Patent Publication Number: US-2021161499-A1

Title: Radiation detection apparatus

Description:
BACKGROUND 
     Radiation detectors may be devices used to measure the flux, spatial distribution, spectrum or other properties of radiations. Radiation detectors may be used for many applications. One important application is imaging. Radiation imaging is a radiography technique and can be used to reveal the internal structure of a non-uniformly composed and opaque object such as the human body. 
     Early radiation detectors for imaging include photographic plates and photographic films. A photographic plate may be a glass plate with a coating of light-sensitive emulsion. Although photographic plates were replaced by photographic films, they may still be used in special situations due to the superior quality they offer and their extreme stability. A photographic film may be a plastic film (e.g., a strip or sheet) with a coating of light-sensitive emulsion. 
     In the 1980s, photostimulable phosphor plates (PSP plates) became available. A PSP plate may contain a phosphor material with color centers in its lattice. When the PSP plate is exposed to radiation, electrons excited by radiation are trapped in the color centers until they are stimulated by a laser beam scanning over the plate surface. As the plate is scanned by laser, trapped excited electrons give off light, which is collected by a photomultiplier tube. The collected light is converted into a digital image. In contrast to photographic plates and photographic films, PSP plates can be reused. 
     Another kind of radiation detectors are radiation image intensifiers. Components of a radiation image intensifier are usually sealed in a vacuum. In contrast to photographic plates, photographic films, and PSP plates, radiation image intensifiers may produce real-time images, i.e., do not require post-exposure processing to produce images. radiation first hits an input phosphor (e.g., cesium iodide) and is converted to visible light. The visible light then hits a photocathode (e.g., a thin metal layer containing cesium and antimony compounds) and causes emission of electrons. The number of emitted electrons is proportional to the intensity of the incident radiation. The emitted electrons are projected, through electron optics, onto an output phosphor and cause the output phosphor to produce a visible-light image. 
     Scintillators operate somewhat similarly to radiation image intensifiers in that scintillators (e.g., sodium iodide) absorb radiation and emit visible light, which can then be detected by a suitable image sensor for visible light. In scintillators, the visible light spreads and scatters in all directions and thus reduces spatial resolution. Reducing the scintillator thickness helps to improve the spatial resolution but also reduces absorption of radiation. A scintillator thus has to strike a compromise between absorption efficiency and resolution. 
     Semiconductor radiation detectors largely overcome this problem by direct conversion of radiation into electric signals. A semiconductor radiation detector may include a semiconductor layer that absorbs radiation in wavelengths of interest. When a particle of radiation is absorbed in the semiconductor layer, multiple charge carriers (e.g., electrons and holes) are generated and swept under an electric field towards electric contacts on the semiconductor layer. 
     SUMMARY 
     Disclosed herein is an apparatus, comprising: a platform configured to support a human body on a first surface of the platform; a first set of radiation detectors arranged in a first layer, wherein the radiation detectors of the first set are attached to a second surface of the platform opposite the first surface; wherein the radiation detectors of the first set are configured to detect radiation from a radiation source inside the human body. 
     According to an embodiment, each of the radiation detectors of the first set is configured to detect an image of the radiation. 
     According to an embodiment, the first set of radiation detectors comprises two members, an area of the first layer between which is devoid of any radiation detector. 
     According to an embodiment, the radiation is beta rays or gamma rays. 
     According to an embodiment, at least one radiation detector of the radiation detectors of the first set comprises a first radiation absorption layer configured to absorb the radiation and generate electrical signals from the radiation. 
     According to an embodiment, the first radiation absorption layer comprises silicon or GaAs. 
     According to an embodiment, the apparatus further comprises a second set of radiation detectors arranged in a second layer; wherein the radiation detectors of the second set are farther away from the second surface of the platform than the radiation detectors of the first set; wherein the radiation detectors of the second set are configured to detect radiation from the radiation source. 
     According to an embodiment, each of the radiation detectors of the second set is spaced apart from the second surface of the platform by a same distance. 
     According to an embodiment, each of the radiation detectors of the second set is configured to detect an image of the radiation. 
     According to an embodiment, the second set of radiation detectors comprises two members, an area of the second layer between which is devoid of any radiation detector. 
     According to an embodiment, the radiation detectors of the second set comprise a second radiation absorption layer configured to absorb the radiation and generate electrical signals from the radiation. 
     According to an embodiment, the second radiation absorption layer comprises silicon or GaAs. 
     According to an embodiment, the apparatus further comprises a processor configured to determine a spatial distribution of the radiation source in the human body based on the radiation detected by the first set of radiation detectors. 
     According to an embodiment, the first radiation absorption layer comprises an electric contact. 
     According to an embodiment, the at least one radiation detector comprises: a first voltage comparator configured to compare a voltage of the electric contact 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 incident on a pixel of the at least one radiation detector; a controller; wherein 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; wherein the controller is configured to activate the second voltage comparator during the time delay; wherein the controller is configured to cause the at least one of the numbers to increase by one, when the second voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the second threshold. 
