Patent Publication Number: US-2021169430-A1

Title: Apparatus and method for imaging an object using radiation

Description:
BACKGROUND 
     Image sensors based on radiation detectors may be devices used to measure the flux, spatial distribution, spectrum or other properties of radiation such as X-rays. These image sensors may be used for many applications. One important application is medical imaging in which the internal structure of a non-uniformly composed and opaque object such as the human body may be revealed. 
     SUMMARY 
     Disclosed herein is an apparatus comprising: a radiation source configured to produce a beam of radiation toward an object; an image sensor comprising a plurality of radiation detectors spaced apart from one another. The image sensor is configured to move between a first position and a second position, relative to the object. The radiation source is configured to move along a path relative to the object. 
     According to an embodiment, the image sensor is configured to capture a first set of images of the object, by using the radiation detectors and with the beam of radiation, while the image sensor is at the first position and the radiation source is respectively at a first plurality of positions on the path. The image sensor is configured to capture a second set of images of the object, by using the radiation detectors and with the beam of radiation, while the image sensor is at the second position and the radiation source is respectively at a second plurality of positions on the path. 
     According to an embodiment, the first plurality of positions on the path and the second plurality of positions on the path are the same. 
     According to an embodiment, the apparatus further comprises a processor configured to stitch at least one image in the first set and at least one image in the second set. 
     According to an embodiment, the apparatus further comprises a processor configured to determine a three-dimensional structure of the object based on the first set of images or the second set of images. 
     According to an embodiment, the object is a breast of a human. 
     According to an embodiment, the radiation source is configured to rotate with respect to the object, while the radiation source moves along the path. 
     According to an embodiment, the path is an arc around the object. 
     According to an embodiment, the image sensor comprises a collimator with a plurality of radiation transmitting zones and a radiation blocking zone. The radiation blocking zone is configured to block radiation that would otherwise incident on a dead zone of the image sensor, and the radiation transmitting zones are configured to transmit at least a portion of radiation that would incident on active areas of the image sensor. 
     According to an embodiment, at least some of the plurality of radiation detectors are arranged in staggered rows. 
     According to an embodiment, wherein radiation detectors in a same row are uniform in size; wherein a distance between two neighboring radiation detectors in a same row is greater than a width of one radiation detector in the same row in an extending direction of the row and is less than twice that width. 
     According to an embodiment, at least some of the plurality of radiation detectors are rectangular in shape. 
     According to an embodiment, at least some of the plurality of radiation detectors are hexagonal in shape. 
     According to an embodiment, the beam of radiation is a divergent beam of radiation. 
     According to an embodiment, the radiation is X-ray. 
     According to an embodiment, at least one of the plurality of radiation detectors comprises a radiation absorption layer and an electronics layer. The radiation absorption layer comprises an electrode. The electronics layer comprises an electronic system. The electronic system comprises: a first voltage comparator configured to compare a voltage of the electrode to a first threshold, a second voltage comparator configured to compare the voltage to a second threshold, a counter configured to register a number of particles of radiation reaching the radiation absorption layer, and a controller. The controller is configured to start a time delay from a time at which the first voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the first threshold. The controller is configured to activate the second voltage comparator during the time delay. The controller is configured to cause the number registered by the counter to increase by one, if the second voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the second threshold. 
     According to an embodiment, the electronic system further comprises an integrator electrically connected to the electrode, wherein the integrator is configured to collect charge carriers from the electrode. 
     According to an embodiment, the controller is configured to activate the second voltage comparator at a beginning or expiration of the time delay. 
     According to an embodiment, the electronic system further comprises a voltmeter, wherein the controller is configured to cause the voltmeter to measure the voltage upon expiration of the time delay. 
     According to an embodiment, the controller is configured to determine an energy of particles of radiation based on a value of the voltage measured upon expiration of the time delay. 
     According to an embodiment, the controller is configured to connect the electrode to an electrical ground. 
     According to an embodiment, a rate of change of the voltage is substantially zero at expiration of the time delay. 
     According to an embodiment, a rate of change of the voltage is substantially non-zero at expiration of the time delay. 
