JP2016217875A - Radiation imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation imaging system capable of reducing degradation in resolution of a peripheral part due to oblique entering of a radiation ray.SOLUTION: The radiation imaging system includes a laminated scintillator 102 comprising a plurality of scintillator layers (104-106), a photodetector array 103 that detects a ray from the laminated scintillator, and image processing means that uses a result of detection by the photodetector array to obtain information on a radiation image. The plurality of scintillator layers are laminated in a normal direction of a light-receiving surface of the photodetector array, and emit scintillation light of wavelengths different from each other. The photodetector array is capable of discriminating and detecting the scintillation light from the laminated scintillator. The image processing means acquires information on a radiation image by subjecting the detection results discriminated by the photodetector array to correction according to an incident angle of the radiation, and superposing the detection results with each other.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a radiation imaging system.

  In radiation detectors used at medical sites, radiation that has passed through a subject is converted into visible light by a scintillator, and the emitted light is detected by a two-dimensional array of light receiving elements arranged in two directions in a plane, and a radiation image is obtained. Is getting. When such a radiation detector is combined with a radiation source to acquire a radiation image, the incident angle of radiation shifts from the vertical at the periphery of the radiation detector due to the positional relationship between the radiation source and the radiation detector. For this reason, there is a problem that the resolution of the peripheral portion is lowered with respect to the vicinity of the center of the imaging range.

  Patent Document 1 describes an example in which the resolution in the peripheral portion is improved by tilting the CsI needle crystal so that the CsI needle crystal functioning as a scintillator faces the incident direction of radiation.

JP 2009-236704 A

  As in the scintillator described in Patent Document 1, the method of forming the scintillator by tilting in the radiation incident direction is performed when the positional relationship between the radiation source and the radiation detector is a preset positional relationship. Resolution is improved. However, when the positional relationship is moved from a preset positional relationship, there is a problem that it is difficult to improve the resolution.

  Accordingly, the present invention provides a radiation imaging system capable of reducing a reduction in resolution of the peripheral portion due to oblique incidence of radiation even when the positional relationship between the radiation source and the radiation detector is set at an arbitrary position. The purpose is to provide.

  According to one aspect of the present invention, a radiation imaging system includes a stacked scintillator having a plurality of scintillator layers, a light receiving element array in which light receiving elements for detecting light from the stacked scintillator are arranged in two directions, and the light receiving element. Image processing means for acquiring radiation image information using detection results from the array, wherein the plurality of scintillator layers are stacked in the normal direction of the light receiving surface of the light receiving element array, and the plurality of scintillator layers Have different wavelengths of scintillation light emitted from each other, the light receiving element array can detect the scintillation light from the laminated scintillator by wavelength, and the image processing means can detect the detection result discriminated by the light receiving element array. The information of the radiation image is obtained by superimposing each other with correction according to the incident angle of the radiation. And acquiring.

  Other aspects of the present invention will be described in a mode for carrying out the invention.

  According to the present invention, even when the positional relationship between the radiation source and the radiation detector is set at an arbitrary position, radiation imaging that can reduce the resolution reduction in the peripheral portion due to the oblique incidence of radiation. A system can be provided.

The schematic diagram of the radiation detector which concerns on embodiment. The schematic diagram of an example of the scintillator layer which concerns on embodiment. The figure which shows the outline | summary of the position shift correction which concerns on embodiment. The X-ray excitation light emission spectrum of an example of the scintillator layer which concerns on embodiment. The figure which shows the fall of the resolution by a radiation detection element injecting diagonally.

  The radiation imaging system of this embodiment includes a radiation detector and image processing means. The radiation detector has a laminated scintillator in which a plurality of scintillator layers having different scintillation light wavelengths are laminated, and a two-dimensional array of light receiving elements capable of discriminating the scintillation light of each scintillator layer. Detect for each scintillator layer. The image processing means is connected to the radiation detector, uses the detection result of each scintillator layer by the radiation detector, corrects the positional deviation between the detection results of each scintillator layer according to the incident angle of radiation, and then detects each detection result. Superimpose each other. Thereby, the fall of the resolution in a peripheral part can be reduced irrespective of the position of a radiation source and a radiation imaging system.

  Hereinafter, embodiments for carrying out the present invention will be described more specifically with reference to the drawings. FIG. 1 shows a schematic diagram of a radiation detector 101 of the present embodiment. The radiation detector 101 includes a laminated scintillator 102 and a light receiving element array 103. The laminated scintillator 102 has a plurality of scintillator layers 104, 105, and 106 having different emission wavelengths, and these scintillator layers are laminated in the normal direction of the light receiving surface of the light receiving element array 103. The light receiving element array 103 includes a substrate 112 and light receiving elements 122 (so-called area sensors) arranged in two directions intersecting on the substrate. Each of the light receiving elements 122 includes a color filter layer 107 and a photoelectric conversion element 111 so that light from the laminated scintillator 102 can be discriminated and detected by wavelength. Thus, by configuring the radiation detector 101, it is possible to detect how much scintillation light is generated in which layer of the laminated scintillator 102. Although details will be described later, when radiation is incident obliquely on the laminated scintillator 102, the radiation is incident on a position farther from the position where the radiation is vertically incident toward the scintillator layer on the downstream side (light receiving element side) of the radiation. . However, the position where the radiation is incident vertically refers to the position where the radiation is incident vertically when nothing is arranged between the radiation source and the detector, and is generally at the center of the imaging range. is there. In the present invention and the present specification, this position may be simply referred to as the center of the scintillator layer. Therefore, the scintillation light is detected for each scintillator layer by the radiation detector 101, and the image processing means (not shown) performs correction according to the incident angle, so that the scintillator layer is a single layer radiation detector. Can also improve the resolution. The correction according to the incident angle by the image processing means reduces the radiation image by the scintillator layer on the downstream side of the radiation (detection result of scintillation light generated in the scintillator layer) relative to the radiation image by the upstream scintillator layer. This can be done.

