Patent Application
20090101844
1. Field of the Invention
The present invention relates to a scintillator panel that converts a radiation image used for medical or industrial radiography or the like to an optical image.
2. Related Background Art
Conventionally, X-ray sensitive films have been used for medical or industrial radiography, however, radiation imaging systems using radiation detectors have become widespread from the aspects of convenience and storage stability of radiographic results. In such a radiation imaging system, two-dimensional image data by radiation is obtained as an electrical signal by the radiation detector, and this signal is processed by a processor and displayed on a monitor.
As a typical radiation detector, a radiation detector exists, having a structure for which a radiation image converting panel (hereinafter, referred to as a “scintillator panel”) with a scintillator to convert radiation to a visible light formed on a substrate made of aluminum, glass, fused silica or the like is formed, and this is combined with an image pickup device. In this radiation detector, the scintillator converts radiation that is made incident to an optical image (not always of a visible light) in a wavelength range detectable by the image pickup device, and the image pickup device detects the optical image.
As scintillator panels structured as such, scintillator panels disclosed in International Publications WO 00/63722, WO 02/23219 A1, WO 02/23220 A1, etc., have been known. The scintillator panel disclosed in WO 00/63722 is one for which, on a radiation-transmissive substrate, a thin reflective metal film and a protective film are laminated, and a scintillator is deposited. The scintillator panel disclosed in WO 02/23219 A1 is one for which provided on a heat-resisting radiation-transmissive substrate is a dielectric multilayer film, and thereon a scintillator is deposited. The scintillator panel disclosed in WO 02/23220 A1 is one for which provided on a light-transmissive substrate is a dielectric multilayer film, and thereon a scintillator is deposited, and provided on either one or both of an interface between the dielectric multilayer film and light-transmissive substrate and a surface of the light-transmissive substrate opposite to the dielectric multilayer film is a light incidence/reflection inhibiting member that inhibits incidence/reflection of an unnecessary light. In these scintillator panels, radiation that has been transmitted through the substrate is made incident into the scintillator, converted to an optical image, and outputted from a surface thereof opposite to the incident surface of radiation, and a reflecting material provided between the scintillator and substrate reflects light emitted from the scintillator toward the substrate, whereby a radiation image with a high intensity can be obtained.
In the scintillator panels described above, the higher the reflectance of the reflector, the higher the intensity of a radiation image can be obtained. In the case of a dielectric multilayer film, a high reflectance can be realized by increasing the number of layers thereof.
In recent years, scintillator panels have been increased in size in application to chest radiography and the like, however, it is difficult in terms of mass production to laminate multiple layers of dielectric films with a uniform film thickness on such large-sized panels, and this causes an increase in the manufacturing cost. In addition, for obtaining a radiation image with a high intensity while inhibiting the amount of radiation exposure, it is desirable to reduce a loss due to absorption by the substrate etc., until radiation reaches the scintillator to the minimum, so that it is preferable to provide the substrate etc., as thin as possible. The more the substrate etc., is increased in size to become thinner, the more difficult it becomes to form multilayered dielectric films.
While the applicant has proceeded with development for mass production of a large-sized thin scintillator panel, the inventors have discovered in the course of development that a warp on the order of a few millimeters to a few tens of millimeters can occur to the substrate as a result of a scintillator formation when the substrate is thinned and increased in size. It was found that, when a warp occurred as such, an inorganic material that forms the dielectric film is low in resistance to deformation produced by this warp since this is poor in flexibility, elasticity, and ductility, and if the dielectric film layer is thickened, an inconvenience such that a crack is produced in the dielectric film occurs.
Furthermore, when an image sensor is adhered to the scintillator panel to form a radiation image sensor, it is necessary to flatten an adhering surface of the scintillator panel, and if such a warp as mentioned above has occurred in the scintillator panel, applying a stress to eliminate the warp results in a repeated stress application to the substrate, which makes a crack more likely to occur.
It is therefore an object of the present invention to provide a large-sized and thin radiation image converting panel, scintillator panel, and radiation image sensor which allow obtaining a radiation image with a high intensity and can be easily mass-produced.