     Disclosed herein is a method comprising: positioning an image sensor at a first position relative to an object, the image sensor comprising a plurality of radiation detectors spaced apart from one another; capturing a first set of images of the object, by using the radiation detectors and with a beam of radiation from a radiation source, while moving the radiation source among a first plurality of positions on a path, relative to the object; positioning the image sensor at a second position relative to the object; capturing a second set of images of the object, by using the radiation detectors and with the beam of radiation, while moving the radiation source among a second plurality of positions on the path, relative to the object. 
     According to an embodiment, the first plurality of positions on the path and the second plurality of positions on the path are the same. 
     According to an embodiment, the method further comprises stitching at least one image in the first set and at least one image in the second set. 
     According to an embodiment, the method further comprises determining a three-dimensional structure of the object based on the first set of images or the second set of images. 
     According to an embodiment, the object is a breast of a human. 
     According to an embodiment, moving the radiation source comprises rotating the radiation source with respect to the object. 
     According to an embodiment, the path is an arc around the object. 
     According to an embodiment, the image sensor comprises a collimator with a plurality of radiation transmitting zones and a radiation blocking zone. The radiation blocking zone is configured to block radiation that would otherwise incident on a dead zone of the image sensor, and the radiation transmitting zones are configured to transmit at least a portion of radiation that would incident on active areas of the image sensor. 
     According to an embodiment, at least some of the plurality of radiation detectors are arranged in staggered rows. 
     According to an embodiment, radiation detectors in a same row are uniform in size; wherein a distance between two neighboring radiation detectors in a same row is greater than a width of one radiation detector in the same row in an extending direction of the row and is less than twice that width. 
     According to an embodiment, at least some of the plurality of radiation detectors are rectangular in shape. 
     According to an embodiment, at least some of the plurality of radiation detectors are hexagonal in shape. 
     According to an embodiment, the beam of radiation is a divergent beam of radiation. 
     According to an embodiment, the radiation is X-ray. 
     According to an embodiment, at least one of the plurality of radiation detectors comprises a radiation absorption layer and an electronics layer. The radiation absorption layer comprises an electrode. The electronics layer comprises an electronic system. The electronic system comprises: a first voltage comparator configured to compare a voltage of the electrode to a first threshold, a second voltage comparator configured to compare the voltage to a second threshold, a counter configured to register a number of particles of radiation reaching the radiation absorption layer, and a controller. The controller is configured to start a time delay from a time at which the first voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the first threshold. The controller is configured to activate the second voltage comparator during the time delay. The controller is configured to cause the number registered by the counter to increase by one, if the second voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the second threshold. 
     According to an embodiment, the electronic system further comprises an integrator electrically connected to the electrode, wherein the integrator is configured to collect charge carriers from the electrode. 
     According to an embodiment, the controller is configured to activate the second voltage comparator at a beginning or expiration of the time delay. 
     According to an embodiment, the electronic system further comprises a voltmeter, wherein the controller is configured to cause the voltmeter to measure the voltage upon expiration of the time delay. 
     According to an embodiment, the controller is configured to determine an energy of particles of radiation based on a value of the voltage measured upon expiration of the time delay. 
     According to an embodiment, the controller is configured to connect the electrode to an electrical ground. 
     According to an embodiment, a rate of change of the voltage is substantially zero at expiration of the time delay. 
     According to an embodiment, a rate of change of the voltage is substantially non-zero at expiration of the time delay. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  schematically shows an apparatus, according to an embodiment. 
         FIG. 2A  schematically shows a cross-sectional view of a radiation detector of an image sensor of the apparatus, according to an embodiment. 
         FIG. 2B  schematically shows a detailed cross-sectional view of the radiation detector, according to an embodiment. 
         FIG. 2C  schematically shows an alternative detailed cross-sectional view of the radiation detector, according to an embodiment. 
         FIG. 3  schematically shows that the radiation detector may have an array of pixels, according to an embodiment. 
         FIG. 4A  schematically shows a top view of a package including the radiation detector and a printed circuit board (PCB). 
         FIG. 4B  schematically shows a cross-sectional view of the image sensor, where a plurality of the packages of  FIG. 4A  are mounted to another PCB. 
         FIG. 5  schematically shows a collimator of the image sensor, according to an embodiment. 
         FIG. 6  schematically shows that the image sensor may capture multiple sets of images when it is respectively at multiple positions relative to the object. 
         FIG. 7A  schematically shows the image of an object can be formed by stitching images of multiple different portions of an object, according to an embodiment. 