  Although FIG. 1 shows a case where the laminated scintillator 102 includes three layers of the first scintillator layer 104, the second scintillator layer 105, and the third scintillator layer 106, it is sufficient that the scintillator layer has two or more layers. . When the radiation 113 enters each of the scintillator layers, the first scintillation light 115, the second scintillation light 116, and the third scintillation light 117 having different wavelengths at the light emission point 114 (114a to f are collectively referred to as 114). Is generated. The first to third scintillation lights 115, 116, 117 are incident on the light receiving element array 103, and the scintillation light transmitted through the color filter layer 107 is incident on the photoelectric conversion element 111. In the present invention and this specification, the scintillator layer having the radiation incident surface 124 of the laminated scintillator, that is, the scintillator layer on the most upstream side with respect to the radiation is defined as the first scintillator layer 104. Then, the scintillator layer adjacent to the first scintillator layer is defined as the second scintillator layer 105. Hereinafter, the third scintillator layer 106 and the fourth scintillator layer are sequentially arranged on the downstream side (that is, the light receiving element array side). ... and so on. That is, one scintillator layer disposed on the light receiving element side of the first scintillator is the second scintillator layer, two scintillator layers disposed on the light receiving element side are the third scintillator layer, and (n-1) light is received. The scintillator layer disposed on the element side is defined as an nth scintillator layer. Members other than the scintillator layer (for example, a substrate) may be disposed between the scintillator layers.

  The laminated scintillator 102 has a plurality of scintillator layers. Each scintillator layer is laminated in the normal direction (vertical direction in FIG. 1) of the light receiving surface of the light receiving element array so that radiation is incident on each scintillator layer. Even if the side surfaces of the scintillator layers are shifted and stacked, they are stacked in the normal direction, and the radiation may be incident on each scintillator layer when the radiation is incident.

  The first to third scintillator layers are not particularly limited as long as they generate different scintillation lights. In order to generate scintillation light having different wavelengths, the material of the base material of the scintillator constituting the scintillator layer may be changed, and the type and concentration of the emission center added to the base material may be changed.

  Each scintillator layer included in the stacked scintillator 102 preferably has a function of guiding scintillation light. Each of the scintillator layers guides the scintillation light, so that the first to third scintillation lights 115, 116, and 117 are incident on the light receiving element array 103 arranged in a plane substantially perpendicularly. Therefore, since most of the scintillation light is incident on the light receiving element arranged at the position where the light emitting point is projected from the normal direction of the light receiving surface of the light receiving element array, the resolution can be improved.

  The color filter layer is configured by arranging first to third color filters 108a, 108b, and 108c. Each of the first to third color filters 108a, 108b, and 108c has only a specific wavelength range corresponding to the first to third scintillation lights 115, 116, and 117 (may not include other scintillation lights). It is provided so as to be transparent. Thereby, the light receiving element array 103 can discriminate and detect the first to third scintillation lights.

  As the photoelectric conversion element 111, an element that can convert light transmitted through the color filter layer 107 into an electric signal may be used. For example, an optical sensor such as a CCD or CMOS can be used. In FIG. 1, one photoelectric conversion element and one color filter (any one of the first to third) correspond to each other at 1: 1 to constitute the light receiving element 112.

  Since the light receiving element array 103 has such a configuration, it is possible to discriminate and detect scintillation light generated in each scintillator layer (104, 105, 106). However, the light receiving element array 103 is not limited to such a configuration as long as it can discriminate and detect scintillation light from each scintillator layer. For example, it is possible to use a light receiving element that can separate information of different wavelengths in the thickness direction of the element (the direction in which light is incident).

  As described above, the radiation detector includes a plurality of scintillator layers that generate scintillation light having different wavelengths, and a light receiving element array that can discriminate and detect scintillation light generated in different scintillator layers. Thereby, the radiation detector can acquire the detection result of a radiation for every thickness of a scintillator.

  As described above, each of the scintillator layers constituting the laminated scintillator preferably has a function of guiding scintillation light. FIG. 2 shows a schematic diagram of a specific example of a scintillator layer having a function of guiding scintillation light. This schematic diagram shows one scintillator layer constituting the laminated scintillator 102. The scintillator layer 201 has a phase separation structure having a plurality of first phases 202 and a second phase 203 covering the side surfaces of the first phase. The first phase 202 and the second phase 203 have different refractive indexes. It is preferable that the phase having a relatively high refractive index functions as a scintillator so that the scintillation light is guided in the first phase 202 or the second phase 203 by total reflection. The phase having a relatively low refractive index may function as a scintillator or may not function. Hereinafter, the case where the first phase 202 has a relatively high refractive index will be described as an example. When the refractive index of the first phase is higher than the refractive index of the second phase, the scintillation light is guided while being confined in the first phase, like an optical fiber. The first phase 202 has a cylindrical shape and functions as a scintillator. Of the scintillation light generated in the first phase 202, the scintillation light 206 that is incident on the boundary surface between the first phase 202 and the second phase 203 at a critical angle or more is in the first phase 202 while repeating total reflection. Is propagated in the waveguide direction 209 and emitted from the light extraction surface 208. Here, the waveguide direction 209 of light emission is the longitudinal direction of the first phase 202 and is substantially parallel to the central axis of the cylinder. When the diameter of the first phase is smaller than the wavelength of the emitted light to be guided, the component that the scintillation light is transmitted without reflecting is increased. Therefore, it is desirable that the period 204 of the first phase and the diameter 205 of the first phase are larger than the wavelength of the scintillation light. In this embodiment, since a scintillator that emits light in the ultraviolet region from 300 nm may be used, the diameter 205 of the first phase is desirably 300 nm or more. On the other hand, if the diameter 205 of the first phase is equal to or larger than the thickness 207 of the scintillator layer, the effect of confining light is reduced, so the diameter 205 of the first phase is larger than the thickness 207 of the scintillator layer. Small is desirable. The above-described scintillator having a waveguide function such as an optical fiber can have a resolution (spatial resolution) of about 10 μm even if the thickness is 1 mm, and the pixel pitch of the light receiving element is also set to a size of about 10 μm accordingly. Can be set. Therefore, when the diameter of the fiber is larger than 10 μm, light leaks to adjacent pixels. Therefore, the diameter of the first phase is desirably 10 μm or less.

