Patent Publication Number: US-2022221596-A1

Title: Method of reading out data in a radiation detector, radiation detector and imaging apparatus

Description:
The present application claims priority to International Application No. PCT/EP2019/062150 filed on May 13, 2019, titled “Method of Reading Out Data in a Radiation Detector, Radiation Detector and Imaging Apparatus,” which is incorporated by reference herein, and is assigned to the assignee of the present invention. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure generally relates to radiation detectors. In particular, a method of reading out data in a radiation detector, a radiation detector and an imaging apparatus, are provided. 
     BACKGROUND 
     Various radiation detectors for detecting ionizing radiation are known in the art. A radiation source transmits radiation through an object, such as a patient, and the radiation detector measures the attenuated radiation. The radiation is converted to an electrical signal, a control unit processes these signals and the desired images can be provided. 
     In some applications, for example in computed tomography (CT) applications, there is a benefit from using multiple energies for the acquisition of frame data. Multiple energy imaging enables improved reconstructions. Multiple energy imaging however generates very large data sets. Moreover, the readout of data indicative of radiation at different energy levels requires longer readout periods. The readout periods are typically required to be twice as long for reading out two energy levels in comparison with reading out a single energy level. 
     US 2016106386 A1 discloses methods and systems for weighting material density images based on the material imaged. In one embodiment, a method for dual energy imaging of a material comprises generating an odd material density image, generating an even material density image, applying a first weight to the odd material density image and a second weight to the even material density image, and generating a material density image based on a combination of the weighted odd material density image and the weighted even material density image. 
     SUMMARY 
     One object of the present disclosure is to provide a method of reading out data in a radiation detector, which method reduces the amount of data read out in the radiation detector. 
     A further object of the present disclosure is to provide a method of reading out data in a radiation detector, which method provides an efficient readout of data. 
     A still further object of the present disclosure is to provide a method of reading out data in a radiation detector, which method provides a fast readout of data. 
     A still further object of the present disclosure is to provide a method of reading out data in a radiation detector, which method enables high quality imaging. 
     A still further object of the present disclosure is to provide a method of reading out data in a radiation detector, which method enables reliable operation. 
     A still further object of the present disclosure is to provide a method of reading out data in a radiation detector, which method enables a simple and/or compact design of the radiation detector. 
     A still further object of the present disclosure is to provide a method of reading out data in a radiation detector, which method solves several or all of the foregoing objects in combination. 
     A still further object of the present disclosure is to provide a radiation detector solving one, several or all of the foregoing objects. 
     A still further object of the present disclosure is to provide an imaging apparatus solving one, several or all of the foregoing objects. 
     According to one aspect, there is provided a method of reading out data in a radiation detector, wherein the radiation detector comprises a plurality of pixels and a plurality of readout circuits associated with the pixels, and wherein each readout circuit comprises at least one register, the method comprising detecting radiation by means of the pixels and storing data indicative of the radiation in one or more of the at least one register of each readout circuit, each time during a plurality of data acquisition periods; and reading out data from one or more of the at least one register of each readout circuit, each time during a plurality of readout periods, each readout period following a data acquisition period, and each readout period being either a low energy readout period or a high energy readout period; wherein only data from a single register of each readout circuit indicative of radiation energy above a low energy level is read out during each low energy readout period; and wherein data indicative of radiation energy above a high energy level, higher than the low energy level, is read out during each high energy readout period. 
     In many applications, it is enough to read out data, i.e. to sample, only relatively few measurement frames with multiple energies, for example in order to detect contrast media or for beam hardening calculations. Dual and multi-energy photon counting CT can reduce beam hardening and provide better tissue contrast. A measurement frame comprises one data acquisition period and one following readout period. By only reading out data from a single register of each readout circuit indicative of radiation energy above a low energy level during some readout periods (i.e. the low energy readout periods of the method) the amount of data read out can be reduced. In many implementations, also the effective dead time over a complete scan can thereby be reduced. In other words, the method provides a sparse readout of data that is only indicative of radiation energy above a high energy level. Thus, in comparison with a method where data from several different registers of each readout circuit is read out during each readout period, the method provides for a reduced amount of data and a faster scanning. 
