Patent Publication Number: US-2015063527-A1

Title: Conventional imaging with an imaging system having photon counting detectors

Description:
The following generally relates to conventional imaging with an imaging system having photon counting detectors and finds particular application to computed tomography (CT); however, the following is also amenable to other imaging including, but not limited to, x-ray and mammography. 
     A conventional (integrating) computed tomography (CT) scanner includes an x-ray tube supported by a rotating frame. The rotating frame and hence the x-ray tube rotate around an examination region, and the x-ray tube emits polychromatic radiation that traverses the examination region and a subject and/or object disposed therein. A radiation sensitive detector is located opposite the x-ray tube, across the examination region, and detects radiation that traverses the examination region and the subject and/or object. The radiation sensitive detector includes a one or two dimensional array of integrating detector pixels, such as scintillator/photosensor based pixels along with corresponding integrating electrical circuitry. Generally, the scintillator produces light in response to absorbing incident photons, the photosensor produces electrical charge indicative of the absorbed photons in response to receiving the light, and the integrating electrical circuitry accumulates the charge and generates projection data indicative of the detected radiation. 
     A reconstructor reconstructs the projection data and generates volumetric image data indicative of the subject and/or object. An image processor can be used to process the volumetric image data and generate one or more images indicative of the subject and/or object. Generally, the volumetric image data/image include voxels/pixels that are represented in terms of gray scale values corresponding to relative radiodensity. Such information reflects the x-ray attenuation characteristics of the scanned subject and/or object, and generally shows structure such as anatomical structures within a subject, physical structures within an inanimate object, etc. However, since the absorption of a photon by a material is dependent on the energy of the photon traversing the material, the detected radiation also includes spectral information, which provides additional information indicative of an elemental composition (e.g., atomic number) of the tissue and/or material. Unfortunately, the volumetric image data generated in conventional (integrating) CT does not reflect the spectral characteristics, as the signal generated by the detector is proportional to the energy fluence integrated over the energy spectrum. 
     A spectral CT scanner, in addition to the components discussed above, includes one or more components that capture the spectral characteristics of the detected radiation. An example of such a component(s) is a photon-counting detector including a direct conversion semiconductor material such as Cadmium Telluride (CdTe), Cadmium Zinc Telluride (CZT) or the like, and corresponding processing circuitry. With such a detector, each pixel produces an electrical signal for each photon it detects, and the electrical signal is indicative of the energy of that photon. An amplifier amplifies the signal, and a signal shaper shapes the amplified signal, forming an electrical pulse having a height or peak indicative of the energy of the photon. A discriminator compares the amplitude of the pulse with one or more energy thresholds that are set in accordance with different energy levels corresponding to mean emission levels of the x-ray tube. A counter counts, for each threshold, the number of times the amplitude exceeds the threshold, and a binner bins or assigns a detected photon to an energy range based on the counts. The resulting energy-resolved detected radiation can be reconstructed using a spectral and/or conventional reconstruction algorithm, producing spectral and/or conventional image data and/or images. 
     The behavior of a CdTe or CZT based photon-counting detector can be modeled with sufficient accuracy either using a paralyzable detector model or a non-paralyzable detector model, depending on the sensor and detector electronics. Below we discuss difficulties in the reconstruction of the incident rate from ambiguous measurements of the output count rate in a detector system that can be described by the paralyzable detector model. Generally, a paralyzable detector is one in which each detected photon has a non-zero (e.g., 10 to 100 nanosecond) resolving or dead time such that if another photon is detected during the dead time, the detector will not be able to resolve the individual photon detections and the resulting pulse will have amplitude depending on the amplitudes of the individual pulses and the time difference between their arrivals. The dead time is a function of a width of the pulses generated by the shaper, and the output photon count rate of a paralyzable detector is a function of a number of incident x-ray photons per time (the input photon count rate) and the dead time. The output photon count rate can be expressed as shown in EQUATION 1: 
         m=r·e   −r·τ   EQUATION 1:
 
