JPH01318986A - Radiation detector - Google Patents

Abstract

PURPOSE:To obtain a uniform sensitivity characteristic by providing a reflection preventing film having a specified thickness which comprises a light transmitting material whose refractive index is smaller than that of a filter between a scintillator element and optoelectronic transducer. CONSTITUTION:A radiation shielding filter 3 shields radiation which is transmitted through a scintillator 2 (block) and acts so as to transmits only light from the scintillator 2. A light transmitting material whose refractive index for the scintillator light is smaller than that of the radiation shielding filter 3 is deposited on the light emitting surface of the radiation shielding filter to a thickness of, e.g. 1/4 of the wavelength of the scintillator light. Therefore, the transmittance of the scintillator light at the light emitting surface of the radiation shielding filter is improved. In this way, dispersion in sensitivity distribution between the scintillator elements 2 due to sloping-down of the sensitivity distribution in the direction of slice width in the scintillator element 2 is eliminated. The characteristic for every scintillator element 2 becomes uniform.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides a scintillator (block) that emits light due to radiation, and a scintillator (block) that is laminated on the opposite side of the radiation incident side to block the radiation. A radiation source comprising a scintillator element (block) consisting of a filter that transmits light, and a photoelectric conversion element (array) that converts light from the light emitting surface of the filter of this scintillator element (block) into an electrical signal. It is related to the detector.

[Conventional technology]

A device using this type of radiation detector, here X-ray CT
An example of the device will be briefly described. In other words, an X-ray CT device consists of an X-ray source that emits a fan-shaped X-ray beam, and an X-ray detector that is a radiation detector.
A radiation detector, here a multi-element X-ray detector comprising a large number of X-ray detection elements (an element consisting of a scintillator element and a photoelectric conversion element), is arranged facing each other at a predetermined interval.
A subject is positioned within the fan-shaped X-ray beam irradiation range between the radiation source and the X-ray detector, and the X-ray source and X-ray detector are rotated integrally around the subject to irradiate the subject with X-rays from multiple directions. It is primarily a medical imaging device that obtains a tomographic image of a subject by emitting X-rays, measuring the intensity distribution of transmitted X-rays, and processing the measurement signals using an electronic computer.

The Xll1I source and the X-ray detector are fixed, and even if the subject is rotated, the same tomographic image can be obtained, and it can also be used as a non-medical imaging device.

In such an apparatus, if there are variations in the sensitivity distribution of the X-ray detection elements constituting the X-ray detector, a concentric false image (ring-shaped artifact) will occur on the reproduced image. Hereinafter, variations in the sensitivity distribution of the X-ray detection element will be explained. FIG. 4 is a diagram for explaining the X-ray measurement principle of the XMCT apparatus. In this figure, the X-ray source 10
The fan-shaped X-ray beam 1 emitted from the object 11 is irradiated with the width of the slice at the position of the object 11, and the X-ray intensity distribution after passing through the object 11 is determined by a large number of X-rays placed opposite the X-ray source 10. It is measured by a multi-element X-ray detector 12 comprising detection elements. The transmitted X-ray intensity to be measured will be determined when the subject 11 is as shown in FIG. When the object slice has a cylindrical shape as shown in FIG. 6(A), the width is constant in the width direction of the object slice as shown in FIG. 6(A), and when it has a truncated conical shape as shown in FIG. The characteristics are inclined as shown in (B).

Here, the slice width on the scintillator element of the X-ray detector 12 becomes αD, which is expanded by the expansion rate α. This X
The sensitivity of each point within the slice width αD of the line detector 12 is not constant, but has a varying sensitivity distribution. Figure 7 illustrates the sensitivity distribution of three typical types of scintillator elements. Figure 7 (A) shows a flat sensitivity distribution, Figure 7 (CB) shows a lower left shoulder, and Figure 7 (C) shows a right shoulder. The sensitivity distribution of the decline is shown in each case. In this way, slice width α
When a fan-shaped X-ray beam with intensity changes in the slice width direction is measured using a multi-element X-ray detector that has a sensitivity distribution that varies in the D direction and in which the state of the sensitivity distribution differs for each scintillator element, the output The values vary depending on the X-ray detection element, and errors that cannot be removed by simple correction processing remain. Therefore, a scintillator element whose sensitivity distribution has a particularly large change contains an error of a certain value or more, and a ring-shaped artifact is generated at a position corresponding to that element.

