Patent Publication Number: US-2007098138-A1

Title: X-ray detector and x-ray ct apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Japanese Application No. 2005-318303 filed Nov. 1, 2005.  
     BACKGROUND OF THE INVENTION  
      The present invention relates to an X-ray detector and an X-ray CT apparatus each having solid state detectors which are repeatedly arranged two-dimensionally with gaps therebetween on a plane board on which X-rays are incident.  
     BACKGROUND ART  
      In recent years, as X-ray detectors used for an X-ray CT apparatus, solid state detectors which are arranged two-dimensionally in a channel direction and a slice direction are used. The number of channels in the scan direction of the x-ray detectors and the number of X-ray detectors in the slice direction are increasing. For example, the number of X-ray detectors in the channel direction is about 1,000, and the number of X-rays in the slice direction is tens (refer to, for example, Japanese Patent Laid-Open No. 2004-093489 (p. 1 and FIG. 4).  
      Under the circumstances, the size of an X-ray receiving surface of a solid state detector is decreasing to a few mm 2 . On the other hand, the width of each of gaps between solid state detectors, which are developed when the solid state detectors are arranged two-dimensionally is about 0.2 mm to 0.4 mm. The width of the gap is not largely changed with increase in the number of solid state detectors in the channel and slice directions but is constant more or less.  
      In the background art, however, efficiency for X-ray utilization of the solid state detectors arranged two-dimensionally deteriorates. Specifically, as the solid state detectors arranged two-dimensionally become finer, the proportion of the gaps increases as compared with the X-ray receiving surfaces of the solid state detectors, and the ratio of X-rays which pass without being detected by the solid state detectors increases.  
      In particular, the gaps of the solid state detectors are created in a process of producing a two-dimensional array of the solid state detectors and also play the role of preventing leakage (crosstalk) of fluorescence generated by X-rays among the solid state detectors. Therefore, it is not easy to reduce the size of the gap from the viewpoint of precision of a machine tool for processing the solid state detectors and performance of the solid state detectors.  
      It is consequently important to realize an X-ray detector and an X-ray CT apparatus with improved efficiency for X-ray utilization while leaving the gaps existing among the solid state detectors arranged two-dimensionally.  
      The present invention has been achieved to solve the problems of the background art and an object of the invention is to provide an X-ray detector and an X-ray CT apparatus with improved efficiency for X-ray utilization while leaving gaps existing among the solid state detectors which are arranged two-dimensionally.  
     SUMMARY OF THE INVENTION  
      To solve the problems and achieve the object, the invention according to a first aspect provides an X-ray detector in which a plurality of solid state detectors each having a parallelepiped shape are arranged in a two-dimensional array with gaps therebetween on a plane board facing an X-ray incidence direction. Two parallel faces orthogonal to the incidence direction, of each of the parallelepipeds in the solid state detector have a positional deviation in the plane direction of the faces.  
      In the invention according to the first aspect, in the solid state detector, the gap portion is covered by the positional deviation in a plane direction between the two parallel faces which are facing the incidence direction.  
      According to the invention of a second aspect, in the X-ray detector according to the invention of the first aspect, the positional deviation is provided in at least one of a channel direction and a slice direction of the two-dimensional array.  
      In the invention according to the second aspect, the positional deviation exists in an arbitrary direction orthogonal to the X-ray incidence direction.  
      An X-ray detector according to the invention of a third aspect is characterized in that, in the invention of the first aspect, the positional deviation has a dimension exceeding width of the gap in the plane direction.  
      In the invention according to the third aspect, the plane board viewed from the X-ray incidence direction is covered with the solid state detectors.  
      An X-ray detector according to the invention of a fourth aspect is characterized in that, in the invention of the first aspect, the solid state detector is a scintillator.  
      In the invention according to the fourth aspect, the solid state detector detects an X-ray efficiently.  
      An X-ray detector according to the invention of a fifth aspect is characterized in that, in the invention of the fourth aspect, the plane board has a photodiode for detecting fluorescence generated by the scintillator.  
      In the invention according to the fifth aspect, the plane board efficiently converts fluorescence into an electric signal by the photodiode.  