  From the above, the diameter 205 of the first phase is preferably in the range of 300 nm to 10 μm. Further, the shape of the first phase 202 may be a polygonal column other than the column. The first phase 202 is preferably continuous in a straight line, but is interrupted or branched in the middle, a plurality of crystal phases are integrated, the diameter of the crystal phase is changed, or is not linear. Non-linear portions may be included.

  The scintillator layer having such a phase separation structure has a function of guiding light emission, and the scintillation light generated by the incidence of radiation travels in the scintillator layer while being prevented from diffusing, and enters the light extraction surface 208. To reach. The scintillation light that has reached the light extraction surface of the nth scintillator layer is incident on the (n + 1) th scintillator layer adjacent to the downstream side, and is guided to the light extraction surface. By repeating this process, the scintillation light generated in each scintillator layer is propagated to the light receiving element array 103 without diffusing as compared with the case where each scintillator layer does not have waveguide properties, that is, without losing position information. Is done. That is, when a scintillator having a phase separation structure is used, a radiation detector that realizes high resolution can be obtained. On the other hand, a decrease in resolution due to oblique incidence of X-rays is more noticeable in a radiation detector having a high resolution.

  When a general acicular scintillator is used, light scattering occurs due to disturbance of the surface of the acicular crystal and the resolution increases as the thickness increases. Therefore, when the thickness is 1 mm, the limit of the spatial resolution is about 100 μm. Therefore, the pixel pitch of the light receiving element is set to a size of about 100 μm. In this case, as described above, the positional shift caused by the oblique incidence of X-rays is about 50 μm even when incident at θ = 3 ° with a 1 mm thick scintillator, which is a pixel of the two-dimensional light receiving element. Since it is relatively smaller than the pitch, the influence on the resolution is relatively small. On the other hand, a scintillator having a phase separation structure can have a resolution (spatial resolution) of about 10 μm even if its thickness is 1 mm, and the pixel pitch of the light receiving element can be set to a size of about 10 μm accordingly. it can. However, when the pixel pitch is set to be small as described above, the positional shift caused by the oblique incidence of X-rays becomes larger than the pixel pitch. Therefore, the unevenness in resolution between the central portion and the peripheral portion becomes larger than when the positional deviation is smaller than the pixel pitch. That is, this embodiment is more effective when applied to a scintillator layer capable of realizing high resolution.

As the scintillator layer as shown in FIG. 2, for example, a scintillator having a eutectic phase separation structure can be used. The eutectic phase separation structure refers to a phase separation structure as shown in FIG. 2 in which the first phase and the second phase constitute a eutectic. As an example of the material system of the eutectic phase separation structure, there is a eutectic phase separation structure of a perovskite oxide material (GdAlO ₃ ) containing Gd and alumina (Al ₂ O ₃ ). In the eutectic phase separation structure of this material system, the refractive index of the first phase (GdAlO ₃ : refractive index 2.05) is higher than that of the second phase (Al ₂ O ₃ : refractive index 1.79). High and the first phase functions as a scintillator. Therefore, the waveguide property is particularly high among the eutectic phase separation structures. What is important in forming a eutectic phase separation structure having two phases of the first crystal phase and the second crystal phase covering the side surface of the first crystal phase is the first crystal phase. The composition ratio of the material to be formed and the material constituting the second crystal phase. In order to obtain a scintillator crystal having a good phase separation structure as shown in the schematic diagram of FIG. 2, generally, the first phase material and the second phase material have a eutectic composition ratio (GdAlO ₃ : Al ₂ O ₃ = 46: 54 (mol%)). However, the composition ratio between the material of the first phase and the material of the second phase should not deviate from the eutectic composition, and the range of the eutectic composition ± 5 mol% with respect to this composition ratio is an allowable range. can do. More preferably, the composition ratio between the first phase material and the second phase material is in the range of eutectic composition ± 3 mol%. By performing unidirectional solidification using a melt in which the material of the first phase and the material of the second phase are mixed in the vicinity of the eutectic composition ratio (± 5 mol%), a high quality as shown in FIG. A crystal having a phase separation structure can be obtained. When the composition ratio of the material of the first phase and the material of the second phase is out of the range of the eutectic composition ± 5 mol%, one of the crystal phases is precipitated first. From the viewpoint, it becomes a factor that disturbs the favorable phase separation structure of the scintillator crystal. However, even if the composition ratio of the first phase material and the second phase material deviates from the range of the eutectic composition ± 5 mol%, the composition ratio is within the range of the eutectic composition ± 10 mol%. In some cases, a scintillator crystal having a good phase separation structure may be obtained. Therefore, even if the composition ratio of the material of the first phase and the second phase is outside the range of the eutectic composition ratio ± 5 mol%, the first phase and the second phase constitute a eutectic, If a separated structure is formed, the structure is regarded as a eutectic phase separated structure.

