Patent Publication Number: US-2021172887-A1

Title: Imaging method

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 method comprising: while an image sensor is at a first position relative to a radiation source, capturing a first set of images of portions of a scene respectively when the image sensor and the radiation source are collectively rotated relative to the scene about a first axis to a plurality of rotational positions; while the image sensor is at a second position relative to the radiation source, capturing a second set of images of portions of the scene respectively when the image sensor and the radiation source are collectively rotated relative to the scene about the first axis to the plurality of rotational positions; and forming an image of the scene by stitching an image of the first set and an image of the second set. 
     According to an embodiment, the method further comprises moving the image sensor from the first position relative to the radiation source to the second position relative to the radiation source by translating or rotating the image sensor relative to the radiation source. 
     According to an embodiment, the first axis is near or on a radiation-receiving surface of the image sensor. 
     According to an embodiment, the image sensor is 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 a radiation-receiving surface of the image sensor. 
     According to an embodiment, the image sensor is 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 image sensor is configured to move relative to the radiation source by rotating about a second axis. 
     According to an embodiment, the image sensor is configured to move relative to the radiation source by rotating about a third axis; wherein the third axis is different from the second axis. 
     According to an embodiment, the 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 from the radiation source. 
     According to an embodiment, the planar surface of the first radiation detector and the planar surface of the second radiation detector are not parallel. 
     According to an embodiment, the first axis is near or on the planar surface of the first radiation detector. 
     According to an embodiment, a relative position of the first radiation detector with respect to the second radiation detector remains the same. 
     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 the 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 second axis, wherein the radiation source is on the second axis. 
     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 third axis; wherein the third axis is different from the second axis. 
     According to an embodiment, the first radiation detector and the second radiation detector each comprise an array of pixels. 
     According to an embodiment, the first radiation detector is rectangular in shape. 
     According to an embodiment, the first radiation detector is hexagonal in shape. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1A - FIG. 1H  schematically show a method of imaging a scene, according to an embodiment. 
         FIG. 2A  schematically shows a portion of an image sensor, according to an embodiment. 
         FIG. 2B  schematically shows another view of the image sensor of  FIG. 2A . 
         FIG. 3A  schematically shows a cross-sectional view of a radiation detector, according to an embodiment. 
         FIG. 3B  schematically shows a detailed cross-sectional view of the radiation detector, according to an embodiment. 
         FIG. 3C  schematically shows an alternative detailed cross-sectional view of the radiation detector, according to an embodiment. 
         FIG. 4  schematically shows that the radiation detector may have an array of pixels, according to an embodiment. 
         FIG. 5  schematically shows a functional block diagram of the image sensor, according to an embodiment. 
         FIG. 6  schematically shows the image sensor capturing images of portions of a scene, according to an embodiment. 
         FIG. 7A-7C  schematically show arrangements of the radiation detectors in the image sensor, according to some embodiments. 
         FIG. 8  schematically shows an image sensor with plurality of radiation detectors that are hexagonal in shape, according to an embodiment. 
         FIG. 9  schematically shows a system comprising the image sensor described herein, suitable for medical imaging such as chest Radiation radiography, abdominal Radiation radiography, etc., according to an embodiment 
         FIG. 10  schematically shows a system comprising the image sensor described herein suitable for dental Radiation radiography, according to an embodiment. 
         FIG. 11  schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the image sensor described herein, according to an embodiment. 
         FIG. 12  schematically shows a full-body scanner system comprising the image sensor described herein, according to an embodiment. 
         FIG. 13  schematically shows a radiation computed tomography (Radiation CT) system comprising the image sensor described herein, according to an embodiment. 
         FIG. 14A  and  FIG. 14B  each show a component diagram of an electronic system of the radiation detector in  FIG. 3A ,  FIG. 3B  and  FIG. 3C , according to an embodiment. 
         FIG. 15  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 - FIG. 1H  schematically show a method of imaging a scene  50 , according to an embodiment. A plurality of sets of images of portions of the scene  50  may be captured when an image sensor  9000  and a radiation source  109  are collectively rotated to a plurality of rotational positions about a first axis  501  relative to the scene  50 . 