     Furthermore, by reading out data also indicative of radiation energy above a high energy level, higher than the low energy level, during some readout periods (i.e. the high energy readout periods of the method), the method enables high quality imaging and provides for an efficient data handling for multiple energy imaging. The method thus increases the efficiency of the data readout for imaging apparatuses having data acquisition periods and readout periods. 
     During each readout period, such as the low energy readout periods and the high energy readout periods, data indicative of charges freed in, and transported through, a conversion element of the radiation detector in response to photons being absorbed, may be read out. The data may contain a number of charge pulses of photons being absorbed by the conversion element. 
     The method comprises reading out data indicative of radiation energy at two or more different energy levels. The method may thus be implemented in dual-energy imaging, but also in multiple energy imaging, for example with six different energy levels. 
     The radiation detector may be a photon counting direct conversion pixelated detector. The readout circuits may alternatively be referred to as readout cells. 
     The low energy level may be constituted by one or more low energy bands or one or more open low energy intervals above one or more low energy threshold values. The high energy level may be constituted by one or more high energy bands or one or more open high energy intervals above one or more high energy threshold values. 
     Throughout the present disclosure, a low energy readout period and a high energy readout period may alternatively be referred to as a first energy readout period and a second energy readout period, respectively, and a low energy level and a high energy level may alternatively be referred to as a first energy level and a second energy level, respectively. 
     According to one variant, data from a first register of each readout circuit indicative of radiation energy above the low energy level, and data from at least one second register of each readout circuit indicative of radiation energy above at least one high energy level, higher than the low energy level, is read out during each high energy readout period. Thus, in this variant, the high energy readout period constitutes a multiple energy readout period and the low energy readout period constitutes a single energy readout period. 
     The low energy level may be defined by a first threshold value and the at least one high energy level may be defined by at least one second threshold value, higher than the first threshold value. Thus, only the number of photon events with an energy level above the respective threshold value is stored in the respective register. The threshold value for a low energy readout period may be set prior to, or at start of, a data acquisition period immediately before the low energy readout period. The threshold value for a high energy readout period may be set prior to, or at start of, a data acquisition period immediately before the high energy readout period. One or more of the at least one second threshold value may be variable and set to different values in one or more of the high energy readout periods. 
     One or more of the at least one second threshold value may be variable and set to different values in one or more of the high energy readout periods. Each threshold value, either static or variable, may be set by means of a threshold setting device according to the present disclosure. The second threshold value may vary cyclically. For example, a second energy level may be read out during a first high energy readout period, a third energy level, higher than the second energy level, may be read out during a second high energy readout period, a fourth energy level, higher than the third energy level, may be read out during a third high energy readout period, and the first high energy readout period may again be read out during a fourth high energy readout period, and so on. Alternatively, or in addition, the first threshold value may be variable and set to different values in one or more of the low energy readout periods. 
     According to one variant, each of the low energy level and the high energy level is defined by a variable threshold value and wherein only data from a single register of each readout circuit is read out during each low energy readout period and during each high energy readout period. In this way, the number of registers in each readout circuit can be reduced, for example to one register per readout circuit. 
     A plurality of low energy readout periods may be provided between a first high energy readout period and a next high energy readout period following the first high energy readout period. A sequence of readout periods may thus be: “n” number of low energy readout periods, where “n” is a positive integer, one high energy readout period, “n” number of low energy readout periods, one high energy readout period, and so on. 
     According to a further aspect, there is provided a radiation detector configured to perform any of the methods according to the present disclosure. 
     According to a further aspect, there is provided a radiation detector comprising a plurality of pixels configured to detect radiation energy; and a plurality of readout circuits associated with the pixels, and each readout circuit comprises at least one comparator configured to compare an electrical signal representative of the radiation energy from one of the pixels against at least two threshold values including a first threshold value and a second threshold value, and at least one register configured to store low energy data indicative of the electrical signal from the one of the pixels above the first threshold value representing the radiation energy above a low energy level and store high energy data indicative of the electrical signal from the one of the pixels above the second threshold value representing the radiation energy above a high energy level, and the at least one register configured to readout the low energy data and the high energy data. 
     Each readout circuit may be associated with one of the pixels. Alternatively, a plurality of readout circuits may be associated with one of the pixels, or vice versa. The readout circuits may be provided in a common readout substrate, such as an application-specific integrated circuit (ASIC). 