  Another example of the scintillator layer having waveguide properties is a porous scintillator. The porous scintillator is a scintillator in which a plurality of columnar holes are formed, and has a structure in which the first phase 202 in the eutectic phase separator shown in FIG. 2 is replaced with holes. Such a porous scintillator can be formed, for example, by selectively removing the first phase after forming the eutectic phase separation structure. In addition, a scintillator in which a plurality of columnar crystals (also referred to as needle crystals) of cesium iodide are arranged on a substrate can be used as the waveguide scintillator layer. In the case of a columnar crystal of cesium iodide, the longitudinal direction of the columnar crystal is the waveguide direction, and the waveguide direction of the columnar crystals arranged on the substrate is substantially aligned, so that the scintillator layer has waveguide properties.

  As described above, each scintillator layer included in the laminated scintillator has a different emission wavelength. Since it is known that the emission wavelength varies depending on the material of the base material of the scintillator, the kind of the emission center to be added, and the concentration of the emission center, the emission wavelength of each scintillator layer is set using this. The emission wavelength of each scintillator layer is selected so that the spectrum of the emission wavelength overlaps little or not, and the intensity distribution of each wavelength can be detected independently.

In the case of GdAlO ₃ described above, the emission wavelength varies depending on the type of emission center. Specifically, rare earth elements Tb ³⁺ , Eu ³⁺ , Pr ³⁺ , and Ce ³⁺ can be used as the emission center. Moreover, these luminescent centers are contained 0.001 mol% or more so that it may function effectively as a scintillator. When a plurality of types of emission centers are added, the total amount of emission centers may be 0.001 mol% or more. The additive element serving as the emission center is added so as to replace the Gd site of GdAlO ₃ which is the first phase. When the additive element is represented by the general formula RE, Gd _1-x RE _x AlO ₃ and Al ₂ O ₃ The composition ratio is 46:54 (mol%).

As shown in FIG. 4, when Tb ³⁺ is used as the emission center, a green emission peak is shown in the vicinity of 545 nm. Further, when Eu ³⁺ is used, a red light emission peak is shown in the vicinity of 615 nm. Further, when Pr ³⁺ is used, a red emission peak is shown in the vicinity of 500 nm. In addition, when Ce ³⁺ is used, broad ultraviolet light emission is observed in the vicinity of 360 nm. Thus, scintillators with various emission wavelengths can be obtained by appropriately selecting the additive elements. Also, other rare earth elements (Nd, Pm, Sm, Dy, Ho, Er, Tm, Yb) can be selected as the additive element.

  In this way, by combining a composite scintillator with different emission wavelengths by each scintillator layer and a light receiving element that discriminates for each emission wavelength and acquires an intensity distribution, the emission intensity distribution by each scintillator layer can be acquired independently. Can do.

  The image processing means uses the detection result of each scintillator layer by the radiation detector, corrects the positional deviation between the detection results of each scintillator layer according to the incident angle of the radiation, and then superimposes the detection results.

  A method for correcting a decrease in resolution that occurs when radiation is incident on the composite scintillator obliquely will be described with reference to FIGS. 1 and 3.

  First, the reason why the resolution is lowered by the oblique incidence of radiation on the composite scintillator will be described.

  As shown in FIG. 1, when radiation 113b is incident on the laminated scintillator 102 at the center of the laminated scintillator and perpendicular to the laminated scintillator (that is, the normal direction of the laminated scintillator surface and the radiation incident direction are parallel). think of.

  At this time, when the light emitting points 114d to 114f in each of the first scintillator layer 104, the second scintillator layer 105, and the third scintillator layer 106 are projected onto the light receiving element array 103, the projected images are spatially at the same positions. Formed. Therefore, when each scintillator layer has an ideal optical waveguide characteristic and the scintillator light travels substantially parallel to the thickness direction (vertical direction in the figure) of the laminated scintillator, a radiation image obtained at the center portion 118 substantially coincides with an image of radiation actually incident on the surface of the laminated scintillator. Therefore, when one beam-like radiation 113b is incident on the laminated scintillator substantially perpendicularly as shown in FIG. 1, the radiation intensity is detected as one point reflecting the cross section of the radiation 113b (circular in FIG. 1). Can do.

  On the other hand, the radiation 113a incident on the peripheral portion is incident obliquely on the laminated scintillator 102 at an incident angle θ. Therefore, when the light emitting points 114a to 114c in the first scintillator layer 104, the second scintillator layer 105, and the third scintillator layer 106 are projected onto the light receiving element array 103, the projected images are spatially shifted positions. Formed. That is, the lower the scintillator layer (layer closer to the light receiving element array), the farther the emission point is from the center of the scintillator layer. Therefore, when the scintillator light travels substantially parallel to the thickness direction of the laminated scintillator, the radiation images 119, 120, 121 by the respective scintillator layers are formed so as to be spatially shifted. Note that the sizes of the color filter 108 (108a to 108c) and the photoelectric conversion element 111 in FIG. 1 are schematically shown, and are actually sufficiently small with respect to the positional deviation amount, and the deviation amount can be identified quantitatively. Use a size.