      The invention according to a sixth aspect provides an X-ray CT apparatus having an X-ray detector in which a plurality of solid state detectors each having a parallelepiped shape are arranged in a two-dimensional array with gaps therebetween on a plane board facing an X-ray incidence direction, wherein a relative position in the X-ray incidence direction of two parallel faces of each of the parallelepipeds in the solid state detector have a positional deviation in the plane direction of the faces.  
      In the invention according to the sixth aspect, the solid state detectors cover gaps by the positional deviation in a plane direction between the two parallel faces which are facing the incidence direction.  
      An X-ray CT apparatus according to the invention of a seventh aspect is characterized in that, in the invention of the sixth aspect, the positional deviation has a dimension exceeding width of the gap in the plane direction.  
      In the invention according to the seventh aspect, the plane board viewed from the X-ray incidence direction is covered with the solid state detectors.  
      An X-ray CT apparatus according to the invention of an eighth aspect is characterized in that, in the invention of the sixth aspect, the solid state detector is a scintillator.  
      In the invention according to the eighth aspect, the solid state detector detects an X-ray efficiently.  
      An X-ray detector according to the invention of a ninth aspect is characterized in that, in the invention of the eighth aspect, the plane board has a photodiode for detecting fluorescence generated by the scintillator.  
      In the invention of the ninth aspect, the plane board efficiently converts fluorescence into an electric signal by the photodiode.  
      The invention of a tenth aspect provides an X-ray CT apparatus comprising: an X-ray tube that generates an X-ray; and an X-ray detector in which a plurality of solid state detectors each having a rectangular parallelepiped shape are arranged in a two-dimensional array with gaps therebetween on a plane board facing the X-ray incidence direction, wherein the plane board being tilted with respect to a direction orthogonal to the incidence direction.  
      In the invention according to the tenth aspect, the plane board has a tilt with respect to an X-ray incidence direction and the gaps between the solid state detectors are positioned in the shadow of the incident X-ray.  
      An X-ray CT apparatus according to the invention of an eleventh aspect is characterized in that, in the invention of the tenth aspect, the plane board being tilted so the X-ray projection of the rectangular parallelepiped as to exceed the gap and overlaps an adjacent rectangular parallelepiped.  
      In the invention according to the eleventh aspect, the tilt is set so that the projection of the rectangular parallelepiped covers the gap.  
      An X-ray CT apparatus according to the invention of a twelfth aspect is characterized in that, in the invention of the tenth aspect, the solid state detector is a scintillator.  
      In the invention according to the twelfth aspect, the solid state detector detects an X-ray efficiently.  
      An X-ray CT apparatus according to the invention of a thirteenth aspect is characterized in that, in the invention of the twelfth aspect, the plane board has a photodiode for detecting fluorescence generated by the scintillator.  
      In the invention according to the thirteenth aspect, the plane board efficiently converts fluorescence into an electric signal by the photodiode.  
      The invention according to a fourteenth aspect provides an X-ray detector in which a plurality of solid state detectors each having a rectangular parallelepiped shape are arranged in a two-dimensional array with gaps therebetween on a plane board facing an X-ray incident direction, wherein the X-ray detector has a multilayer solid state detector in which a plurality of the solid state detectors in the two-dimensional array are stacked in the incidence direction, and relative positions of the stacked solid-state detectors are deviated in direction orthogonal to the stacked direction.  
      In the invention according to the fourteenth aspect, the multilayer solid state detector has a configuration in which a plurality of two-dimensional arrays of the solid state detectors whose relative positions are deviated preferably only by the width of the gap are stacked in the incidence direction.  
      An X-ray detector according to the invention of a fifteenth aspect is characterized in that, in the invention of the fourteenth aspect, the two-dimensional array has the relative position which varies in at least one of a channel direction and a slice direction as two arrangement directions of the two-dimensional array.  
      In the invention of the fifteenth aspect, the relative positions of the two-dimensional arrays vary in an arbitrary direction orthogonal to the X-ray incidence direction.  
      An X-ray detector according to the invention of a sixteenth aspect is characterized in that, in the invention of the fifteenth aspect, the multilayer solid state detector has first, second, third, and fourth solid state detectors whose relative positions are different from each other.  
