Patent Publication Number: US-2021169434-A1

Title: Imaging system

Description:
BACKGROUND 
     Radiation detectors may be devices used to measure the flux, spatial distribution, spectrum or other properties of radiations. 
     Radiation detectors may be used for many applications. One important application is imaging. Radiation imaging is a radiography technique and can be used to reveal the internal structure of a non-uniformly composed and opaque object such as the human body. 
     Early radiation detectors for imaging include photographic plates and photographic films. A photographic plate may be a glass plate with a coating of light-sensitive emulsion. Although photographic plates were replaced by photographic films, they may still be used in special situations due to the superior quality they offer and their extreme stability. A photographic film may be a plastic film (e.g., a strip or sheet) with a coating of light-sensitive emulsion. 
     In the 1980s, photostimulable phosphor plates (PSP plates) became available. A PSP plate may contain a phosphor material with color centers in its lattice. When the PSP plate is exposed to radiation, electrons excited by radiation are trapped in the color centers until they are stimulated by a laser beam scanning over the plate surface. As the plate is scanned by laser, trapped excited electrons give off light, which is collected by a photomultiplier tube. The collected light is converted into a digital image. In contrast to photographic plates and photographic films, PSP plates can be reused. 
     Another kind of radiation detectors are radiation image intensifiers. Components of a radiation image intensifier are usually sealed in a vacuum. In contrast to photographic plates, photographic films, and PSP plates, radiation image intensifiers may produce real-time images, i.e., do not require post-exposure processing to produce images. Radiation first hits an input phosphor (e.g., cesium iodide) and is converted to visible light. The visible light then hits a photocathode (e.g., a thin metal layer containing cesium and antimony compounds) and causes emission of electrons. The number of emitted electrons is proportional to the intensity of the incident radiation. The emitted electrons are projected, through electron optics, onto an output phosphor and cause the output phosphor to produce a visible-light image. 
     Scintillators operate somewhat similarly to radiation image intensifiers in that scintillators (e.g., sodium iodide) absorb radiation and emit visible light, which can then be detected by a suitable image sensor for visible light. In scintillators, the visible light spreads and scatters in all directions and thus reduces spatial resolution. Reducing the scintillator thickness helps to improve the spatial resolution but also reduces absorption of radiation. A scintillator thus has to strike a compromise between absorption efficiency and resolution. 
     Semiconductor radiation detectors largely overcome this problem by direct conversion of radiation into electric signals. A semiconductor radiation detector may include a semiconductor layer that absorbs radiation in wavelengths of interest. When a radiation particle is absorbed in the semiconductor layer, multiple charge carriers (e.g., electrons and holes) are generated and swept under an electric field towards electric contacts on the semiconductor layer. Cumbersome heat management required in currently available semiconductor radiation detectors (e.g., Medipix) can make a detector with a large area and a large number of pixels difficult or impossible to produce. 
     SUMMARY 
     Disclosed herein is a system comprising: a radiation source; a marker; a first image sensor; and a second image sensor; wherein the first image sensor is configured to capture images of the marker; wherein the second image sensor is configured to move between a first position relative to the radiation source and a second position relative to the radiation source; wherein the second image sensor is configured to capture, with radiation from the radiation source, a first set of images of portions of a scene when the second image sensor is at the first position relative to the radiation source; wherein the second image sensor is configured to capture, with the radiation from the radiation source, a second set of images of portions of the scene when the second image sensor is at the second position relative to the radiation source; wherein the second image sensor and the radiation source are configured to collectively rotate relative to the scene; wherein the second image sensor is configured to form an image of the scene by selecting an image from the first set based on the images of the marker and selecting an image from the second set based on the images of the marker, and stitching the image selected from the first set and the image selected from the second set. 
     According to an embodiment, the marker is stationary relative to the scene; and wherein a relative position of the first image sensor with respect to the radiation source is fixed. 
     According to an embodiment, the first image sensor is stationary relative to the scene; and wherein a relative position of the marker with respect to the radiation source is fixed. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source and the second position relative to the radiation source by translating along a first direction relative to the radiation source. 
     According to an embodiment, the first direction is parallel to a radiation-receiving surface of the image sensor. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source and the second position relative to the radiation source by translating along a second direction relative to the radiation source; wherein the second direction is different from the first direction. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source relative to the radiation source by rotating about a first axis relative to the radiation source. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source and the second position relative to the radiation source by rotating about a second axis relative to the radiation source; wherein the second axis is different from the first axis. 
     According to an embodiment, the radiation source is on the first axis. 
     According to an embodiment, the second image sensor and the radiation source are configured to collectively rotate relative to the scene about one or more axes. 
     According to an embodiment, at least one of the one or more axes is on the second image sensor. 
     According to an embodiment, a first rotational position which the radiation source is at when the image selected from the first set is captured and a second rotational position which the radiation source is at when the image selected from the second set is captured are the same. 
     According to an embodiment, the images of the marker comprise a first image of the marker and a second image of the marker; wherein rotational positions which the radiation source is at when the image selected from the first set is captured and when the first image of the marker is captured are the same; wherein rotational positions which the radiation source is at when the image selected from the second set is captured and when the second image of the marker is captured are the same; wherein the first image of the marker and the second image of the marker are identical. 
     According to an embodiment, the second image sensor comprises a first radiation detector and a second radiation detector. 
     According to an embodiment, the first radiation detector and the second radiation detector respectively comprise a planar surface configured to receive the radiation; wherein the planar surface of the first radiation detector and the planar surface of the second radiation detector are not coplanar. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by translating along a first direction relative to the radiation source. 
     According to an embodiment, the first direction is parallel to the planar surface of the first radiation detector but not parallel to the planar surface of the second radiation detector. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by translating along a second direction relative to the radiation source; wherein the second direction is different from the first direction. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by rotating about a first axis relative to the radiation source. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by rotating about a second axis relative to the radiation source; wherein the second axis is different from the first axis. 
     According to an embodiment, the radiation source is on the first axis. 
     Disclosed herein is a system comprising: a radiation source; a marker; a first image sensor; and a second image sensor; wherein the second image sensor is configured to move between a first position relative to the radiation source and a second position relative to the radiation source; wherein the second image sensor is configured to capture, with radiation from the radiation source, an image of first portions of a scene, when the second image sensor is at the first position relative to the radiation source and the first image sensor captures a first image of the marker that matches one of a set of reference images; wherein the second image sensor is configured to capture, with the radiation from the radiation source, an image of second portions of the scene, when the second image sensor is at the second position relative to the radiation source and the first image sensor captures a second image of the marker that matches one of the set of reference images; wherein the second image sensor and the radiation source are configured to collectively rotate relative to the scene; wherein the second image sensor is configured to form an image of the scene by stitching the image of the first portions and the image of the second portions if the first image of the marker and the second image of the marker are identical. 
