Abstract:
An apparatus for detecting X-rays comprises a scintillator which emits a plurality of photoelectrons upon being impacted by an X-ray photon. The photoelectrons are amplified in a gas electron multiplier and the resultant photoelectrons are accumulated on a two dimensional array of charge collection electrodes. Electrical signals are produced which indicate the quantity of photoelectrons which strike each charge collection electrode. A processor determines a location of the X-ray photon strike by analyzing the spatial distribution of the photoelectrons accumulated by the array of charge collection electrodes. The intensity of the X-ray photon is determined from the number of accumulated photoelectrons.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention relates to X-ray imaging apparatus; and more particularly to X-ray detectors which produce electrical image signal in such apparatus.  
           [0005]    2. Description of the Related Art  
           [0006]    Conventional X-ray imaging equipment includes a source for projecting a beam of X-rays through an object being imaged, such as a medical patient. The portion of the beam which passes through the patient impinges upon an X-ray detector which converts the X-rays attenuated by the patient into photons which then are converted into an electric image signal. One type of X-ray detector has a combination of a scintillator in front of a two dimensional array of photodetectors. Each photodetector integrates the energy of the impacting X-ray photons over the period of X-ray exposure time to produce a signal that is proportional to the X-ray energy integral or X-ray intensity. The electrical signal from each photodetector forms a picture element, commonly referred to as a pixel, which are processed and combined to form an image that is displayed on a video monitor. The resolution of the resultant X-ray image was adversely affected by the diversion, or spreading, of the light within the scintillator. In order to increase X-ray detection efficiency, it is desirable to increase the thickness of the scintillator, however increased thickness also increases the light spread.  
           [0007]    U.S. Pat. No. 6,011,265 discloses a detector which can be used for X-rays or gamma rays. The radiation enters the detector through an inlet window and interacts with a gas to generate primary electrons. Those electrons pass through a cascaded series of gas electron multipliers (GEMs). Ultimately striking a linear set of charge collection electrodes. The charge collection electrodes are connected to read-out electronics which produce a pixel from the signal from each electrode.  
           [0008]    The resolution of the resultant X-ray data is limited by the pitch, or spacing, of the charge collection electrodes. Thus, the ability to physically construct the electrode array and read-out electronics connected thereto, limits the resolution of the X-ray detector. Although advances in microelectronics enable formation of finer electrodes and denser electronic read-out circuitry to increase the image resolution, such increased resolution comes with a significant cost increase. Therefore, it is desirable to increase the X-ray image resolution without paying the price of increased density of the charge collection electrodes and electronics.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention relates to forming an X-ray image by sensing each impact of an X-ray photon, known as a photon event, on a detection apparatus. The location of the photon event is determined and the number of photon events at each defined location on the apparatus are counted for use in constructing the X-ray image.  
           [0010]    That apparatus for detecting the X-rays comprises a scintillator which emits a plurality of photoelectrons upon being impacted by an X-ray photon. That impact is referred to as an X-ray photon event. A gas electron multiplier, with a plurality of stages, is adjacent the scintillator to receive the photoelectrons. A two dimensional array of charge collection electrodes is positioned to receive photoelectrons emitted by the gas electron multiplier in response to receipt of the plurality of photoelectrons from the scintillator. Each charge collection electrode produces an electrical signal indicating the quantity of photoelectrons which have struck that respective charge collection electrode.  
           [0011]    The electrical signals from the array of charge collection electrodes are fed to a signal processor. The signal processor analyzes the electrical signals and defines a two dimensional matrix of the charge collection electrodes in the two dimensional array. Preferably a square matrix is defined that is centered about the charge collection electrode that produced the electrical signal indicating the greatest number of photoelectron strikes. The analysis of the electrical signals from the charge collection electrodes in the matrix determines a location of the X-ray photon event. Therefore, the adverse effect on image resolution that results from light spread in the scintillator is reduced by locating the X-ray photon event with more precision according to the present technique. This allows a thicker scintillator to be employed for increased X-ray detection efficiency without a significant decrease in image resolution.  