     where m represents the output photon count rate, r represents the input photon count rate, and τ represents the dead time. An example of this behavior is shown graphically in  FIG. 1  through curve  100 , where a y-axis  102  represents the average output photon count rate m and an x-axis  104  represents the input photon count rate r as a function of the dead time τ. In  FIG. 1 , a peak  106  of the curve  100  represents a maximum output photon count rate  108 , which occurs at an input photon count rate  110  that corresponds to r MAX =1/τ. For output photon count rates (e.g., an output photon count rate  112 ) less than the maximum output photon count rate  108 , there exist two possible input photon count rates (e.g., input photon count rate  114  and input photon count rate  116 ), one less than r MAX  and one greater than r MAX . 
     In order to correctly reconstruct the data corresponding to the output photon count rate  112  to generate conventional image data, the correct input photon count rate  114  or  116  corresponding to the data needs to be known. Reconstructing the data with the incorrect input photon count rate introduces artifact into the images, which may render the images unsuitable for diagnostic purposes. By way of example,  FIG. 2  shows an image reconstructed from simulated data for a conventional integrating detector, and  FIG. 3  shows an image reconstructed from simulated data for a counting detector where the correct input photon count rate is known. Note that visually the image of  FIG. 3  is very similar to the image of  FIG. 2 , but has a slightly higher noise level. In contrast,  FIG. 4  shows an image reconstructed from simulated data for a counting detector under the assumption that r is greater than r MAX  in all cases, and  FIG. 5  shows an image reconstructed from simulated data for a counting detector under the assumption that r is smaller than r MAX  in all cases.  FIGS. 4 and 5  visually show that reconstructing images using the incorrect input photon count rate introduces artifact. 
     Aspects described herein address the above-referenced problems and others. 
     In one aspect, an imaging system includes a radiation source that emits polychromatic radiation that traverses an examination region and a detector array located opposite the radiation source, across the examination region, which includes a paralyzable photon counting detector pixel that detects photons of the radiation that traverse the examination region and illuminate the detector pixel and that generates a signal indicative of each detected photon. An output photon count rate to input photon count rate map includes at least one map which maps multiple input photon count rates of the detector pixel to a single output photon count rate of the detector pixel, and an input photon count rate determiner identifies one input photon count rate of the multiple input photon count rates of the map as a correct input photon count rate for the detector pixel. A reconstructor that reconstructs the signal based on the identified input photon count rate. 
     In another aspect, a method includes receiving an output signal of a paralyzable photon counting detector pixel that is receiving photons at an input photon count rate. The method further includes determining an output photon count rate of the detector pixel. The method further includes identifying an input photon count rate, from multiple candidate input photon count rates for the output photon count rate, as the input photon count rate corresponding to the detector pixel and the output photon count rate. The method further includes reconstructing the output signal based on the identified input photon count rate. 
    
    
     
       The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. 
         FIG. 1  graphically illustrates example of the output photon count rate behavior of a paralyzable photon counting detector as a function of input photon count rate and pulse shaping dead time. 
         FIG. 2  illustrates an image produced based on simulated data from a conventional detector. 
         FIG. 3  illustrates a conventional image produced based on simulated data from a paralyzable photon counting detector and using the correct input photon count rate. 
         FIG. 4  illustrates a conventional image produced based on simulated data from a paralyzable photon counting detector and using an input photon count rate that is less than r MAX  when the correct input photon count rate is greater than r MAX . 
         FIG. 5  illustrates a conventional image produced based on simulated data from a paralyzable photon counting detector and using an input photon count rate that is greater than r MAX  when the correct input photon count rate is less than r MAX . 
         FIG. 6  schematically illustrates an example CT imaging system having a photon counting detector and in connection with an input photon count rate determiner. 
         FIG. 7  schematically illustrates an example of the input photon count rate determiner of  FIG. 6  which uses a time over threshold value to determine the correct input photon count rate. 
         FIG. 8  graphically illustrates an example of a measured output photon count rate of a detector pixel for an integration period for a lower input photon count rate. 
         FIG. 9  graphically illustrates an example of a measured output photon count rate, which has the same value as that of  FIG. 8 , of a detector pixel for an integration period for a relatively higher input photon count rate. 
         FIG. 10  graphically illustrates the amount of time pulses are above a given threshold as a function of input photon count rate. 
         FIG. 11  schematically illustrates an example of the input photon count rate determiner of  FIG. 6  which uses a pulse pile-up count to determine the correct input photon count rate. 
         FIG. 12  schematically illustrates an example of the input photon count rate determiner of  FIG. 6  which uses information from at least two different size detector pixels to determine the correct input photon count rate. 
         FIG. 13  schematically illustrates an example of the input photon count rate determiner of  FIG. 6  which uses at least two different shaping times to generate information to determine the correct input photon count rate. 
         FIG. 14  schematically illustrates an example of the input photon count rate determiner of  FIG. 6  which uses information generated from using at least two different flux rates to generate information to determine the correct input photon count rate. 
         FIG. 15  graphically illustrates an example of the counts in a bin as a function of the input photon count rate in connection with  FIG. 1 . 
         FIG. 16  illustrates an example method. 
     