(Problem to be Solved by the Invention) The above-mentioned conventional technology does not take into consideration the sensitivity distribution in the slice width direction of the scintillator element constituting the X-ray detector, and the state of the sensitivity distribution differs for each element. Therefore, the measured values include errors, and when images are created using these measured values, ring-shaped artifacts tend to occur.

An object of the present invention is to provide a radiation detector that eliminates the decline in the sensitivity distribution of scintillator elements, obtains uniform sensitivity characteristics, and in particular, is configured in a multi-element configuration to obtain good images without ring artifacts. It is in.

[Means to solve the problem]

In the scintillator element 20 shown in Fig. 3, the sensitivity distribution in the slice width direction (horizontal direction in the figure) is not flat as shown in Fig. 7 (A), but has a downward shoulder as shown in Fig. 7 (B) and (C). The reason for this condition is as follows. That is,
The light (scintillator light) 7 from the scintillator 2 generated by the radiation 1 is emitted from the slice width side end surface 8 of the radiation blocking filter 3, and the scintillator light 7 from the light emitting surface 6 in contact with the photoelectric conversion element is This is because it decreases closer to the end face 8. For this reason, a method was devised in which a light reflection process is applied to the slice width side end face 8 of the radiation blocking filter 3 to prevent the scintillator light 7 from being emitted from there.
Patent Application No. 61-301283 (filed by Hitachi Medical Co., Ltd.)) However, even if the end surface 8 is subjected to light reflection treatment, the reflected light 5 therefrom will be incident on the light emitting surface 6 at an angle, so that the critical angle In this case, the light is reflected by the light emitting surface 6 and is not emitted to the outside. This phenomenon becomes larger as it gets closer to the slice width side end face 8, and as a result, the sensitivity distribution also has a lower shoulder, and since there is a difference in the reflectance of the slice width side end face 8, the lowering state of both shoulders is not the same. Figure 7 (B
) and (C), each scintillator element 20 has a different sensitivity distribution state. This condition can be improved by reducing the light reflectance at the light emitting surface 6, in other words by increasing the transmittance of the scintillator light 7.

The spectrum of the scintillator light 7 is a profile that shows a maximum value at a specific wavelength λ due to the constituent materials of the scintillator 2. Therefore, in the present invention, the thickness has a predetermined ratio (according to the theory of transmission and reflection in a single-layer optical thin film, λ/4 is preferable to obtain an antireflection effect) with respect to this wavelength λ, and A translucent material (antireflection film 9) whose refractive index for scintillator light 7 is smaller than that of the radiation blocking filter 3 is placed between the scintillator element 20 and the photoelectric conversion element, for example, between the scintillator element 20 and the filter. The light emitting surface 6 of No. 3 was coated and interposed therebetween. As a result, the transmittance of the scintillator light 7 increases, and the downward slope of the sensitivity distribution in the vicinity of the slice width side end face 8 of the scintillator element 20 is eliminated, thereby achieving the above-mentioned objective.

Further, a scintillator element block formed by arranging a plurality of the scintillator elements 20, and a photoelectric conversion element for converting light from the light emitting surface of the radiation blocking filter of this scintillator element block into an electrical signal are arranged opposite to each scintillator. In a multi-element radiation detector comprising a photoelectric conversion element array, a light-transmitting element having a refractive index for light from the scintillator that is smaller than that of the filter is provided between the scintillator element block and the photoelectric conversion element array. By interposing an anti-reflection film made of a reflective substance with a thickness having a predetermined ratio to the wavelength of light from the scintillator, the transmittance of scintillator light is increased and The objective of eliminating the downward slope of the sensitivity distribution and obtaining a good image without ring-shaped artifacts is achieved.