           [0012]    In the preferred embodiment of the present apparatus, the signal processor determines the location of the X-ray photon event by deriving intensity weighted means of the electrical signals in two orthogonal dimensions in the matrix of charge collection electrodes. For example, the orthogonal coordinates x, y for the X-ray photon event location of the X-ray photon event can be derived according to the equations:  
             x   =           ∑   i   m                       n   i          x   i             ∑   i   m                     n   i         =         ∑   i   m                       n   i          x   i           N   m                     y   =           ∑   i   m                       n   i          y   i             ∑   i   m                     n   i         =         ∑   i   m                       n   i          y   i           N   m                                     
 
           [0013]    where x is a coordinate of the pixel location along a first axis of the matrix, y is a coordinate of the pixel location along a second axis which is orthogonal to the first axis, i is an integer designating one of the charge collection electrodes, n i  is a number of photoelectrons collected by the ith charge collection electrode in the matrix, x i  is the coordinate of the ith charge collection electrode in the matrix, M is the number of charge collection electrodes in the matrix, N m  is the sum of the photoelectrons collected by the matrix, and y i  is the coordinate of the ith charge collection electrode in the matrix.  
           [0014]    In another aspect of the present invention the signal processor determines an intensity value for the X-ray photon event in response to the electrical signals from the charge collection electrodes in the matrix. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a block schematic diagram of an X-ray imaging system incorporating the present invention,  
         [0016]    [0016]FIG. 2 is a schematic cross sectional diagram of the X-ray detector in FIG. 1,  
         [0017]    [0017]FIG. 3 illustrates signal formation in the X-ray detector,  
         [0018]    [0018]FIG. 4 depicts the two dimensional distribution of photoelectrons on a three by three electrode matrix defined in the X-ray detector, and  
         [0019]    [0019]FIG. 5 illustrates a sub-pixel displacement detection error for an X-ray event occurring off-center in the electrode matrix. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    With initial reference to FIG. 1, an X-ray imaging system  10 , such as used for medical imaging, has an X-ray source  12  that projects a cone beam of X-rays  14  toward a detector array  16  on the opposite side of the medical patient being imaged. The detector array  16  is formed by two dimensional array of a plurality of detector elements  18  which together sense the projected X-rays that pass through a patient  15 . The impact of an X-ray photon on the detector array  16  is known as a photon event and produces electrical signals from several of the detector elements  18  as will be described. The detector array  16  has circuits which digitize the detector element signals.  
         [0021]    Operation of the X-ray source  12  is governed by a control and image processing system  20  which includes an X-ray controller  22  that provides power and timing signals to the X-ray source  12 . A data acquisition system (DAS)  24  samples data produced by detector elements  18 . Operation of the X-ray controller  22  and the data acquisition system  24  are governed by a computer system  25  which receives commands and exposure parameters from an operator via a console  26  that has a keyboard and a display monitor which allows the operator to observe the X-ray image and operational data for the control and image processing system  20 . The computer system  25  processes the data from the detector array  16  to determine the location of each photon event and count the photon events at each defined location on the array. That information is stored the X-ray image data in a mass storage device  28  for subsequent use in constructing an X-ray image.  
         [0022]    With reference to FIG. 2, the detector array  16  and associated signal processing circuits count the number of X-ray photons impacting the detector and the location of each impact which information is used to form the X-ray image. The detector array  16  includes a scintillator  30  which has a layer of scintillation material  32 , such as sodium iodide or cesium iodide. A surface of the scintillator  30  which faces the source of X-rays is coated with a film  34  that reflects light produced within the scintillator material  32  so that the light travels toward the opposite surface. That opposite surface is coated with a conductive film  36  which in turn is coated with a photocathode film  38 . The conductive film  36  is connected to a voltage divider  42  which applies a relatively negative high voltage (e.g. −2800 volts) to the conductive film. The conductive film  36  is relatively thin and is highly transmissive to light at the wavelengths generated in the scintillation material  32 . The desired light spread within the scintillator  30  is controlled by varying its thickness or utilizing columnar or pixellation structures, as have been used in previous detectors. The light intensity at the photocathode film  38  due to a single X-ray photon impinging the scintillation material  32  has a spatial distribution which can be measured. In the following description, a Gaussian point spread function is assumed. The photocathode film  38  emits photoelectrons  40  upon the impingement of light from the scintillator material  32 .  