Here, first, a comparative example of this embodiment will be described with reference to FIGS. FIG. 5A is a schematic diagram for explaining how radiation enters a scintillator 311 having a single layer structure. FIG. 5B shows an intensity distribution detected by a light receiving element array (not shown) arranged below the scintillator 311 when radiation is incident on the scintillator 311 as shown in FIG. Show. Consider a scintillator 311 composed of only one layer with a thickness T ₀ = 900 μm. The positional deviation amount d ₀ at the peripheral portion is d ₀ = T ₀ × tan θ, and when the incident angle is θ = 1 °, d ₀ = 16 μm, and when θ = 3 °, d ₀ = 47 μm. The scintillator here has ideal optical waveguide characteristics. First, as shown in FIG. 5A, a case will be described in which beam-like radiation 302 is evenly incident from a radiation source 301 to a total of nine points in the central portion and the peripheral portion of one scintillator 311. Each cross section of the beam of radiation is a circle. Since the radiation 302 is perpendicularly incident on the scintillator 311 at the center, there is no displacement in the radiation image 312a by each scintillator layer (FIG. 5B). On the other hand, since radiation is incident obliquely at the peripheral portion, the radiation image 312b spreads outward as the distance from the central portion increases. Such image blur is caused by the fact that radiation does not enter perpendicularly, and cannot be eliminated even if the scintillator has ideal optical waveguide characteristics. In the present embodiment, the positional deviation can be improved by dividing and obtaining the light emission points in the thickness direction and correcting the image by the image processing means.

  FIG. 3A is a schematic diagram for explaining a state in which radiation enters a scintillator 311 having a single layer structure. FIG. 3B shows the intensity detected by a light receiving element array (not shown) disposed below the laminated scintillator 102 when radiation is incident on the laminated scintillator 102 as shown in FIG. Show the distribution. FIG. 3C shows an intensity distribution in which the positional deviation is reduced by performing correction by the image processing unit using FIG.

Here, the thicknesses of the first to third scintillator layers 104, 105, and 106 of the laminated scintillator 102 are T ₁ , T ₂ , and T ₃ , respectively. Then, if the spatial spread projected on the light receiving element array 103 is expressed as d ₁ , d ₂ , d ₃ , where d ₁ = T ₁ × tan θ, d ₂ = T ₂ × tan θ, and d ₃ = T ₃ × tan θ. For example, T ₁ = 300 μm, T ₂ = 300 μm, and T ₃ = 300 μm. In this case, the amount of misregistration generated in each layer is d ₁ = d ₂ = d ₃ = 5 μm when the incident angle is θ = 1 °, and d ₁ = d ₂ = d ₃ = 16 μm when θ = 3 °. Compared to the case where it is configured, it is improved to 1/3 (FIG. 3B). However, the total amount of misalignment generated in each layer is the same as when the scintillator is composed of only one layer. Therefore, image processing is performed by the image processing means to correct a positional shift between the plurality of radiation images and generate a single radiation image.

  The positional deviation correction performed by the image processing unit will be described. Here, as shown in FIG. 3A, a case will be described in which dotted radiation 302 is evenly incident from a radiation source 301 to a total of nine points in the central portion and the peripheral portion of the laminated scintillator 102. FIG. 3B shows a radiation image 119 from the first scintillator layer 104, a radiation image 120 from the second scintillator layer 105, and a radiation image 121 from the third scintillator layer 106, which are output as a single detection result. It is. As can be seen from FIG. 3B, the scintillator layer disposed closer to the light receiving element array is enlarged toward the outside. Therefore, with respect to the radiation image 119 of the scintillator layer having the radiation incident surface, the radiation image 120 of the scintillator layer disposed immediately below the radiation image 119 has a position corresponding to the center of the imaging range (position where the radiation is vertically incident). Two radiation images are superimposed after being reduced as the center. When there are two or more scintillator layers, the radiographic images of the scintillator layer may be superimposed after being relatively reduced in accordance with the position of the scintillator layer. Relatively reducing includes not only reducing the radiation image of the scintillator layer disposed under the scintillator layer having the incident surface, but also enlarging the radiation image of the scintillator layer having the incident surface. When there are two scintillator layers, only the detection result detected by one scintillator layer may be corrected, or the detection result detected by both scintillator layers may be corrected. That is, the positional deviation correction according to the incident angle of radiation by the image processing means may be performed on at least one detection result. Therefore, to superimpose the discriminated detection results by performing correction according to the incident angle of radiation means to superimpose the detection results after correcting at least one detection result. A more specific correction method will be described.

It is assumed that the light emitting points of each layer are formed approximately at the middle position of the thickness of each layer. The correction amounts for correcting the positional deviation between the radiation image 120 by the second scintillator layer and the radiation image 119 by the first scintillator layer are H ₁ and ₂ . The radiation image 120 is reduced and superimposed on the radiation image 119 so that the position of each point of the radiation image 120 moves toward the center by H _1,2 = [(T ₁ + T ₂ ) / 2] × tan θ. There is a need. On the other hand, the correction amount for correcting the positional deviation between the radiation image 121 by the third scintillator layer and the radiation image 307 by the first scintillator layer is set to _H1,3 . After reducing the radiation image so that each point of the radiation image 121 moves toward the center by H _1,3 = [(T ₁ + T ₃ ) / 2 + T ₂ ] × tan θ, the radiation image 119 and the reduced radiation image 120 are reduced. And superimpose. When the laminated scintillator has three or more scintillator layers, assuming that the number of scintillator layers is n, a general formula for the correction amount is expressed by the following formula.