      In the invention according to the sixteenth aspect, the gaps viewed from the X-ray incidence direction are covered in the multilayer solid state detector.  
      An X-ray detector according to the invention of a seventeenth aspect is characterized in that, in the invention according to the fourteenth aspects, the solid state detector is a scintillator.  
      In the invention according to the seventeenth aspect, the solid state detector detects an X-ray efficiently.  
      The invention according to an eighteenth aspect provides an X-ray CT apparatus comprising an X-ray detector in which a plurality of solid state detectors each having a rectangular parallelepiped shape are arranged in a two-dimensional array with gaps therebetween on a plane board facing an incident X-ray, wherein the X-ray detector has a multilayer solid state detector in which a plurality of the solid state detectors in the two-dimensional array are stacked in the incidence direction, and relative positions of the stacked solid-state detectors are deviated in a direction orthogonal to the stacked direction.  
      In the invention according to the eighteenth aspect, the multilayer solid state detector has a configuration in which a plurality of two-dimensional arrays of solid state detectors whose relative positions are different preferably only by width of the gap are stacked in the incidence direction.  
      An X-ray detector according to the invention of a nineteenth aspect is characterized in that, in the invention of the eighteenth aspect, the relative position in the multilayer solid state detector varies in at least one of a channel direction and a slice direction as two arrangement directions of the two-dimensional array.  
      In the invention according to the nineteenth aspect, the relative position of the two-dimensional array varies in an arbitrary direction orthogonal to the X-ray incidence direction.  
      An X-ray detector according to the invention of a twentieth aspect is characterized in that, in the invention of the eighteenth aspect, the solid state detector is a scintillator.  
      In the invention according to the twentieth aspect, the solid state detector detects an X-ray efficiently.  
      According to the present invention, in the solid state detector, the gap portions are covered by the positional deviation in a plane direction of two parallel faces which are facing the incidence direction. Thus, the side on which an X-ray is incident of the plane board is covered with the solid state detectors, thereby eliminating an X-ray insensitive region. Thus, efficiency for X-ray utilization improves and, moreover, X-ray detectivity and the picture quality can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing a general configuration of an X-ray CT apparatus.  
       FIG. 2  is an outside diagram of an X-ray detector of a first embodiment.  
       FIG. 3  is an outside diagram of a plane block of the first embodiment.  
       FIG. 4  is a cross section of the plane block of the first embodiment.  
       FIGS. 5A and 5B  are explanatory diagrams illustrating operations of the plane block of the first embodiment.  
       FIGS. 6A and 6B  are an outside diagram and a cross section, respectively, of a plane block of a second embodiment.  
       FIG. 7  is an explanatory diagram showing operation of the plane block of the second embodiment.  
       FIGS. 8A and 8B  are a cross section and an outside diagram, respectively, of a plane block of a third embodiment.  
       FIGS. 9A  to  9 D are plan views of multilayer scintillators constructing the plane block of the third embodiment.  
       FIG. 10  is an explanatory diagram showing operation of the plane block of the third embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Best modes for carrying out an X-ray detector and an X-ray CT apparatus according to the present invention will be described below with reference to the appended drawings. The present invention, however, is not limited to the best modes.  
     First Embodiment  
      A general configuration of an X-ray CT apparatus according to a first embodiment will be described.  FIG. 1  is a block diagram of an X-ray CT apparatus. As shown in  FIG. 1 , the apparatus has a scan gantry  10  and an operation console  6 .  
      The scan gantry  10  has an X-ray tube  20 . A not-shown X-ray emitted from the X-ray tube  20  spreads, for example, in a fan shape having a thickness and is shaped into a conical X-ray beam by a collimator  22  and is emitted to an X-ray detector  24 .  
      The X-ray detector  24  has a plurality of scintillators arranged in a matrix in the spread direction of the fan-shaped X-ray beam. The X-ray detector  24  is a multi-channel detector having a width in which a plurality of scintillators are arranged two-dimensionally in a matrix in the channel direction and the slice direction.  
      In the X-ray detector  24 , an X-ray incident surface curved in a concave shape as a whole is formed. The X-ray detector  24  is obtained by combining, for example, scintillators as solid state detectors made of inorganic crystal and a photodiode as a photoelectric converter.  