    
    
     The following describes an approach for generating conventional images with a spectral imaging system having paralyzable photon-counting detectors where the images have an image quality that is similar to an image quality of images generated with a conventional (non-spectral) imaging system. 
     Initially referring to  FIG. 6 , an example spectral CT scanner  600  is illustrated. The CT scanner  600  includes a generally stationary gantry  602  and a rotating gantry  604 , which is rotatably supported by the stationary gantry  602  and rotates around an examination region  606  about a z-axis. A radiation source  608 , such as an x-ray tube, is rotatably supported by the rotating gantry  604 , rotates with the rotating gantry  604 , and emits polychromatic radiation that traverses the examination region  606 . 
     A radiation sensitive detector array  610  subtends an angular arc opposite the radiation source  608  across the examination region  606 . The radiation sensitive detector array  610  detects radiation traversing the examination region  606  and generates a signal indicative thereof for each detected photon. In the illustrate embodiment, the radiation sensitive detector array  610  is a photon-counting detector array with a one or two dimensional array of photon-counting detector pixels  611  that include direction conversion material such as CdTe, CZT, and/or other paralyzable direct conversion material. 
     For each detector pixel  611 , an optional amplifier  612  amplifies the signal. A shaper  614  processes the amplified signal and generates a pulse such as voltage or other pulse indicative of the energy of the detected photon. A discriminator  616  energy discriminates the pulse. In the illustrated example, the energy discriminator  616  includes one or more comparators  618  that compare the amplitude of the pulse with different energy thresholds, which correspond to different energies of interest. The discriminator  616  produces an output (e.g., high or low, 0 or 1, etc.) that indicates whether, for each threshold, the amplitude exceeds the threshold. 
     A counter  620  increments a count value for each threshold based on the output of the discriminator  616 . For instance, when the output of the comparator  618  for a particular threshold indicates that the amplitude of the pulse exceeds the corresponding threshold, the count value for that threshold is incremented. A binner  622  energy bins the signals and, hence, the photons into two or more energy bins based on the counts. Generally, an energy bin encompasses an energy range or window. For example, a bin may be defined for the energy range between two thresholds, where a photon resulting in a count for the lower threshold but not for higher threshold would be assigned to that bin. 
     An input photon count rate determiner  624  determines the correct input photon count rate, from multiple candidate input photon count rates, for each detector pixel  611  each integration period from at least an output photon count rate to input photon count rate map  626 , which includes a first sub-map  626   1  corresponding to r&lt;r MAX  ( FIG. 1 ) and a second sub-map  626   2  corresponding to r&gt;r MAX  ( FIG. 1 ). The map  626  can be generated based on an air scan using different and known flux rates and/or a series of calibration scans using various thicknesses of tissue equivalent materials of known attenuation properties. 
     As described in greater detail below, the input photon count rate determiner  624  determines the correct input photon count rate based on one or more approaches including, but not limited to, an amount time pulses generated in response to detecting photons exceed a threshold during an integration period, a number of detected pulse pile-ups in an integration period, a ratio of input photon count rates for different detectors having different and known radiation sensitive areas, a ratio of input photon count rates for different radiation source emission fluxes, estimates based on a sinogram, a distribution of count values of an energy bin, and/or otherwise. 
     A reconstructor  628  reconstructs the data based on the input photon count rate determined by the input photon count rate determiner  624 , generating volumetric image data, which can be processed to produce one or more conventional images. As discussed herein, reconstructing the data using the correct one of the multiple candidate input photon count rates mitigates artifact introduced by reconstructing the data based on an incorrect one of the input photon count rates, and facilitates generating images having an image quality comparable to an image quality of images generated with a conventional CT scanner. 
     A subject support  630 , such as a couch, supports an object or subject in the examination region  606 . A general-purpose computing system or computer serves as an operator console  632 . The console  632  includes a human readable output device such as a monitor and an input device such as a keyboard, mouse, etc. Software resident on the console  632  allows the operator to interact with and/or operate the scanner  600  via a graphical user interface (GUI) or otherwise. 
       FIG. 7  illustrates a non-limiting example of the input photon count rate determiner  624  for one of the detector pixels  611 . 
     A shaper  700  receives the output signal from the detector pixel  611  and generates, for each detected photon, a pulse such as voltage or other pulse having a peak height indicative of an energy of the detected photon. A comparator  702  compares an amplitude of the pulses with a photon detection identifying threshold (TH PDI )  704  and produces an output (e.g., high or low, 0 or 1, etc.) that indicates whether the amplitude exceeds the threshold. In one instance, the value of the threshold  704  is at or just above a noise level of the detector pixel  611 , which facilitates discriminating between detected photons and noise. 