       FIG. 1A  and  FIG. 1B  each schematically show that the image sensor  9000  and the radiation source  109  are collectively at two different rotational positions relative to the scene  50 , and that the image sensor  9000  is at the first position (e.g.,  910  in  FIG. 1C ) relative to the radiation source  109 . The first axis  501  is near or on a radiation-receiving surface of the image sensor  9000 .  FIG. 1A  schematically shows the radiation source  109  and the image sensor  9000  at the first rotational position  510 .  FIG. 1B  schematically shows that the radiation source  109  and the image sensor  9000  are collectively rotated to a second rotational position  511  about the first axis  501  relative to the scene  50 , from the first rotational position  510 . The image sensor  9000  may remain at the first position relative to the radiation source  109  during this collective rotation. The first axis  501  may be stationary relative to the scene  50 . At the first rotational position  510  and the second rotational position  511 , the radiation from the radiation source  109  may pass through different portions of the scene  50 . A first set of images of portions of the scene  50  are captured respectively when the radiation source  109  and the image sensor  9000  are collectively rotated to a plurality of rotational positions about the first axis  501  relative to the scene  50 , while the image sensor  9000  is at the first position relative to the radiation source  109 . For example, the first set of images may include an image the image sensor  9000  captured at the first rotational position  510  shown in  FIG. 1A  or an image the image sensor  9000  captured at the second rotational position  511  shown in  FIG. 1B . 
     The image sensor  9000  may be moved from the first position relative to the radiation source  109  to a second position relative to the radiation source  109 .  FIG. 1C  schematically shows that image sensor  9000  may move relative to the radiation source  109  by translating relative to the radiation source  109 , according to an embodiment. In the example shown in  FIG. 1C , the image sensor  9000  may move from the 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  504  may be parallel to a radiation-receiving surface of the image sensor  9000 . 
       FIG. 1C  also shows that the image sensor  9000  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 . 
       FIG. 1D  and  FIG. 1E  each schematically show that the image sensor  9000  and the radiation source  109  are collectively at two different rotational positions relative to the scene  50  after the image sensor  9000  has moved to the second position (e.g.,  920  in  FIG. 1C ) relative to the radiation source  109  by translating relative to the radiation source  109 .  FIG. 1D  schematically shows the radiation source  109  and the image sensor  9000  at the first rotational position  510 .  FIG. 1E  schematically shows that the radiation source  109  and the image sensor  9000  are collectively rotated to the second rotational position  511  about the first axis  501  relative to the scene  50 , from the first rotational position  510 . The image sensor  9000  may remain at the second position relative to the radiation source  109  during this collective rotation. A second set of images of portions of the scene  50  are captured respectively when the radiation source  109  and the image sensor  9000  are collectively rotated to a plurality of rotational positions about the first axis  501  relative to the scene  50 , while the image sensor  9000  is at the second position relative to the radiation source  109 . For example, the second set of images may include an image the image sensor  9000  captured at the first rotational position  510  shown in  FIG. 1D  or an image the image sensor  9000  captured at the second rotational position  511  shown in  FIG. 1E . 
       FIG. 1F  schematically shows that the image sensor  9000  may move relative to the radiation source  109  by rotating relative to the radiation source  109 , according to an embodiment. In the example shown in  FIG. 1F , the image sensor  9000  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 second axis  902  relative to the radiation source  109 . The second axis  902  may be parallel to a radiation-receiving surface of the image sensor  9000 . The radiation source  109  may be on the second axis  902 . 
       FIG. 1F  also shows that the image sensor  9000  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 third axis  903  relative to the radiation source  109 . The third axis  903  is different from the second axis  902 . For example, the third axis  903  may be perpendicular to the second axis  902 . The radiation source  109  may be on the third axis  903 . 