     According to an embodiment, the marker is stationary relative to the scene; and wherein a relative position of the first image sensor with respect to the radiation source is fixed. 
     According to an embodiment, the first image sensor is stationary relative to the scene; and wherein a relative position of the marker with respect to the radiation source is fixed. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source and the second position relative to the radiation source by translating along a first direction relative to the radiation source. 
     According to an embodiment, the first direction is parallel to a radiation-receiving surface of the image sensor. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source and the second position relative to the radiation source by translating along a second direction relative to the radiation source; wherein the second direction is different from the first direction. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source and the second position relative to the radiation source by rotating about a first axis relative to the radiation source. 
     According to an embodiment, the second image sensor is configured to move between the first position relative to the radiation source and the second position relative to the radiation source by rotating about a second axis relative to the radiation source; wherein the second axis is different from the second axis. 
     According to an embodiment, the radiation source is on the first axis. 
     According to an embodiment, the second image sensor and the radiation source are configured to collectively rotate relative to the scene about one or more axes. 
     According to an embodiment, at least one of the one or more axes is on the second image sensor. 
     According to an embodiment, a first rotational position which the radiation source is at when the first image of the marker is captured and a second rotational position which the radiation source is at when the second image of the marker is captured are the same. 
     According to an embodiment, the second image sensor is configured to determine the first rotational position based on the first image of the marker. 
     According to an embodiment, the second image sensor comprises a first radiation detector and a second radiation detector. 
     According to an embodiment, the first radiation detector and the second radiation detector respectively comprise a planar surface configured to receive the radiation; wherein the planar surface of the first radiation detector and the planar surface of the second radiation detector are not coplanar. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by translating along a first direction relative to the radiation source. 
     According to an embodiment, the first direction is parallel to the planar surface of the first radiation detector but not parallel to the planar surface of the second radiation detector. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by translating along a second direction relative to the radiation source; wherein the second direction is different from the first direction. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by rotating about a first axis relative to the radiation source. 
     According to an embodiment, the first radiation detector and the second radiation detector are configured to move relative to the radiation source by rotating about a second axis relative to the radiation source; wherein the second axis is different from the first axis. 
     According to an embodiment, the radiation source is on the first axis. 
     Disclosed herein is a method comprising when a radiation source is at a first rotational position relative to a scene, capturing an image of first portions of the scene with radiation from the radiation source and capturing a first image of a marker; when the radiation source is at a second rotational position relative to the scene, capturing an image of second portions of the scene with the radiation from the radiation source and capturing a second image of the marker; determining whether the first rotational position and the second rotational position are the same based on the first image of the marker and the second image of the marker; upon determining that the first rotational position and the second rotational position are the same, forming an image of the scene by stitching the image of the first portions and the image of the second portions. 
     According to an embodiment, the marker is stationary relative to the scene; wherein the first image of the marker and the second image of the marker are captured by a first image sensor whose relative position with respect to the radiation source is fixed. 
     According to an embodiment, the first image of the marker and the second image of the marker are captured by a first image sensor that is stationary relative to the scene; wherein a relative position of the marker with respect to the radiation source is fixed. 
     According to an embodiment, the image of the first portions of the scene is captured by a second image sensor when the second image sensor is at a first position relative to the radiation source; wherein the image of the second portions of the scene is captured by the second image sensor when the second image sensor is at a second position relative to the radiation source. 
     According to an embodiment, the second image sensor and the radiation source are configured to collectively rotate relative to the scene. 
     According to an embodiment, determining whether the first rotational position and the second rotational position are the same based on the first image of the marker and the second image of the marker comprises: determining the first rotational position based on the first image of the marker and determining the second rotational position based on the first image of the marker. 
     According to an embodiment, determining whether the first rotational position and the second rotational position are the same based on the first image of the marker and the second image of the marker comprises: determining whether the first image of the marker and the second image of the marker are identical. 
     Disclosed herein is a method comprising when a first image of a marker is captured and matches one of a set of reference images, capturing, with radiation from a radiation source, an image of first portions of a scene; when a second image of a marker is captured and matches one of a set of reference images, capturing, with the radiation from the radiation source, an image of second portions of the scene; determining whether the first image of the marker and the second image of the marker are identical; upon determining that the first image of the marker and the second image of the marker are identical, forming an image of the scene by stitching the image of the first portions and the image of the second portions. 
     According to an embodiment, the marker is stationary relative to the scene; wherein the first image of the marker and the second image of the marker are captured by a first image sensor whose relative position with respect to the radiation source is fixed. 
     According to an embodiment, the first image of the marker and the second image of the marker are captured by a first image sensor that is stationary relative to the scene; wherein a relative position of the marker with respect to the radiation source is fixed. 
     According to an embodiment, the image of the first portions of the scene is captured by a second image sensor when the second image sensor is at a first position relative to the radiation source; wherein the image of the second portions of the scene is captured by the second image sensor when the second image sensor is at a second position relative to the radiation source. 
     According to an embodiment, the second image sensor and the radiation source are configured to collectively rotate relative to the scene. 
     According to an embodiment, a first rotational position which the radiation source is at when the first image of the marker is captured and a second rotational position which the radiation source is at when the second image of the marker is captured are the same. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1A  schematically shows a portion of a system, according to an embodiment. 
         FIG. 1B  and  FIG. 1C  each schematically show movements of the second image sensor in the system of  FIG. 1A , relative to the radiation source, according to an embodiment. 
         FIG. 2A  and  FIG. 2B  each schematically show movement of the marker and the first image sensor in the system of  FIG. 1A , when the second image sensor and the radiation source in the system of  FIG. 1A  are collectively rotated relative to the scene, according to an embodiment. 
         FIG. 3A  and  FIG. 3B  each schematically show operation of the system, according to an embodiment. 
         FIG. 4A  and  FIG. 4B  schematically show operation of the system, according to an embodiment. 
         FIG. 5A  schematically shows that the second image sensor may have a plurality of radiation detectors, according to an embodiment. 
         FIG. 5B  schematically shows an example of a perspective view of the first image sensor and the second image sensor, with respect to the scene, the marker and the radiation source. 
