JP2002196079A - Radiation detector, and x-ray ct equipment using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation detector of high light transmittance, capable of increase a grooving working speed without generating lowering of partial radiation detection sensitivity even when using a hard ceramic scintillator, and capable of enhancing productivity thereby, and an X-ray CT equipment capable of providing a tomographic image of high quality free from an artifact using the radiation detector. SOLUTION: This radiation detector is constituted of phosphor elements of a short optical path length having μ=0.7 mm-1 or less of photoabsorption coefficient in a visible ray range, and having a surface roughness Ra (average center line roughness) ranging within 0.1 μm<=Ra<=2 μm in at least one surface (where μ is the absorption coefficient when photoabsorption is expressed as I (d)=I0e-μd(d is a thickness of an absorber)), photoelectric transfer elements for detecting light emission by the phosphor elements, and an element array substrate arrayed with the phosphor elements and the photoelectric transfer elements, and the radiation detector is used as an X-ray detector for the X-ray CT equipment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation detector for detecting X-rays, .gamma.-rays, etc., and more particularly to a radiation detector suitable for improving productivity without lowering radiation detection sensitivity and using the detector. X-ray CT apparatus capable of obtaining high quality tomographic images.

[0002]

2. Description of the Related Art Conventionally, as a radiation detector used in an X-ray CT apparatus, a xenon gas chamber, a combination of bismuth germanate (BGO single crystal) and a photomultiplier, a CsI: Tl single crystal or CdWO are used. Combinations of _four single crystals and photodiodes have been used. In recent years, rare-earth phosphors with high conversion efficiency from radiation to light have been developed, and radiation detectors combining such phosphors and photoelectric conversion elements, so-called solid-state detectors, have been put into practical use. Have been.

In general, a phosphor material used for a radiation detector is required to have high luminous efficiency, short afterglow, large X-ray stopping power, and the like. As the radiation detector having the high luminous efficiency, a radiation detector using a phosphor element having Ce as a light emitting element and at least Gd, Al, Ga, and O is disclosed in PCT / JP98 / 05806 by the present inventors. Have been. In the radiation detector combining the above-mentioned ceramic scintillator and Si photodiode, a plate-shaped scintillator is adhered to a substrate on which a photodiode array is lined in order to divide the channels of the scintillator, and then the photo-detector is sliced by an outer edge slicer or the like. The scintillator was grooved according to the diode. At that time, in order to prevent the variation of radiation sensitivity from occurring due to the influence of irregular reflection of light on the bonding surface between the scintillator and the photodiode,
A method for controlling the surface roughness of a scintillator is disclosed in
It is disclosed in 5-209969. This method is used when a rare earth oxysulfide scintillator with a long optical path length is used.
This is based on the fact that sensitivity variations due to irregular reflection of light on the bonding surface are offset and alleviated due to the long optical path length.

[0004]

The rare earth oxysulfide-based ceramic scintillator has high luminous efficiency but low light transmittance. Therefore, if the scintillator is made thicker to sufficiently absorb radiation, it will be sufficiently absorbed by the scintillator by self-absorption. There is a problem that a proper output cannot be obtained. On the other hand, for example
The rare earth oxide-based ceramic scintillator described in PCT / JP98 / 05806 etc. has a light absorption coefficient μ in the visible light region of 0.
7 mm ⁻¹ or less, the light transmittance is high, and the above problem can be solved. However, since the ceramic scintillator is hard, there is a problem in workability.

[0005] In the groove cutting of the solid state detector, the workability of the ceramic scintillator greatly affects the workability. The rare earth oxysulfide-based ceramic scintillator described in Japanese Patent Publication No. 60-4856 etc. has a Vickers hardness Hv of about 5 GPa, which is relatively soft, and can perform grooving at about 50 mm / min. However, the rare earth oxide-based ceramic scintillator described in PCT / JP98 / 05806 or the like has a Vickers hardness Hv of about 15 GPa, which is very hard as compared with the rare earth oxysulfide-based ceramic scintillator. Therefore, the workability deteriorates and the grooving speed has to be reduced, and as a result, the processing cost increases.