         FIG. 7B  schematically shows the image of an object can be formed by stitching images of multiple different portions of an object, according to an embodiment. 
         FIG. 8A - FIG. 8C  schematically show arrangements of the detectors in the image sensor, according to some embodiments. 
         FIG. 9  schematically shows detectors that are hexagonal in shape, according to an embodiment. 
         FIG. 10  schematically shows the flowchart of a method, according to an embodiment. 
         FIG. 11A  and  FIG. 11B  each show a component diagram of an electronic system of the detector in  FIG. 2A ,  FIG. 2B  and  FIG. 2C , according to an embodiment. 
         FIG. 12  schematically shows a temporal change of the electric current flowing through an electrode (upper curve) of a diode or an electrical contact of a resistor of a radiation absorption layer exposed to radiation, the electric current caused by charge carriers generated by a particle of radiation incident on the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode (lower curve), according to an embodiment. 
         FIG. 13  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current), and a corresponding temporal change of the voltage of the electrode (lower curve), in the electronic system operating in the way shown in  FIG. 12 , according to an embodiment. 
         FIG. 14  schematically shows a temporal change of the electric current flowing through an electrode (upper curve) of the radiation absorption layer exposed to radiation, the electric current caused by charge carriers generated by a particle of radiation incident on the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode (lower curve), when the electronic system operates to detect incident particles of radiation at a higher rate, according to an embodiment. 
         FIG. 15  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current), and a corresponding temporal change of the voltage of the electrode (lower curve), in the electronic system operating in the way shown in  FIG. 14 , according to an embodiment. 
         FIG. 16  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a series of particles of radiation incident on the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode, in the electronic system operating in the way shown in  FIG. 14  with RST expires before t e , according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows an apparatus  10 , according to an embodiment. The apparatus  10  comprises a radiation source  12  and an image sensor  9000 . The radiation source  12  produces a beam  13  of radiation toward an object  50 . The beam  13  may be a divergent beam, a convergent beam, a parallel beam or another suitable beam. The radiation from the radiation source  12  may be X-ray or another suitable radiation such as gamma ray. The object  50  may be a human organ or tissue. For example, the object  50  may be a breast of a human. The image sensor  9000  has a plurality of radiation detectors  100  spaced apart from one another. Here, two radiation detectors  100  being “spaced apart” means that a portion of the dead zone of the image sensor  9000  is between the two radiation detectors. The term “dead zone” is explained below. The image sensor  9000  is able to move between multiple positions (e.g., between a first position  14 A and a second position  14 B, or the first position  14 A, the second position  14 B and a third position  14 C) relative to the object  50 . The multiple positions (e.g., the first position  14 A, the second position  14 B and the third position  14 C) may or may not be on the same straight line. Namely, when the image sensor  9000  move between the multiple positions, the image sensor  9000  may move along different directions at different time. The radiation source  12  is able to move along a path  15  relative to the object  50 . The radiation source  12  may rotate with respect to the object  50  while it moves along the path  15 . The path  15  may be an arc around the object  50 . The center of the arc may be on the object  50 , between the object  50  and the image sensor  9000 , on the image sensor  9000 , or on an opposite side of the image sensor  9000  with respect to the object  50 . 
       FIG. 2A  schematically shows a cross-sectional view of one radiation detector  100  of the image sensor  9000 , according to an embodiment. 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 produced by the radiation sources in the apparatus  10 . 
     As shown in a detailed cross-sectional view of the radiation detector  100  in  FIG. 2B , 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 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 particle of radiation hits the radiation absorption layer  110  including diodes, the particle of radiation may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of radiation 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. Electrical contact  119 A is electrically connected with the first doped region  111 . Electrical contact  119 B may include discrete portions each of which is electrically connected with the discrete regions  114 . In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of radiation 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 particle of radiation 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 particle of radiation incident therein 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. 2C , 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 produced by the radiation source  12 . 
     When a particle of radiation 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 particle of radiation may generate  10  to  100000  charge carriers. The charge carriers may drift to the electrical contacts  119 A and  119 B under an electric field. The field may be an external electric field. The electrical 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 particle of radiation are not substantially shared by two different discrete portions of the electrical 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 particle of radiation incident around the footprint of one of these discrete portions of the electrical contact  119 B are not substantially shared with another of these discrete portions of the electrical contact  119 B. A pixel  150  associated with a discrete portion of the electrical 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 particle of radiation incident therein flow to the discrete portion of the electrical 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 electrical contact  119 B. 