In order to achieve the above object, for a radiation image converting panel according to the present invention, in a radiation image converting panel including a converting portion that converts a radiation image to an optical image on a support, the support includes: (1) a metal reflector; and (2) a dielectric film mirror including at least a first dielectric layer which is in contact with the metal reflector and formed thereon and a second dielectric layer which is formed on the first dielectric layer and has a higher refractive index than that of the first dielectric layer to light of an optical image outputted from the converting portion, and the converting portion is formed on the dielectric film mirror side.
In addition, for a scintillator panel according to the present invention, a scintillator is used as the converting portion of the above radiation image converting panel, and in a scintillator panel for which a scintillator composed of a plurality of needle crystals is formed on a support, the support includes: (1) a metal reflector; and (2) a dielectric film mirror including at least a first dielectric layer which is in contact with the metal reflector and formed thereon and a second dielectric layer which is formed on the first dielectric layer and has a higher refractive index than that of the first dielectric layer to light emitted from the scintillator, and the scintillator is formed on the dielectric film mirror side.
According to the inventor's knowledge, when combining a plurality of dielectric films different in the refractive index to compose a mirror, in the case of two layers, the refractive index as a whole becomes higher if a layer lower in the refractive index is provided as a first layer with reference to the side opposite to a light incident surface and thereon a layer higher in the refractive index than the first layer is laminated as the second layer. The present invention is based on this knowledge. By employing such construction as in the above, a high light reflectance can be obtained even with a small number of layers, and not only can lamination be favorably performed even when the substrate is thin, but occurrence of a crack in the dielectric layers can also be prevented even when the substrate has been warped in a subsequent process, and thus a radiation image with a high intensity can be obtained, and mass production thereof also becomes easier.
It is preferable that the first dielectric layer contains SiO2, and the second dielectric layer contains at least one material from TiO2, Nb2O5, Ta2O5, HfO2, and ZrO2. Since SiO2 is lower in the refractive index than any of TiO2, Nb2O5, Ta2O5, HfO2, and ZrO2 and also has a large difference in the refractive index, a high reflectance can be realized.
More preferably, the dielectric film mirror is formed by laminating three or more layers of the first dielectric layer and the second dielectric layer alternately. By laminating three or more layers, the reflectance can be further improved.
However, when the number of laminations is increased, although the reflectance is improved, the difficulty in lamination is increased as described above, and resistance to a warp that can possibly be produced in a subsequent process is also lowered, and thus in consideration of simultaneous pursuit of a high reflectance and mass producibility, the number of laminations of the dielectric film mirror is 10 layers or less, or the film thickness thereof as a whole is 1 μm or less.
The scintillator panel may further include a transparent organic film which is formed, on the dielectric film mirror, at least between the same and the scintillator, and has a lower refractive index than that of the second dielectric layer to light emitted by the scintillator. By such configuration, not only is the dielectric film mirror protected by the transparent organic film, but an effect to further improve the light reflectance can also be obtained.
As the metal reflector, a thin metal film or a metal substrate can be used, and when the metal reflector is a thin metal film, handling thereof becomes easy if the scintillator panel further includes a supporting substrate that supports the metal reflector. If these types of metal reflectors are formed of aluminum, silver, or gold, since the light reflectance of the reflectors themselves can be increased, the light reflectance as a whole is improved, which is preferable.
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. To facilitate the comprehension of the explanation, the same reference numerals denote the same parts, where possible, throughout the drawings, and a repeated explanation will be omitted. In addition, dimensions in the respective drawings have been overdrawn in part for the sake of description, and are not always coincident with those in reality.
The aluminum substrate 10 functions as a supporting substrate that supports the whole. When the thickness of the aluminum substrate 10 is less than 0.3 mm, the scintillator 40 tends to become easily peeled off due to curving of the aluminum substrate 10. On the other hand, when the thickness of the aluminum substrate 10 exceeds 1.0 mm, its radiation transmittance tends to decline. In order to secure intensity of radiation that is transmitted through this aluminum substrate 10 and made incident into the scintillator 40 while reliably forming the scintillator 40 on the substrate, it is preferable to provide the thickness of the aluminum substrate 10 as 0.3 mm or more and 11.0 mm or less.