_Tm is the thickness of the mth scintillator layer. The corrected radiographic image is superimposed after correcting (reducing) the radiographic image of each scintillator layer so that the pixel value of the radiographic image approaches the center of H1 _{, n} indicated by the above formula. Thereby, as shown in FIG. 3C, the radiographic image 310 in which the center positions of the radiographic images 119, 120, and 121 of the respective layers coincide with each other and the positional deviation is corrected can be obtained. Note that tan θ may be set to tan θ = D / L, where L is the distance D from the center of each of the radiation images (119, 120, 121) and the distance between the radiation source and the radiation incident surface 124 of the laminated scintillator layer. it can.

Further, a radiation shielding member serving as a reference may be provided on the radiation incident surface 124, and the correction amount of each layer may be acquired from the position of the radiation shielding member in the radiation image of each scintillator layer. An appropriate reduction ratio can be obtained by using the positional deviation amounts Hc _{1, 2,} Hc _{1, 3,} Hc _{1, n} of the radiation shielding member in each radiation image and the distance D ₀ between the radiation shielding member and the center. it can. Further, assuming that the distance between the center and the point whose position deviation is to be corrected in the radiographic image is D, H _1,2 = D / D _c × Hc _1,2 , H _1,3 = D / D _c × Hc _{1, 3} . Thus, if a radiation shielding member is used, an image can be corrected without using L or θ. As the radiation shielding member, a radiation shielding member made of lead, tungsten, or the like having a dotted, linear, or concentric pattern can be used.

  The image processing means can be constituted by a general-purpose computer having hardware resources such as a CPU (Central Processing Unit), a RAM (Random Access Memory), and an auxiliary storage device. The above-described image processing method may be realized by the CPU reading and executing a program stored in the auxiliary storage device. Note that some or all of the functions of the image processing means can be configured by a circuit such as an ASIC (Application Specific Integrated Circuit). The image processing means may be integrated with the radiation detector. For example, it may be arranged on the back side of the light receiving element of a so-called flat panel detector (the surface on which the scintillator is arranged is the front). As described above, regardless of whether or not the radiation detector and the image processing unit are integrated, a unit including the scintillator, the light receiving element array, and the image processing unit is referred to as a radiation imaging system in the present invention and the present specification.

Although the present embodiment has been described above, T ₁ , T ₂ , and T ₃ can be set to arbitrary values. Since the scintillator layer relatively positioned in the upper layer absorbs low energy radiation and the lower layer absorbs high energy radiation, the lower layer may be thicker. However, the thicker the layer, the larger the amount of deviation that occurs in the layer. In addition, if the number of layers is two or more, it is effective, and if a scintillator layer that emits light at four or more different emission wavelengths and a color filter corresponding to the emission wavelength are used, the number of layers is four or more. It is also possible to further reduce the amount of positional deviation. In FIG. 1, a color filter and a two-dimensional light receiving element are arranged below the scintillator layer (on the side facing the incident direction of radiation) to discriminate light emission. However, light emission may be discriminated by arranging a color filter and a two-dimensional light receiving element above the scintillator layer (on the side on which radiation is incident). In addition, the first scintillation layer does not enter the second scintillator layer, and the second scintillation light does not enter the first scintillator layer, so that the first scintillation layer does not enter the first scintillator layer. When a layer for separating the light of the reflective film layer is provided, and the two-dimensional light receiving element is disposed above the first scintillator layer and below the second scintillator layer, the first scintillation light and the first scintillation light are separated from each other without discriminating wavelengths. Two scintillation lights can be discriminated. At this time, it is not necessary for the wavelengths of the first scintillation light and the second scintillation light to be different.

  Hereinafter, a specific example of this embodiment will be described.

  The present embodiment relates to a radiation imaging system using a laminated scintillator having three scintillator layers.

Each scintillator layer GdAlO ₃ as a material of the plurality of first phase is a eutectic phase separation scintillator with Al ₂ O ₃ as a material of the second phase, the first to third scintillator layer, respectively Tb ³⁺ , Eu ³⁺ and Ce ³⁺ are contained as the emission center.

First, Gd ₂ O ₃ , Tb ₄ O ₇ , Al ₂ O so that the composition ratio of the material in which 8 mol% of Tb ³⁺ is added to GdAlO ₃ and Al ₂ O ₃ is 46:54 (mol%). ₃ were weighed and prepared as a well-mixed material powder. Similarly, were prepared raw material powder added 6 mol% relative to the GdAlO ₃ and ^{Eu 3+,} and a raw material powder added 0.1 mol% relative to the GdAlO ₃ and ^{Ce 3+.} As raw materials, Eu ₂ O ₃ was used as Eu ³⁺ , and CeO ₂ was used as Ce ³⁺ .

Each of these raw material powders is put into an Ir crucible, and the crucible is heated to 1700 ° C. by induction heating, held for 30 minutes after the entire sample is dissolved, and then subjected to unidirectional solidification at a speed of 54 mm / h. Nurtured. Three types of samples thus prepared were cut out at 5 mm × 5 mm × thickness of 300 μm, and both surfaces were polished to prepare samples. When the sample was observed with a scanning electron microscope, it was confirmed that it was a phase separation structure in which a GdAlO ₃ columnar structure having a diameter of about 680 nm was embedded in the Al ₂ O ₃ phase.