       FIG. 1G  and  FIG. 1H  each schematically show that the image sensor  9000  and the radiation source  109  are collectively at two different rotational positions relative to the scene  50  after the image sensor  9000  has moved to the fourth position (e.g.,  940  in  FIG. 1F ) relative to the radiation source  109  by rotating relative to the radiation source  109 .  FIG. 1G  schematically shows the radiation source  109  and the image sensor  9000  at the first rotational position  510 .  FIG. 1H  schematically shows that the radiation source  109  and the image sensor  9000  are collectively rotated to the second rotational position  511  about the first axis  501  relative to the scene  50 , from the first rotational position  510 . The image sensor  9000  may remain at the fourth position relative to the radiation source  109  during this collective rotation. A second set of images of portions of the scene  50  are captured respectively when the radiation source  109  and the image sensor  9000  are collectively rotated to a plurality of rotational positions about the first axis  501  relative to the scene  50 , while the image sensor  9000  is at the fourth position relative to the radiation source  109 . For example, the second set of images may include an image the image sensor  9000  captured at the first rotational position  510  shown in  FIG. 1G  or an image the image sensor  9000  captured at the second rotational position  511  shown in  FIG. 1H . 
       FIG. 2A  schematically shows that the image sensor  9000  may have a plurality of radiation detectors (e.g., a first radiation detector  100 A, a second radiation detector  100 B). The image sensor  9000  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. 2A . The first radiation detector  100 A may have a first planar surface  103 A configured to receive radiation from a 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. 2B  schematically shows a perspective view of the image sensor  9000  depicted in  FIG. 2A , with respect to the scene  50  and the radiation source  109 . 
     The first axis  501  may be parallel to 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. The first axis  501  may be near or on the planar surface of the first radiation detector  100 A. A relative position of the first radiation detector  100 A with respect to the second radiation detector  100 B may remain unchanged when the image sensor  9000  moves relative to the radiation source  109  and when the image sensor  9000  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 image sensor  9000 . Therefore, the first radiation detector  100 A and the second radiation detector  100 B may move relative to the radiation source  109  with the image sensor  9000  by translating along the first direction  904  or the second direction  905  relative to the radiation source  109  or by rotating about the second axis  902  or the third 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. 3A  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 sensor  9000 , for example as the first radiation detector  100 A or the second radiation detector  1008 . 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. 3B , 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. 2B , 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. 2B , 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. 3C , 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. 4  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. 
     In an embodiment, the radiation detectors  100  (e.g.,  100 A and  100 B) of the image sensor  9000  can move to multiple positions, relative to the radiation source  109 . The image sensor  9000  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 image sensor  9000  can stitch these images to form an image of the entire scene  50 . As shown in  FIG. 5 , according to an embodiment, the image sensor  9000  may include an actuator  500  configured to move the radiation detectors  100  to the multiple positions. The actuator  500  may include a controller  600 . The image sensor may include a collimator  200  that only allows radiation to reach active area of the radiation detectors  100 . Active areas of the radiation detectors  100  are areas of the radiation detectors  100  that are sensitive to the radiation. The actuator  500  may move the collimator  200  together with the radiation detectors  100 . The positions may be determined by the controller  600 . 
       FIG. 6  schematically shows capturing images of portions of the scene  50  by the image sensor  9000 . In the example shown in  FIG. 6 , the radiation detectors  100  move to three positions relative to the radiation source  109 , for example, the first position  510 , the second position  520 , for example, by using the actuator  500 . Respectively at the positions  510 ,  520 , the image sensor  9000  captures a first set of image  51 A, a second set of images  51 B of portions of the scene  50  when the image sensor  9000  and the radiation source  109  are collectively rotated relative to the scene  50  about a first axis  501  to a plurality of rotational positions (e.g.,  511 ,  521 ). The image sensor  9000  can stitch the image of the first set  51 A and the image of the second set  51 B of the portions to form an image of the scene  50 . The images  51 A,  51 B of the portions may have overlap among one another to facilitate stitching. Every portion of the scene  50  may be in at least one of the images captured when the detectors are at the multiple positions. Namely, the images of the portions when stitched together may cover the entire scene  50 . 
     The radiation detectors  100  may be arranged in a variety of ways in the image sensor  9000 .  FIG. 7A  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. This arrangement allows imaging of the scene as shown in  FIG. 6 , and an image of the scene may be obtained from stitching three images of portions of the scene captured at three positions spaced apart in the X direction. 