As the metal reflection film 11, a thin film of gold, silver, aluminum, or the like can be used. The aluminum substrate 10 is usually formed by rolling, striped streaks are formed in the rolling process, and it is difficult to completely remove the streaks even by subsequent polishing. When the thickness of the metal reflection film 11 is less than 50 nm, unevenness tends to occur on the reflection surface under the influence of the stripes. On the other hand, when the thickness thereof exceeds 200 nm, its radiation reflectance tends to decline. In consideration of balance between the light reflectance of the metal reflection film 11 and intensity of radiation made incident into the scintillator 40, it is preferable to provide the thickness of the metal reflection film 11 as 50 nm or more and 200 nm or less. In the present embodiment, a thin aluminum film with a thickness of 70 nm is used, however, this may be analyzed as an incomplete aluminum oxide by an AES analysis (Auger Electron Spectroscopy).
Both of the SiO2 film 21 and the TiO2 film 22 compose a dielectric film mirror 2.
The reflection protective film 30 has a function to further prevent erosion and deterioration of the metal reflection film 11 due to the constituent of the scintillator 40. The SiO2 film 21 and TiO2 22 described above are formed by vapor deposition as will be described later, and fine pinholes may occur at that time. In such a case, if the scintillator 40 is formed directly on the TiO2 film 22, a constituent of the scintillator 40 reaches the metal reflection film 11 through the fine pinholes existing in the SiO2 film 21 and TiO2 film 22 to cause erosion and deterioration thereof. In the present embodiment, the reflection protective film 30 exists, so that even when pinholes have occurred at the time of a dielectric film formation, these openings can be effectively blocked to effectively inhibit the constituent of the scintillator 40 from reaching the metal reflection film 11. In addition, this can also efficiently inhibit the constituent of the scintillator 40 from reaching the metal reflection film 11 from the lateral side of the aluminum substrate 10.
On the other hand, since the scintillator 40 is generally formed of a deliquescent material, when the scintillator 40 is formed of such a deliquescent material, it is desirable to seal the scintillator 40 by the moisture-resistant protective film 50.
As the reflection protective film 30 and moisture-resistant protective film 50, an organic film or an inorganic film can be used, and different materials or the same material may be used. In the present embodiment, both films 30 and 50 are made from, for example, polyparaxylylene, but may be made from a xylylene-based material such as polymonochloroparaxylylene, polydichloroparaxylylene, polytetrachloroparaxylylene, polyfluoroparaxylylene, polydimethylparaxylylene, or polydiethylparaxylylene. Alternatively, the reflection protective film 30 and moisture-resistant protective film 50 may be made from, for example, polyurea, polyimide, or the like, or may be made from an inorganic material such as LiF, MgF2, SiO2, Al2O3, TiO2, MgO, or SiN. Further alternatively, these may be formed in combination of an inorganic film and an organic film. In the present embodiment, the thickness of each protective film 30, 50 is 10 μm. The protective film 30 has a refractive index to light with a wavelength of 560 nm of 1.64, which is lower than the refractive index of the TiO2 22 being the second layer and higher than the refractive index of the SiO2 film 21 being the first layer.
The scintillator 40 is smaller than the aluminum substrate 10 when viewed from the thickness direction of the aluminum substrate 10. In other words, the scintillator 40 is formed not on the whole of a scintillator 40 forming surface of the aluminum substrate 10 but in a partial region on the surface. The region of the surface of the aluminum substrate 10 formed with the scintillator 40 may be surrounded by regions formed without the scintillator 40 on four sides, however, regions formed without the scintillator 40 may exist only at three or two sides thereof and the scintillator 40 may be formed up to the ends of the substrate 10 on the other sides.
The scintillator 40 is made of, for example, a phosphor that converts radiation to a visible light, and is composed of CsI columnar crystals or the like doped with Tl, Na, or the like. The scintillator 40 has a construction bristled with a plurality of needle crystals. The scintillator 40 may be made of NaI doped with Tl, KI doped with Tl, or LiI doped with Eu. Alternatively, the scintillator 40 may be of a type that emits infrared or ultraviolet light other than visible light. The thickness of the scintillator 40 is preferably 100 μm to 1000 μm, and more preferably, 450 μm to 550 μm. The average needle diameter of the needle crystals that compose the scintillator 40 is preferably 3 μm to 10 μm.