[Function] In the radiation detector described above, the radiation blocking filter functions to block radiation transmitted through the scintillator (block) and to transmit only light from the scintillator (block). In addition, on the light emitting surface of the radiation blocking filter, a translucent material whose refractive index for scintillator light is smaller than that of the radiation blocking filter is placed at the wavelength λ of the scintillator light.
Since the film is deposited to a thickness of, for example, 1/4 of that of the radiation blocking filter, the transmittance of the scintillator light on the light emitting surface of the radiation blocking filter is improved. This reduces the emission of scintillator light from the slice width side end face of the scintillator element (block), eliminates variations in sensitivity distribution between scintillator elements due to a drop in the sensitivity distribution in the slice width direction, and makes the characteristics of each scintillator element uniform. In particular, when applied to a multi-element type, a good image without ring-shaped artifacts can be obtained.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the drawings. 1st
The figure is a perspective view showing a main part of an embodiment of a radiation detector according to the present invention, and 2 in the figure is a scintillator made of a plate-shaped phosphor. The phosphor may be any material as long as it absorbs radiation and emits light, but X4ICT
As a phosphor for use in a multi-element detector for equipment, Japanese Patent Laid-Open No. 5
G d 20 described in Publication No. 6-151376
2S; P r' t F rCe is preferable because it has high X-ray light conversion efficiency and short afterglow time. The scintillator 2 is produced by molding such a phosphor into a plate shape using a transparent epoxy resin or by molding it into a plate shape by high-pressure firing. scintillator 2
The optimum thickness of the phosphor plate constituting the phosphor plate is determined from the relationship between X-ray blocking ability (ability to block X-rays) and light transmittance characteristics. Approximately 1 for a board
++m is optimal.

As shown in the figure, on the opposite side of the scintillator 2 from the incident side of the X-ray beam 1, a filter that blocks the X-rays 1 and transmits the light, in this case a lead glass plate (lead glass plate for blocking X-rays) 3 is laminated. and is bonded with adhesive 4. This lead glass plate 3 has a high lead content since its purpose is to block X-rays, and is made of lead glass to which Ce is added to prevent coloring due to X-rays. The thickness of the lead glass plate 3 is preferably set so that the amount of transmitted X-rays is 1% or less; in this embodiment, the thickness is 21 m1. moreover,
The surface (light emitting surface 6) of this lead glass plate 3 facing a photoelectric conversion element described later is finished to a mirror surface. This mirror-finished surface (light emitting surface 6) has a 1, for example, 540r
The thickness has a predetermined ratio to the wavelength of v, here 1
An antireflection film 9 with a thickness of /4 is applied. As the material for the antireflection film 9, a material having a refractive index smaller than 1.84, which is the refractive index of the lead glass plate 3 for the scintillator light 7, and is translucent is used. Materials suitable for this purpose include:
MgF2 is generally used, but since the anti-reflection film 9 is required to have appropriate strength, Si with a refractive index of 1.5 is used here.
O was used. Further, the film thickness was 130 nm, and the mirror-finished surface (light emitting surface 6) of the lead glass plate 3 was deposited by vacuum evaporation. According to this, the transmittance of the scintillator light 7 increases by 10% compared to the state without the antireflection film 9, and increases by 93%.
It became.

FIG. 2 is a perspective view showing an example of an X-ray detector block formed by arranging a plurality of scintillator elements 20 (hereinafter simply referred to as scintillator elements 20) each coated with an antireflection film 9 in this manner. As shown in FIG. 2, a plurality of scintillator elements 20 are arranged with a metal separator plate 21 for optically separating the elements 20 from each other, and an adhesive 22 is applied at the end of each element 20. They are joined together to form one scintillator element block 23. This scintillator element block 23 is disposed opposite to a photoelectric conversion element array 25 which is optically separated into the same number of photoelectric conversion elements 24 as the scintillator elements 20 . In this case, the pixel elements 20 and 24 are arranged and joined with sufficiently high alignment accuracy.