         FIG. 6A  schematically shows a cross-sectional view of a radiation detector, according to an embodiment. 
         FIG. 6B  schematically shows a detailed cross-sectional view of the detector, according to an embodiment. 
         FIG. 6C  schematically shows an alternative detailed cross-sectional view of the detector, according to an embodiment. 
         FIG. 7  schematically shows that the radiation detector may have an array of pixels, according to an embodiment. 
         FIG. 8  schematically shows a functional block diagram of the system, according to an embodiment. 
         FIG. 9A - FIG. 9C  schematically show arrangements of the detectors in the image sensor, according to some embodiments. 
         FIG. 10  schematically shows an image sensor with plurality of detectors that are hexagonal in shape, according to an embodiment. 
         FIG. 11  and  FIG. 12  each schematically show a flowchart for a method, according to an embodiment. 
         FIG. 13  schematically shows a system comprising the system described herein, suitable for medical imaging such as chest radiation radiography, abdominal radiation radiography, etc., according to an embodiment 
         FIG. 14  schematically shows a system comprising the system described herein suitable for dental radiation radiography, according to an embodiment. 
         FIG. 15  schematically shows a cargo scanning or non-intrusive inspection (NII) system comprising the image sensor described herein, according to an embodiment. 
         FIG. 16  schematically shows a full-body scanner system comprising the system described herein, according to an embodiment. 
         FIG. 17  schematically shows a radiation computed tomography (Radiation CT) system comprising the system described herein, according to an embodiment. 
         FIG. 18A  and  FIG. 18B  each show a component diagram of an electronic system of the radiation detector in  FIG. 6A ,  FIG. 6B  and  FIG. 6C , according to an embodiment. 
         FIG. 19  schematically shows a temporal change of the electric current flowing through an electrode (upper curve) of a diode or an electric contact of a resistor of a radiation absorption layer exposed to radiation, the electric current caused by charge carriers generated by a radiation particle incident on the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode (lower curve), according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  schematically shows a portion of a system  9000  comprising a radiation source  109 , a marker  60 , a first image sensor  9001 , and a second image sensor  9002 , according to an embodiment. The first image sensor  9001  is configured to capture images of the marker  60 , e.g., using the radiation from the radiation source  109 . The second image sensor  9002  and the radiation source  109  may collectively rotate to a plurality of rotational positions relative to the scene  50 . The second image sensor  9002  may move between multiple positions relative to the radiation source  109 . At one of the multiple positions relative to the radiation source  109 , the second image sensor  9002  may capture a set of images of portions of the scene  50  using the radiation from the radiation source  109 ; at another of the multiple positions relative to the radiation source  109 , the second image sensor  9002  may capture another set of images of portions of the scene  50  using the radiation from the radiation source  109 , e.g., respectively when the second image sensor  9002  and the radiation source  109  are at the plurality of rotational positions relative to the scene  50 . The second image sensor  9002  may comprise a radiation-receiving surface configured to receive radiation, e.g., radiation that is from the radiation source  109  and may have passed through the scene  50 . 
       FIG. 1B  and  FIG. 1C  each schematically show movements of the second image sensor  9002  relative to the radiation source  109 , according to an embodiment. In the example shown in  FIG. 1B , the second image sensor  9002  may move from a first position  910  relative to the radiation source  109  to a second position  920  relative to the radiation source  109  by translating along a first direction  904  relative to the radiation source  109 . The first direction  904  may be parallel to a radiation-receiving surface of the second image sensor  9002 . 
       FIG. 1B  also shows that the second image sensor  9002  may move from the first position  910  relative to the radiation source  109  to a third position  930  relative to the radiation source  109  by translating along a second direction  905  relative to the radiation source  109 . The second direction  905  is different from the first direction  904 . 
     In the example shown in  FIG. 1B , according to an embodiment, a first set  1010  of images of portions of the scene  50  are captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the first position  910  relative to the radiation source  109 . A second set  1020  of images of portions of the scene  50  are captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the second position  920  relative to the radiation source  109 . The images  1000  of marker  60  are captured by the first image sensor  9001 , e.g., with radiation from the radiation source  109 . 
     In the example shown in  FIG. 1C , according to an embodiment, the second image sensor  9002  may move from the first position  910  relative to the radiation source  109  to a fourth position  940  relative to the radiation source  109  by rotating about a first axis  902  relative to the radiation source  109 . The first axis  902  may be parallel to the radiation-receiving surface of the second image sensor  9002 . The radiation source  109  may be on the first axis  902 . 
       FIG. 1C  also shows that the second image sensor  9002  may move from the first position  910  relative to the radiation source  109  to a fifth position  950  relative to the radiation source  109  by rotating about a second axis  903  relative to the radiation source  109 . The second axis  903  is different from the first axis  902 . For example, the second axis  903  may be perpendicular to the first axis  902 . The radiation source  109  may be on the second axis  903 . 
     In the example shown in  FIG. 1C , according to an embodiment, a third set of images  1030  of portions of the scene  50  are captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the fourth position  940  relative to the radiation source  109 . A first set  1010  of images of portions of the scene  50  are captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the first position  910  relative to the radiation source  109 . The images  1000  of marker are captured by the first image sensor  9001  with radiation from the radiation source  109 . 
       FIG. 2A  and  FIG. 2B  each schematically show movement of the marker  60  or the first image sensor  9001  when the second image sensor  9002  and the radiation source  109  collectively rotate relative to the scene  50 , according to an embodiment. In the example shown in  FIG. 2A , the marker  60  is stationary relative to the scene  50 , and a relative position of the first image sensor  9001  with respect to the radiation source  109  is fixed when the second image sensor  9002  and the radiation source  109  collectively rotate relative to the scene  50  about one or more axes, e.g., an axis  501 . At least one of the one or more axes, e.g., the axis  501 , may be on the second image sensor  9002 . Namely, the first image sensor  9001 , the second image sensor  9002  and the radiation source  109  collectively rotate relative to the scene  50  about the axis  501 . 
     In the example shown in  FIG. 2B , the first image sensor  9001  is stationary relative to the scene  50 , and a relative position of the marker  60  with respect to the radiation source  109  is fixed when the second image sensor  9002  and the radiation source  109  collectively rotate relative to the scene  50  about one or more axes, e.g., the axis  501 . At least one of the one or more axes, e.g., the axis  501 , may be on the second image sensor  9002 . Namely, the marker  60 , the second image sensor  9002  and the radiation source  109  collectively rotate relative to the scene  50  about the axis  501 . 