     A counter  706  increments a count value each time the output of the comparator  702  transitions from indicating the output is below the threshold  704  to indicating the output has exceeded the threshold  704 . As discussed herein, such a transition may be indicative of an individual photon detection or multiple piled up (overlapping) photons. The counter  706  resets each integration period, for example, upon receiving an integration period (IP) trigger signal, and outputs the count value, which is a measure of the output photon count rate for the corresponding integration period. The integration period time can be measured, or a predetermined static value can be used. 
     Briefly turning to  FIGS. 8 and 9 , examples of two different input photon count rates resulting in a same measured output photon count rate, due to pule pile-up, are shown. In  FIG. 8 , a lower input photon count rate of six photons within a predetermined period of time (e.g., an integration period) results in an output photon count rate of five photons with pulses for two of the detected photons overlapping such that they cannot be individually resolved. In  FIG. 9 , a higher input photon count rate of fifteen photons within the same period of time also results in an output photon count rate of five photons within the same period of time due to overlapping pulses which cannot be individually resolved. As a result, the correct input photon count rate of the multiple candidate input photon count rates of the map  626  cannot be determined from the measured output photon count rate alone. 
     Returning to  FIG. 7 , a timer  708  times the amount of time the amplitude of the output of the comparator  702  indicates the threshold  704  is exceeded. That is, the timer  708  is activated in response to the output of the comparator  702  rising to or above the threshold  704  and continues until the output falls below the threshold  704 . The timer  708  resets each subsequent integration period, for example, upon receiving the IP trigger signal, and outputs a time over threshold value. Briefly turning to  FIG. 10 , a time over threshold curve  1000  is graphically illustrated as a function of input photon count rate, in which a y-axis  1002  represents time over threshold within one integration period, and an x-axis  1004  represents the input photon count rate. As shown, the time over threshold increases monotonically, unlike the measured output photon count rate ( FIG. 1 ), even once r max  is reached and exceeded. 
     Returning to  FIG. 7 , logic  710  receives the time over threshold value, compares it with an input photon time-over-threshold level (TH TOTL )  712 , and identifies the sub-map  626   1  as the correct sub-map in response to the time over threshold value falling under the threshold  712  or the sub-map  626   2  as the correct sub-map in response to the time over threshold value meeting or exceeding the threshold  712 . The logic  710  populates a two-dimensional matrix, which corresponds to the sinogram, which indicates the correct sub-map for each data point in the sinogram. 
     In this embodiment, the logic  710  utilizes the matrix, the measured output photon count rate, and the map  626  to obtain the input photon count rate for reconstruction. In another embodiment, the reconstructor  628 , the console  632  and/or other component utilizes the matrix, the measured output photon count rate, and the map  626  to obtain the input photon count rate for reconstruction. In such an embodiment, the counter  706  can be omitted from the input photon count rate determiner  624 . For sake of brevity and clarity, the counter  706  is not shown in the following embodiments, but can be included therewith. Wherein included, the logic in the following embodiments can employ the output thereof as discussed in connection with  FIG. 7  to determine the input photon count rate and/or otherwise. 
       FIG. 11  illustrates another non-limiting example of the input photon count rate determiner  624 . In this example, a shaper  1100  and a comparator  1102  operate substantially similar to the shaper  700  and the comparator  702  of  FIG. 7 . However, in this example, the comparator  1102  compares the amplitude of the output of the shaper  1100  with pulse pile-up identifier threshold (TH PPI )  1104 , which has a value that is larger than the radiation source emission voltage. As such, the amplitude of the output of the shaper  1100  will only exceed TH PPI  when there is a pulse pile-up event in which individual pulses overlap and combine to produce an amplitude that exceeds TH PPI . A counter  1106  counts pulse pile-up events and the logic  1108  compares the count value with a pulse pile-up level threshold (TH PPL )  1110 . Logic  1108  operates substantially similar to the logic  710  of  FIG. 7  and at least identifies the correct sub-map based on the comparison. 
       FIG. 12  schematically illustrates a non-limiting example of the input photon count rate determiner  624  in connection with at least two detector pixels  1202  and  1204 , which have different size radiation sensitive areas. In this example, a first processing chain  1206  processes data corresponding to the detector pixel  1202  and a second processing chain  1208  processes data corresponding to a detector pixel  1204 . The processing chains  1206  and  1208  respectively include shapers  1210  and  1212 , comparators  1214  and  1216 , and counters  1218  and  1220 , which operate substantially similar to the shaper  700 , the comparator  702 , and the counter  706  of  FIG. 7 . The comparators  1214  and  1216  can employ a threshold (TH) similar to that of  FIG. 7  or  11 , or a different threshold. 
     In this example, the detector pixel  1202  has a radiation sensitive area that is x (where x is a real number greater than zero) times the size of the radiation sensitive area of the detector pixel  1204 . Both detector pixels  1202  and  1204  are irradiated by the same input photon count rate (IPCR) per mm 2 , but due to their different size radiation sensitive areas, they will see different IPCR&#39;ss; namely, the smaller area detector pixel  1204  will see IPCR s  and the larger area detector pixel  1202  will see IPCR b =x·IPCR s . As such, there will be two different output photon count rates (OPCR) measured by the counters  1218  and  1220 , namely m b  and m s . IPCR s  can be expressed as shown in EQUATION 2: 
     