Next, description will be given of a manufacturing process of this scintillator panel 100. First, the aluminum substrate 10 is prepared. Next, on this aluminum substrate 10, the metal reflection film 11 is formed by a vacuum vapor deposition method. Subsequently, the SiO2 film 21 and TiO2 film 22 are laminated similarly by a vacuum vapor deposition method. Then, the reflection protective film 30 is formed by a CVD method so as to seal the aluminum substrate 10, the metal reflection film 11, and the dielectric film mirror 2 as a whole. Subsequently, by a vapor deposition method, the scintillator 40 is formed at a predetermined position on the reflection protective film 30. Next, the moisture-resistant protective film 50 is formed by a CVD method so as to seal the whole of the aluminum substrate 10, the metal reflection film 11, and dielectric film mirror 2 sealed by the reflection protective film 30 and the scintillator 40 formed thereon altogether. In this manner, the scintillator panel 100 is manufactured. Here, sealing of the protective film 30, 50 can be realized by making the opposite surface side of the aluminum substrate 10 to the scintillator forming surface float up from a substrate holder that supports the substrate in the apparatus at the time of CVD. Such methods include a method described in, for example, the specification of U.S. Pat. No. 6,777,690. According to this method, pins are used to float the aluminum substrate 10. In this case, no protective film is formed on a minute contact surface between the aluminum substrate 10 and pins.
Next, we will describe actions and effects of this scintillator panel 100. This scintillator panel 100 is used as a part of a radiation image sensor.
This radiation image sensor 400 includes the scintillator panel 100 and an image pickup device 470 that converts an optical image IL emitted from the scintillator 40 of the scintillator panel 100 to an electrical signal IE. Between the image pickup device 470 and the scintillator panel 100, arranged is a mirror 450 and a lens 460 as a reducing optical system that demagnifies an optical image IL and guides the image to an imaging plane of the image pickup device 470. The reducing optical system is not limited to the illustrated construction but may be composed only of either the mirror or lens, and the mirror and lens may be either a single or a plurality of mirrors and lenses. In addition, a prism and other optical components may be used besides the mirror and lens.
To describe the case of chest radiography for example, located as a subject is a reagent (not shown) between the surface (hereinafter, referred to as a “radiation input surface”) of the scintillator panel 100 on the side where the scintillator 40 has not been formed and a radiation source 340. Radiation emitted from the radiation source 340 toward the reagent partially passes through the reagent, and an obtained radiation image Ix is made incident into the radiation input surface of the scintillator panel 100. The radiation made incident into the scintillator panel 100 passes through the moisture-resistant protective film 50, reflection protective film 30, the aluminum substrate 10, the metal reflection film 11, the dielectric film mirror 2, and the reflection protective film 30, and is made incident into the scintillator 40. The scintillator 40 emits visible light according to the radiation made incident. Thereby, a visible optical image IL according to the radiation image IX is generated.
A part of the visible light emitted from the scintillator 40 passes through the moisture-resistant protective film 50 and is outputted from the surface (hereinafter, referred to as a “optical image output surface”) of the scintillator panel 100 on the side opposite to the radiation input surface. In addition, a part is outputted in the opposite direction, that is, to the dielectric film mirror 2 side, but is reflected by the reflection protective film 30, the dielectric film mirror 2, and the metal reflection film 11 to return to the scintillator 40 side, and is finally outputted from the optical image output surface after passing through the scintillator 40 and the moisture-resistant protective film 50.
Since the scintillator panel 100 of the present embodiment employs the thin aluminum substrate 10, this allows securing intensity of radiation made incident into the scintillator 40 to obtain a radiation image (in actuality, the optical image IL corresponding to the radiation image IX) with a high intensity. In addition, since light emitted from the scintillator 40 is effectively guided by the reflection protective film 30, the dielectric film mirror 2, and the metal reflection film 11 to the optical image output surface to be outputted, intensity of the output optical image can be further increased. Since this allows obtaining an optical image corresponding to a radiation image with a sufficient intensity even when the intensity of light made incident is low, the amount of radiation made incident into the subject can be reduced, and an effect to lower the amount of radiation exposure of the subject can also be obtained.