     Each register may be configured to temporarily store converted electrical signals from an associated pixel. Throughout the present disclosure, the registers may alternatively be referred to as storage units. Furthermore, each register may comprise, or be constituted by, a counter configured to count the number of photon pulses above a given energy level, which corresponds to a given comparator threshold, and optionally also configured to count the number of photon pulses within a given energy range. 
     The at least one comparator may comprise a first comparator configured to compare the electrical signal from the one of the pixels against the first threshold value, and a second comparator configured to compare the electrical signal from the one of the pixels against the second threshold value; and wherein the at least one register comprises a first register configured to store low energy data indicative of the electrical signal from the one of the pixels above the first threshold value, and a second register configured to store high energy data indicative of the electrical signal from the one of the pixels above the second threshold value representing the radiation energy above a high energy level. 
     The at least one comparator may comprise a third comparator configured to compare the electrical signal from the one of the pixels against the third threshold value; and wherein the at least one register comprises a third register configured to store high energy data indicative of the electrical signal from the one of the pixels above the third threshold value representing the radiation energy above a second high energy level and the third register is serially coupled to the second register. 
     Each readout circuit may further comprise an amplifier configured to receive and amplify the electrical signal from the one of the pixels, and a pulse shaper configured to shape the waveform of the electrical signal from the one of the pixels, and the pulse shaper is operatively coupled to at least one input of the at least one comparator. 
     The radiation detector may further comprise a threshold setting device configured to set the at least two threshold values for each readout circuit, and optionally, the threshold setting device comprises at least one digital to analog converter (DAC) configured to set the first threshold value or the second threshold value. 
     Each readout circuit may comprise a switch switchable between a first state decoupling the second register from a serial output, and a second state coupling the second register to the serial output, wherein the switch is coupled to a serial data output of the second register and a serial data input of the first register. 
     According to one variant, the switch is a single pole, double throw (SPDT) switch; or the switch is in the first state for a duration at least 50% longer than a second state; or each readout circuit between a first readout circuit and a last readout circuit includes the serial data output coupled to a serial data input of a next readout circuit; or the at least one DAC variably sets the second threshold value to different values in one or more readout periods; or the one of the pixels comprises a conversion element including cadmium telluride (CdTe); or the high energy level is higher than the low energy level. 
     According to a further aspect, there is provided an imaging apparatus comprising the radiation detector according to the present disclosure; and a control unit operatively connected to the radiation detector and configured to read radiation data from the radiation detector. 
     The imaging apparatus may be a multiple-energy scanning based imaging apparatus. For example, the imaging apparatus may comprise a radiation source and a multiple-energy filter device having a variable spectral transmission characteristic. The radiation detector may for example be one dimensional or two dimensional. Each pixel may be configured to directly convert radiation into an electric charge. 
     The control unit may comprise a data processing device and a memory having a computer program stored thereon, the computer program comprising program code which, when executed by the data processing device, causes the data processing device to perform, or command performance of, one, several or all steps of the methods according to the present disclosure. The control unit may for example be arranged in the radiation detector or outside the radiation detector. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       Further details, advantages and aspects of the present disclosure will become apparent from the following embodiments taken in conjunction with the drawings, wherein: 
         FIG. 1  schematically represents an imaging apparatus comprising a radiation detector. 
         FIG. 2  schematically represents a partial cross-sectional view of the radiation detector comprising a readout substrate. 
         FIG. 3  schematically represents a partial view of the readout substrate. 
         FIG. 4  schematically represents a partial view of an alternative example of a readout substrate. 
         FIG. 5  graphically represents one example of a method of reading out data in the radiation detector. 
         FIG. 6  graphically represents a further example of a method of reading out data in the radiation detector. 
         FIG. 7  graphically represents a further example of a method of reading out data in the radiation detector. 
         FIG. 8  graphically represents a further example of a method of reading out data in the radiation detector. 
         FIG. 9  graphically represents a further example of a method of reading out data in the radiation detector. 
         FIG. 10  graphically represents a further example of a method of reading out data in the radiation detector. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, a method of reading out data in a radiation detector, a radiation detector and an imaging apparatus, will be described. The same reference numerals will be used to denote the same or similar structural features. 
