Patent Publication Number: US-2021169436-A1

Title: Radiation detector with automatic exposure control and a method of automatic exposure control

Description:
TECHNICAL FIELD 
     The disclosure herein relates to radiation detectors, particularly relates to radiation detectors with automatic exposure control. 
     BACKGROUND 
     A radiation detector is a device that measures a property of a radiation. Examples of the property may include a spatial distribution of the intensity, phase, and polarization of the radiation. The radiation may be one that has interacted with a subject. For example, the radiation measured by the radiation detector may be a radiation that has penetrated or reflected from the subject. The radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray or γ-ray. The radiation may be of other types such as α-rays and β-rays. One type of radiation detectors is based on interaction between the radiation and a semiconductor. For example, a radiation detector of this type may have a semiconductor layer that absorbs the radiation and generate charge carriers (e.g., electrons and holes) and circuitry for detecting the charge carriers. 
     An automatic exposure control (AEC) device (e.g., a physically thin radiation ionization chamber) may be positioned between a radiation source (e.g., X-ray source) and a radiation detector, and be used to adjust (e.g., terminate, reduce) radiation exposure when a predetermined amount of radiation has been received by the radiation detector. AEC devices may be used to significantly reduce, or eliminate variations in image quality between different images, also to reduce the variation in radiation doses to different sized object (e.g., a patient if used for a medical radiography) cross sections. 
     SUMMARY 
     Disclosed herein is a method comprising: determining doses of radiation received by a first set of pixels of a radiation detector; determining that the doses satisfy a criterion; adjusting exposure of the radiation detector to the radiation in response to the doses satisfying the criterion; and forming an image based on radiation received by a second set of pixels of the radiation detector. 
     According to an embodiment, the second set does not include any member of the first set. 
     According to an embodiment, the first set is a subset of the second set. 
     According to an embodiment, the first set of pixels are in a same row or a same column of an array. 
     According to an embodiment, determining the doses is based on electrical signals generated from the radiation received by the first set of pixels. 
     According to an embodiment, the criterion is that a statistical characteristic of the doses is above a threshold. 
     According to an embodiment, the statistical characteristic is a median of the doses. 
     According to an embodiment, the statistical characteristic is a mean of the doses. 
     According to an embodiment, the statistical characteristic is a ratio of doses exceeding a limit to the doses received by the first set of pixels. 
     According to an embodiment, adjusting exposure of the radiation detector to the radiation in response to the doses satisfying the criterion comprises preventing the radiation from reaching the radiation detector. 
     According to an embodiment, adjusting exposure of the radiation detector to the radiation in response to the doses satisfying the criterion comprises stopping producing the radiation. 
     According to an embodiment, the first set of pixels are distributed among a set of chips; wherein each chip in the set comprises a subset of pixels of the first set. 
     According to an embodiment, determining that the doses satisfy a criterion comprises counting a number of subsets whose doses received by the pixels of each of the subsets satisfy a condition; wherein the criterion is that ratio of the number of subsets to the total number of subsets in the plurality is above a threshold. 
     According to an embodiment, the condition is that a statistical characteristic of the doses received by the pixels of each of the subsets is above another threshold. 
     Disclosed herein is an apparatus comprising: a radiation source; a radiation detector comprising a processor; wherein the processor is configured: to determine doses of radiation received by a first set of pixels of the radiation detector, to determine that the doses satisfy a criterion, to adjust exposure of the radiation detector to the radiation in response to the doses satisfying the criterion, and to form an image based on radiation received by a second set of pixels of the radiation detector. 
     According to an embodiment, the second set does not include any member of the first set. 
     According to an embodiment, the first set is a subset of the second set. 
     According to an embodiment, the first set of pixels are in a same row or a same column of an array. 
     According to an embodiment, the criterion is that a statistical characteristic of the doses is above a threshold. 
     According to an embodiment, the statistical characteristic is a median of the doses. 