     The electronics layer  120  may include an electronic system  121  suitable for processing or interpreting signals generated by particles of radiation 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 circuit such as a microprocessor and a 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. 3  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 particle of radiation incident thereon, measure the energy of the particle of radiation, or both. For example, each pixel  150  may be configured to count numbers of particles of radiation 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 particles of radiation 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 particle of radiation 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 particle of radiation incident thereon. Each pixel  150  may be configured to deduct the contribution of the dark current from the energy of the particle of radiation incident thereon. The pixels  150  may be configured to operate in parallel. For example, when one pixel  150  measures an incident particle of radiation, another pixel  150  may be waiting for a particle of radiation to arrive. The pixels  150  may be but do not have to be individually addressable. 
     The radiation detectors  100  of the image sensors  9000  may be arranged in any suitable fashion.  FIG. 4A  and  FIG. 4B  show an example of the arrangement of the radiation detectors  100  in the image sensor  9000 . One or more of the radiation detectors  100  may be mounted on a printed circuit board (PCB)  400 . The term “PCB” as used herein is not limited to a particular material. For example, a PCB may include a semiconductor. The radiation detector  100  is mounted to the PCB  400 . The wiring between the radiation detectors  100  and the PCB  400  is not shown for the sake of clarity. The PCB  400  and the radiation detectors  100  mounted thereon may be called a package  200 . The PCB  400  may have an area not covered by the radiation detectors  100  (e.g., an area for accommodating bonding wires  410 ). Each of the radiation detector  100  may have an active area  190 , which is where the pixels  150  are located. Each of the radiation detector  100  may have a perimeter zone  195  near the edges. The perimeter zone  195  has no pixels and particles of radiation incident on the perimeter zone  195  are not detected. 
       FIG. 4B  schematically shows that the image sensor  9000  may have a system PCB  450  with multiple packages  200  mounted on it. The image sensor  9000  may include one or more such system PCBs  450 . The electrical connection between the PCBs  400  in the packages  200  and the system PCB  450  may be made by bonding wires  410 . In order to accommodate the bonding wires  410  on the PCB  400 , the PCB  400  has an area  405  not covered by the radiation detectors  100 . In order to accommodate the bonding wires  410  on the system PCB  450 , the packages  200  have gaps in between. The active areas  190  of the radiation detectors  100  in the image sensor  9000  are collectively called the active area of the image sensor  9000 . The other areas of the image sensor  9000 , radiation incident on which cannot be detected by the image sensor  9000 , such as the perimeter zones  195 , the area  405  or the gaps between the packages  200 , are collectively called the dead zone of the image sensor  9000 . 
       FIG. 5  schematically shows that the image sensor  9000  may comprise a collimator  2000 , according to an embodiment. The collimator  2000  comprises a plurality of radiation transmitting zones  2002  and a radiation blocking zone  2004 . The radiation blocking zone  2004  substantially blocks radiation that would otherwise incident on the dead zone  9004  of the image sensor  9000 , and the radiation transmitting zones  2002  allow at least a portion of radiation that would incident on the active areas  9002  of the image sensor  9000  to pass. The radiation transmitting zones  2002  may be holes through the collimator  2000  and the rest of the collimator  2000  may function as the radiation blocking zone  2004 . The material for the collimator  2000 , for example, may be lead or other suitable material which can efficiently absorb the radiation produced by the radiation source  12 . 
     As schematically shown in  FIG. 6 , the image sensor  9000  may capture a first set  19 A of images of the object  50 , by using the radiation detectors  100  and with the radiation, while the image sensor  9000  is at the first position  14 A and the radiation source  12  is respectively at a first plurality of positions on the path  15 . For example, the radiation source  12  may scan continuously along the path  15  and the image sensor  9000  may capture images at multiple points of time during the scan of the radiation source  12 . The image sensor  9000  may capture a second set  19 B of images of the object  50 , by using the radiation detectors  100  and with the radiation, while the image sensor  9000  is at the second position  14 B and the radiation source  12  is respectively at a second plurality of positions on the path  15 . If the image sensor  9000  can move to additional positions (e.g., the third position  14 C), the image sensor  9000  may capture additional sets of images (e.g., the third set of images  19 C) of the object  50 , by using the radiation detectors  100  and with the radiation, while the image sensor  9000  is respectively at the additional positions and the radiation source  12  is respectively at a plurality of positions on the path  15 . The first plurality of positions on the path  15  and the second plurality of positions on the path  15  may be the same. For example, the first set  19 A and the second set  19 B may each contain an image that was captured when the radiation source  12  is at the same position along the path  15  relative to the object  50 . 
