Abstract:
A method for determining a particle impact on a sensor device comprising M sensing areas, each impact causing a variation of at least one physical magnitude of several sensing portions, said method comprising the steps of: determining an impact center sensing area, where the variation of the at least one physical magnitude is substantially maximal, allocating a result of a sum of the physical magnitude variations of a first set of N sensing areas, said set including the impact center area and other sensing areas neighboring said impact center sensing area, where N is at least 2 and is strictly less than M.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to methods for determining a particle impact and sensor device for sensing and determining particle impacts. 
       BACKGROUND OF THE INVENTION 
       [0002]    In imaging systems, for example adapted for X-ray, the sensor device comprises an array of sensing areas, for sensing a photon impact. The impact of a photon causes a variation of a physical magnitude, for instance the charge or the voltage of a sensing area. In single photon counting sensing devices, each impinging photon is counted and allocated to the area on which it impacted. However, for sensing device with high resolution, and where the pixel pitch is similar to or smaller than the sensor device thickness, the energy deposited by the impinging photon is shared by a plurality of sensing areas. Consequently, the variation of physical magnitude of the impact center sensing area is not exactly representative of the energy of the impinging photon. For example, the spectrum resulting of this sensing device is erroneous and does not correspond to the actual X-ray emission received by the sensing device. 
         [0003]    Moreover, a single X-ray photon impinging in the vicinity of the edge of a sensing area could be detected as two incoming photons with only half energy. 
         [0004]    It is thus an object of the present invention to provide a sensing device and a method of determining a particle impact, like X-ray photons which can output an energy spectrum which is representative of the actual X-ray radiation. 
         [0005]    Another object of the invention is to provide a method of determining X-ray photons impinging on a sensor device, with a high signal to noise ratio. 
         [0006]    The invention proposes a method for determining a particle impact on a sensor device comprising M sensing areas to said impact center, each impact causing a variation of at least one physical magnitude of several sensing portions, 
         [0000]    said method comprising the steps of:
       (a) determining an impact center sensing area, where the variation of the at least one physical magnitude is substantially maximal;   (b) allocating a result of a sum of the physical magnitude variations of a first set of N sensing areas to said impact center, said set including the impact center area and other sensing areas neighboring said impact center sensing area, where N is at least  2  and is strictly less than M.       
 