     According to an embodiment, the statistical characteristic is a mean of the doses. 
     According to an embodiment, the statistical characteristic is a ratio of doses exceeding a limit to the doses received by the first set of pixels. 
     According to an embodiment, the apparatus further comprises a shutter configured to prevent the radiation from reaching the radiation detector when the processor adjusts exposure of the radiation detector to the radiation in response to the doses satisfying the criterion. 
     According to an embodiment, the radiation source is configured to stop producing the radiation when the processor adjusts exposure of the radiation detector to the radiation in response to the doses satisfying the criterion. 
     According to an embodiment, the first set of pixels are distributed among a set of chips; wherein each chip in the set comprises a subset of pixels of the first set. 
     According to an embodiment, the processor comprises a counter configured to count a number of subsets whose doses received by the pixels of each of the subsets satisfy a condition; wherein the criterion is that ratio of the number of subsets to the total number of subsets in the plurality is above a threshold. 
     According to an embodiment, the condition is that a statistical characteristic of the doses received by the pixels of each of the subsets is above another threshold. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1A  and  FIG. 1B  schematically show a system, according to an embodiment. 
         FIG. 2A  schematically shows the radiation detector, according to an embodiment. 
         FIG. 2B  schematically shows an alternative example of the radiation detector, according to an embodiment. 
         FIG. 3A  schematically shows a cross-sectional view of the radiation detector, according to an embodiment. 
         FIG. 3B  shows a flow chart for a method suitable for AEC, for example, using the system as shown in  FIG. 1A  and  FIG. 1B . 
         FIG. 4A  schematically shows a detailed cross-sectional view of the radiation detector shown in  FIG. 3A , according to an embodiment. 
         FIG. 4B  schematically shows a detailed cross-sectional view of the radiation detector shown in  FIG. 3A , according to an embodiment. 
         FIG. 5A  and  FIG. 5B  each show a component diagram of a portion of a processor configured to measure doses of radiation, according to an embodiment. 
         FIG. 6  schematically shows the system  100  described herein being used for medical imaging such as dental X-ray radiography. 
         FIG. 7  schematically shows a full-body scanner system comprising the system  100  described herein. 
         FIG. 8  schematically shows an X-ray computed tomography (X-ray CT) system comprising the system  100  described herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  and  FIG. 1B  schematically show a system  100 , according to an embodiment. The system  100  may have a radiation source  110  and a radiation detector  300 . The radiation source  110  may be configured to generate radiation directed through an object  130  toward the radiation detector  300 . The radiation detector  300  may detect the radiation from the radiation source  110  and process the signals produced by the radiation to generate an image of the object  130 . An image of the object  130  may be a set of position-dependent data (e.g., intensity of the radiation) produced by the radiation detector  300  after processing the radiation incident thereon. The radiation detector  300  may be configured to adjust (e.g., terminate or reduce) exposure of the radiation detector  300  to the radiation from the radiation source  110  when certain criteria are met. For instance, a processor (e.g., processor  322  in  FIG. 3A ) may measure doses of radiation received by the radiation detector  300  (e.g., doses of radiation transmitted through the object  130 ), and adjust the exposure (e.g., turn off the radiation source  110  or block the radiation therefrom) when the doses satisfy a criterion. For instance, the radiation source  110  may be controlled by the processor (e.g., processor  322  in  FIG. 3A ) such that the processor may turn the radiation source  110  off. As shown in the example of  FIG. 1B , the system  100  may comprise a shutter  140  configured to prevent the radiation from reaching the radiation detector  300  or the object  130 . The processor may control the operation of the shutter  140 . The shutter  140  may comprise a material with a high mass attenuation coefficient for the radiation from the radiation source  110 . 