      To the X-ray detector  24 , a data collector  26  is connected. The data collector  26  collects detection information of each of the scintillators of the X-ray detector  24 . The irradiation of the X-ray from the X-ray tube  20  is controlled by an X-ray controller  28 . The connection relation between the X-ray tube  20  and the X-ray controller  28  and the connection relation between the collimator  22  and a collimator controller  30  are not shown. The collimator  22  is controlled by the collimator controller  30 .  
      The above-described components from the X-ray tube  20  to the collimator controller  30  are mounted on a rotation part  34  of the scan gantry  10 . A subject or a phantom is placed on an image capture table  4  in a bore  29  positioned in the center of the rotation part  34 . The rotation part  34  rotates while being controlled by a rotation controller  36 , an X-ray is emitted from the X-ray tube  20 , and the X-ray detector  24  detects the X-ray passed through the subject or phantom as projection information of each view according to the rotation angle. The connection relation between the rotation part  34  and the rotation controller  36  is not shown.  
      The operation console  6  has a data processor  60 . The data processor  60  is constructed by, for example, a computer. To the data processor  60 , a control interface  62  is connected. To the control interface  62 , the scan gantry  10  is connected. The data processor  60  controls the scan gantry  10  via the control interface  62 .  
      The data collector  26 , X-ray controller  28 , collimator controller  30 , and rotation controller  36  in the scan gantry  10  are controlled via the control interface  62 . The connection between each of those components and the control interface  62  is not shown here.  
      To the data processor  60 , a data collection buffer  64  is connected. The data collection buffer  64  is connected to the data collector  26  in the scan gantry  10 . Data collected by the data collector  26  is input to the data processor  60  via the data collection buffer  64 .  
      The data processor  60  reconstructs an image by using a transmission X-ray signal, that is, projection information collected via the data collection buffer  64 . To the data processor  60 , a storage  66  is connected. The storage  66  stores the projection information collected by the data collection buffer  64 , reconstructed slice image information, a program for realizing the functions of the apparatus, and the like.  
      To the data processor  60 , a display  68  and an operating device  70  are connected. The display  68  displays the slice image information and other information which is output from the data processor  60 . The operating device  70  is operated by an operator and inputs various instructions, information, and the like to the data processor  60 . The operator operates the apparatus interactively by using the display  68  and the operating device  70 . The scan gantry  10 , the image capture table  4 , and the operation console  6  radiograph the subject or phantom to obtain slice images.  
       FIG. 2  is an outside drawing showing three-dimensional layout of the X-ray tube  20 , the X-ray detector  24 , and the data collector  26 . The X-ray detector  24  includes scintillators  41  for detecting a conical X-ray beam generated by the X-ray tube  20 , a photodiode  42  as a photoelectric converter for detecting light emission of the scintillators  41 , a reflection film  48 , and a plane board  43 . Although the reflection film  48  exists on the two-dimensional array of the scintillators  41 , it is not shown here.  
      The scintillators  41  are arranged two-dimensionally on the surface facing the conical X-ray beam and emit light when the X-ray enters. Approximately  64  scintillators  41  are arranged in the slice direction as the thickness direction of the conical X-ray beam and approximately 1,000 scintillators  41  are arranged in the channel direction as a spread direction of the fan shape of the X-ray beam.  
      The photodiode  42  is formed on the plane board  43  and detects light emission of the scintillators  41 . On the plane board  43  as a single plane board, the scintillators  41  and the photodiodes  42  corresponding to a plurality of channels and a plurality of slices are formed. By the scintillators  41 , the photodiodes  42 , and the plane board  43  formed in an integral structure, a single plane block  47  is formed. By combination of a plurality of plane blocks  47 , the X-ray detector  24  having an almost concave shape is constructed. In the example of  FIG. 2 , the plane blocks  47  of four channels and three slices are formed. The plane blocks  47  are arranged on a concave surface which is almost orthogonal to the incident conical X-ray beam.  
      The data collector  26  includes flexible printed boards  44 , printed boards  45 , and electric cables  46 . The flexible printed board  44  transmits an analog signal of the X-ray detected by the photodiode  42  to the printed board  45 .  