       FIG. 1  schematically represents an imaging apparatus  10 . The imaging apparatus  10  comprises a radiation detector  12  and a control unit  14 . The imaging apparatus  10  of this example is a multiple-energy imaging apparatus for computed tomography (CT) scanning. 
     The imaging apparatus  10  further comprises a radiation source  16 , such as an X-ray tube, for emitting X-rays that are transmitted through an object  18  to be imaged, for example through the body of a patient. After transmission through the object  18 , the X-rays reach the radiation detector  12  where the X-rays are detected and converted into signals representing a spatially resolved projection image of the object  18 . 
     The control unit  14  is operatively connected to the radiation detector  12 . The control unit  14  is configured to read radiation data from the radiation detector  12 . The control unit  14  may be configured to acquire 2D projection images. The acquired 2D images may be used to reconstruct, for example 3D images, of the object  18  according to known principles of computed tomography. 
       FIG. 2  schematically represents a partial cross-sectional view of the radiation detector  12  in  FIG. 1 . The radiation detector  12  comprises a conversion element  20 , for example a cadmium telluride (CdTe) crystal, and a readout substrate  24 , for example a readout ASIC substrate. The radiation detector  12  further comprises a support substrate  22 . 
     The conversion element  20  comprises a plurality of pixels  26 - 1 ,  26 - 2 ,  26 - 3 ,  26 - 4 ,  26 - n . Each pixel  26 - 1 ,  26 - 2 ,  26 - 3 ,  26 - 4 ,  26 - n  may also be referred to with reference numeral “26”. The pixels  26  are evenly distributed over at least a major part of the radiation detector  12 , such as over the entire radiation detector  12 . In this example, the conversion element  20  is two-dimensional, i.e. comprising a two-dimensional array of pixels  26 . 
     The conversion element  20  may be constituted by at least one semiconductor substrate, such as a CdTe or cadmium zinc telluride (CdZnTe or CZT) substrate. The conversion element  20  may comprise a continuous conversion substrate or several discrete conversion portions. 
     The conversion element  20  of this example further comprises a plurality of charge collection electrodes  28 , here implemented as contact pads. Each pixel  26  is defined by a charge collection electrode  28 . 
     When X-rays (or other type of ionizing radiation) impinges on the conversion element  20 , electron-hole pairs are created inside the conversion element  20  (thus the term “direct conversion”) in response to the absorbed energy. Under the influence of an electric field applied across the conversion element  20 , these electrons (holes) are transferred to associated charge collection electrodes  28 . Thus, the conversion element  20  is configured to produce one or more charge carriers in response to incident radiation. For example, the conversion element  20  can capture and convert incident X-ray photons directly into electric charge. 
     The readout substrate  24  comprises a plurality of readout circuits  30 - 1 ,  30 - 2 ,  30 - 3 ,  30 - 4 ,  30 - n . Each readout circuit  30 - 1 ,  30 - 2 ,  30 - 3 ,  30 - 4 ,  30 - n  may also be referred to with reference numeral “30”. Each readout circuit  30  comprises a readout electrode  32 , here implemented as a contact pad. Each readout circuit  30  is associated with a pixel  26 . 
     The radiation detector  12  further comprises a plurality of interconnections  34 . Each pair of one pixel  26  and one readout circuit  30  is connected by means of an interconnection  34 . In  FIG. 2 , the interconnections  34  are exemplified as solder bumps between the charge collection electrodes  28  and the associated readout electrodes  32 . Each readout electrode  32  thereby acts as the input to the associated readout circuit  30 . Other types of interconnections  34  are however conceivable. 
     Each readout circuit  30  further comprises at least one electronic component with a function specific for the associated pixel  26 . The readout circuit  30  are arranged to process signals generated by the radiation incident on the conversion element  20 . 
     The radiation detector  12  is configured to detect radiation repeatedly in measurement frames, where each measurement frame comprises a data acquisition period and a readout period. 
       FIG. 3  schematically represents a partial view of the readout substrate  24 . Each readout circuit  30  comprises at least one register (or counter). The register may be a special purpose register or counter. In the example in  FIG. 3 , each readout circuit  30  comprises a first register (or first counter)  36   a  and a second register (or second counter)  36   b . Each register  36   a ,  36   b  may also be referred to with reference numeral “36”. However, only one register  36 , or more than two registers  36  may alternatively be provided in each readout circuit  30 . Each register  36  is configured to temporarily store values corresponding to a converted electrical signal of at least one incoming radiation event from an associated pixel  26 . 