       FIG. 2A  schematically shows the radiation detector  300 , according to an embodiment. The radiation detector  300  has an array of pixels including a first set of pixels  204  and a second set of pixels  202 . Each pixel in the array may be configured to detect radiation from the radiation source  110  incident thereon and may be configured measure a characteristic (e.g., the energy of the particles, the wavelength, and the frequency) of the radiation. For example, each pixel is 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 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. When the incident radiation particles have similar energy, the pixels may be simply configured to count numbers of radiation particles incident thereon within a period of time, without measuring the energy of the individual radiation particles. Each pixel 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, or to digitize an analog signal representing the total energy of a plurality of incident radiation particles into a digital signal. The pixels may be configured to operate in parallel. For example, when one pixel measures an incident radiation particle, another pixel may be waiting for a radiation particle to arrive. 
     The array may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. The second set may or may not include any member of the first set. For instance, the first set may be a subset of the second set. The first set of pixels  204  may be in the same row or the same column of the array but is not necessarily so arranged. In the example of  FIG. 2A , the first set of pixels  204  are in the same row and no pixel of the first set of pixels  204  belongs to the second set of pixels  202 . In an example, the second set of pixels  202  may include some or all of the pixels of the first set of pixels  204 , i.e., the first set of pixels  204  being a subset of the second set of pixels  202 . 
       FIG. 2B  schematically shows an alternative example of the radiation detector  300 , according to an embodiment. The radiation detector  300  may include a set of chips  210 . The first set of pixels  204  may be distributed among the set of chips  210 . In other words, each of the chips  210  may comprise a subset of the first set, and each of the subsets may include one or more pixels  204  of the first set. The number and distribution of pixels  204  in one subset is not necessarily the same as that of another subset. 
       FIG. 3A  schematically shows a cross-sectional view of the radiation detector  300 , according to an embodiment. The radiation detector  300  may comprise a radiation absorption layer  310  and an electronics layer  320  (e.g., an ASIC) for processing or analyzing electrical signals incident radiation generates in the radiation absorption layer  310 . The radiation absorption layer  310  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 of interest. The radiation absorption layer  310  may include one or more diodes (e.g., p-i-n or p-n) or resistors, as shown in  FIG. 4A  or  FIG. 4B . The radiation detector  300  may or may not include a scintillator. 
     When the radiation hits the radiation absorption layer  310 , it may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of the radiation may generate  10  to  100000  charge carriers. The charge carriers may drift to the electrical contacts  319 A and  319 B under an electric field. The field may be an external electric field. The charge carriers may drift in directions such that the charge carriers generated by a single particle of the radiation are not substantially shared by two different pixels (e.g., 204 or 202) (“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 pixels than the rest of the charge carriers). 
     The radiation detector  300  may include a processor  322  configured to determine doses of radiation received by the first set of pixels  204  and to adjust exposure of the radiation detector  300  to the radiation, as shown in  FIG. 1A  and  FIG. 1B . The processor  322  may be in the electronics layer  320 . In an embodiment, the processor  322  may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as microprocessors, and memory. The processor  322  may include one or more ADCs. The processor  322  may include components shared by the pixels or components dedicated to a single pixel. In the case of  FIG. 2B , the processor  322  may comprise components shared by the set of chips  210  or components dedicated to a single chip  210 . The processor  322  may be electrically connected to the pixels by vias  331 . Space among the vias may be filled with a filler material  330 , which may increase the mechanical stability of the connection of the electronics layer  320  to the radiation absorption layer  310 . Other bonding techniques are possible to connect the processor  322  to the pixels without using vias. 
       FIG. 3B  shows a flow chart for a method suitable for AEC, for example, using the system  100  as shown in  FIG. 1A  and  FIG. 1B . 
     In procedure  31 , doses of radiation received by the first set of pixels  204  are determined determine, e.g., using the processor  322 . In an embodiment, the doses are determined based on electrical signals generated from the radiation received by the first set of pixels  204 . 