      The electric cable  46  is a flat cable electrically connected from an end in the slice direction to the printed board  45 . The printed board  45  is electrically connected to the data collection buffer  64  via the electric cable  46 .  
       FIGS. 3 and 4  are diagrams showing the scintillators  41 , the photodiode  42 , and the plane board  43  constructing the plane block  47 . In the following, the case where the plane block  47  is positioned in a YZ plane and the X-ray incidence direction is the X-axis direction will be described.  
       FIG. 3  is a plan view showing the plane block  47  viewed from the X axis direction as the X-ray incidence direction. Although the reflection film  48  which will be described later exists on the scintillators  41  of the plane block  47 , it is not shown in  FIG. 3  in order to clearly show the two-dimensional array of the scintillators  41 . In  FIG. 3 , as an example, dot lines as hidden lines are shown only in the upper left scintillator  41 .  
      Each of the scintillators  41  has a parallelepiped shape. The scintillators  41  having the same structure are repeatedly arranged two-dimensionally in the channel direction and the slice direction with gaps  50  therebetween. It is assumed here that the length of the gap  50  in the channel direction is l 1 , and the length of the gap  50  in the slice direction is l 2 .  
      A top face “a” orthogonal to the incident X-ray of the parallelepiped constructing the scintillator  41  and an under face “c” indicated by the dot lines in  FIG. 3  are position-deviated from each other in the channel and slice directions. When the size of the positional deviation between the top face “a” and the under face “c” is d 1  in the channel direction and is d 2  in the slice direction, the following expressions are satisfied.  
      d 1  &gt;l 1  in the channel direction  
      d 2  &gt;l 2  in the slice direction  
       FIG. 4  is a cross section taken along line A-A′ when the scintillators  41  arranged two-dimensionally and shown in  FIG. 3  are viewed from the z axis direction. On the scintillators  41  on the photodiode  42 , the reflection film  48  which is not shown in  FIG. 3  is illustrated. The reflection film  48  is made of a resin filler containing metal powders and is filled on the top of the scintillators  41  and in the gaps  50 .  FIG. 4  also shows an anode  51  of the photodiode  42 . The anode  51  serves as a light receiving surface of the photodiode  42  and overlaps the under face “c” of the scintillator  41 .  
      Scintillation light generated in the scintillator  41  by incidence of the X-ray is confined in the scintillator  41  by the reflection film  48  and detected by the anode  51 . Leaked light among the scintillators  41  is also prevented by the reflection film  48  in the parts of the gaps  50 .  
      As described above, the top face “a” is deviated from the under face “c” in the channel direction only by the amount d 1 . Since the amount is larger than the amount l 1  of the gap  50  in the channel direction, when the plane block  47  is viewed from the X-ray incidence direction, the gap  50  cannot be seen except for the peripheral portions of the two-dimensional array.  
      Next, the operation of the scintillators  41  according to the first embodiment will be described with reference to  FIGS. 5A and 5B .  FIG. 5A  is an explanatory cross section taken along A-A′ line of  FIG. 3  like  FIG. 4 . The scintillator  41  has a parallelepiped shape, and the top face “a” and the under face “c” are deviated from each other only by the amount d 1  in the channel direction. Therefore, when it is assumed that the length in the channel direction of the top face “a” or the under face “c” is “s”, the length of an X-ray sensitive area in the channel direction of the scintillator  41  with respect to the X-ray entering from above is equal to s+d 1 . When the length s+d 1  is compared with the length s+l 1  (the length “s” in the channel direction of the under face “c” and the width l 1  of the gap  50 ), the following expression is obtained.  
      s+d 1  &gt;s+l 1   
      Therefore, the whole plane block  47  viewed from the X-ray incidence direction is covered with the X-ray sensitive areas of the scintillators  41 , so that efficiency for X-ray utilization improves.  
      The X-ray sensitive areas of neighboring scintillators  41  overlap each other and hide the gap  50 . Therefore, in end portions of the scintillators  41  under which the gap  50  exists, the scintillator length in the X-ray incident direction decreases, and the probability of absorbing incident X-rays decreases. In other words, the probability that the incident X-ray passes through the end portion of the scintillator  41  is high. To lessen the phenomenon, the height “h” in the X-ray incidence direction of the scintillator  41  is increased or the distance d 1  of the deviation in the channel direction between the top face “a” and the under face “c” is increased, thereby narrowing the width of the gap  50  in the X-ray incidence direction or the like.  