     According to one embodiment, the second image sensor  9002  and the radiation source  109  may collectively rotate about one or more axes, for example, the axis  501  in  FIG. 2A  and  FIG. 2B . The second image sensor  9002  and the radiation source  109  may collectively rotate about other axes that are different than the axis  501 . The axes, including axis  501 , may be on the second image sensor  9002 . 
       FIG. 3A  and  FIG. 3B  each schematically shows operation of the system  9000 , according to an embodiment. In the example shown in  FIG. 3A , the second image sensor  9002  is at the first position  910  relative to the radiation source  109  (see  FIG. 1B ). According to one embodiment, the marker  60  remains stationary relative to the scene  50 , while the radiation source  109 , the first image sensor  9001 , the second image sensor  9002  collectively rotate about the axis  501  relative to the scene  50  to a first rotational position  510 . At the first rotational position  510 , a first image  1001  of the marker  60  is captured by the first image sensor  9001 , and an image  1011  of a portion of the scene  50 , which belongs to the first set  1010  of images, is captured by the second image sensor  9002 . The capturing of the first image  1001  of the marker  60  and the capturing of the image  1011  of the portion of the scene  50  may or may not be at the same time. The second image sensor  9002  may be configured to continue the collective rotation with the radiation source  109  and complete capturing the first set  1010  of images of the portion of the scene  50 . 
     In the example shown in  FIG. 3B , the second image sensor  9002  is at the second position  920  relative to the radiation source  109  (see  FIG. 1B ). According to one embodiment, the marker  60  remains stationary relative to the scene  50 , while the radiation source  109 , the first image sensor  9001 , the second image sensor  9002  collectively rotate about the axis  501  relative to the scene  50  to a second rotational position  520 . At the second rotational position  520 , a second image  1002  of the marker  60  is captured by the first image sensor  9001 , and an image  1021  of a portion of the scene  50 , which belongs to the second set  1020  of images, is captured by the second image sensor  9002 . The capturing of the second image  1002  of the marker  60  and the capturing of the image  1021  of the portion of the scene  50 , may or may not be at the same time. The second image sensor  9002  may be configured to continue the collective rotation with the radiation source  109  and complete capturing the second set  1020  of images of the portion of the scene  50 . 
       FIG. 4A  and  FIG. 4B  each schematically shows operation of the system  9000 , according to an embodiment. In the example shown in  FIG. 4A , the second image sensor  9002  is at the first position  910  relative to the radiation source  109  (see in  FIG. 1B ). According to one embodiment, the first image sensor  9001  remains stationary relative to the scene  50 , and the radiation source  109 , the marker  60 , the second image sensor  9002  collectively rotate about the axis  501  relative to the scene  50  to a first rotational position  510 . At the first rotational position  510 , a first image  1001  of the marker  60  is captured by the first image sensor  9001 , and an image  1011  of the first portion of the scene  50 , which belongs to the first set  1010  of images, is captured by the second image sensor  9002 . The capturing of the first image  1001  of the marker  60  and the capturing of the image  1011  of the portion of the scene  50  may or may not be at the same time. The second image sensor  9002  may be configured to continue the collective rotation with the radiation source  109  and complete capturing the first set  1010  of images of the portion of the scene  50 . 
     In the example shown in  FIG. 4B , the second image sensor  9002  is at the second position  920  relative to the radiation source  109  (see  FIG. 1B ). According to one embodiment, the first image sensor  9001  remains stationary relative to the scene  50 , and the radiation source  109 , the marker  60 , the second image sensor  9002  collectively rotate about the axis  501  relative to the scene  50  to a second rotational position  520 . At the second rotational position  520 , a second image  1002  of the marker  60  is captured by the first image sensor  9001 , and an image  1021  of the second portion of the scene  50 , which belongs to the second set  1020  of images, is captured by the second image sensor  9002 . The capturing of the second image  1002  of the marker  60  and the capturing of the image  1021  of the portion of the scene  50 , may or may not be at the same time. The second image sensor  9002  may be configured to continue the collective rotation with the radiation source  109  and complete capturing the second set  1020  of images of the portion of the scene  50 . 
     According to an embodiment, the second image sensor  9002  is configured to form an image of the scene  50  by selecting an image (e.g., image  1011  in  FIG. 3A  or  FIG. 4A ) from the first set  1010  of images based on the images of the marker  60 , selecting an image (e.g., image  1021  in  FIG. 3B  or  FIG. 4A ) from the second set  1020  of images based on the images of the marker  60 , and stitching the two selected images together. The rotational position which the radiation source  109  is at when the image selected from the first set  1010  is captured and the rotational position which the radiation source  109  is at when the image selected from the second set  1020  is captured may be the same. According to one embodiment, selecting the images from the first set and the second set may be by comparing images of the marker  60 . In the example in  FIG. 3A  and  FIG. 3B , or in the example in  FIG. 4A  and  FIG. 4B , if the image  1001  of the marker  60  and the image  1002  of the marker  60  are identical, rotational position  510  and rotational position  520  are the same and image  1011  and image  1021  can be respectively selected from the first set and the second set, and stitched. 
     In the example shown in  FIG. 1B , according to an embodiment, a first image  1310  of portions of the scene  50  is captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the first position  910  relative to the radiation source  109  and the first image sensor  9001  captures an image  1311  of the marker  60  that matches one of a set of reference images  1111 . A second image  1320  of portions of the scene  50  is captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the second position  920  relative to the radiation source  109  and the first image sensor  9001  captures an image  1312  of the marker  60  that matches one of the set of reference images  1111 . 
     In the example shown in  FIG. 1C , according to an embodiment, a third image  1330  of portions of the scene  50  is captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the fourth position  940  relative to the radiation source  109  and the first image sensor  9001  captures an image  1413  of the marker  60  that matches one of a set of reference images  1111 . A first image  1310  of portions of the scene  50  is captured by the second image sensor  9002  with radiation from the radiation source  109 , when the second image sensor  9002  is at the first position  910  relative to the radiation source  109  and the first image sensor  9001  captures an image  1411  of the marker  60  that matches one of a set of reference images  1111 . 