     In procedure  32 , whether the doses satisfying a criterion is determined, e.g., using the processor  322 . The criterion may be that a statistical characteristic of the doses is above a threshold. The value of the threshold may be pre-determined and set based on the specific needs of applications. In an embodiment, the statistical characteristic may be a median of the doses or a mean of the doses. In an embodiment, the statistical characteristic maybe a ratio of doses exceeding a limit to the doses received by the first set of pixels  204 . For instance, a number of doses that exceed the limit is counted by a counter and used to calculate the ratio. The limit may be a percentage value such as 80%, 90%, 100% or any other suitable value. In the case shown in  FIG. 2B , the chips  210  in the set each comprises a subset of the first set, and a number of subsets (i.e., a number of chips  210 ) whose doses satisfy a condition may be counted by the counter. The condition may be that a statistical characteristic of the doses of a subset is above a threshold. The criterion may be that the ratio of the number of subsets to the total number of subsets in the plurality is above another threshold. For instance, the other threshold may be a percentage value such as 80%, 90%, 100% or any other suitable value. 
     In procedure  33 , exposure of the radiation detector  300  to the radiation is adjusted in response to the doses satisfying the criterion. In an embodiment, the radiation source  110  may stop producing the radiation once the doses satisfy the criterion. For instance, the processor  322  may connect to the radiation source  110  (as shown in  FIG. 1A ) and send signals to the radiation source  110  when the criterion is met. The radiation source  110  may be automatically turned off after receiving the signals. In an embodiment, the radiation may be physically blocked (e.g., by the shutter  140  as shown in  FIG. 1B ) once the doses satisfy the criterion. For instance, the processor  322  may connect to the shutter  140 , and control the movement of the shutter to prevent the radiation from reaching the radiation detector  300  or the object  130  when the criterion is met. 
     In procedure  34 , an image is formed based on radiation received by the second set of pixels  202  of the radiation detector  300  before the exposure is adjusted above. The image may be a set of position-dependent data generated by the radiation detector  300 . The image may be formed based on the doses of radiation transmitted through the object  130  and received by the second set of pixels  202 , and may reflect attenuation characteristics (e.g., composition and thickness) of different parts of the object  130 . In an embodiment, the signals (e.g., doses of radiation) from the first set of pixels  204  may also be used to generate portions of the image. 
       FIG. 4A  schematically shows a detailed cross-sectional view of the radiation detector  300  shown in  FIG. 3A , according to an embodiment. The radiation absorption layer  310  may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region  311 , one or more discrete regions  314  of a second doped region  313 , and an electrical contact  319 B. The second doped region  313  may be separated from the first doped region  311  by an optional the intrinsic region  312 . The discrete regions  314  are separated from one another by the first doped region  311  or the intrinsic region  312 . The first doped region  311  and the second doped region  313  have opposite types of doping (e.g., region  311  is p-type and region  313  is n-type, or region  311  is n-type and region  313  is p-type). Each of the discrete regions  314  of the second doped region  313  forms a diode with the first doped region  311  and the optional intrinsic region  312 . Namely, the radiation absorption layer  310  has a plurality of diodes having the first doped region  311  as a shared electrode. The first doped region  311  may also have discrete portions. The electrical contact  319 B may include discrete portions each of which is in electrical contact with the discrete regions  314 . 
     In an embodiment, charge carriers generated by a particle of radiation incident around the footprint of one of these discrete regions  314  are not substantially shared with another of these discrete regions  314 . The portion of the radiation absorption layer  310  associated with a pixel ( 202  or  204 ) may an area around the discrete region  314  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 the radiation incident therein flow to the discrete region  314 . Namely, less than 2%, less than 1%, less than 0.1%, or less than 0.01% of these charge carriers flow beyond the pixel ( 202  or  204 ) associated with the discrete region  314 . 
     As shown in an alternative detailed cross-sectional view of the radiation detector  300  in  FIG. 4B , according to an embodiment, the radiation absorption layer  310  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 of interest. 