       FIG. 5B  is an explanatory diagram showing an example of the case where scintillators  40  each having a rectangular parallelepiped shape are arranged on the anode  51  for comparison with  FIG. 5A . The length of the X-ray sensitive area in the channel direction of the scintillator  40  with respect to an X-ray entering from above is “s”. On the other hand, a gap  49  having a width l 1  between the scintillators  40  is a complete X-ray insensitive area. Therefore, efficiency for X-ray utilization is about s/(s+l 1 ) and is lower than that in the case of  FIG. 5A .  
      Although the X-ray sensitive areas in the channel direction of the scintillators  41  are shown as an example in  FIGS. 5A and 5B , there is similarly no X-ray insensitive area also in the slice direction, and efficiency for X-ray utilization improves.  
      As described above, in the first embodiment, the scintillator  41  has a parallelepiped structure in which the top face “a” and the under face “c” are deviated from each other in the channel and slice directions only by the amounts d 1  and d 2  exceeding the width in the orthogonal direction of the gap  50 , thereby eliminating X-ray insensitive areas when viewed from the X-ray incidence direction. Thus, efficiency for X-ray utilization can be improved and, moreover, X-ray detectivity and the picture quality of a slice image captured can be improved.  
     Second Embodiment  
      In the foregoing first embodiment, the scintillator  41  has a parallelepiped structure in which the top face “a” and the under face “c” are deviated from each other only by the amounts exceeding the width of the gap  50 , thereby eliminating X-ray insensitive areas when viewed from the X-ray incidence direction, typified by the gaps  50 . Alternately, by forming the scintillator in a rectangular parallelepiped structure and tilting the plane block on which the scintillators are mounted with respect to the incident X-ray, X-ray insensitive areas when viewed from the X-ray incidence direction can be eliminated. In a second embodiment, the case where the scintillator has a rectangular parallelepiped structure and the plane block is tilted with respect to the incident X-ray will be described. Since the general configuration of the invention according to the second embodiment is the same as that shown in  FIG. 1 , its detailed description will not be repeated here.  
       FIGS. 6A and 6B  are diagrams showing the configuration of a plane block  77  according to the second embodiment. The plane block  77  corresponds to the plane block  47  including the scintillators  41 , the photodiode  42 , and the plane board  43  shown in  FIG. 2 . Since the other configuration is the same as that shown in  FIG. 2 , its detailed configuration will not be repeated.  
      The plane block  77  includes a reflection film  75 , scintillators  70 , a photodiode  72 , and a plane board  73 . Each of the scintillators  70  repeatedly arranged two-dimensionally in the channel direction and the slice direction has a rectangular parallelepiped shape, and emits light by incidence of an X-ray. The photodiode  72  converts the light emitted from the scintillator  70  into an electric signal when the scintillator  70  is mounted on an anode  71  as a photodetector. The scintillators  70  and the photodiode  72  are mounted on the plane board  73 , and the plane board  73  is disposed at a predetermined tilt θ from the channel direction orthogonal to the incident X-ray.  
       FIG. 6A  is a plan view showing the plane block  77  viewed from the X-axis direction as an X-ray incident direction. Although the reflection film  75  which will be described later exists on the scintillators  70  of the plane block  77 , it is not shown in  FIG. 6A  so that the two-dimensional array of the scintillators  70  is clearly shown.  
      The scintillator  70  has a rectangular parallelepiped shape. The scintillators  70  having the same structure are repeatedly arranged with gaps  74  in the channel and slice directions.  
       FIG. 6B  is a cross section taken along B-B′ line of the scintillators  70  arranged two-dimensionally and shown in  FIG. 6A  when viewed from the z-axis direction. The reflection film  75  which is not shown in  FIG. 6A  is shown on the scintillators  70  on the photodiode  72 . Like the reflection film  48 , the reflection film  75  confines scintillation light within the scintillator  70  and prevents leakage of light among the scintillators  70 . The plane block  77 , that is, the plane board  73  is tilted only by a predetermined tilt θ from the orthogonal direction orthogonal to the incident X-ray.  