     Disclosed herein is a method comprising: detecting radiation from a radiation source inside a human body using a first set of radiation detectors arranged in a first layer; detecting radiation from the radiation source using a second set of radiation detectors arranged in a second layer; determining a spatial distribution of the radiation source based on the radiation detected using the first set of radiation detectors and the radiation detected using the second set of radiation detectors; wherein the first layer and the second layer are at different distances from the human body. 
     According to an embodiment, the method of detecting the radiation from the radiation source using the first set of radiation detectors comprises detecting an image of the radiation. 
     According to an embodiment, the first set of radiation detectors comprises two members, an area of the first layer between which is devoid of any radiation detectors. 
     According to an embodiment, the radiation detectors of the first set comprise a first radiation absorption layer configured to absorb the radiation and generate electrical signals from the radiation. 
     According to an embodiment, the first radiation absorption layer comprises silicon or GaAs. 
     According to an embodiment, the method of detecting the radiation from the radiation source using the second set of radiation detectors comprises detecting an image of the radiation. 
     According to an embodiment, each of the radiation detectors of the second set is spaced apart from the second surface of the platform by a same distance. 
     According to an embodiment, the second set of radiation detectors comprises two members, an area of the second layer between which is devoid of any radiation detectors. 
     According to an embodiment, the radiation detectors of the second set comprise a radiation absorption layer configured to absorb the radiation and generate electrical signals from the radiation. 
     According to an embodiment, the radiation absorption layer comprises silicon or GaAs. 
     According to an embodiment, the radiation is beta rays or gamma rays. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1A ,  FIG. 1B , and  FIG. 2  each schematically show a view of an apparatus, according to an embodiment. 
         FIG. 3  schematically shows that the apparatus has a radiation detector with an array of pixels, according to an embodiment. 
         FIG. 4A  shows a cross-sectional schematic of the radiation detector, according to an embodiment. 
         FIG. 4B  shows a detailed cross-sectional schematic of the radiation detector, according to an embodiment. 
         FIG. 4C  shows an alternative detailed cross-sectional schematic of the radiation detector, according to an embodiment. 
         FIG. 5A  and  FIG. 5B  each show a component diagram of an electronic system of the radiation detector, according to an embodiment. 
         FIG. 6  schematically shows a temporal change of the electric current flowing through an electric contact (upper curve) of the radiation absorption layer of the radiation detector, and a corresponding temporal change of the voltage on the electric contact (lower curve). 
         FIG. 7  schematically shows a flowchart for a method, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A ,  FIG. 1B  and  FIG. 2  each schematically show an apparatus  500  from a different view.  FIG. 1A  shows a first surface  531  of a platform  530  of the apparatus  500 . The first surface  531  of the platform  530  may support a human body.  FIG. 1B  schematically shows a second surface  532  of the platform  530 . The second surface  532  is opposite the first surface  531 . The apparatus  500  has a first set of radiation detectors  521  arranged in a first layer  510 , as shown in  FIG. 1B  and  FIG. 2 . The first set of radiation detectors  521  are attached to the second surface  532 . The first set of radiation detectors  521  may include at least two radiation detectors  521 . The areas of the first layer  510  between the radiation detectors  521  may be devoid of any radiation detector. 
     As schematically shown in the  FIG. 1B  and  FIG. 2 , the apparatus  500  has a second set of radiation detectors  522  arranged in a second layer  520 , according to an embodiment. The second set of radiation detectors  522  are farther away from the second surface  532  than the first set of radiation detectors  521 . Each of the second set of radiation detectors  522  may be spaced apart from the second surface  532  by the same distance. Namely, the second layer  520  may be parallel to the second surface  532 . The second set of radiation detectors  522  may comprise at least two radiation detectors  522 . The areas of the second layer  520  between the radiation detectors  522  may be devoid of any radiation detector. The platform  530  is between the human body and the radiation detectors  521  and  522 . The radiation detectors  521  and  522  may detect radiation from a radiation source inside the human body. The radiation detectors  521  and  522  may detect images of the radiation. The radiation may be beta rays or gamma rays. 
     As shown in  FIG. 2 , the first layer  510  and the second layer  520  are stacked. The second set of radiation detectors  521  in the second layer  520  may be aligned with the areas of the first layer  510  that are devoid of any radiation detectors. 
     The radiation source inside the human body may be a radioactive substance introduced into the human body for medical purposes. Examples of the radioactive substance may include iodine-131 ( 131 I), iodine-123 ( 123 I), and iodine-125 ( 125 I). In an example,  131 I is used for treating thyrotoxicosis (hyperthyroidism) and some types of thyroid cancer because the thyroid can absorb iodine. In an example,  131 I is used as a radioactive label for certain radiopharmaceuticals (e.g.,  131 I-metaiodobenzylguanidine (131I-MIBG) for imaging and treating pheochromocytoma and neuroblastoma). Knowing the spatial distribution of the radioactive substance thus may facilitate using the radioactive substance for diagnosing or treating certain diseases. The apparatus  500  may have a processor  550  that can determine the spatial distribution of the radiation source based on the radiation detected by the first set of radiation detectors  521  or the second set of radiation detectors  522 . 
       FIG. 3  schematically shows that a radiation detector  100 , which is one among the radiation detectors  521  and  522 , may have an array of pixels  150 , according to an embodiment. The array of the pixels  150  may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. The radiation detector  100  may count the numbers of particles of radiation incident on the pixels  150 , within a period of time. An example of the particles of radiation is gamma ray photons. Each pixel  150  may be configured to measure its dark current, such as before or concurrently with each particle of radiation incident thereon. The pixels  150  may be configured to operate in parallel. For example, the radiation detector  100  may count one particle of radiation incident on one pixel  150  before, after or while the radiation detector  100  counts another particle of radiation incident on another pixel  150 . The pixels  150  may be individually addressable. 