     As shown in  FIG. 1 , the apparatus  10  may have a processor  8000  that can stitch at least one image in the first set  19 A and at least one image in the second set  19 B. For example, the at least one image in the first set  19 A and the at least one image in the second set  19 B may be captured when the radiation source  12  is at the same position along the path  15  relative to the object  50 . The processor  8000  may be able to determine a three-dimensional structure of the object  50  based on the first set  19 A of images, the second set  19 B of images, or both. A suitable algorithm such as the Fourier-Domain Reconstruction Algorithm, the Back Projection Algorithm, the Iterative Reconstruction Algorithm, and the Fan-Beam Reconstruction may be used to calculate the three-dimensional structure of the object  50 . 
     In an example schematically shown in  FIG. 7A , image  51 A is one in the first set  19 A of images; image  51 B is one in the second set  19 B of images. The images  51 A and  51 B may be captured when the radiation source  12  was at the same position along the path  15 . An image of the object  50  may be formed by stitching the images  51 A and  51 B (e.g., using the processor  8000 ). 
     In an example schematically shown in  FIG. 7B , image  52 A is one in the first set  19 A of images; image  52 B is one in the second set  19 B of images; and image  52 C is one in the third set of images  19 C. The images  52 A,  52 B and  52 C may be captured when the radiation source  12  was at the same position along the path  15 . An image of the object  50  may be formed by stitching the images  52 A,  52 B and  52 C (e.g., using the processor  8000 ). 
     The radiation detectors  100  may be arranged in a variety of ways in the image sensor  9000 .  FIG. 8A  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 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 detector in the same column. 
       FIG. 8B  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. 8A , without radiation detectors  100 C,  100 D,  100 G, or  100 H in  FIG. 8A . 
     Other arrangements may also be possible. For example, in  FIG. 8C , 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 detector Y 1 . 
     The radiation detectors  100  in the image sensor  9000  have any suitable sizes and shapes. According to an embodiment (e.g., in  FIG. 8A - FIG. 8C ), at least some of the radiation detectors  100  are rectangular in shape. According to an embodiment, as shown in  FIG. 9 , at least some of the radiation detectors are hexagonal in shape. 
       FIG. 10  schematically shows a flowchart of a method, according to an embodiment. In procedure  1010 , the image sensor  9000  is positioned at the first position  14 A relative to the object  50 . In procedure  1020 , the first set  19 A of images of the object  50  is captured, by using the radiation detectors  100  of the image sensor  9000  and with the beam  13  of radiation from the radiation source  12 , while moving the radiation source  12  among the first plurality of the positions on the path  15 , relative to the object  50 . In procedure  1030 , the image sensor  9000  is positioned at the second position relative to the object  50 . In procedure  1040 , the second set  19 B of images of the object  50  is captured, by using the radiation detectors  100  of the image sensor  9000  and with the beam  13  of radiation from the radiation source  12 , while moving the radiation source  12  among the second plurality of positions on the path  15 , relative to the object  50 . Here, capturing the image “while” the radiation source  12  moves along the path  15  does not imply that the radiation source  12  is in motion relative to the object  50  when the image sensor  9000  captures an image. Instead, the radiation source  12  may be still relative to the object  50  when the image sensor  9000  captures an image, then move to the next position on the path  15 , and then remain still at that next position when the image sensor  9000  captures the next image. In optional procedure  1050 , at least one image in the first set  19 A and at least one image in the second set  19 B are stitched. In optional procedure  1060 , a three-dimensional structure of the object  50  is determined based on the first set  19 A of images or the second set  19 B of images. 
       FIG. 11A  and  FIG. 11B  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 , a voltmeter  306  and a controller  310 . 