       FIG. 7  is an explanatory diagram illustrating the magnitude of the tilt θ of the plane block  77 .  FIG. 7  is a cross section which is taken along line B-B′ shown in  FIG. 6A  in a manner similar to  FIG. 6B . The height of the scintillator  70  having a rectangular parallelepiped from the photodiode  72  is set as “h” and the width of the gap  74  between the scintillators  70  is set as l 3 .  
      It is assumed that the shade of the scintillator  71  projected onto the photodiode  72  has a distance d 3  from the end portion of the scintillator  71 . In this case, the distance d 3  is expressed as follows:
 
 d 3= h× tan(θ)
 
      The tilt (θ) is set so that d 3 &gt;l 3 , that is, h×tan(θ)&gt;l 3  is satisfied and no X-ray insensitive area exists when seen from the X-ray incidence direction.  
      As described above, in the second embodiment, the plane block  77  in which the scintillators  71  each having a rectangular parallelepiped shape are arranged two-dimensionally is tilted only by the tilt θ from the orthogonal direction which is orthogonal to the incident X-ray. Consequently, when viewed from the direction of the incident X-ray, there is no X-ray insensitive area due to the existence of the gaps  74  and the X-ray sensitive areas can be provided in almost the entire surface of the plane block  77 . Thus, efficiency for X-ray utilization can be improved.  
     Third Embodiment  
      In the first embodiment, the scintillator  41  has a parallelepiped structure in which the top face “a” and an under face “c” are deviated from each other by an amount exceeding the width of the gap  50 , thereby eliminating the X-ray insensitive area when viewed from the X-ray incidence direction. Alternately, by using a multilayer scintillator as a multilayer solid state detector in which a plurality of scintillators each having a rectangular parallelepiped structure are stacked, similarly, an X-ray insensitive area of a two-dimensional scintillator array can be eliminated when viewed from the X-ray incidence direction. In the third embodiment, a multilayer scintillator in which a number of scintillators each having a rectangular parallelepiped structure are stacked will be disclosed. Since a general configuration of the invention according to the third embodiment is the same as that shown in  FIG. 1 , its detailed description will not be repeated.  
       FIG. 8A  is a cross section in the XY axis showing the configuration of a plane block  98  according to the third embodiment. The plane block  98  corresponds to the plane block  47  shown in  FIG. 2 , and the other configuration is the same as that shown in  FIG. 2 . The plane block  98  includes a reflection film  85 , first to fourth layers  86  to  89  of multilayer scintillators, a photodiode  82 , an anode  81 , and a plane board  83 . Since the reflection film  85 , the photodiode  82 , the anode  81 , and the plane board  83  are the same as the reflection film  48 , the photodiode  42 , the anode  51 , and the plane board  43  shown in  FIG. 4 , respectively, their description will not be repeated.  
      The first to fourth layers  86  to  89  of the multilayer scintillators form a multilayer solid state detector, and each of the scintillators has a rectangular parallelepiped shape. The first to fourth layers  86  to  89  are two-dimensionally-arranged four layers stacked in the X-ray incidence direction and whose relative positions are different from each other in the Y-axis or Z-axis direction.  FIGS. 9A  to  9 D show the positions of the four multilayer scintillators from the X-axis direction as the X-ray incidence direction. The first to fourth layers  86  to  89  of  FIGS. 9A  to  9 D are shown in a common frame, and the relative positions in the vertical and horizontal directions are shown.  
       FIG. 9A  shows the first layer  86  from the X-axis direction as the X-ray incidence direction. The first layer  86  includes scintillators  90  each having a rectangular parallelepiped shape, which are arranged two-dimensionally, and gaps  94  between the scintillators.  FIG. 9B  shows the second layer  87  from the X-axis direction as the X-ray incidence direction. The second layer  87  includes scintillators  91  each having a rectangular parallelepiped shape, which are arranged two-dimensionally, and gaps  95  between the scintillators. The scintillator  91  has the same size as the scintillator  90 , the gap  95  has the same width as the gap  94 , and the scintillators  91  and the gaps  95  are moved only by the amount of the width of the gap  94  in the channel direction.  
