the detector ring is larger, the field of view radius becomes larger, the transmittance of gamma rays passing through the center of one field of view further decreases, and the coincidence rate of the peripheral part of the field of view increases relatively. In order to reduce the coincidence rate in the peripheral part of the field of view to s-, which is in harmony with the central part of the field of view, the ring diameter must be longer than the length of the tungsten plate in a small detector system.

The width of the lentilator currently used is controlled by the size of the photomultiplier tube (PMT) to which it is connected. The minimum diameter PMT on the market is φ1/2", and the minimum width compatible with this is about 7", which is the practical limit. For this minimum width scintillator, the scintillator surface has an optimal tungsten length of about 71 The condition of protruding by 4 or more is satisfied.

As mentioned above, a tungsten plate with a length of +4- or more reduces the coincidence rate at the peripheral part of the visual field and is effective in smoothing the projection data. However, as shown in Figure 5f,
As the length of the tungsten plate becomes longer, the coincidence rate not only in the peripheral part of the visual field but also in the central part decreases, causing a problem of lower system sensitivity to gamma rays. This could be prevented by setting the upper limit of the protruding length of the tungsten plate to 30 m.

Gamma ray absorber l is not only tungsten plate but also 511
Heavy metals such as lead, which have a high absorption coefficient for KI annihilation gamma rays, and compounds such as mercury nitrate can also be applied, and the same effect as tungsten can be obtained.

As detailed above, according to the single-shot 911, by inserting a gamma ray absorber between the scintillators, the chance coincidence rate that causes front image noise can be reduced, and the linearity of the tightening wheel can be improved. Positron CT equipment can be provided.

[Brief explanation of drawings]

Figure 1 is a diagram showing the probability that a pair of annihilation gamma rays will reach the gamma ray detector without being absorbed within the field of view when the source is uniformly distributed within the field of view, and Figures 3m and 2b are conventional 3 is a diagram showing coincidence projection data obtained by the conventional method when a radiation source is uniformly distributed over the entire field of view, and FIG. 4 is a diagram showing a radiation detector system according to the present invention. Figures 5a to 5f showing the system are diagrams showing coincidence projection data by the detector system according to the present invention, respectively, and Figure 6 is a diagram showing the relationship between the tungsten plate length and the coincidence rate. Patent Applicant Makoto Ishiita, Director of the Agency of Industrial Science and Technology - 'vJ1 Figure ¥1 211 Mouth, vi Zb Figure 3 Figure '¥ 4 Figure fJ 5α Figure Inner Flame VJ sb Figure Nishi Tomo'jj, Sc Mouth 45 Sunset 5e Figure

Claims (1)

[Scope of Claims] 1. Positron CT that obtains an in-vivo immersion distribution image of positron radioactive isotopes by detecting a pair of annihilation gamma rays generated when positrons (positrons) generated in the positron decay process are annihilated. In the device, the above extinguishing gun i
A positron CT device comprising a gamma ray absorber that protrudes toward the center of the visual field from the surface of a scintillator for detecting rays and absorbs a portion of annihilated gamma rays. 2. The positron CT apparatus according to claim 1, wherein the length of the portion of the gamma ray absorber protruding from the surface of the scintillator along the direction of the center of the visual field is from 4 to 30 cm. 3. The positron CT apparatus according to claim 1 or 2, wherein the thickness of the gamma ray absorber is 4 mm or less. 4. The thickness of the gamma ray absorber is 0.5■ or more 2■
A positive) cyn CT apparatus according to claim 1 or #I2, characterized in that: & The positron CT apparatus according to any one of claims fiIIJ to 4, wherein the gamma ray absorber is made of tungsten.