     The first voltage comparator  301  is configured to compare the voltage of an electrode of a diode  300  to a first threshold. The diode may be a diode formed by the first doped region  111 , one of the discrete regions  114  of the second doped region  113 , and the optional intrinsic region  112 . Alternatively, the first voltage comparator  301  is configured to compare the voltage of an electrical contact (e.g., a discrete portion of electrical contact  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 diode or electrical contact 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  configured as a continuous comparator reduces the chance that the system  121  misses signals generated by an incident particle of radiation. The first voltage comparator  301  configured as a continuous comparator is especially suitable when the incident radiation intensity is relatively high. The first voltage comparator  301  may be a clocked comparator, which has the benefit of lower power consumption. The first voltage comparator  301  configured as a clocked comparator may cause the system  121  to miss signals generated by some incident particles of radiation. When the incident radiation intensity is low, the chance of missing an incident particle of radiation is low because the time interval between two successive particles is relatively long. Therefore, the first voltage comparator  301  configured as a clocked comparator is especially suitable when the incident radiation intensity is relatively low. 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 in the diode or the resistor. The maximum voltage may depend on the energy of the incident particle of radiation (i.e., the wavelength of the incident 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 activated 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 signs. 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 in the diode or resistor. 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 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 system  121  to operate under a high flux of incident radiation. However, having a high speed is often at the cost of power consumption. 
     The counter  320  is configured to register a number of particles of radiation reaching the diode or resistor. 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 cutting 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 voltmeter  306  to measure the voltage upon expiration of the time delay. The controller  310  may be configured to connect the electrode to an electrical ground, so as to reset the voltage and discharge any charge carriers accumulated on the electrode. In an embodiment, the electrode is connected to an electrical ground after the expiration of the time delay. In an embodiment, the electrode is connected to an electrical ground for a finite reset time period. The controller  310  may connect the electrode 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 system  121  may include an integrator  309  electrically connected to the electrode of the diode  300  or which electrical contact, wherein the integrator is configured to collect charge carriers from the electrode. The integrator 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 electrode accumulate on the capacitor over a period of time (“integration period”) (e.g., as shown in  FIG. 12 , between t 0  to t 1 , or t 1 -t 2 ). After the integration period has expired, the capacitor voltage is sampled and then reset by a reset switch. The integrator can include a capacitor directly connected to the electrode. 
       FIG. 12  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a particle of radiation incident on the diode or the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve). The voltage may be an integral of the electric current with respect to time. At time t 0 , the particle of radiation hits the diode or the resistor, charge carriers start being generated in the diode or the resistor, electric current starts to flow through the electrode of the diode or the resistor, and the absolute value of the voltage of the electrode or electrical contact 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  causes the number registered by the counter  320  to increase by one. At time t e , 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. In the example of  FIG. 12 , 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 . The rate of change of the voltage is thus substantially zero at t s . 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 controller  310  may be configured to cause the voltmeter  306  to measure the voltage upon expiration of the time delay TD 1 . In an embodiment, the controller  310  causes the voltmeter  306  to measure the voltage after the rate of change of the voltage becomes substantially zero after the expiration of the time delay TD 1 . The voltage at this moment is proportional to the amount of charge carriers generated by a 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 based on voltage the voltmeter  306  measures. One way to determine the energy is by binning the voltage. The counter  320  may have a sub-counter for each bin. When the controller  310  determines that the energy of the particle of radiation falls in a bin, the controller  310  may cause the number registered in the sub-counter for that bin to increase by one. Therefore, the system  121  may be able to detect a radiation image and may be able to resolve particle of radiation energies of each particle of radiation. 
     After TD 1  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. After RST, the system  121  is ready to detect another incident particle of radiation. Implicitly, the rate of incident particles of radiation the system  121  can handle in the example of  FIG. 12  is limited by 1/(TD 1 +RST). 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. 
       FIG. 13  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current, background radiation, scattered radiations, fluorescent radiations, shared charges from adjacent pixels), and a corresponding temporal change of the voltage of the electrode (lower curve), in the system  121  operating in the way shown in  FIG. 12 . At time t 0 , the noise begins. If the noise is not large enough to cause the absolute value of the voltage to exceed the absolute value of V 1 , the controller  310  does not activate the second voltage comparator  302 . If the noise is large enough to cause the absolute value of the voltage to exceed the absolute value of V 1  at time t 1  as determined by the first voltage comparator  301 , 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 . During TD 1  (e.g., at expiration of TD 1 ), the controller  310  activates the second voltage comparator  302 . The noise is very unlikely large enough to cause the absolute value of the voltage to exceed the absolute value of V 2  during TD 1 . Therefore, the controller  310  does not cause the number registered by the counter  320  to increase. At time t e , the noise ends. At time t s , the time delay TD 1  expires. The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 1 . The controller  310  may be configured not to cause the voltmeter  306  to measure the voltage if the absolute value of the voltage does not exceed the absolute value of V 2  during TD 1 . After TD 1  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode as a result of the noise to flow to the ground and reset the voltage. Therefore, the system  121  may be very effective in noise rejection. 