     According to an embodiment, the second image sensor  9002  is configured to form an image of the scene  50  by stitching the first image  1310  and the second image  1320 , if the image  1311  and the image  1312  are identical. The image  1311  and the image  1312  being identical indicates that the rotational position which the radiation source  109  is at when the image  1311  is captured and the rotational position which the radiation source  109  is at when the image  1312  is captured are the same. According to an embodiment, the second image sensor  9002  is configured to form an image of the scene  50  by stitching the first image  1310  and the third image  1330 , if the image  1411  and the image  1413  are identical. The image  1411  and the image  1413  being identical indicates that the rotational position which the radiation source  109  is at when the image  1411  is captured and the rotational position which the radiation source  109  is at when the image  1413  is captured are the same. 
       FIG. 5A  schematically shows that the second image sensor  9002  may have a plurality of radiation detectors (e.g., a first radiation detector  100 A, a second radiation detector  100 B). The second image sensor  9002  may have a support  107  with a curved surface  102 . The plurality of radiation detectors may be arranged on the support  107 , for example, on the curved surface  102 , as shown in the example of  FIG. 6A . The first radiation detector  100 A may have a first planar surface  103 A configured to receive radiation from the radiation source  109 . A second radiation detector  100 B may have a second planar surface  103 B configured to receive the radiation from the radiation source  109 . The first planar surface  103 A of the first radiation detector  100 A and the second planar surface  103 B of the second radiation detector  100 B may be not parallel. The radiation from the radiation source  109  may have passed through the scene  50  (e.g., a portion of a human body) before reaching the first radiation detector  100 A or the second radiation detector  100 B. 
       FIG. 5B  schematically shows an example of a perspective view of the first image sensor  9001  and the second image sensor  9002  depicted in  FIG. 5A , with respect to the scene  50 , the marker  60  and the radiation source  109 . 
     A relative position of the first radiation detector  100 A with respect to the second radiation detector  100 B may remain unchanged when the second image sensor  9002  moves relative to the radiation source  109  and when the second image sensor  9002  and the radiation source  109  collectively rotate relative to the scene  50 . The first radiation detector  100 A and the second radiation detector  100 B remain stationary relative to the second image sensor  9002 . Therefore, the first radiation detector  100 A and the second radiation detector  100 B may move relative to the radiation source  109  with the second image sensor  9002  by translating along the first direction  904  or the second direction  905  relative to the radiation source  109  or by rotating about the first axis  902  or the second axis  903  relative to the radiation source  109 . The first direction  904  or the second direction  905  may be parallel to both, either or neither of the first planar surface  103 A and the second planar surface  103 B. For example, the first direction  904  may be parallel to the first planar surface  103 A, but not parallel to the second planar surface  103 B. 
       FIG. 6A  schematically shows a cross-sectional view of a radiation detector  100 , according to an embodiment. The radiation detector  100  may be used in the image sensors in the system  9000 , for example as the first radiation detector  100 A or the second radiation detector  1006 . The radiation detector  100  may include a radiation absorption layer  110  and an electronics layer  120  (e.g., an ASIC) for processing or analyzing electrical signals incident radiation generates in the radiation absorption layer  110 . In an embodiment, the radiation detector  100  does not comprise a scintillator. The radiation absorption layer  110  may include a semiconductor material such as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. The semiconductor may have a high mass attenuation coefficient for the radiation energy of interest. The surface  103  of the radiation absorption layer  110  distal from the electronics layer  120  is configured to receive radiation. 
     As shown in a detailed cross-sectional view of the radiation detector  100  in  FIG. 6B , according to an embodiment, the radiation absorption layer  110  may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region  111 , one or more discrete regions  114  of a second doped region  113 . The second doped region  113  may be separated from the first doped region  111  by an optional the intrinsic region  112 . The discrete regions  114  are separated from one another by the first doped region  111  or the intrinsic region  112 . The first doped region  111  and the second doped region  113  have opposite types of doping (e.g., region  111  is p-type and region  113  is n-type, or region  111  is n-type and region  113  is p-type). In the example in  FIG. 6B , each of the discrete regions  114  of the second doped region  113  forms a diode with the first doped region  111  and the optional intrinsic region  112 . Namely, in the example in  FIG. 6B , the radiation absorption layer  110  has a plurality of diodes having the first doped region  111  as a shared electrode. The first doped region  111  may also have discrete portions. 
     When a radiation particle hits the radiation absorption layer  110  including diodes, the radiation particle may be absorbed and generate one or more charge carriers by a number of mechanisms. A radiation particle may generate 10 to 100000 charge carriers. The charge carriers may drift to the electrodes of one of the diodes under an electric field. The field may be an external electric field. The electric contact  119 B may include discrete portions each of which is in electrical contact with the discrete regions  114 . In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single radiation particle are not substantially shared by two different discrete regions  114  (“not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow to a different one of the discrete regions  114  than the rest of the charge carriers). Charge carriers generated by a radiation particle incident around the footprint of one of these discrete regions  114  are not substantially shared with another of these discrete regions  114 . A pixel  150  associated with a discrete region  114  may be an area around the discrete region  114  in which substantially all (more than 98%, more than 99.5%, more than 99.9%, or more than 99.99% of) charge carriers generated by a radiation particle incident therein at an angle of incidence of 0° flow to the discrete region  114 . Namely, less than 2%, less than 1%, less than 0.1%, or less than 0.01% of these charge carriers flow beyond the pixel. 
     As shown in an alternative detailed cross-sectional view of the radiation detector  100  in  FIG. 6C , according to an embodiment, the radiation absorption layer  110  may include a resistor of a semiconductor material such as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does not include a diode. The semiconductor may have a high mass attenuation coefficient for the radiation energy of interest. 
     When a radiation particle hits the radiation absorption layer  110  including a resistor but not diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A radiation particle may generate 10 to 100000 charge carriers. The charge carriers may drift to the electric contacts  119 A and  119 B under an electric field. The field may be an external electric field. The electric contact  119 B includes discrete portions. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single radiation particle are not substantially shared by two different discrete portions of the electric contact  119 B (“not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers). Charge carriers generated by a radiation particle incident around the footprint of one of these discrete portions of the electric contact  119 B are not substantially shared with another of these discrete portions of the electric contact  119 B. A pixel  150  associated with a discrete portion of the electric contact  119 B may be an area around the discrete portion in which substantially all (more than 98%, more than 99.5%, more than 99.9% or more than 99.99% of) charge carriers generated by a radiation particle incident at an angle of incidence of 0° therein flow to the discrete portion of the electric contact  119 B. Namely, less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow beyond the pixel associated with the one discrete portion of the electric contact  119 B. 