     In an embodiment, the electrical contact  319 B includes discrete portions. The charge carriers generated by a particle of radiation incident around the footprint of one of these discrete portions of the electrical contact  319 B are not substantially shared with another of these discrete portions of the electrical contact  319 B. The portion of the radiation absorption layer  310  associated with a pixel ( 202  or  204 ) may 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 the radiation incident therein flow to the discrete portion of the electrical contact  319 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 ( 202  or  204 ) associated with the discrete portion of the electrical contact  319 B. 
       FIG. 5A  and  FIG. 5B  each show a component diagram of a portion of the processor  322  configured to measure doses of radiation, according to an embodiment. The portion of processor  322  includes a capacitor module  409  electrically connected to an electrode of a diode  400  or an electrical contact, wherein the capacitor module is configured to collect charge carriers from the electrode. The capacitor module can include a capacitor and charge carriers from the electrode 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 capacitor module can include a capacitor directly connected to the electrode. The capacitor may be 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. 
     In addition to the capacitor module  409 , the portion of processor  322  may further include a first voltage comparator  401 , a second voltage comparator  402 , a counter  420 , a switch  405 , a voltmeter  406  and a controller  410 , as shown in  FIG. 5A  and  FIG. 5B . 
     The first voltage comparator  401  is configured to compare the voltage of an electrode of a diode  400  to a first threshold. The diode may be a diode formed by the first doped region  311 , one of the discrete regions  314  of the second doped region  313 , and the optional intrinsic region  312 . Alternatively, the first voltage comparator  401  is configured to compare the voltage of an electrical contact (e.g., a discrete portion of electrical contact  319 B) to a first threshold. The first voltage comparator  401  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  401  may be controllably activated or deactivated by the controller  410 . The first voltage comparator  401  may be a continuous comparator. Namely, the first voltage comparator  401  may be configured to be activated continuously, and monitor the voltage continuously. The first voltage comparator  401  configured as a continuous comparator reduces the chance that the portion of processor  322  misses signals generated by an incident radiation particle. The first voltage comparator  401  configured as a continuous comparator is especially suitable when the incident radiation intensity is relatively high. The first voltage comparator  401  may be a clocked comparator, which has the benefit of lower power consumption. The first voltage comparator  401  configured as a clocked comparator may cause the portion of processor  322  to miss signals generated by some incident radiation particles. When the incident radiation intensity is low, the chance of missing an incident radiation particle is low because the time interval between two successive particles is relatively long. Therefore, the first voltage comparator  401  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 radiation particle may generate in the diode or the resistor. The maximum voltage may depend on the energy of the incident radiation particle (i.e., the wavelength of the incident radiation), the material of the radiation absorption layer  310 , and other factors. For example, the first threshold may be 50 mV, 100 mV, 150 mV, or 200 mV. 
     The second voltage comparator  402  is configured to compare the voltage to a second threshold V 2 . The second voltage comparator  402  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  402  may be a continuous comparator. The second voltage comparator  402  may be controllably activate or deactivated by the controller  410 . When the second voltage comparator  402  is deactivated, the power consumption of the second voltage comparator  402  may be less than 1%, less than 5%, less than 10% or less than 20% of the power consumption when the second voltage comparator  402  is activated. The absolute value of the second threshold is greater than the absolute value of the first threshold. As used herein, the term “absolute value” or “modulus” |x| of a real number x is the non-negative value of x without regard to its sign. Namely, 
     
       
         
           
             
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     The second threshold may be 200%-300% of the first threshold. The second threshold may be at least 50% of the maximum voltage one incident radiation particle 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  402  and the first voltage comparator  401  may be the same component. Namely, the portion of processor  322  may have one voltage comparator that can compare a voltage with two different thresholds at different times. 