       FIG. 9C  shows the third layer  88  from the X-axis direction as the X-ray incidence direction. The third layer  88  includes scintillators  92  each having a rectangular parallelepiped shape, which are arranged two-dimensionally, and gaps  96  between the scintillators. The scintillator  92  has the same size as the scintillator  90 , the gap  96  has the same width as the gap  94 , and the scintillators  92  and the gaps  96  are moved only by the amount of the width of the gap  94  in the channel and slice directions.  FIG. 9D  shows the fourth layer  89  from the X-axis direction as the X-ray incidence direction. The fourth layer  89  includes scintillators  93  each having a rectangular parallelepiped shape, which are arranged two-dimensionally, and gaps  97  between the scintillators. The scintillator  93  has the same size as the scintillator  90 , the gap  97  has the same width as the gap  94 , and the scintillators  93  and the gaps  94  are moved only by the amount of the width of the gap  94  in the slice direction.  
       FIG. 8B  is a diagram showing the plane block  98  in which the first to fourth layers  86  to  89  of the multilayer scintillators illustrated in  FIGS. 9A  to  9 D are stacked, viewed from the X-ray incidence direction. The reflection film  85  covering the scintillators  90  to  93  is not shown in order to clearly show the positions of the first to fourth layers  86  to  89  viewed from the X-ray incidence direction.  
      When viewed from the X-ray incidence direction, the gaps  97  between the scintillators  93  (fourth layer  89 ) positioned in the uppermost layer in the X-ray incidence direction are covered with the scintillators  92  (third layer  88 ), the scintillators  91  (second layer  87 ), and the scintillators  90  (first layer  86 ) positioned in the lower layers. When viewed in the X-ray incidence direction, the X-ray insensitive areas in which there are no scintillators but the photodiode  82  is directly seen are only the peripheral parts of the two-dimensional array.  
      The operations performed by the first to fourth layers  86  to  89  of the multilayer scintillators when an X-ray is incident will be described with reference to  FIG. 10 .  FIG. 10  is a section in the channel direction like  FIG. 9A , showing, as an example, the case where an X-ray enters the gap  97  portion between the scintillators  93 . The X-ray entering the gap  97  portion between the scintillators  93  is incident on at least one of the scintillators  92  and  91 . By mutual action with one of the scintillators  92  and  91 , fluorescence is generated. The fluorescence is multiple-reflected by the reflection film  85  surrounding the scintillators  90 ,  91 ,  92 , and  93 , finally absorbed by the anode  81 , and converted to an electric signal. Although there is a portion partially in contact with the adjacent channel, since the contact portion is linear, it is considered that light leaked to the adjacent channel occurring in this portion is small.  
      The case where an X-ray enters the gap  96  portion between the scintillators  92 , the case where an X-ray enters the gap  95  portion between the scintillators  91 , and the case where an X-ray enters the gap  94  portion between the scintillators  90  are quite similar to the above case. Therefore, when the plane block  98  is viewed from the X-ray incidence direction, the X-ray insensitive areas exist only in the peripheries of the first to fourth layers  86  to  89  of the multilayer scintillators arranged two-dimensionally.  
      The thickness of the first to fourth layers  86  to  89  of the multilayer scintillators in the X-ray incidence direction is set to be optimum in consideration of the X-ray detection efficiency, weight, price, and the like. To be specific, since the scintillator itself of each layer is thin, the efficiency of detecting the X-rays entering the gaps  94  to  97  is low. Consequently, by increasing the detection efficiency by increasing the thickness in the X-ray incidence direction of each of the first to fourth layers  86  to  89  of the multilayer scintillators, the efficiency for X-ray utilization can be further increased.  
      As described above, in the third embodiment, the first to fourth layers  86  to  89  of the multilayer scintillators each having a rectangular parallelepiped shape are overlapped in a state where the relative positions of the layers are moved only by the widths of the gaps  94  to  97  in the channel direction and the slice direction, thereby eliminating the X-ray insensitive area in the plane block  98  when viewed from the X-ray incidence direction. Thus, an X-ray can be prevented from being undetected due to the gap between scintillators and, moreover, efficiency for X-ray utilization can be improved.