       FIG. 4A  shows a cross-sectional schematic of the radiation detector  100 , 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 particles of radiation generate in the radiation absorption layer  110 . The radiation detector  100  may or may not include a scintillator. The radiation absorption layer  110  may include a semiconductor material such as single-crystalline silicon. The semiconductor may have a high mass attenuation coefficient for the radiation of interest. 
     As shown in a more detailed cross-sectional schematic of the radiation detector  100  in  FIG. 4B , according to an embodiment, the radiation absorption layer  110  may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region  111 , one or more discrete regions  114  of a second doped region  113 . The second doped region  113  may be separated from the first doped region  111  by an optional the intrinsic region  112 . The discrete regions  114  are separated from one another by the first doped region  111  or the intrinsic region  112 . The first doped region  111  and the second doped region  113  have opposite types of doping (e.g., region  111  is p-type and region  113  is n-type, or region  111  is n-type and region  113  is p-type). In the example in  FIG. 4B , 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. 4B , 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. The radiation absorption layer  110  may have an electric contact  119 A in electrical contact with the first doped region  111 . The radiation absorption layer  110  may have multiple discrete electric contacts  119 B, each of which is in electrical contact with the discrete regions  114 . 
     When particles of radiation hit the radiation absorption layer  110  including diodes, the particles of radiation may be absorbed and generate one or more charge carriers by a number of mechanisms. The charge carriers may drift to the electric contacts  119 A and  119 B under an electric field. The field may be an external electric field. In an embodiment, 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 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 the 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 the 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  150 . 
     As shown in an alternative detailed cross-sectional schematic of the radiation detector  100  in  FIG. 4C , according to an embodiment, the radiation absorption layer  110  may include a resistor of a semiconductor material such as single-crystalline silicon but does not include a diode. The semiconductor may have a high mass attenuation coefficient for the radiation of interest. The radiation absorption layer  110  may have an electric contact  119 A in electrical contact with the semiconductor on one surface of the semiconductor. The radiation absorption layer  110  may have multiple electric contacts  119 B on another surface of the semiconductor. 
     When particles of radiation hit the radiation absorption layer  110  including a resistor but not diodes, the particles of radiation 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  119 A and  119 B under an electric field. The field may be an external electric field. In an embodiment, 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 electrical contacts  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 the radiation incident around the footprint of one of the electrical contacts  119 B are not substantially shared with another of the electrical contacts  119 B. A pixel  150  associated with one of the electrical contacts  119 B may be an area around it 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 that one 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 that one electrical contact  119 B. 
     The electronics layer  120  may include an electronic system  121  suitable for processing or interpreting signals generated by the 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 circuitry such as a microprocessor, and memory. The electronic system  121  may include one or more ADCs. 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  150  and a microprocessor shared among all the pixels  150 . 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. 5A  and  FIG. 5B  each show a component diagram of the electronic system  121 , according to an embodiment. The electronic system  121  may include a first voltage comparator  301 , a second voltage comparator  302 , a counter  320 , a switch  305 , an optional voltmeter  306  and a controller  310 . 
     The first voltage comparator  301  is configured to compare the voltage of at least one of the electric contacts  119 B to a first threshold. The first voltage comparator  301  may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the electrical contact  119 B over a period of time. The first voltage comparator  301  may be controllably activated or deactivated by the controller  310 . The first voltage comparator  301  may be a continuous comparator. Namely, the first voltage comparator  301  may be configured to be activated continuously and monitor the voltage continuously. The first voltage comparator  301  may be a clocked comparator. The first threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the maximum voltage one incident particle of radiation may generate on the electric contact  119 B. The maximum voltage may depend on the energy of the incident particle of radiation, the material of the radiation absorption layer  110 , and other factors. For example, the first threshold may be 50 mV, 100 mV, 150 mV, or 200 mV. 
     The second voltage comparator  302  is configured to compare the voltage to a second threshold. The second voltage comparator  302  may be configured to monitor the voltage directly or calculate the voltage by integrating an electric current flowing through the diode or the electrical contact over a period of time. The second voltage comparator  302  may be a continuous comparator. The second voltage comparator  302  may be controllably activate or deactivated by the controller  310 . When the second voltage comparator  302  is deactivated, the power consumption of the second voltage comparator  302  may be less than 1%, less than 5%, less than 10% or less than 20% of the power consumption when the second voltage comparator  302  is activated. The absolute value of the second threshold is greater than the absolute value of the first threshold. As used herein, the term “absolute value” or “modulus” |x| of a real number x is the non-negative value of x without regard to its sign. Namely, 
     