       FIG. 7B  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. 7A , without radiation detectors  100 C,  100 D,  100 G, or  100 H in  FIG. 8A . 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. 7C , the radiation detectors  100  may span the whole width of the image sensor  9000  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. 7 ), at least some of the radiation detectors are rectangular in shape. According to an embodiment, as shown in  FIG. 8 , at least some of the radiation detectors are hexagonal in shape. 
     The image sensor  9000  described above may be used in various systems such as those provided below. 
       FIG. 9  schematically shows a system comprising the image sensor  9000  as described in relation to  FIG. 1 - FIG. 8 . The system may be used for medical imaging such as chest radiation radiography, abdominal radiation radiography, etc. The system comprises a radiation source  1201 . Radiation emitted from the Radiation source  1201  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 image sensor  9000 . The image sensor  9000  forms an image by detecting the intensity distribution of the radiation. 
       FIG. 10  schematically shows a system comprising the image sensor  9000  as described in relation to  FIG. 1 - FIG. 8 . The system may be used for medical imaging such as dental Radiation radiography. The system comprises a radiation source  1301 . Radiation emitted from the Radiation source  1301  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 image sensor  9000 . The image sensor  9000  forms an image by detecting the intensity distribution of the Radiation. 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. 11  schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the image sensor  9000  as described in relation to  FIG. 1 - FIG. 8 . The system may be used for luggage screening at public transportation stations and airports. The system comprises a radiation source  1501 . Radiation emitted from the radiation source  1501  may penetrate a piece of luggage  1502 , be differently attenuated by the contents of the luggage, and projected to the image sensor  9000 . The image sensor  9000  forms an image by detecting the intensity distribution of the transmitted radiation. The system may reveal contents of luggage and identify items forbidden on public transportation, such as firearms, narcotics, edged weapons, flammables. 
       FIG. 12  schematically shows a full-body scanner system comprising the image sensor  9000  as described in relation to  FIG. 1 - FIG. 8 . The full-body scanner system may detect objects on a person&#39;s body for security screening purposes, without physically removing clothes or making physical contact. The full-body scanner system may be able to detect non-metal objects. The full-body scanner system comprises a radiation source  1601 . Radiation emitted from the radiation source  1601  may backscatter from a human  1602  being screened and objects thereon, and be projected to the image sensor  9000 . The objects and the human body may backscatter Radiation differently. The image sensor  9000  forms an image by detecting the intensity distribution of the backscattered radiation. The image sensor  9000  and the radiation source  1601  may be configured to scan the human in a linear or rotational direction. 
       FIG. 13  schematically shows a radiation computed tomography (Radiation CT) system. The Radiation CT system 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 CT system comprises the image sensor  9000  as described in relation to  FIG. 1 - FIG. 8  and a radiation source  1701 . The image sensor  9000  and the radiation source  1701  may be configured to rotate synchronously along one or more circular or spiral paths. 
     The image sensor  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 image sensor  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. 14A  and  FIG. 14B  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, 
     
       
         
           
             
                
               x 
                
             
             = 
             
               { 
               
                 
                   
                     
                       x 
                       , 
                       
                         
                           if 
                            
                           
                               
                           
                            
                           x 
                         
                         ≥ 
                         0 
                       
                     
                   
                 
                 
                   
                     
                       
                         - 
                         x 
                       
                       , 
                       
                         
                           if 
                            
                           
                               
                           
                            
                           x 
                         
                         ≤ 
                         0. 
                       
                     
                   
                 
               
             
           
         
       
     
     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  310  may be the same component. Namely, the 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 at least one of 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. 15  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.