         [0023]    The photoelectrons  40  emitted by the scintillator  30  enter a gas electron multiplier (GEM)  44  having three stages  45 ,  46  and  47 . The details and functionality of a gas electron multiplier  44  are well known, such as described in the aforementioned U.S. patent. Each GEM stage  45 - 47  has an electrical insulator layer with major surfaces clad with metal and have an array of electric field condensing areas formed by a plurality of through holes  54  extending through the multiplier stage. Specifically, the first GEM stage  45  has an electrical insulator material  48  sandwiched between metal layers  50  and  52 . Each of the metal cladding layers  50  and  52  is connected to different points on the voltage divider  42  so that a potential difference exists across the multiplier stage thereby creating an electric field condensing area as shown by the electric field lines in the left section of the drawing, Similar electric fields are created at each hole in the multiplier stages. The metal cladding layers of each GEM stage  45 - 47  have progressively less negative voltage applied to them going away from the scintillator  30 . The first GEM stage  45 , has relatively small holes  54  as compared to the holes in subsequent stages and also has a unity or small gain which is chosen to minimize gas scintillated photon and ion feedback to the photocathode  38 . In other words, the first GEM stage  45  serves as an electron extraction and feedback blocking function.  
         [0024]    The second and third GEM stages  46  and  47  have a similar physical construction to the first GEM stage  45 . In particular an insulator layer  56  of the second GEM stage  46  is clad with metal layers  60  and  62 , and the third GEM stage  47  is clad with metal layers  64  and  66 . Each of these metal cladding layers  60 - 64  is connected to successive taps of the voltage divider  42  to create an increasingly less negative bias on those conductive layers. The signal gain desired for the GEM  44  is provided by the second and third stages  46  and  47 , each providing a gain between 10 and 100. Because high GEM gains have an adverse impact on the stability and counting rate capability, it is preferred that these gains be kept relatively moderate. As is well known, the gains are determined based on the required X-ray counting rate (with lower gains required for higher rates), the read-out electronic noise level, and the photoelectron production from the scintillator  30  (with lower photoelectron production requiring higher gain). Additional GEM stages can be inserted if greater gain is required.  
         [0025]    The photoelectrons flowing from the third GEM stage  47  travel toward a read-out stage  70  which comprises a two dimensional array of charge collection electrodes  72  separated in both dimensions by a focusing grid  74 . The focusing grid  74  is connected to a final tap of the voltage divider  42  thereby being biased to attract the photoelectrons from the third GEM stage  47 . Each charge collection electrode  72  receives incoming photoelectrons from the gas electron multiplier  44  and is connected via a preamplifier  76  to the digital acquisition system  24  in FIG. 1. When the pulse from an individual preamplifier exceeds a predetermined level, the pulse signal is digitized by an analog to digital converter (ADC)  77  with at adequate resolution (e.g. three-bits). Then the DAS  24  defines a matrix of 3×3 (or 5×5) charge collection electrodes  72  having the greatest signal values.  
         [0026]    The readout circuitry and the digital acquisition system  24  operate with sufficient speed so as to sense photoelectrons impinging the collection electrodes  72  resulting from a single X-ray photon striking the scintillator  30 . In other words, when the signal from a given charge collection electrode  42  is read out, that signal level corresponds to a single X-ray photon event. Furthermore, reference to FIG. 2 also shows that there are several channels through the GEM  42  for each charge collection electrode  72 . It should be understood that an X-ray photon event occurring at one point in the scintillator  30 , results in photoelectrons from the photocathode  38  entering several of these channels. In fact, as shown in FIG. 3, an X-ray photon  80  striking the scintillator  30  produces light photons which strike an area of the photocathode  38  thereby producing a cloud of primary photoelectrons  82 . The scintillator light point spread function at the photocathode  38 , as well as the photoelectron distribution in the cloud  82 , has a Gaussian distribution in two dimensions about the path the X-ray photon  80 . The photoelectrons in the cloud  82  enter the GEMs  44  and are multiplied as they travel toward the charge collection electrodes  72 . A single X-ray photon event produces a flow of photoelectrons through the GEMs  42  which impact a plurality of the charge collection electrodes  72  in a two dimension region of the read-out stage  70 .  