     The first voltage comparator  401  or the second voltage comparator  402  may include one or more op-amps or any other suitable circuitry. The first voltage comparator  401  or the second voltage comparator  402  may have a high speed to allow the portion of processor  322  to operate under a high flux of incident radiation. However, having a high speed is often at the cost of power consumption. 
     The counter  420  is configured to register a number of radiation particles reaching the diode or resistor. The number of radiation particles counted is considered a dose of radiation received by a pixel ( 202  or  204 ). In an embodiment, the counter  420  is further configured to count the number of doses that exceed the limit or a number of subsets whose doses satisfy the condition as in procedure  32  of  FIG. 3B . The counter  420  may count and register a plurality of numbers at the same time. The counter  420  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  410  may be a hardware component such as a microcontroller and a microprocessor. The controller  410  is configured to start a time delay from a time at which the first voltage comparator  401  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  410  may be configured to keep deactivated the second voltage comparator  402 , the counter  420  and any other circuits the operation of the first voltage comparator  401  does not require, before the time at which the first voltage comparator  401  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  410  may be configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay. In an embodiment, the controller  410  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  410  itself may be deactivated until the output of the first voltage comparator  401  activates the controller  410  when the absolute value of the voltage equals or exceeds the absolute value of the first threshold. 
     The controller  410  may be configured to cause the number registered by the counter  420  to increase by one, if, during the time delay, the second voltage comparator  402  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold. 
     The controller  410  may be configured to cause the voltmeter  406  to measure the voltage upon expiration of the time delay. The controller  410  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  410  may connect the electrode to the electrical ground by controlling the switch  405 . The switch may be a transistor such as a field-effect transistor (FET). 
     In an embodiment, the portion of processor  322  has no analog filter network (e.g., a RC network). 
     In an embodiment, the portion of processor  322  has no analog circuitry. 
     The voltmeter  406  may feed the voltage it measures to the controller  410  as an analog or digital signal 
       FIG. 6  schematically shows the system  100  described herein being used for medical imaging such as dental X-ray radiography. The radiation source  110  is an X-ray source. X-ray emitted from the X-ray source penetrates an object  630  that is part of a mammal (e.g., human) mouth. The object  630  may include a maxilla bone, a palate bone, a tooth, the mandible, or the tongue. The X-ray is attenuated by different degrees by the different structures of the object  630  and is projected to the radiation detector  300 . The radiation detector  300  forms an image by detecting the intensity distribution of the X-ray and automatically control X-ray exposure of the object  630 . Teeth absorb X-ray more than dental caries, infections, periodontal ligament. The dosage of X-ray radiation received by a dental patient is typically small (around 0.150 mSv for a full mouth series). 
       FIG. 7  schematically shows a full-body scanner system comprising the system  100  described herein. 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 radiation source  110  is an X-ray source. X-ray emitted from the X-ray source may backscatter from a human  730  being screened and objects thereon, and be projected to the radiation detector  300 . The objects and the human body may backscatter X-ray differently. The radiation detector  300  forms an image by detecting the intensity distribution of the backscattered X-ray, and automatically control X-ray exposure of the human  730 . The radiation detector  300  and the X-ray source may be configured to scan the human in a linear or rotational direction. 
       FIG. 8  schematically shows an X-ray computed tomography (X-ray CT) system. The X-ray CT system uses computer-processed X-rays 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 X-ray CT system comprises the system  100  described herein. The radiation source  110  is an X-ray source. The radiation detector  300  and the X-ray source may be configured to rotate synchronously along one or more circular or spiral paths. 
     The system  100  described here may have other applications such as in an X-ray telescope, X-ray mammography, industrial X-ray defect detection, X-ray microscopy or microradiography, X-ray casting inspection, X-ray non-destructive testing, X-ray weld inspection, X-ray digital subtraction angiography, etc. It may be suitable to use this system  100  in place of a photographic plate, a photographic film, a PSP plate, an X-ray image intensifier, a scintillator, or another semiconductor X-ray detector. 
     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.