     Each readout circuit  30  is configured to process an input analog signal. As shown in  FIG. 3 , each readout circuit  30  comprises an amplifier  38 , a pulse shaper  40 , and two comparators  42   a ,  42   b , one associated with each register  36   a ,  36   b . The amplifier  38  is configured to receive and amplify the electrical signal from the associated pixels  26 . The pulse shaper  40  is configured to shape the waveform of the electrical signal from the associated pixel  26 . The pulse shaper  40  is operatively coupled to at least one input of each comparators  42   a ,  42   b.    
     Each amplifier  38  is configured to receive and amplify the electrical signal from the readout electrode  32 , which electrical signal is generated in response to incident single x-ray events. Thus, the incoming radiation hits for each photon are converted into an electrical signal, which is then amplified by the amplifier  38 . 
     The pulse shaper  40  of each readout circuit  30  is configured to shape the waveform of the electric pulse from the conversion element  20 . The pulse shaper  40  may for example create a bandwidth-limited semi-gaussian output pulse and may act as a noise filter. Each comparator  42   a ,  42   b  is connected to the pulse shaper  40 . Each comparator  42   a ,  42   b  is configured to compare the peak value of an electric pulse from the pulse shaper  40 , i.e. the energy of an X-ray photon detected by the conversion element  20 , with a threshold value. 
     The output of each comparator  42   a ,  42   b  is connected to an associated register  36   a ,  36   b . When the comparator  42   a ,  42   b  determines that the energy of an X-ray photon detected by the conversion element  20  has a peak value above the threshold value set in the comparator  42   a ,  42   b , the comparator  42   a ,  42   b  outputs an electric pulse to the register  36   a ,  36   b . The two comparators  42   a ,  42   b  are thereby configured to compare an electrical signal representative of the radiation energy from one of the pixels  26  against a first threshold value and a second threshold value. 
     Each register  36   a ,  36   b  of this example comprises a counter configured to count the number of photon pulses above a given energy level set by the associated comparator  42   a ,  42   b . Each register  36   a ,  36   b  will count the number of photon pulses in an energy range which corresponds to a given threshold value in the associated comparator  42   a ,  42   b.    
     The registers  36   a ,  36   b  count electric pulses from the comparators  42   a ,  42   b  during each data acquisition period. In response to receiving a readout trigger, e.g. from the control unit  14 , the registers  36   a ,  36   b  start counting electric pulses input from the comparator  42 . Every time an electric pulse is generated, the register  36   a ,  36   b  increments a stored number by one. In response to receiving the next readout trigger, the register  36   a ,  36   b  reads out data (count data) of the count number stored, and resets the data of the count number in the internal memory to an initial value (e.g. 0). 
     The imaging apparatus  10  further comprises a threshold setting device  44 . In the example in  FIG. 3 , the threshold setting device  44  is provided in the readout substrate  24 . The threshold setting device  44  is configured to set a threshold value for each register  36   a ,  36   b  in each readout circuit  30 . To this end, the threshold setting device  44  comprises a first digital to analog (DA) converter (DAC)  46   a  arranged to set a threshold value in the first comparator  42   a  of each readout circuit  30 , and a second DA converter  46   b  arranged to set a threshold value in the second comparator  42   b  of each readout circuit  30 . The threshold setting device  44  is controlled by the control unit  14  via a signal line  48 . 
     Each readout circuit  30  in  FIG. 3  further comprises a switch  50 . The switch  50  can be a single pole, double throw (SPDT) switch. Each switch  50  is switchable between a first state  52  and a second state  54 . The switching of all switches  50  is controlled by the control unit  14  via a signal line  56 . 
       FIG. 3  further shows a signal line  58 . The readout circuit  30 - 2  receives serial data from the previous readout circuit  30 - 1  via the signal line  58 . Data from the readout circuit  30 - 2  is then added to the serial data and passed on to the next readout circuit  30 - 3  and so on until serial data  60  from the last readout circuit  30 -n is read out. Thus, each readout circuit  30  between a first readout circuit  30 -α and a last readout circuit  30 -n includes the serial data output coupled to a serial data input of a next readout circuit. 