Each output signal from the photoelectric conversion element array 25 is transmitted to the substrate 26.
It is bonded to the wiring butt of and taken out from the connector 27.

The photoelectric conversion element array 25 preferably has a spectral sensitivity similar to the spectrum of the scintillator light from the scintillator element block 23, and in this embodiment, it is constructed using a PIN type silicon photodiode.

By arranging a plurality of such X-ray detector blocks on an arc centered on the X-ray source 10 as shown in FIG. 4, the multi-element X-ray detector 12 in the X-ray CT apparatus
is configured.

〔Effect of the invention〕

According to the present invention, when light from a scintillator passes through a radiation blocking filter, the decrease in scintillator light near the end face on the slice width side is reduced, and it is possible to eliminate a drop in the sensitivity distribution and achieve a uniform This has the effect of providing sensitivity characteristics. Therefore, when this is applied to a multi-element X-ray detector, it is possible to reduce variations in the sensitivity distribution in the slice width direction between scintillator elements, and in particular, it is possible to reduce X-ray measurement of objects whose shape changes in the slice width direction. Also, there is an effect that a good image without ring-shaped artifacts can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the main parts of an embodiment of the detector of the present invention;
Fig. 2 is a perspective view showing an example of an X-ray detector block formed by arranging a plurality of scintillator elements coated with anti-reflection films, Fig. 3 is a schematic explanatory diagram of the detector of the present invention, and Fig. 4 is xN
Diagram 5 for explaining the X-ray measurement principle of the IAcT device
The figure is a diagram showing an example of the shape of an object, FIG. 6 is a graph showing the X-ray intensity distribution after passing through the object, and FIG. 7 is a graph for explaining the variation in sensitivity distribution in the slice width direction of one scintillator element. . 1...Fan-shaped X-ray beam (radiation), 2...Scintillator, 3...Lead glass plate (X-ray blocking filter), 6
...Light emitting surface, 7...Scintillator light, 8...Slice width side end face, 9...Anti-reflection film, 10...X-ray source, 11...Subject, 12...Multiple element x-ray detector,
20...Scintillator element, 21...Metal separator plate, 23...Scintillator block, 24...
Photoelectric conversion element, 25... Photoelectric conversion element array, 26.
... Board, 27... Connector. Patent Applicant Hitachi Medical Co., Ltd. Patent Attorney Masami Akimoto (1 other person) A) (B) Seventh! !
! !

Claims (1)

[Scope of Claims] 1. A scintillator element consisting of a scintillator that emits light due to radiation and a filter that is laminated on the side opposite to the radiation incident side of the scintillator and blocks the radiation and transmits the light, and this scintillator element A radiation detector comprising a photoelectric conversion element that converts light from the light emitting surface of the filter into an electrical signal, wherein a refractive index for the light from the scintillator is located between the scintillator element and the photoelectric conversion element. A radiation detector characterized in that an antireflection film made of a light-transmitting material smaller than that of the filter is interposed with a thickness having a predetermined ratio to the wavelength of light from the scintillator. 2. A scintillator element block comprising a scintillator block formed by arranging a plurality of scintillators that emit light due to radiation, and a filter laminated on the side opposite to the radiation incident side of this scintillator block to block the radiation and transmit light. , a multi-element radiation detector comprising: a photoelectric conversion element array in which photoelectric conversion elements for converting light from the light emitting surface of the filter of the scintillator element block into electrical signals are arranged opposite to each of the scintillators; between the scintillator element block and the photoelectric conversion element array, an anti-reflection film made of a translucent material whose refractive index for light from the scintillator is smaller than that of the filter for the wavelength of light from the scintillator is provided. A radiation detector characterized in that a radiation detector is provided with a thickness having a predetermined ratio.