         [0027]    The processing of data from the X-ray detector  16  utilizes signal samples from a square matrix of charge collection electrodes  72  to determine the intensity and location of each X-ray photon striking the detector. The intensity and location determination is based on the signal samples from a square matrix of charge collection electrodes  72  that is defined by the computer system  25  for each X-ray photon event. The processing will be described in the context of a three by three matrix with the understanding that a five by five or larger square matrix may be employed.  
         [0028]    [0028]FIG. 4 depicts the two dimensional distribution of the photoelectrons striking a three by three matrix  86  of charge collection electrodes  72  as a result of an X-ray photon event occurring directly above the midpoint of the central electrode in that matrix. Assuming that 624 photoelectrons were emitted by the photocathode  38  as a result of that single X-ray photon impact, the distribution of photoelectrons striking the nine charge collection electrodes  72  in the matrix  86  is indicated by the numbers n i  within each matrix square where n is the number of primary photoelectrons, i designates the particular charge collection electrode, and G is the total gain of the GEMs  44 . Thus, the number of photoelectrons striking each charge collection electrode  72  has a substantially Gaussian distribution about the center of the matrix  86 , which in this case corresponds to the location of the X-ray photon event that occurred in the scintillator  30  directly above the matrix center. A precise Gaussian distribution is the ideal case and the actual number of photoelectrons striking each charge collection electrode differs from the ideal due to noise and other factors. Nevertheless, a substantially Gaussian distribution occurs. The impact of photoelectrons causes a charge to accumulate on the affected charge collection electrodes  72 .  
         [0029]    The DAS  24  continuously receives signals from plurality of preamplifiers  76  and ADC&#39;s  77  and stores digital signal samples denoting the magnitude of charge on each charge collection electrode  72 . Upon receiving the signal samples from the DAS  24 , the computer system  25  selects the charge collection electrode  72  which produced the largest signal sample as being the central electrode of the processing matrix  86 . The remainder of that three by three matrix  86  is formed by the eight charge collection electrodes  72  that surround the selected central electrode. The coordinates (x i , y i ) of each charge collection electrode in the defined matrix  86  is designated based on an origin at the midpoint of the central electrode, as depicted in FIG. 4. Furthermore, by knowing the gain of the GEMs the number of primary photoelectrons for each charge collection electrode  72  can be derived from the total signal produced by that electrode.  
         [0030]    The example depicted in FIG. 4 assumes that the X-ray photon event occurred directly above the midpoint of the central charge collection electrode in the designated matrix  86 . However, it is more likely that the X-ray photon event will be offset from the midpoint of a charge collection electrode  72 . As seen in FIG. 5, the X-ray photon event is likely to occur above some location  90  that is offset from the center (0,0) of a charge collection electrode  72 . As a result, the peak of the Gaussian distribution of photoelectrons impacting the charge collection electrodes is shifted to coincide with that location  90 .  
         [0031]    Heretofore, the image processing identified the X-ray photon event as being located at the position of the charge collection electrode  72  that produced the largest signal. Thus the resolution of the X-ray detector was equal to the pitch of the charge collection electrodes. The computer system  25  in the present imaging system  10  is able to determine the location of the X-ray photon with finer resolution by determining that location within the area of the central electrode in the defined square matrix  86 . That determination is based on the signal samples produced by the charge collection electrode  72  in that matrix.  