     The switch  50  is coupled to a serial data output of the second register  36   b  and a serial data input of the first register  36   a . The switch  50  is also coupled to the signal line  58  from the previous readout circuit  30 - 1 . 
     When the switch  50  is in the illustrated second state  54 , data from both the first register  36   a  and the second register  36   b  of each readout circuit  30  is added to the serial data. The second state  54  of the switch  50  thus corresponds to a multiple energy readout mode (dual energy readout mode in this example). In the second state  54  of the switch  50 , the second register  36   b  is coupled to the output of serial data  60  from the last readout circuit  30 -n. 
     When the switch  50  is in the first state  52 , only data from the first register  36   a  of each readout circuit  30  is added to the serial data. The first state  52  of the switch  50  thus corresponds to a single energy readout mode. By means of the switch  50 , each readout circuit  30  is configured to selectively bypass at least one register  36   b  of the readout circuit  30 . In the first state  52  of the switch  50 , the second register  36   b  is decoupled from the output of serial data  60  from the last readout circuit  30 -n. The switch  50  may be in the first state  52  for a duration at least 50% longer, such as at least 100% longer, than the second state  54 . 
       FIG. 4  schematically represents a partial view of an alternative example of a readout substrate  24 . Mainly differences with respect to  FIG. 3  will be described. In the example in  FIG. 4 , each readout circuit  30  comprises a first register (or first counter)  36   a , a second register (or second counter)  36   b  and a third register (or third counter)  36   c . Furthermore, each readout circuit  30  comprises an amplifier  38 , a pulse shaper  40 , and three comparators  42   a ,  42   b ,  42   c , one associated with each register  36   a ,  36   b ,  36   c.    
     In  FIG. 4 , each comparator  42   a ,  42   b ,  42   c  is connected to the pulse shaper  40 . Each comparator  42   a ,  42   b ,  42   c  is configured to compare the peak value of an electric pulse from the pulse shaper  40 , i.e. the energy of an X-ray photon detected by the conversion element  20 , with a threshold value. The three comparators  42   a ,  42   b ,  42   c  are thereby configured to compare an electrical signal representative of the radiation energy from the associated pixel  26  against a first threshold value, a second threshold value, and a third threshold value. The third register  36   c  is configured to store high energy data indicative of the electrical signal from the associated pixels  26  above the third threshold value representing the radiation energy above a second high energy level. The third register  36   c  is serially coupled to the second register  36   b.    
     The output of each comparator  42   a ,  42   b ,  42   c  is connected to an associated register  36   a ,  36   b ,  36   c . When the comparator  42   a ,  42   b    42   c  determines that the energy of an X-ray photon detected by the conversion element  20  has a peak value above the threshold value set in the comparator  42   a ,  42   b ,  42   c , the comparator  42   a ,  42   b ,  42   c  outputs an electric pulse to the register  36   a ,  36   b ,  36   c.    
     The threshold setting device  44  of the example in  FIG. 4  is configured to set a threshold value for each register  36   a ,  36   b ,  36   c  in each readout circuit  30 . To this end, the threshold setting device  44  comprises a first DAC  46   a  arranged to set a first threshold value in the first comparator  42   a  of each readout circuit  30 , a second DAC  46   b  arranged to set a second threshold value in the second comparator  42   b  of each readout circuit  30 , and a third DAC  46   c  arranged to set a third threshold value in the third comparator  42   b  of each readout circuit  30 . 
     When the switch  50  in  FIG. 4  is in the illustrated second state  54 , data from each of the first register  36   a , the second register  36   b  and the third register  36   c  of each readout circuit  30  is added to the serial data. The second state  54  of the switch  50  thus corresponds to a multiple energy readout mode (three energy level readout mode in this example). When the switch  50  is in the first state  52 , only data from the first register  36   a  of each readout circuit  30  is added to the serial data. The first state  52  of the switch  50  thus corresponds to a single energy readout mode. By means of the switch  50 , each readout circuit  30  is configured to selectively bypass the second register  36   b  and the third register  36   c  of the readout circuit  30 . 