Further, when the groove cutting speed is increased in order to reduce the processing cost, there is a problem that the scintillator is partially peeled off from the photodiode substrate due to the stress due to the processing, and the X-ray detection sensitivity is reduced only in that portion. Was. If the detector sensitivity is partially reduced, an artifact occurs in a reconstructed image of the X-ray CT apparatus or the like, so that the partial reduction in sensitivity must be suppressed.

[0007] Therefore, an object of the present invention is to solve the above-mentioned problems, and even when a hard ceramic scintillator having a high light transmittance is used, the grooving speed can be reduced without causing a partial decrease in radiation detection sensitivity. It is to provide a radiation detector capable of improving productivity and improving productivity.
Another object of the present invention is to provide an X-ray CT apparatus that detects X-rays transmitted through a subject using the radiation detector and obtains high-quality tomographic images without artifacts. .

[0008]

The above object is achieved by the following means.

(1) A radiation detector comprising a phosphor element, a photoelectric conversion element for detecting light emission by the phosphor element, and an element array substrate on which the phosphor element and the photoelectric conversion element are arranged in an array. Thus, as the phosphor element,
A phosphor element having a short optical path length having a light absorption coefficient of μ = 0.7 mm ⁻¹ or less in a visible light region is used, and surface roughness Ra (center line average roughness) of at least one surface of the phosphor element is used.
Is 0.1 μm ≦ Ra ≦ 2 μm. Where μ is the light absorption
It is the absorption coefficient μ when I (d) = I ₀ e− ^μd (d is the thickness of the absorber).

(2) As the phosphor element described in (1) above, a phosphor element having a host crystal having a garnet structure containing Ce as a light emitting element and containing at least Gd, Al, Ga, and O is used. It is a radiation detector.

(3) An X-ray source, an X-ray detector placed opposite to the X-ray source, holding the X-ray source and the X-ray detector, and driven to rotate around the subject. An X-ray CT apparatus comprising: a rotating disk; and image reconstruction means for reconstructing a tomographic image of the subject based on the intensity of X-rays detected by the X-ray detector. An X-ray CT apparatus is configured using the radiation detectors described in (1) and (2) above.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [1] Radiation detector An embodiment of the radiation detector of the present invention will be described in detail with reference to FIGS.

FIG. 1 shows a cross section of the radiation detector of the present invention. In FIG. 1, reference numeral 1 denotes a phosphor element, 2 denotes a photoelectric conversion element bonded to the phosphor element 1 by an optical adhesive that does not absorb light emitted from the phosphor element 1, and 3 denotes a photoelectric conversion element 2.
An element array substrate provided with a plurality of one-dimensional or two-dimensional,
4 is an adhesion surface of the phosphor element 1 adhered to the photoelectric conversion element 2, 5 is a channel separation groove for dividing the phosphor element, and 6 is inserted into the channel separation groove 5 to prevent crosstalk between channels. It is a light reflection plate. Phosphor element 1 has
For example, a ceramic scintillator having a high light transmittance and having a host crystal having a garnet structure containing at least Gd, Al, Ga, and O with Ce as a light emitting element is used. For the photoelectric conversion element 2, a Si-PIN photodiode having a high response speed or the like is used, and for the light reflection plate 6, a Mo plate or the like is used.

Next, the procedure for assembling the radiation detector of the present invention will be described.

First, a plate-shaped ceramic scintillator 1 is bonded to a photodiode 2 with an adhesive. Next, the channel separation groove 5 is cut in the X direction in FIG. 1 (perpendicular to the paper surface) by an outer peripheral slicer or the like, and the light reflection plate 6 is inserted into the channel separation groove. Also, when the channels are separated in the Y direction in FIG.