     The electronics layer  120  may include an electronic system  121  suitable for processing or interpreting signals generated by radiation particles incident on the radiation absorption layer  110 . The electronic system  121  may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessor, and memory. The electronic system  121  may include components shared by the pixels or components dedicated to a single pixel. For example, the electronic system  121  may include an amplifier dedicated to each pixel and a microprocessor shared among all the pixels. The electronic system  121  may be electrically connected to the pixels by vias  131 . Space among the vias may be filled with a filler material  130 , which may increase the mechanical stability of the connection of the electronics layer  120  to the radiation absorption layer  110 . Other bonding techniques are possible to connect the electronic system  121  to the pixels without using vias. 
       FIG. 7  schematically shows that the radiation detector  100  may have an array of pixels  150 . The array may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. Each pixel  150  may be configured to detect a radiation particle incident thereon, measure the energy of the radiation particle, or both. For example, each pixel  150  may be configured to count numbers of radiation particles incident thereon whose energy falls in a plurality of bins, within a period of time. All the pixels  150  may be configured to count the numbers of radiation particles incident thereon within a plurality of bins of energy within the same period of time. Each pixel  150  may have its own analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident radiation particle into a digital signal. The ADC may have a resolution of 10 bits or higher. Each pixel  150  may be configured to measure its dark current, such as before or concurrently with each radiation particle incident thereon. Each pixel  150  may be configured to deduct the contribution of the dark current from the energy of the radiation particle incident thereon. The pixels  150  may be configured to operate in parallel. For example, when one pixel  150  measures an incident radiation particle, another pixel  150  may be waiting for another radiation particle to arrive. The pixels  150  may be but do not have to be individually addressable. 
     According to an embodiment, the second image sensor  9002  can move to multiple positions, relative to the radiation source  109 . The second image sensor  9002  may use the radiation detectors  100  and with the radiation from the radiation source  109  to capture images of multiple portions of the scene  50  respectively at the multiple positions. The second image sensor  9002  can compare the images of marker captured by the first image sensor  9001  and stitch the images of portions of the scene  50  captured by the second image sensor to form an image of the entire scene  50 . As shown in  FIG. 8 , according to an embodiment, the system  9000  may include an actuator  500  configured to move the second image sensor  9002  to the multiple positions. The second image sensor  9002  may include a processor  200  that compares images of marker to determine the rotational positions of the second image sensor  9002 . The processor  200  may be used to stitch the images of portions of the scene  50 . The rotational positions of the second image sensor  9002  and the radiation source  109  may be controlled by the actuator  500 . The rotational positions of the first image sensor  9001  and the marker  60  may be optionally controlled by the actuator  500 . 
     The radiation detectors  100  may be arranged in a variety of ways in the second image sensor  9002 .  FIG. 9A  schematically shows one arrangement, according to an embodiment, where the radiation detectors  100  are arranged in staggered rows. For example, radiation detectors  100 A and  100 B are in the same row, aligned in the Y direction, and uniform in size; radiation detectors  100 C and  100 D are in the same row, aligned in the Y direction, and uniform in size. Radiation detectors  100 A and  100 B are staggered in the X direction with respect to radiation detectors  100 C and  100 D. According to an embodiment, a distance X 2  between two neighboring radiation detectors  100 A and  100 B in the same row is greater than a width X 1  (i.e., dimension in the X direction, which is the extending direction of the row) of one radiation detector in the same row and is less than twice the width X 1 . Radiation detectors  100 A and  100 E are in a same column, aligned in the X direction, and uniform in size; a distance Y 2  between two neighboring radiation detectors  100 A and  100 E in the same column is less than a width Y 1  (i.e., dimension in the Y direction) of one radiation detector in the same column. An image of the scene using this arrangement may be obtained from stitching three images of portions of the scene captured at three positions spaced apart in the X direction. 
       FIG. 10B  schematically shows another arrangement, according to an embodiment, where the radiation detectors  100  are arranged in a rectangular grid. For example, the radiation detectors  100  may include radiation detectors  100 A,  100 B,  100 E and  100 F as arranged exactly in  FIG. 10A , without radiation detectors  100 C,  100 D,  100 G, or  100 H in  FIG. 9A . This arrangement allows imaging of the scene by taking images of portions of the scene at six positions. For example, three positions spaced apart in the X direction and another three positions spaced apart in the X direction and spaced apart in the Y direction from the first three positions. 
     Other arrangements may also be possible. For example, in  FIG. 9C , the radiation detectors  100  may span the whole width of the image sensor  9001  or  9002  in the X-direction, with a distance Y 2  between two neighboring radiation detectors  100  being less than a width of one radiation detector Y 1 . Assuming the width of the detectors in the X direction is greater than the width of the scene in the X direction, the image of the scene may be stitched from two images of portions of the scene captured at two positions spaced apart in the Y direction. 
     The radiation detectors  100  described above may be provided with any suitable size and shapes. According to an embodiment (e.g., in  FIG. 9A ,  FIG. 9B , and  FIG. 9C ), at least some of the radiation detectors are rectangular in shape. According to an embodiment, as shown in  FIG. 10 , at least some of the radiation detectors are hexagonal in shape. 
       FIG. 11  schematically shows a flowchart for a method, according to an embodiment. In procedure  151 , an image of first portions of the scene  50  is captured with radiation from the radiation source  109  (e.g., by the second image sensor  9002  when it is at the first position  910  relative to the radiation source  109 ) and a first image of the marker  60  is captured (e.g., by the first image sensor  9001 ), when the radiation source  109  is at the first rotational position  510  relative to the scene  50 . The first image of the marker  60  is not necessarily captured at the same time as the first of first portions of the scene  50 . In procedure  152 , an image of second portions of the scene  50  is captured (e.g., by the second image sensor  9002  when it is at the second position  920  relative to the radiation source  109 ) with radiation from the radiation source  109  and a second image of the marker  60  is captured (e.g., by the first image sensor  9001 ), when the radiation source  109  is at the second rotational position  520  relative to the scene  50 . The second image of the marker  60  is not necessarily captured at the same time as the image of the second portions of the scene  50 . In procedure  153 , whether the first rotational position  510  and the second rotational position  520  are the same is determined based on the first image of the marker and the second image of the marker. In an example, whether the first rotational position  510  and the second rotational position  520  are the same involves determining the first rotation position  510  based on the first image of the marker and determining the second rotational position  520  based on the second image of the marker. In an example, whether the first rotational position  510  and the second rotational position  520  are the same involves determining whether the first image of the marker and the second image of the marker are identical. In procedure  154 , an image of the scene  50  is formed by stitching the image of the first portions and the image of the second portions, upon determining that the first rotational position  510  and the second rotational position  520  are the same. 