       
         
           
             
               
                 
                   
                     
                       IPCR 
                       s 
                     
                     = 
                     
                       
                         ln 
                          
                         
                           ( 
                           
                             ms 
                             xmb 
                           
                           ) 
                         
                       
                       
                         
                           ( 
                           
                             1 
                             - 
                             x 
                           
                           ) 
                         
                          
                         τ 
                       
                     
                   
                   , 
                 
               
               
                 
                   EQUATION 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
     and IPCR b  can be expressed as shown in EQUATION 3: 
       IPCR b   =x· IPCR s   EQUATION 3:
 
     In this example, the output photon count rate to input photon count rate maps  626  can include separate maps for the different size detector pixels  1202  and  1204 , or separate maps  626  can be created for the different size detector pixels  1202  and  1204 . Logic  1222  determine the correct input photon count rate from the output photon count rate to input photon count rate maps  626  of the detector pixels  1202  and  1204  and the measurements m b  and m s  by selecting the input photon count rates that satisfies EQUATION 3. Where the smaller area detector pixel  1204  always detects an incoming rate below r max , a well-defined solution exists for the input photon count rate. 
       FIG. 13  schematically illustrates another non-limiting example of the input photon count rate determiner  624 . In this example, a shaper  1300 , a comparator  1302 , and a counter  1304  operate substantially similar to the shaper  700 , the comparator  702 , and the counter  706  of  FIG. 7 . However, in this example, the input photon count rate determiner  624  further includes a shaper controller  1306 , which switches a shaping time of the shaper  1300  between at least two different shaping times each integration period. By way of non-limiting example, in one instance, the shaper controller  1306  switches the shaping time between τ 1  and τ 2 . As a result, there will be two curves  100  ( FIG. 1 ) and two different r max &#39;s for each detector pixel, one for τ 1  and τ 2 . In addition, there will be two different output photon count rates (e.g., m 1  and m 2 ). The logic  1308  determines the input photon count rate based on τ 1 , τ 2 , m 1  and m 2  as shown in EQUATION 4: 
     
       
         
           
             
               
                 
                   
                     IPCR 
                     = 
                     
                       
                         ln 
                          
                         
                           ( 
                           
                             
                               m 
                               2 
                             
                             
                               m 
                               1 
                             
                           
                           ) 
                         
                       
                       
                         
                           τ 
                           1 
                         
                         - 
                         
                           τ 
                           2 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   EQUATION 
                    