     Although  FIG. 4  illustrates three registers  36   a ,  36   b ,  36   c  and comparator  42   a ,  42   b ,  42   c  in each readout circuit  30  and three DACs  46   a ,  42   b ,  42   c  in the threshold setting device  44 , in alternate examples, additional register(s) (represented as ellipses) and additional comparator(s) (represented as ellipses) and additional DAC(s) (represented as an ellipsis) may be used. The additional register(s) can be coupled between the registers  36   b  and  36   c  and the additional comparator(s) can be coupled to the additional register(s) similar to the coupling of registers  36   b  and  36   c  to comparators  42   b  and  42   c , respectively. The additional DAC(s) can be coupled to the additional comparator(s) similar to the coupling of DACs  42   b  and  42   c  to comparators  42   b  and  42   c . The additional DAC(s) can also be coupled to the signal line  48  similar to DACs  42   b  and  42   c.    
     The additional register(s), additional comparator(s), and additional DAC(s) can be used to read out multiple high energy levels with multiple different threshold values during each high energy readout period. For example, with the additional register(s), additional comparator(s), and additional DAC(s), four high energy levels can be read out with four different threshold values during each high energy readout period. 
       FIG. 5  graphically represents one example of a timing diagram of a method of reading out data in the radiation detector  12 .  FIG. 5  is a diagram where the ordinate shows the energy E and the abscissa shows the time t.  FIG. 5  shows a plurality of low energy readout periods  62  and a plurality of high energy readout periods  64 . In  FIG. 5 , every third readout period is a high energy readout period  64 . However, any “n” readout period may be a high energy readout period  64 , where “n” is a positive integer. In  FIG. 5 , two low energy readout periods  62  are provided between each pair of most adjacent high energy readout periods  64 . Each readout period  62 ,  64  is following a data acquisition period  66 . Each pair of data acquisition period  66  and a following readout period  62 ,  64  constitutes a measurement frame. 
     In the example in  FIG. 5 , acquisition of data is not performed during the readout period (dead time). Although the readout periods  62 ,  64  are illustrated with the same width as the data acquisition periods  66 , the data acquisition periods  66  are typically much longer, e.g. ten times longer, than the readout periods  62 ,  64 . The data acquisition periods  66  may be shorter than 10 ms, such as shorter than 5 ms, such as shorter than 1 ms, such as shorter than 0.5 ms. The readout periods  62 ,  64  may be shorter than 5 ms, such as shorter than 1 ms, such as shorter than 0.5 ms, such as shorter than 0.1 ms. 
     A low energy level  68  is set by means of a first threshold value and a high energy level  70  is set by means of a second threshold value. The first and second threshold values may be set by the first and second comparators  42   a ,  42   b , respectively (see  FIG. 3 ). According to one non-limiting example, the low energy level  68  may be 6 keV, and the high energy level  70  may be 35 keV. The first register  36   a  is configured to store low energy data indicative of the electrical signal from the associated pixel  26  above the first threshold value representing the radiation energy above the low energy level  68 . The second register  36   b  is configured to store high energy data indicative of the electrical signal from the associated pixel  26  above the second threshold value representing the radiation energy above the high energy level  70 . The registers  36   a ,  36   b  are further configured to readout the low energy data and the high energy data. 
     During each data acquisition period  66 , data indicative of the radiation detected by the pixels  26  is stored in the registers  36   a ,  36   b  of readout circuits  30  associated with the pixels  26 . During each readout period  62 ,  64 , data is read out from one or more registers  36   a ,  36   b  of each readout circuit  30 . During each low energy readout period  62 , only data indicative of radiation energy above the low energy level  68  is read out from each first register  36   a . During each high energy readout period  64 , data indicative of radiation energy above the high energy level  70  is read out. In the example in  FIG. 5 , data indicative of radiation energy above the low energy level  68  is read out from the first register  36   a , and data indicative of radiation energy above the high energy level  70  is read out from the second register  36   b , during each high energy readout period  64 . Thus, in  FIG. 5 , the high energy readout periods  64  constitute multiple energy readout periods. 
     The provision of the low energy readout periods  62 , during which only data indicative of radiation energy above the low energy level  68  is read out, enables the amount of data read out to be reduced. In addition, the low energy readout periods  62  can be shortened and the scanning can consequentially be made faster. Due to the provision of the high energy readout periods  64 , high quality multiple energy imaging (dual energy imaging in  FIG. 5 ) is still enabled. 