         [0009]    Thus, in this method, the magnitude variations of a limited set of sensing areas may be taken into account. This method provides then signals which really correspond to the impinging particle, without adding too much noise, since the number of sensing areas is limited. Moreover, the resolution of an image obtained by this method is improved since each impinging particle is allocated to one pixel. Consequently, the image quality is increased. 
         [0010]    According to a first embodiment, the method further comprises the step (c) of selecting the first set of sensing areas, among a plurality of possible sets of sensing areas before the step (b) of allocating the result. Thus, the set is selected on the basis of several criteria. This enables to choose the best set among a plurality of possible sets, increasing then the quality of the resulting signal. The possible sets of sensing areas comprise all the possible combinations of N sensing areas including the center sensing area and other sensing areas neighboring the impact center area. Since the impinging particle will affect mainly the impact sensing area and its neighboring areas, these possible sets provides generally better results. 
         [0011]    According to variant of the invention, if the particle impact is detected as a high energy particle impact, the method further comprises the step of (d) allocating a result of a sum of the physical magnitude variations of a second set of P sensing areas, said second set including the impact center area and other sensing areas neighboring said impact center sensing area, where P is greater than n. Indeed, if the energy of the particle is high, the impact will affect a larger number of sensing areas. Consequently, a larger set of sensing areas will be selected in such a case, providing a signal more representative of the high energy particle. 
         [0012]    By taking N=4, the resulting signal is closer to the total signal deposited in the sensor, but does not increase in a too large amount the noise of the signal. The signal to noise ratio is then maintained acceptable with such a compromise. 
         [0013]    The invention also concerns a sensor device for determining a particle impact comprising M sensing areas, each impact causing a variation of at least one physical magnitude of several sensing areas, 
         [0000]    said sensor device further comprising:
       means for determining an impact center sensing area, where the variation of the at least one physical magnitude is substantially maximal;   means for summing the physical magnitude variations of a first set of N sensing areas including the impact center area and other sensing areas neighboring said impact center sensing area, where N is at least 2 and strictly less than M and providing a result;   means for allocating to said impact center area the result corresponding to said first set.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1A-1D  illustrate a sensor device according to the invention; 
           [0018]      FIGS. 2A and 2B  illustrate a sensor device according to a variant of the invention; 
           [0019]      FIGS. 3A-3C  illustrate a sensor device according to a further variant of the invention; 
           [0020]      FIG. 4  represents schematically the electronic circuit of the device of  FIGS. 2A and 2B ; 
           [0021]      FIG. 5  is a flowchart of a method of detecting particle impact according to the invention; 
           [0022]      FIG. 6  is a flowchart of a method of detecting an particle impact according to a variant of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    On  FIGS. 1A to 1D , a detection matrix  1  of a sensor device in conformity with the invention is illustrated. This detection matrix  1  comprises M sensing areas, or pixels  2 . Such a matrix is arranged to detect and to sense a particle impact on each of the M pixels. For instance, a particle impact is schematically represented by the “X”, with the reference  4 , on an impact center pixel. 
         [0024]    When a particle, like a photon for example, impacts on the detection matrix  1 , a physical magnitude of pixels is affected by the energy absorption due to the particle impact and any diffusion of the signal in the sensor. However, as illustrated by the arrows  5  on  FIG. 1A , the impact energy may be transmitted to a plurality of pixels. This physical magnitude may be the voltage, the charge, the temperature or likewise. 
         [0025]    Consequently, according to the invention, and as illustrated on each  FIG. 1A to 1D , the sensor device senses the magnitude variation of a set of sensing areas or set of pixels,  3   a  to  3   d.  These sets of 3 pixels may be constituted of the impact center pixel and two neighboring pixels. The  FIGS. 1A to 1D  introduce all the possible sets of 3 pixels according to this embodiment. According to the invention, the best set of pixels is selected, and the sum of the physical magnitude variation of the set pixels is affected to the impact center pixel. The best set is the set whose physical magnitude variation sum result is maximal. 
         [0026]    As illustrated by  FIGS. 2A and 2B , the number of pixels in a set may vary, dependant on the desired accuracy, and on the signal to noise ratio limit. On these figures, the shown sets of pixels  6   a  and  6   b  comprise 4 pixels, including the impact center pixel. It is also possible to have sets of 5 or more pixels, including the impact center pixel, depending on the requirements of accuracy and signal to noise ratio. 
         [0027]    According another embodiment shown on  FIGS. 3A and 3B , the sensing device comprises a detection matrix  1 ′, where each column is shifted of a half pixel relating to the preceding column. In such a case, an adapted choice of the number of pixels by set may be 3, as shown with the sets  7   a  and  7   b  respectively on  FIGS. 3A and 3B . These sets permit to obtain further information of the distance between the impact center and a corner. A variant of this embodiment is shown by  FIG. 3C , where the set of pixels  8  is constituted of the 6 pixels surrounding the impact center pixel. In this example, the accuracy of the signal is improved, but the signal to noise ratio is reduced. 
         [0028]    The  FIG. 4  is a schematic view of the electronic circuit of the detection matrix  1  of  FIG. 2A  or  2 B. This detection matrix  1  comprises pixels P 1  to P 9 . For each pixel P 1  to P 9 , an electronic detection circuit may be formed. This detection circuit comprises for instance a photodiode,  10  which turns the radiation received from the particle impact into an electric current. This current may load a capacitance  12 , which is connected in parallel to an Operational Transconductance Amplifier (OTA)  11 . The OTA  11  allows the current resulting from the particle impact on the photodiode to be integrated on capacitance  12  producing a voltage Vp 1  which is proportional to the total charge deposited by the incoming particle in the single photodiode. This voltage is then transformed into a current Ip 1  by the transconductance stage gm. 
         [0029]    This detection circuit provides then a signal which is representative of the energy transmitted by the impact in the considered pixel P 1 . Such a detection circuit is provided for each pixel P 1  to P 9 . 
         [0030]    For each possible set of 4 pixels, two circuits  14  and  15  are built, and the corresponding detection circuits of the respective pixels of the sets provide their signals Vp 1 , Vp 2 , Vp 4 , Vp 5 , and Ip 1 , Ip 2 , Ip 4 , Ip 5  to these circuits  14  and  15 . 
         [0031]    The circuit  14  permits to determine whether an impact occurred in this set, and whether this set is the most representative set of the detection matrix  1 . Indeed, this circuit may compare the signals Ip 1 , Ip 2 , Ip 4  and Ip 5  to several thresholds, to determine if an impact occurred, or if a detected impact has to be ignored. A first threshold may be used to eliminate not relevant signals. Moreover, a discrimination may be carried out to maintain only the set in which the best result occurred. Thus, the result of the considered set is compared with the best result of the other sets. If the result of the considered set is greater than the other, this set is selected to provide its signal to a central processing unit for example. 
         [0032]    The circuit  15  is a “Winner Takes All” (WTA) circuit, i.e. this circuit adds all the four input signals, and outputs the result to the output among the four outputs Wp 1  to Wp 4 , corresponding to the greatest input. 
         [0033]    Consequently, this circuit WTA  15  allocates the result of the sum to the impact center pixel, where the energy due to the impact is maximal. Thus, the output signal, allocated to the impact center pixel, takes into account the energy shared by the four pixels P 1 , P 2 , P 4 , and P 5 , and does not add too much noise in the output signal. Indeed, the table below shows that the noise increase is substantially proportional to the square of pixel number. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Configuration 
                 Increase in Noise 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1 pixel 
                 1.0 
               