       
         
           
             
                
               ϰ 
                
             
             = 
             
               { 
               
                 
                   
                     
                       
                         ϰ 
                         , 
                         
                             
                         
                          
                         
                           
                             if 
                              
                             
                                 
                             
                              
                             ϰ 
                           
                           ≥ 
                           0 
                         
                       
                     
                   
                   
                     
                       
                         
                           - 
                           ϰ 
                         
                         , 
                         
                             
                         
                          
                         
                           
                             if 
                              
                             
                                 
                             
                              
                             ϰ 
                           
                           ≤ 
                           0 
                         
                       
                     
                   
                 
                 . 
               
             
           
         
       
     
     The second threshold may be 200%-300% of the first threshold. The second threshold may be at least 50% of the maximum voltage one incident particle of radiation may generate on the electric contact  119 B. For example, the second threshold may be 100 mV, 150 mV, 200 mV, 250 mV or 300 mV. The second voltage comparator  302  and the first voltage comparator  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 particles of radiation. However, having a high speed is often at the cost of power consumption. 
     The counter  320  is configured to register at least a number of particles of radiation incident on the pixel  150  encompassing the electric contact  119 B. The counter  320  may be a software component (e.g., a number stored in a computer memory) or a hardware component (e.g., a  4017  IC and a  7490  IC). 
     The controller  310  may be a hardware component such as a microcontroller and a microprocessor. The controller  310  is configured to start a time delay from a time at which the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold (e.g., the absolute value of the voltage increases from below the absolute value of the first threshold to a value equal to or above the absolute value of the first threshold). The absolute value is used here because the voltage may be negative or positive, depending on whether the voltage of the cathode or the anode of the diode or which electrical contact is used. The controller  310  may be configured to keep deactivated the second voltage comparator  302 , the counter  320  and any other circuits the operation of the first voltage comparator  301  does not require, before the time at which the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold. The time delay may expire before or after the voltage becomes stable, i.e., the rate of change of the voltage is substantially zero. The phase “the rate of change of the voltage is substantially zero” means that temporal change of the voltage is less than 0.1%/ns. The phase “the rate of change of the voltage is substantially non-zero” means that temporal change of the voltage is at least 0.1%/ns. 
     The controller  310  may be configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay. In an embodiment, the controller  310  is configured to activate the second voltage comparator at the beginning of the time delay. The term “activate” means causing the component to enter an operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by providing power, etc.). The term “deactivate” means causing the component to enter a non-operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by cut off power, etc.). The operational state may have higher power consumption (e.g., 10 times higher, 100 times higher, 1000 times higher) than the non-operational state. The controller  310  itself may be deactivated until the output of the first voltage comparator  301  activates the controller  310  when the absolute value of the voltage equals or exceeds the absolute value of the first threshold. 
     The controller  310  may be configured to cause the number registered by the counter  320  to increase by one, if, during the time delay, the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold. 
     The controller  310  may be configured to cause the optional voltmeter  306  to measure the voltage upon expiration of the time delay. The controller  310  may be configured to connect the electric contact  119 B to an electrical ground, so as to reset the voltage and discharge any charge carriers accumulated on the electric contact  119 B. In an embodiment, the electric contact  119 B is connected to an electrical ground after the expiration of the time delay. In an embodiment, the electric contact  119 B is connected to an electrical ground for a finite reset time period. The controller  310  may connect the electric contact  119 B to the electrical ground by controlling the switch  305 . The switch may be a transistor such as a field-effect transistor (FET). 