       FIG. 12  schematically shows a flowchart for a method, according to an embodiment. In procedure  161 , an image of first portions of the scene  50  is captured with radiation from the radiation source  109  (e.g., by the second image sensor  9002  at the first position  910  relative to the radiation source  109 ), when a first image of the marker  60  is captured and matches one of a set of reference images. The first image of the marker  60  is not necessarily captured at the same time as the image of first portions of the scene  50 . In procedure  162 , an image of second portions of the scene  50  is captured (e.g., by the second image sensor  9002  when it is at the second position  920  relative to the radiation source  109 ) with radiation from the radiation source  109  by the second image sensor  9002 , when a second image of the marker  60  is captured matches one of a set of reference images. The second image of the marker  60  is not necessarily captured at the same time as the image of second portion of the scene  50 . In procedure  163 , whether the first image of the marker and the second image of the marker are identical is determined. For example, the rotational position which the radiation source  109  is at when the first image of the marker is captured and the rotational position which the radiation source  109  is at when the second image of the marker is captured are the same. When the first image of the marker and the second image of the marker are identical, the first rotational position and the second rotational position are the same. In procedure  164 , an image of the scene  50  is formed by stitching the image of the first portions and the image of the second portions, upon determining that the first image of the marker and the second image of the marker  60  are identical. 
     The system  9000  described above may be used in various systems such as those provided below. 
       FIG. 13  schematically shows that the system  9000  as described above may be used for medical imaging such as chest radiation radiography, abdominal radiation radiography, etc. Radiation emitted from the radiation source  109  penetrates an object  1202  (e.g., a human body part such as chest, limb, abdomen), is attenuated by different degrees by the internal structures of the object  1202  (e.g., bones, muscle, fat and organs, etc.), and is projected to the second image sensors  9002 . 
       FIG. 14  schematically shows that the system  9000  as described above may be used for medical imaging such as dental radiation radiography. Radiation emitted from the radiation source  109  penetrates an object  1302  that is part of a mammal (e.g., human) mouth. The object  1302  may include a maxilla bone, a palate bone, a tooth, the mandible, or the tongue. The radiation is attenuated by different degrees by the different structures of the object  1302  and is projected to the second image sensors  9002 . Teeth absorb radiation more than dental caries, infections, periodontal ligament. The dosage of radiation received by a dental patient is typically small (around 0.150 mSv for a full mouth series). 
       FIG. 15  schematically shows that the system  9000  as described above may be used for cargo scanning or non-intrusive inspection (NII), e.g., at public transportation facilities. Radiation emitted from the radiation source  109  may penetrate a piece of luggage  1502 , be differently attenuated by the contents of the luggage, and projected to the second image sensor  9002 . The system may reveal contents of luggage and identify items forbidden on public transportation, such as firearms, narcotics, edged weapons, flammables. 
       FIG. 16  schematically shows that the system  9000  as described above may be used as a full-body scanner for detecting metal or non-metal objects on a person&#39;s body for security screening purposes, without physically removing clothes or making physical contact. Radiation emitted from the radiation source  109  may backscatter from a human  1602  being screened and objects thereon and be projected to the second image sensor  9002 . The objects and the human body may backscatter radiation differently. The radiation source  109  may be configured to scan the human in a linear or rotational direction. 
       FIG. 17  schematically shows that the system  9000  as described above may be used for a radiation computed tomography (Radiation CT). Radiation CT uses computer-processed radiations to produce tomographic images (virtual “slices”) of specific areas of a scanned object. The tomographic images may be used for diagnostic and therapeutic purposes in various medical disciplines, or for flaw detection, failure analysis, metrology, assembly analysis and reverse engineering. The radiation source  109  may be configured to rotate synchronously along one or more circular or spiral paths. 
     The system  9000  described here may have other applications such as in a radiation telescope, radiation mammography, industrial radiation defect detection, radiation microscopy or microradiography, radiation casting inspection, radiation non-destructive testing, radiation weld inspection, radiation digital subtraction angiography, etc. It may be suitable to use the system  9000  in place of a photographic plate, a photographic film, a PSP plate, a radiation image intensifier, a scintillator, or another semiconductor radiation detector. 
       FIG. 18A  and  FIG. 18B  each show a component diagram of the electronic system  121 , according to an embodiment. The electronic system  121  may include a first voltage comparator  301 , a second voltage comparator  302 , a counter  320 , a switch  305 , an optional voltmeter  306  and a controller  310 . 
     The first voltage comparator  301  is configured to compare the voltage of at least one of the electric contacts  119 B to a first threshold. The first voltage comparator  301  may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the electrical contact  119 B over a period of time. The first voltage comparator  301  may be controllably activated or deactivated by the controller  310 . The first voltage comparator  301  may be a continuous comparator. Namely, the first voltage comparator  301  may be configured to be activated continuously and monitor the voltage continuously. The first voltage comparator  301  may be a clocked comparator. The first threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the maximum voltage one incident particle of radiation may generate on the electric contact  119 B. The maximum voltage may depend on the energy of the incident particle of radiation, the material of the radiation absorption layer  110 , and other factors. For example, the first threshold may be 50 mV, 100 mV, 150 mV, or 200 mV. 
     The second voltage comparator  302  is configured to compare the voltage to a second threshold. The second voltage comparator  302  may be configured to monitor the voltage directly or calculate the voltage by integrating an electric current flowing through the diode or the electrical contact over a period of time. The second voltage comparator  302  may be a continuous comparator. The second voltage comparator  302  may be controllably activate or deactivated by the controller  310 . When the second voltage comparator  302  is deactivated, the power consumption of the second voltage comparator  302  may be less than 1%, less than 5%, less than 10% or less than 20% of the power consumption when the second voltage comparator  302  is activated. The absolute value of the second threshold is greater than the absolute value of the first threshold. As used herein, the term “absolute value” or “modulus” |x| of a real number x is the non-negative value of x without regard to its sign. Namely, 
     
       
         
           
             
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      The second threshold may be 200%-300% of the first threshold. The second threshold may be at least 50% of the maximum voltage one incident particle of radiation may generate on the electric contact  119 B. For example, the second threshold may be 100 mV, 150 mV, 200 mV, 250 mV or 300 mV. The second voltage comparator  302  and the first voltage comparator  301  may be the same component. Namely, the electronic system  121  may have one voltage comparator that can compare a voltage with two different thresholds at different times. The first voltage comparator  301  or the second voltage comparator  302  may include one or more op-amps or any other suitable circuitry. The first voltage comparator  301  or the second voltage comparator  302  may have a high speed to allow the electronic system  121  to operate under a high flux of incident particles of radiation. However, having a high speed is often at the cost of power consumption. 