               
                   
                 3 pixels sums 
                 1.73 
               
               
                   
                 4 pixels sums 
                 2.0 
               
               
                   
                 5 pixels sums 
                 2.24 
               
               
                   
                 7 pixels sums 
                 2.65 
               
               
                   
                   
               
             
          
         
       
     
         [0034]    In this table, it is shown that the method of summing 4 pixels by set enables to cover a large surface, since the possible sets cover totally 9 pixels. Moreover, the noise increase is 2, which permits a good compromise. 
         [0035]    According to a variant of the invention, the discrimination circuit  14  comprises a second threshold which permits to determine whether an impact is a high energy impact. Such a high energy impact, due to high energy photon, is spread in a greater number of pixels. Consequently, if a signal exceeds this second threshold, for example the sum result and/or the impact center pixel signal, the circuit will allocate the sum of a wider set of pixels, for instance a nine pixel set. 
         [0036]    Other electronic circuits could permit the summing of several pixels magnitude variations, for example to obtain a more flexible computation of the result. Indeed, the number of pixels by set may be chosen case by case, depending of the impact center result or of the operation conditions. 
         [0037]    As shown on  FIG. 5 , the method comprises a first step (S 501 ) of detecting an impact, for instance when the physical magnitude of a pixel exceeds a predetermined threshold. Then, the impact center, where the magnitude variation is maximal, is determined (S 502 ), by comparing the value of pixels or of sets of pixels. The method may comprise a step selecting the number N of pixels for computing the output signal. This number N may be predetermined dependant on the application, for example only a predetermined range of radiations is expected. Instead, it may depend on the value of the impact center result. 
         [0038]    At step S 504 , the sums of all possible sets of N pixels are computed, and only the best result is allocated to the impact center S 505 . 
         [0039]      FIG. 6  illustrates a variant of the invention. In this variant, N is predetermined. At steps  601  and  602 , an impact is detected and the impact center is determined. Then, at Step  603 , the magnitude variations signals of all possible sets of N pixels around the impact center are summed and compared to a threshold at step S 604 . This threshold permits to determine whether the considered impact is a high energy threshold. In such a case, all the possible sets of P pixels are summed, with P greater than N (S 605 ). Consequently, these sums cover a wider area of the detection matrix, since high energy particle are more shared. Eventually, the greatest sum result is allocated to the impact center pixel (S 606 ). 
         [0040]    This method is adapted to different situations, and permits to obtain good results on a larger range of radiation frequencies.