A sample to which Tb ³⁺ was added as the emission center by X-ray irradiation showed a green emission peak in the vicinity of 545 nm. Further, when Eu ³⁺ was used, a red emission peak was observed around 615 nm, and when Ce ³⁺ was used, broad ultraviolet emission was observed near 360 nm. One surface of these three prepared samples was bonded to each other as a light extraction surface, and a 5 mm × 5 mm × 900 μm thick laminated scintillator was manufactured. This multi-layer scintillator having a size of 25 mm × 25 mm arranged in a two-dimensional manner in 5 × 5 rows is arranged so as to face a two-dimensional light receiving element array having a color filter corresponding to each wavelength. The radiation detector was used. In order from the radiation incident side, the first, second, and third scintillator layers were scintillator layers containing Tb ³⁺ , Eu ³⁺ , and Ce ³⁺ as emission centers, respectively. A light receiving element array having a pixel pitch of 2 μm was used.

As the radiation source, a tungsten tube X-ray source was used, the distance between the X-ray source and the radiation detector was 50 cm, and X-rays obtained under conditions of 60 kV, 1 mA, and no Al filter were used for imaging. Then, the X-ray was irradiated to the center and the peripheral part of the imaging range through an opening having a diameter of 50 μm provided on a 2 mm thick tungsten plate, and an X-ray spot image was captured in each imaging region. At this time, imaging was performed by tilting the angle of the tungsten plate in the peripheral portion so that the X-rays pass through the opening. The photoelectric conversion element detected the light which permeate | transmitted the color filter, and acquired the X-ray image in each of the 1st-3rd scintillator layer. As a result, similar to the radiographic image shown in FIG. 3B, X-ray spot images with a diameter of 50 μm of each layer overlapped at the center. On the other hand, in the peripheral part 20 mm away from the center, the X-ray spot image of the first scintillator layer is an X-ray spot image having a major axis length of about 62 μm, a minor axis length of about 50 μm, and a diameter of 50 μm. Was an ellipse extending radially about 12 μm. This is due to the oblique incidence of X-rays, but the influence could be reduced by reducing the thickness of one layer by multilayering as in this embodiment. In the peripheral portion, the center position of the X-ray spot image of the second scintillator layer is located at a position shifted about 12 μm outward from the X-ray spot image of the first scintillator layer, and is similarly an elliptical shape extending about 12 μm. It was. The center position of the X-ray spot image of the third scintillator layer was shifted to the outside by about 24 μm from the X-ray spot image of the first scintillator layer, and was similarly an ellipse extending about 12 μm. . The positional deviation of the X-ray spot image of each layer is corrected so as to approach about 12 μm centering on the X-ray spot image of the second scintillator layer, and so as to approach about 24 μm centering on the X-ray spot image of the third scintillator layer. It can be corrected by correcting. From this imaging result, it can be seen that Hc _1,2 = about 12 μm and Hc _1,3 = about 24 μm, and the first and second detection values (pixel values) at positions 20 mm away from the center approach the respective distances. When the second radiation image is reduced and superimposed, the positional deviation can be corrected. Further, when an arbitrary distance from the center is D (mm), the detection value at that position is the correction amount H _1,2 = D / 20 × 12 (μm) for the second layer, and the correction value for the third layer. The amount H _1,3 = D / 20 × 24 (μm) may be corrected and superimposed so as to approach the center. By using this correction value, it is possible to obtain an X-ray image in which the positional deviation is corrected for any subject. As described above, even if the X-ray source and the radiation detector are arbitrarily arranged, the distance D (mm) from the center portion can be used by first acquiring the amount of deviation of the X-ray image of each layer. An X-ray image in which the positional deviation is corrected can be acquired.

Using the above X-ray image reconstruction process, a 10 μm (line pair / mm) line pattern of 50 μm thick lead was imaged. A pattern of 10 (line pairs / mm) could be clearly resolved at both the center and the periphery of the imaging range. Further, if defined as Contrast Transfer Function (CTF) = (I _max −I _min ) / (I _max + I _min ) as the bright part (I _max ) and dark part (I _min ) of the line imaging region, CTF = 0 in the central part .60, CTF = 0.55 at the periphery. Thus, a high-contrast X-ray image with little decrease in resolution could be obtained even in the peripheral part.

  If a radiation detector having such a laminated scintillator is used, an image corresponding to low energy X-rays can be obtained in the upper scintillator layer, and an image corresponding to high energy X-rays can be obtained in the lower layer. Therefore, the image processing means may reconstruct the energy discrimination image by performing a weighting operation between the acquired X-ray images for each layer and calculating the difference.

  From the above, it was proved that the resolution was improved by dividing and acquiring the radiation images in the radiation penetration thickness direction and correcting and superimposing those positional deviations.

[Comparative Example]
The present embodiment relates to a conventional scintillator having a single layer and is a comparative example corresponding to the embodiment.

The scintillator layer is composed of a perovskite-type oxide material containing Gd as a plurality of unidirectional first phases, and alumina (Al ₂ O ₃ ) as a second phase, and contains Tb ³⁺ as an emission center. It is an example using things.

First, a sample to which 8 mol% of Tb ³⁺ was added was prepared in the same manner as in Example 1, cut out at 5 mm × 5 mm × thickness 900 μm, and polished on both sides to prepare a sample. This scintillator was arranged in a two-dimensional manner in 5 × 5 rows and a laminated scintillator having a size of 25 mm × 25 mm was arranged so as to face the two-dimensional light receiving element, thereby forming a radiation detector.