The optical image IL outputted from the scintillator panel 100 is demagnified by the mirror 450 and the lens 460 and is guided to the imaging plane of the image pickup device 470. As the image pickup device 470, besides, for example, a solid-state image pickup device such as a CCD (Charge-Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) device, a pickup tube or the like can be used. The image pickup device 470 outputs the electrical signal IE according to the optical image IL made incident from the imaging plane. The outputted electrical signal IE is sent to an analyzing workstation 490 through an electronic device 480, and is applied with a predetermined processing to display an image onto a display or accumulate/store image information onto a storage device such as hard disk. The electrical signal IE corresponding to the image may be sent as an analog signal to the workstation 490 or may be converted to a digital signal by the electronic device 480 or the image pickup device 470 itself. The electronic device 480 may apply, without limitation to digitization, other processes to the obtained electrical signal IE. In addition, the electronic device 480 may control operation of the image pickup device 470. In the present embodiment, using the reducing optical system provides an advantage that a small-sized image pickup device can be used as the image pickup device 470.
The radiation image sensor using the scintillator panel 100 of the present embodiment is not limited to the embodiment mentioned above.
Similar to the radiation image sensor of the first embodiment, in the present embodiment as well, a radiation image made incident from the radiation input surface is converted to an electrical signal by being made incident into the scintillator 40, is converted to an optical image, and is guided to the image pickup device 500. By guiding this electrical signal to an unillustrated analyzing device, a display, accumulation/storage, or the like can be performed, and the same effects as those of the radiation image sensor of the first embodiment can be obtained. In the present embodiment, since the scintillator panel 100 and the image pickup device 500 can be integrated, handling thereof becomes easy, and there is also an advantage that omitting an optical system makes an adjustment easy.
Next, we will describe verification results of the difference in effects depending on the existence of the dielectric film mirror 2 of the scintillator panel 100 of the present embodiment.
Next, we will describe the case where the dielectric film mirror 2 is provided.
It can be understood from this table that an effect to improve reflectance when the TiO2 film serving as an even number layer was laminated is larger than the effect to improve reflectance when the SiO2 film 21 serving as an odd number layer was laminated. Therefore, in consideration of cost performance etc., it is preferable to provide the number of laminations as an even number.
As has been described above, although it is preferable that the number of layers of the dielectric films is large for improving reflecting characteristics thereof, since a reflectance of nearly 95% can be obtained even with two layers and a reflectance of 99.85% can be obtained with ten layers, the effect to improve reflectance to be provided by further multilayered dielectric films is small. Meanwhile, the materials composing the dielectric films are poor in flexibility, elasticity, and ductility. During a manufacturing process of the scintillator panel 100 of the present embodiment, a warp on the order of a few millimeters to a few tens of millimeters can occur in the substrate 10 due to a formation of the scintillator 40, however, the dielectric films are poor in resistance to distortion caused by this warp, and an inconvenience such that a crack is produced can occur. The larger the number of layers and the layer thickness of the dielectric films, the more likely a crack will occur, so that it is preferable that the thickness does not exceed approximately 1 μm. Therefore, the dielectric films can realize both mass producibility and a high reflectance when these are provided on the order of ten layers or less in terms of the number of layers and at 1 μm or less in terms of the thickness.
Hereinafter, we will describe some different embodiments of a scintillator panel according to the present invention.
The present invention is not limited to the configurations of the above embodiments, and also includes combinations of the respective embodiments and replacements thereof, etc. For example, in the third and fourth embodiments, the substrate may be replaced by a substrate made of a metal other than aluminum, and further, this can be replaced by another radiation-transmissive substrate such as an amorphous carbon substrate, a silicon substrate, or a glass substrate.
In the above embodiments, SiO2 has been used as the material of the first layer (low-refractive index layer) of the dielectric film layers, and as the material of the second layer (high-refractive index layer), TiO2, however, other materials may be used. As the material of the high-refractive index layer, a material containing at least one of the materials including TiO2 as well as Nb2O5, Ta2O5, HfO2, and ZrO2 can be used, for example.
In addition, stability of the metal reflection film may be improved by, for example, providing an inorganic or organic film such as an oxide film between the substrate and metal reflection film. As a method for forming an oxide film, besides a method for forming an oxide film on the substrate by vapor deposition or the like, if the substrate is made of a metal, an oxide film may be formed by oxidizing the substrate surface.
Although the scintillator panel has herein been exemplified as a radiation image converting panel, a photostimulable phosphor such as CsBr doped with Eu may be used as a converting portion that converts a radiation image to an optical image to replace the scintillator. A radiation image is once converted by the photostimulable phosphor to a latent image, and a visible optical image can be read out by scanning this latent image with a laser light. The visible optical image thus read out is detected by various photodetectors such as, for example, a line sensor, an image sensor, and a photomultiplier.