         [0032]    The x and y coordinates of the X-ray photon event with respect to the midpoint (0,0) of the matrix  86  are derived by the computer system  25  by determining an intensity weighted mean of electron distribution along two orthogonal axes according to equations 1 and 2:  
             x   =           ∑   i   m                       n   i          x   i             ∑   i   m                     n   i         =         ∑   i   m                       n   i          x   i           N   m                 (   1   )               y   =           ∑   i   m                       n   i          y   i             ∑   i   m                     n   i         =         ∑   i   m                       n   i          y   i           N   m                 (   2   )                               
 
         [0033]    where X is a coordinate of the X-ray photon event location along a first axis of the matrix, y is a coordinate of the X-ray photon event location along a second axis which is orthogonal to the first axis, i is an integer designating one of the charge collection electrodes, n i  is a number of primary photoelectrons collected by the ith charge collection electrode in the matrix, x i  is the coordinate of the ith charge collection electrode in the matrix, m is the number of charge collection electrodes in the matrix, N m  is the sum of the primary photoelectrons collected by the matrix, and y i  is the coordinate of the ith charge collection electrode in the matrix.  
         [0034]    The coordinates x, y of the X-ray photon event and photon intensity as denoted by M are stored in the memory of the computer system  25  for subsequent use with similar data from the other X-ray photon events occurring in a given X-ray exposure to construct an image of the object  15 . Thus  
         [0035]    This analysis of the electrical signals from the charge collection electrodes in the matrix determines the location of the X-ray photon event even where the resultant light has spread in the scintillator and produced a sizable cloud of electrons. Therefore, the adverse effect on image resolution that results from light spread in the scintillator is reduced by locating the X-ray photon event according to the present technique. This allows a thicker scintillator to be employed for increased X-ray detection efficiency without a significant decrease in image resolution.  
         [0036]    It should be understood that some of the photoelectrons at the periphery of the cloud  82  may strike the read-out stage  70  outside the square matrix  86 . This effect is of little concern when the X-ray photon event occurs directly over the center of a charge collection electrode  72 , as those outer photoelectrons are evenly distributed in all directions around the matrix. However, the X-ray photon event probably is offset from the center of a charge collection electrode  72 , such as above location  90  in FIG. 5. Therefore, some of the primary photoelectrons in the upper right portion of the cloud  82  will not fall within the three by three electrode matrix  85 . As a consequence, derivation of the X-ray photon event location will be based on non-symmetrical data samples and can produce coordinates for a point  92  which is displaced from the actual X-ray event location  90 . Noise which effects the system also contributes to the displacement Δ. Both quantum noise, due to variation in the number of photoelectrons produced at different sections of the scintillator  30  according to a Poisson distribution, and spatial quantization noise contribute to the displacement of the calculated location from the actual location of the X-ray photon event.  
         [0037]    The displacement error can be corrected by collecting empirical data which quantifies that error. One technique sends X-rays through a fine pin hole to impinge a well-defined known location on the read-out stage  70 . The signals from the charge collection electrodes  72  are processed, as described previously, to calculate the location of the X-ray photon event. The calculated location, (X,Y)_cal, is compared to the actual location, (X,Y)_true, to determine a correction coefficient, (X,Y)_coef=(X,Y)_true−(X,Y)_cal. The correction coefficient for each central charge collection electrode can be derived in this manner and stored in a look-up table. During the real imaging, each calculated location is corrected to produce a corrected location, (X,Y)_corr=(X,Y)_cal+(X,Y)_coef.  
         [0038]    Another calibration technique employs a very large matrix size (e.g. a nine by nine matrix instead of a three by three matrix used during imaging). Very few photoelectrons are undetected with that much larger matrix, and equations (1) and (2) yield substantially the actual location, (X,Y)_true, of the X-ray photon event. Although this much larger matrix could be employed during real imaging, significantly greater signal processing time would be required, for example the processing time is nine times greater for a nine by nine matrix then for a three by three matrix. During this latter calibration technique, the photon event location is calculated twice, once using data from the entire nine by nine matrix and again with the data from only a three by three matrix. The difference in the two calculated locations defines the displacement error for the center charge collection electrode of the matrices and thus the correction coefficient.  
         [0039]    The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.