  As in Example 1, when an X-ray spot image was acquired for the center and the periphery through an opening with a diameter of 50 μm in a 2 mm thick tungsten plate, an X-ray spot image with a diameter of 50 μm was blurred in the center. It was obtained without. On the other hand, in the peripheral part 20 mm away from the central part, the X-ray spot image has a major axis length of about 86 μm and a minor axis length of about 50 μm, and an X-ray spot image having a diameter of 50 μm from the central part is radial. It was an ellipse extending 36 μm. This is caused by the extension of the path length due to the oblique incidence of X-rays. In the case of being configured with only one layer, the image blur is three times as compared with the case of dividing and acquiring three layers in the first embodiment.

  Using the above X-ray image reconstruction process, a 10 μm (line pair / mm) line pattern of 50 μm thick lead was imaged. The center portion was able to clearly resolve a pattern of 10 (line pair / mm), and CTF = 0.60. On the other hand, a decrease in contrast occurred in the peripheral portion, and CTF = 0.40.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

DESCRIPTION OF SYMBOLS 101 Radiation detector 102 Laminated scintillator 103 Light receiving element array 104 1st scintillator layer 105 2nd scintillator layer 106 3rd scintillator layer 107 Color filter layer 108a 1st color filter 108b 2nd color filter 108c 3rd color Filter 111 Photoelectric conversion element 112 Substrate 113 Radiation 114 Light emission point 115 First scintillation light 116 Second scintillation light 117 Third scintillation light 118 Radiation image obtained at the center 119 By the first scintillator layer obtained at the periphery Radiation image 120 Radiation image by the second scintillator layer obtained at the peripheral portion 121 Radiation image by the third scintillator layer obtained at the peripheral portion H _1,2 First scintillator layer and second scintillator Misalignment correction amount with layer H _1,3 Misalignment correction amount between first scintillator layer and third scintillator layer 201 Scintillator layer 202 First phase 203 Second phase 204 First phase period 205 First Phase diameter 206 Scintillation light 207 Scintillator layer thickness 208 Light extraction surface 209 Waveguide direction 301 Radiation source 302 Radiation 307 Radiation image by first scintillator layer 310 Radiation image corrected for misalignment 311 Scintillator 312a Obtained at the center Radiation image obtained 312b Radiation image obtained at the periphery

Claims (13)

A laminated scintillator having a plurality of scintillator layers;
A light receiving element array in which light receiving elements for detecting light from the laminated scintillator are arranged in two directions;
Image processing means for obtaining information of a radiographic image using a detection result by the light receiving element array,
The plurality of scintillator layers are stacked in the normal direction of the light receiving surface of the light receiving element array,
The plurality of scintillator layers have different wavelengths of scintillation light emitted from each other,
The light receiving element array is capable of detecting the scintillation light from the stacked scintillator by discriminating by wavelength,
The image processing means acquires the information of the radiological image by superimposing the detection results discriminated by the light receiving element array by performing correction according to the incident angle of the radiation. . Of the scintillator layers,
A scintillator layer having a radiation incident surface to the laminated scintillator is a first scintillator layer,
When the scintillator layer disposed downstream of the radiation from the first scintillator layer is a second scintillator layer,
Correction according to the incident angle by the image processing means,
For the detection result of the first scintillation light generated in the first scintillator layer,
The radiation imaging system according to claim 1, wherein the detection result of the second scintillation light generated in the second scintillator layer is relatively reduced. The laminated scintillator has three or more scintillator layers,
A scintillator layer having a radiation incident surface in the laminated scintillator is a first scintillator layer,
When the position where the radiation is perpendicularly incident on the laminated scintillator is the center of the imaging range,
The image processing means includes
For the detection result of the nth scintillation light emitted by the nth scintillator layer disposed on the (n−1) light receiving element side of the first scintillator layer,
The correction is performed on the assumption that the detected position is close to the center of the scintillator by H1 _{, n} minutes represented by the following formula,
The radiation imaging system according to claim 1, wherein the detection result of the first scintillation light and the corrected n-th detection result are superimposed.

Where _Tm is the thickness of the mth scintillator layer, and θ is the incident angle of the radiation with respect to the radiation incident surface.   4. The radiation imaging system according to claim 1, wherein each of the plurality of scintillator layers guides the scintillation light. 5.   Each of the plurality of scintillator layers includes a first material having a plurality of first phases and a second phase covering a side surface of the first phase, and forming the plurality of first phases. The radiation imaging system according to claim 4, wherein a refractive index of the second material forming the second phase is different.   6. The scintillator layer, wherein the first material forming the first phase and the second material forming the second phase form a eutectic. The radiation imaging system described in 1.   The first phase is a perovskite oxide material containing Gd, the first phase contains 0.001 mol% or more of a rare earth element as a luminescent center, the second phase is alumina, The radiation imaging system according to claim 5 or 6, wherein the first phase emits scintillation light. The radiation imaging system according to claim 7, wherein the rare earth element is at least one of Tb ³⁺ , Eu ³⁺ , Pr ³⁺ , and Ce ³⁺ .   6. The radiation imaging system according to claim 5, wherein each of the scintillator layers is a porous scintillator having a plurality of holes.   The radiation imaging system according to claim 5, wherein each of the scintillator layers includes a plurality of columnar crystals. In the laminated scintillator,
The radiation imaging system according to claim 1, wherein different scintillator layers include different emission centers. Among the scintillator layers provided in the laminated scintillator,
A scintillator layer having a radiation incident surface of the laminated scintillator is a first scintillator layer,
When the scintillator layer on the light receiving element array side is the scintillator layer on the light receiving element side,
The radiation imaging system according to claim 1, wherein a thickness of the first scintillator layer is thinner than a scintillator layer on the light receiving element side.   The radiation imaging system according to claim 1, wherein the light receiving element includes a color filter and a photoelectric conversion element.

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