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
     where m i =IPCR exp(−IPCR τ i ) and i=1, 2. In one instance, the value of one of τ 1  or τ 2  is such that r max  is greater than the input photon count rate, which allows for a well-defined solution for the input photon count rate. In an alternative embodiment, a second counter can be implemented without energy discrimination and a shorter τ. 
     In a variation of the above, the shaper  1300  includes a plurality of sub-shapers, and at least two of the sub-shapers have different static or switchable shaping times. In one instance, at least two of the plurality of sub-shapers share the comparator  1302  and/or the counter  1304 . In another instance, the comparator  1302  and/or the counter  1304  respectively include two or more sub-comparators and/or sub-counters, and the output of the at least two of the plurality of sub-shapers is processed by different sub-comparators and/or counters. In yet another variation, the input photon count rate determiner  624  includes two or more data pipelines or chains, each including a different shaper  1300 , a different comparator  1302  and/or a different counter  1304 . 
       FIG. 14  schematically illustrates another non-limiting example of the input photon count rate determiner  624 . In this example, a shaper  1400 , a comparator  1402 , and a counter  1404  operate substantially similar to the shaper  700 , the comparator  702 , and the counter  706  of  FIG. 7 . In this example, the imaging system  600  further includes a source controller  1406 , which is configured to switch the x-ray flux of the radiation source  608  between at least two different levels during scanning. Generally, a reduction of the incoming rate leads to an a decrease of the output count rate m, if the rate r is far below r max , and an increase of the output count rate m, if r is far above r max , compared to the signal in the long time period at a high flux. Similar to using two pixels with different areas described above in connection with  FIG. 12 , each of the at least two different fluxes will have a corresponding different output photon count rate (e.g., m h  and m l ), and logic  1408  can determine the input photon count rates based on m 1  and m s  as shown in EQUATIONS 5 and 6: 
     
       
         
           
             
               
                 
                   
                     
                       IPCR 
                       1 
                     
                     = 
                     
                       
                         ln 
                          
                         
                           ( 
                           
                             
                               m 
                               s 
                             
                             
                               xm 
                               l 
                             
                           
                           ) 
                         
                       
                       
                         
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                             1 
                             - 
                             x 
                           
                           ) 
                         
                          
                         τ 
                       
                     
                   
                   , 
                   and 
                 
               
               
                 
                   EQUATION 
                    
                   
                       
                   
                    
                   5 
                 
               
             
             
               
                 
                   
                     
                       IPCR 
                       s 
                     
                     = 
                     
                       x 
                       · 
                       
                         IPCR 
                         1 
                       
                     
                   
                   , 
                 
               
               
                 
                   EQUATION 
                    
                   
                       
                   
                    
                   6 
                 
               
             
           
         
       
     
     where x represents the ratio of the two different flux rates. 
     In another example, a distribution of the counted numbers of photons in an energy bin is used to estimate the amount of pile-ups, which can be used as a measure for the incoming rate r. This is described in connection with  FIG. 15 , which includes  FIG. 1  and additionally a second curve  1500 , which represents the counts in bin, which correspond to a difference between the two counters defining the bin. Note that the curve  1500  increases with increasing IPCR up to a point at which the curve  1500  begins to decreases due to increasing pulse pile up. As such, the count value of one or more energy bins can be monitored and used to facilitate determining the correct input photon count rate for each detector pixel within each integration period. 
     In another example, the decision for every detector pixel is made by looking at the sinogram and using prior knowledge. For example, in one instance, high flux conditions can be assumed to be at a periphery of the sinogram and low flux conditions can be assumed to be at a center region of the sinogram. 
     In another embodiment, a combination of the approaches discussed herein and/or one or more other approaches can be used to facilitate determining the input photon count rate for each detector pixel within each integration period. 
       FIG. 16  illustrates an example method in accordance with the embodiments described herein. 
     It is to be appreciated that the ordering of the acts in the methods described herein is not limiting. As such, other orderings are contemplated herein. In addition, one or more acts may be omitted and/or one or more additional acts may be included. 
     At  1602 , an object and/or subject is scanned with an imaging system, which includes direct conversion material based photon counting detectors, producing projection data indicative of the scanned object and/or subject. 
     At  1604 , one or more output photon count rate to input photon count rate maps are obtained in which a map includes at least two input photon count rates for each output photon count rate. 
     At  1606 , one of the at least two input photon count rate is identified as the correct input photon count rate for each detector pixel each integration period using one or more of the embodiments described herein. 
     At  1608 , the projection data is reconstructed based on the identified input photon count rates. 
     The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.