At this time, for example, a PC used in the present invention
The rare earth oxide ceramic scintillator described in T / JP98 / 05806 or the like, that is, Ce is used as a light emitting element, and at least Gd,
Surface roughness R of the bonding surface 4 of the ceramic scintillator having a host crystal with a garnet structure containing Al, Ga, and O
By setting a (center line average roughness) to 0.1 μm ≦ Ra,
The adhesive strength with the photodiode can be improved. More preferably, by satisfying 0.3 μm ≦ Ra,
The adhesive strength with the photodiode can be improved. This prevents the scintillator from peeling off from the photodiode even when the grooving speed is increased, thereby preventing the X-ray detection sensitivity from being partially reduced. On the other hand, when the surface roughness Ra exceeds 2 μm, bubbles are generated when bonding with the photodiode,
Further, since the mechanical strength of the scintillator is reduced, chipping or the like is likely to occur at the time of grooving, which causes variation in X-ray detection sensitivity. Therefore, it is desirable that the surface roughness Ra of the bonding surface 4 be 0.1 μm ≦ Ra ≦ 2 μm.

Next, the above effects will be described with reference to the experimental results shown in FIG.

FIG. 2 shows the results of examining the rate of decrease in X-ray detection sensitivity due to grooving when the surface roughness of the bonding surface is changed using a Gd3Al3Ga2O12: Ce ceramic scintillator. The surface roughness of the bonding surface is controlled by the particle size of the abrasive grains used for lap polishing.For example, when lap-polishing a Gd3Al3Ga2O12: Ce ceramic scintillator with SiC # 1200, the surface roughness Ra becomes 0.2 μm, and polishing with SiC # 600. Ra is 0.
5 μm. In the first embodiment of FIG. 2, the grooving speed is 50 m.
m / min. Comparative Example 1 in the figure is Gd3Al3Ga2
The groove cutting speed was reduced to 10 mm / min using an O12: Ce ceramic scintillator, and Comparative Example 2 used a G2O2S: Pr ceramic scintillator, which is a rare earth oxysulfide-based phosphor, as a phosphor element. This is the case where the cutting speed is 50 mm / min.

After the ceramic scintillator is bonded to the photodiode, a groove is cut in the X direction shown in FIG.
The measurement was made based on the amount of change in the difference between the maximum value and the minimum value of the output from each photodiode channel when scanning was performed in the X direction in FIG.

FIG. 2 shows that the surface roughness of the bonding surface is 0.1 μm.
When ≦ Ra, more preferably, when 0.3 μm ≦ Ra, the decrease in X-ray detection sensitivity due to processing was reduced by G2 in Comparative Example 2.
It can be seen that it can be suppressed to the same extent as in the case of the O2S: Pr ceramic scintillator. In addition, in the case of Comparative Example 1, the X-ray detection sensitivity reduction rate is almost the same as that of Comparative Example 2, but since the processing speed is reduced to 1/5, the production efficiency is reduced, which is disadvantageous in terms of production cost. Become.

In the first embodiment, the bonded surface of the ceramic scintillator is lapped and polished, and the surface roughness Ra is controlled by the grain diameter of the abrasive. However, a fixed abrasive or a surface grinder may be used.

As described above, Ce has a high light transmittance and a high Vickers hardness, for example, Ce as a light emitting element.
A ceramic scintillator having a host crystal with a garnet structure containing Gd, Al, Ga, and O has an adhesive surface roughness.
When Ra is set to 0.1 μm ≦ Ra ≦ 2 μm, it is possible to increase the grooving speed without partially lowering the X-ray detection sensitivity, thereby improving the productivity of the radiation detector.