     In an embodiment, the system  121  has no analog filter network (e.g., a RC network). In an embodiment, the system  121  has no analog circuitry. 
     The voltmeter  306  may feed the voltage it measures to the controller  310  as an analog or digital signal. 
     The electronic system  121  may include an integrator  309  electrically connected to the electric contact  119 B, wherein the integrator is configured to collect charge carriers from the electric contact  119 B. The integrator  309  can include a capacitor in the feedback path of an amplifier. The amplifier configured as such is called a capacitive transimpedance amplifier (CTIA). CTIA has high dynamic range by keeping the amplifier from saturating and improves the signal-to-noise ratio by limiting the bandwidth in the signal path. Charge carriers from the electric contact  119 B accumulate on the capacitor over a period of time (“integration period”). After the integration period has expired, the capacitor voltage is sampled and then reset by a reset switch. The integrator  309  can include a capacitor directly connected to the electric contact  119 B. 
       FIG. 6  schematically shows a temporal change of the electric current flowing through the electric contact  119 B (upper curve) caused by charge carriers generated by a particle of radiation incident on the pixel  150  encompassing the electric contact  119 B, and a corresponding temporal change of the voltage of the electric contact  119 B (lower curve). The voltage may be an integral of the electric current with respect to time. At time t 0 , the particle of radiation hits pixel  150 , charge carriers start being generated in the pixel  150 , electric current starts to flow through the electric contact  119 B, and the absolute value of the voltage of the electric contact  119 B starts to increase. At time t 1 , the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V 1 , and the controller  310  starts the time delay TD 1  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 1 . If the controller  310  is deactivated before t 1 , the controller  310  is activated at t 1 . During TD 1 , the controller  310  activates the second voltage comparator  302 . The term “during” a time delay as used here means the beginning and the expiration (i.e., the end) and any time in between. For example, the controller  310  may activate the second voltage comparator  302  at the expiration of TD 1 . If during TD 1 , the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold V 2  at time t 2 , the controller  310  waits for stabilization of the voltage to stabilize. The voltage stabilizes at time t e , when all charge carriers generated by the particle of radiation drift out of the radiation absorption layer  110 . At time t s , the time delay TD 1  expires. At or after time t e , the controller  310  causes the voltmeter  306  to digitize the voltage and determines which bin the energy of the particle of radiation falls in. The controller  310  then causes the number registered by the counter  320  corresponding to the bin to increase by one. In the example of  FIG. 6 , time t s  is after time t e ; namely TD 1  expires after all charge carriers generated by the particle of radiation drift out of the radiation absorption layer  110 . If time t e  cannot be easily measured, TD 1  can be empirically chosen to allow sufficient time to collect essentially all charge carriers generated by a particle of radiation but not too long to risk have another incident particle of radiation. Namely, TD 1  can be empirically chosen so that time t s  is empirically after time t e . Time t s  is not necessarily after time t e  because the controller  310  may disregard TD 1  once V 2  is reached and wait for time t e . The rate of change of the difference between the voltage and the contribution to the voltage by the dark current is thus substantially zero at t e . The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 1  or at t 2 , or any time in between. 
     The voltage at time t e  is proportional to the amount of charge carriers generated by the particle of radiation, which relates to the energy of the particle of radiation. The controller  310  may be configured to determine the energy of the particle of radiation, using the voltmeter  306 . 
     After TD 1  expires or digitization by the voltmeter  306 , whichever later, the controller  310  connects the electric contact  119 B to an electric ground for a reset period RST to allow charge carriers accumulated on the electric contact  119 B to flow to the ground and reset the voltage. After RST, the 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. 7  schematically shows a flowchart for a method, according to an embodiment. In procedure  801 , a radiation emitted from a radiation source inside a human body is detected by the first set of radiation detectors  521 . In procedure  802 , the radiation is detected by the second set of radiation detectors  522 . In procedure  803 , a spatial distribution of the radiation source inside the human body is determined, based on the radiation detected by the first set of radiation detectors  521  and the radiation detected by the second set of radiation detectors  521 . 
     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.