       FIG. 6  graphically represents a further example of a timing diagram of a method of reading out data in the radiation detector  12 . Mainly differences with respect to  FIG. 5  will be described. In  FIG. 6 , every fourth readout period is a high energy readout period  64 . 
     In  FIG. 6 , a low energy level  68 , a first high energy level  70   a , higher than the low energy level  68 , and a second high energy level  70   b , higher than the first high energy level  70   a , are used. Each high energy level may also be referred to with reference numeral “70”. In order to carry out the method in  FIG. 6 , each readout circuit  30  comprises three registers  36  with respective threshold values, as illustrated in  FIG. 4 . 
     In each low energy readout period  62 , data indicative of radiation energy above the low energy level  68  is read out. In each high energy readout period  64 , data indicative of radiation energy above the low energy level  68 , data indicative of radiation energy above the first high energy level  70   a , and data indicative of radiation energy above the second high energy level  70   b , is read out. 
       FIG. 7  graphically represents a further example of a timing diagram of a method of reading out data in the radiation detector  12 . Mainly differences with respect to  FIG. 6  will be described. In  FIG. 7 , every fifth readout period is a high energy readout period  64 . 
     In  FIG. 7 , data indicative of radiation energy above the low energy level  68  is read out during each low energy readout period  62 . However, during each high energy readout period  64 , data indicative of radiation energy above the low energy level  68  is not read out. Instead, data indicative of radiation energy above the first high energy level  70   a , and data indicative of radiation energy above the second high energy level  70   b , is read out during each high energy readout period  64 . The method in  FIG. 7  may be carried out with two registers  36  in each readout circuit  30 , one with a static threshold value (e.g. for the second high energy level  70   b ) and one with a variable threshold value (e.g. for the low energy level  68  and the first high energy level  70   a ), for example by means of a readout substrate  24  illustrated in  FIG. 3 . 
       FIG. 8  graphically represents a further example of a timing diagram of a method of reading out data in the radiation detector  12 . Mainly differences with respect to  FIGS. 5 to 7  will be described. In  FIG. 8 , every third readout period is a high energy readout period  64 . 
     The method in  FIG. 8  also employs a third high energy level  70   c , higher than the second high energy level  70   b . The method in  FIG. 8  may be carried out with only two registers  36  in each readout circuit  30  according to  FIG. 3 , one register  36  with a static first threshold value defining the low energy level  68 , and one register  36  with a variable second threshold value that alternatingly defines the first high energy level  70   a , the second high energy level  70   b , and the third high energy level  70   c . In this case, the DAC  46   b  may variably set the second threshold value to different values in one or more readout periods. Also in  FIG. 8 , each high energy readout period  64  is a multiple energy readout period. 
       FIG. 9  graphically represents a further example of a timing diagram of a method of reading out data in the radiation detector  12 . Mainly differences with respect to  FIGS. 5 to 8  will be described. The method in  FIG. 9  employs a low energy level  68  and a high energy level  70 , higher than the low energy level  68 . However, each low energy readout period  62  and each high energy readout period  64  is a single energy readout period. Thus, the method in  FIG. 9  can be carried out with only one single register  36  in each readout circuit  30 . The threshold value in the register  36  is altered between the low energy level  68  for the low energy readout periods  62 , and the high energy level  70  for the high energy readout periods  64 . The switch  50  remains in the first state  52 , so only data from the first register  36   a  of each readout circuit  30  is added to the serial data, as illustrated in  FIGS. 3 and 4 . 
       FIG. 10  graphically represents a further example of a timing diagram of a method of reading out data in the radiation detector  12 . Mainly differences with respect to  FIG. 5  will be described. In the example in  FIG. 10 , each data acquisition period  66  starts before the end of the readout period  62 ,  64  of the immediately preceding measurement frame. More specifically, each data acquisition period  66  starts at the same time as the start of the readout period  62 ,  64  of the immediately preceding measurement frame. Thus, the data acquisition periods  66  and the readout periods  62 ,  64  partly overlap. Each readout period  62 ,  64  still follows a preceding data acquisition period  66 . 
     While the present disclosure has been described with reference to exemplary embodiments, it will be appreciated that the present invention is not limited to what has been described above. For example, it will be appreciated that the dimensions of the parts may be varied as needed. Accordingly, it is intended that the present invention may be limited only by the scope of the claims appended hereto.