     The counter  320  is configured to register at least a number of particles of radiation incident on the pixel  150  encompassing the electric contact  119 B. The counter  320  may be a software component (e.g., a number stored in a computer memory) or a hardware component (e.g., a  4017  IC and a  7490  IC). 
     The controller  310  may be a hardware component such as a microcontroller and a microprocessor. The controller  310  is configured to start a time delay from a time at which the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold (e.g., the absolute value of the voltage increases from below the absolute value of the first threshold to a value equal to or above the absolute value of the first threshold). The absolute value is used here because the voltage may be negative or positive, depending on whether the voltage of the cathode or the anode of the diode or which electrical contact is used. The controller  310  may be configured to keep deactivated the second voltage comparator  302 , the counter  320  and any other circuits the operation of the first voltage comparator  301  does not require, before the time at which the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold. The time delay may expire before or after the voltage becomes stable, i.e., the rate of change of the voltage is substantially zero. The phase “the rate of change of the voltage is substantially zero” means that temporal change of the voltage is less than 0.1%/ns. The phase “the rate of change of the voltage is substantially non-zero” means that temporal change of the voltage is at least 0.1%/ns. 
     The controller  310  may be configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay. In an embodiment, the controller  310  is configured to activate the second voltage comparator at the beginning of the time delay. The term “activate” means causing the component to enter an operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by providing power, etc.). The term “deactivate” means causing the component to enter a non-operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by cut off power, etc.). The operational state may have higher power consumption (e.g., 10 times higher, 100 times higher, 1000 times higher) than the non-operational state. The controller  310  itself may be deactivated until the output of the first voltage comparator  301  activates the controller  310  when the absolute value of the voltage equals or exceeds the absolute value of the first threshold. 
     The controller  310  may be configured to cause the number registered by the counter  320  to increase by one, if, during the time delay, the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold. 
     The controller  310  may be configured to cause the optional voltmeter  306  to measure the voltage upon expiration of the time delay. The controller  310  may be configured to connect the electric contact  119 B to an electrical ground, so as to reset the voltage and discharge any charge carriers accumulated on the electric contact  119 B. In an embodiment, the electric contact  119 B is connected to an electrical ground after the expiration of the time delay. In an embodiment, the electric contact  119 B is connected to an electrical ground for a finite reset time period. The controller  310  may connect the electric contact  119 B to the electrical ground by controlling the switch  305 . The switch may be a transistor such as a field-effect transistor (FET). 
     In an embodiment, the system  121  has no analog filter network (e.g., a RC network). In an embodiment, the system  121  has no analog circuitry. 
     The voltmeter  306  may feed the voltage it measures to the controller  310  as an analog or digital signal. 
     The electronic system  121  may include an integrator  309  electrically connected to the electric contact  119 B, wherein the integrator is configured to collect charge carriers from the electric contact  119 B. The integrator  309  can include a capacitor in the feedback path of an amplifier. The amplifier configured as such is called a capacitive transimpedance amplifier (CTIA). CTIA has high dynamic range by keeping the amplifier from saturating and improves the signal-to-noise ratio by limiting the bandwidth in the signal path. Charge carriers from the electric contact  119 B accumulate on the capacitor over a period of time (“integration period”). After the integration period has expired, the capacitor voltage is sampled and then reset by a reset switch. The integrator  309  can include a capacitor directly connected to the electric contact  119 B. 
       FIG. 19  schematically shows a temporal change of the electric current flowing through the electric contact  119 B (upper curve) caused by charge carriers generated by a particle of radiation incident on the pixel  150  encompassing the electric contact  119 B, and a corresponding temporal change of the voltage of the electric contact  119 B (lower curve). The voltage may be an integral of the electric current with respect to time. At time to, the particle of radiation hits pixel  150 , charge carriers start being generated in the pixel  150 , electric current starts to flow through the electric contact  119 B, and the absolute value of the voltage of the electric contact  119 B starts to increase. At time t 1 , the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V 1 , and the controller  310  starts the time delay TD 1  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 1 . If the controller  310  is deactivated before t 1 , the controller  310  is activated at t 1 . During TD 1 , the controller  310  activates the second voltage comparator  302 . The term “during” a time delay as used here means the beginning and the expiration (i.e., the end) and any time in between. For example, the controller  310  may activate the second voltage comparator  302  at the expiration of TD 1 . If during TD 1 , the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold V 2  at time t 2 , the controller  310  waits for stabilization of the voltage to stabilize. The voltage stabilizes at time t e , when all charge carriers generated by the particle of radiation drift out of the radiation absorption layer  110 . At time t s , the time delay TD 1  expires. At or after time t e , the controller  310  causes the voltmeter  306  to digitize the voltage and determines which bin the energy of the particle of radiation falls in. The controller  310  then causes the number registered by the counter  320  corresponding to the bin to increase by one. In the example of  FIG. 9 , time t s  is after time t e ; namely TD 1  expires after all charge carriers generated by the particle of radiation drift out of the radiation absorption layer  110 . If time t e  cannot be easily measured, TD 1  can be empirically chosen to allow sufficient time to collect essentially all charge carriers generated by a particle of radiation but not too long to risk have another incident particle of radiation. Namely, TD 1  can be empirically chosen so that time t s  is empirically after time t e . Time t s  is not necessarily after time t e  because the controller  310  may disregard TD 1  once V 2  is reached and wait for time t e . The rate of change of the difference between the voltage and the contribution to the voltage by the dark current is thus substantially zero at t e . The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 1  or at t 2 , or any time in between. 
     The voltage at time t e  is proportional to the amount of charge carriers generated by the particle of radiation, which relates to the energy of the particle of radiation. The controller  310  may be configured to determine the energy of the particle of radiation, using the voltmeter  306 . 
     After TD 1  expires or digitization by the voltmeter  306 , whichever later, the controller  310  connects the electric contact  119 B to an electric ground for a reset period RST to allow charge carriers accumulated on the electric contact  119 B to flow to the ground and reset the voltage. After RST, the electronic system  121  is ready to detect another incident particle of radiation. If the first voltage comparator  301  has been deactivated, the controller  310  can activate it at any time before RST expires. If the controller  310  has been deactivated, it may be activated before RST expires. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.