       FIG. 14  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a particle of radiation incident on the diode or the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve), when the system  121  operates to detect incident particles of radiation at a rate higher than 1/(TD 1 +RST). The voltage may be an integral of the electric current with respect to time. At time t 0 , the particle of radiation hits the diode or the resistor, charge carriers start being generated in the diode or the resistor, electric current starts to flow through the electrode of the diode or the electrical contact of resistor, and the absolute value of the voltage of the electrode or the electrical contact 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 a time delay TD 2  shorter than TD 1 , and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 2 . If the controller  310  is deactivated before t 1 , the controller  310  is activated at t 1 . During TD 2  (e.g., at expiration of TD 2 ), the controller  310  activates the second voltage comparator  302 . If during TD 2 , the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold at time t 2 , the controller  310  causes the number registered by the counter  320  to increase by one. At time t e , all charge carriers generated by the particle of radiation drift out of the radiation absorption layer  110 . At time t h , the time delay TD 2  expires. In the example of  FIG. 14 , time t h  is before time t e ; namely TD 2  expires before all charge carriers generated by the particle of radiation drift out of the radiation absorption layer  110 . The rate of change of the voltage is thus substantially non-zero at t h . The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 2  or at t 2 , or any time in between. 
     The controller  310  may be configured to extrapolate the voltage at t e  from the voltage as a function of time during TD 2  and use the extrapolated voltage to determine the energy of the particle of radiation. 
     After TD 2  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. In an embodiment, RST expires before t e . The rate of change of the voltage after RST may be substantially non-zero because all charge carriers generated by the particle of radiation have not drifted out of the radiation absorption layer  110  upon expiration of RST before t e . The rate of change of the voltage becomes substantially zero after t e  and the voltage stabilized to a residue voltage VR after t e . In an embodiment, RST expires at or after t e , and the rate of change of the voltage after RST may be substantially zero because all charge carriers generated by the particle of radiation drift out of the radiation absorption layer  110  at t e . After RST, the 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. 
       FIG. 15  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current, background radiation, scattered radiations, fluorescent radiations, shared charges from adjacent pixels), and a corresponding temporal change of the voltage of the electrode (lower curve), in the system  121  operating in the way shown in  FIG. 14 . At time to, the noise begins. If the noise is not large enough to cause the absolute value of the voltage to exceed the absolute value of V 1 , the controller  310  does not activate the second voltage comparator  302 . If the noise is large enough to cause the absolute value of the voltage to exceed the absolute value of V 1  at time t 1  as determined by the first voltage comparator  301 , the controller  310  starts the time delay TD 2  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 2 . During TD 2  (e.g., at expiration of TD 2 ), the controller  310  activates the second voltage comparator  302 . The noise is very unlikely large enough to cause the absolute value of the voltage to exceed the absolute value of V 2  during TD 2 . Therefore, the controller  310  does not cause the number registered by the counter  320  to increase. At time t e , the noise ends. At time t h , the time delay TD 2  expires. The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 2 . After TD 2  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode as a result of the noise to flow to the ground and reset the voltage. Therefore, the system  121  may be very effective in noise rejection. 
       FIG. 16  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a series of particles of radiation incident on the diode or the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve), in the system  121  operating in the way shown in  FIG. 14  with RST expires before t e . The voltage curve caused by charge carriers generated by each incident particle of radiation is offset by the residue voltage before that particle. The absolute value of the residue voltage successively increases with each incident particle. When the absolute value of the residue voltage exceeds V 1  (see the dotted rectangle in  FIG. 16 ), the controller starts the time delay TD 2  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 2 . If no other particle of radiation incidence on the diode or the resistor during TD 2 , the controller connects the electrode to the electrical ground during the reset time period RST at the end of TD 2 , thereby resetting the residue voltage. The residue voltage thus does not cause an increase of the number registered by the counter  320 . 
     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.