[2] X-ray CT apparatus using the radiation detector of the present invention By arranging a plurality of the above-described radiation detectors of the present invention, an X-ray detector of an X-ray CT apparatus can be obtained. FIG. 3 schematically shows an X-ray CT apparatus of the present invention using the X-ray detector. This apparatus includes a gantry section 18 and an image reconstructing section 22. The gantry section 18 has a rotating disk 19 having an opening 20 into which a subject is carried in, and an X mounted on the rotating disk. A X-ray tube 16, a collimator 17 attached to the X-ray tube and controlling the radiation direction of X-rays, an X-ray detector 15 mounted on a rotating disk facing the X-ray tube, and an X-ray detector 15. A detector circuit 21 for converting a detected X-ray into a specific signal, and a scan control circuit for controlling the rotation of the rotating disk and the width of the X-ray flux
24 and have. X-rays are emitted from an X-ray tube in a state where a subject (not shown) is placed on a bed (not shown) installed in the opening 20. The X-rays have directivity obtained by a collimator, and X-rays transmitted through the subject are detected by an X-ray detector. By rotating the rotating disk around the subject, X-rays are detected while changing the X-ray irradiation direction,
The image reconstruction unit 22 creates a tomographic image and displays it on the monitor 23.

Further, in realizing high image quality in thinning slices by a multi-slice X-ray CT apparatus using a two-dimensional detector array and in high-speed scanning of an X-ray CT apparatus such as 0.5 second or 0.3 second per scan. However, the present invention exerts a greater effect.

[0025]

According to the present invention, in a radiation detector using a hard phosphor element having a high light transmittance, the channel separation groove or the slice separation groove can be formed without partially lowering the radiation detection sensitivity. Since the processing can be performed at high speed, the productivity of the radiation detector can be increased.

By using this radiation detector as an X-ray detector of an X-ray CT apparatus, a high quality tomographic image can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a radiation detector of the present invention.

FIG. 2 is a diagram showing the relationship between the adhesion surface roughness and the X-ray detection sensitivity.

FIG. 3 is a configuration diagram of an X-ray CT apparatus using a radiation detector according to the present invention.

[Explanation of symbols]

1 phosphor element, 2 photoelectric conversion element, 3 element array substrate, 4 adhesive surface, 5 channel separation groove, 6 light reflector, 1
5 X-ray detector, 16 X-ray tube, 17 collimator, 18 gantry, 19 rotating disk, 20 opening, 21 detector circuit,
22 image reconstruction unit, 23 monitor, 24 scan control circuit

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Minoru Yoshida 1-1-1 Uchikanda, Chiyoda-ku, Tokyo F-term in Hitachi Medical Co., Ltd. (Reference) 2G088 EE02 FF02 GG10 GG16 GG19 GG20 JJ04 JJ05 JJ37 LL12 LL15

Claims (3)

[Claims] 1. A radiation detector comprising: a phosphor element; a photoelectric conversion element for detecting light emission by the phosphor element; and an element array substrate on which the phosphor element and the photoelectric conversion element are arranged in an array. A phosphor element having a short light path length having a light absorption coefficient of μ = 0.7 mm ⁻¹ or less in a visible light region, and a surface roughness Ra (center line) of at least one surface of the phosphor element is used as the phosphor element. (Average roughness)
A radiation detector, wherein 0.1 μm ≦ Ra ≦ 2 μm. Here, μ is an absorption coefficient μ when light absorption is expressed as I (d) = I ₀ e− ^μd (d is the thickness of the absorber). 2. The phosphor element according to claim 1, wherein the phosphor element is Ce.
A radiation detector comprising: a phosphor element having a host crystal having a garnet structure containing at least Gd, Al, Ga, and O as a light emitting element. 3. An X-ray source, an X-ray detector placed opposite to the X-ray source, the X-ray source and the X-ray source detector are held, and are rotated around the subject. A rotating disk and the X
Image reconstruction means for reconstructing a tomographic image of the subject based on the intensity of the X-rays detected by the X-ray detector.
3. An X-ray CT apparatus, wherein the radiation detector according to claim 1 is used as said X-ray detector.
apparatus.