Patent Publication Number: US-8983165-B2

Title: System and method for measuring X-ray beam profile using an area detector

Description:
CLAIM OF PRIORITY 
     This application claims priority to the provisional application entitled, “System and Method for Measuring X-Ray Beam Profile Using Area Detector,” filed Jul. 16, 2010, Application No. 61/365,204. 
    
    
     TECHNICAL BACKGROUND 
     The usage of area x-ray beams is common in the field of x-ray imaging and external radiation treatment to generate images of a target volume. For many imaging applications, a beam of x-rays or other radiation particles is directed from a radiation source and through a target volume or object. The traversing particles of the beam are collected in an imager or detector positioned on the far side of the target volume with respect to the source, and the data acquired by the acquisition (reception) of the particles in the imager may be subsequently used to generate an image of the target volume. 
     Certain characteristics of the particular beam used in acquiring the images are also critical to many applications, and may have a significant impact on the quality of the images and the treatment generated by the X-ray beams. For example, many digital X-ray imaging applications which include conventional scatter correction and dose verification techniques require knowledge of the spatial distribution of the flux intensity (also known as the beam profile) of the radiation beam used during the application. In addition, the beam profile data can also be used to facilitate calibration for X-ray detectors (e.g., gain calibration), and flat panel detectors in particular. Unfortunately, the generation of non-uniform beam fields is common in many radiology and medical imaging devices. This non-uniformity, when not properly calibrated for, can cause unintended artifacts in generated images which may significantly impact the quality of a generated image. 
     Non-uniformity of the beam field can be attributed to a variety of factors which include non-uniform directionality of Brehmsstrahlung radiation and the varying attenuation properties of the x-ray tube components (e.g. the Heel effect) and its associated housing. Ideally, in an ideal imager where the detector pixels each have a uniform response, the beam profile should approximate a flood field image (e.g., no object in the path of the beam). In practice however, practically all detectors also have gain variations caused by inherent imperfections in the sensors and the associated electronic circuitry. These gain variations must be calibrated out to create images that are sufficiently uniform and artifact-free. Unfortunately, conventional gain calibration techniques also remove flux intensity variations. Thus, after calibration, the flood field measurement will be a uniform signal, rendering any information regarding flux intensity variations lost. 
     One conventional approach of determining the beam profile involves performing a raster scan of the beam field with a small (e.g., single point) detector, determining the flux intensity of the beam at each designated point, and subsequently combining the results to form the beam profile. However, this approach suffers from several significant drawbacks: the scanning mechanism is cumbersome and the measurement time required is often lengthy since many beam pulses are required. Moreover, geometric pointing inaccuracies and global flux intensity variations from scan-to-scan can limit the accuracy of the final result. Another conventional method of determining the beam profile is by measuring the beam field using a large area of a detector. While more time-efficient than using a single point of a detector, the inherent non-uniformity among one or more pixels of the x-ray detector can detrimentally affect the accuracy of a beam profile thus measured. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that is further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     To overcome the difficulties inherent in traditional beam calibration methods, a new technique for measuring the beam profile is herein provided. This technique uses an area detector to generate two or more spatially correlated images, and mathematically decouples the detector gain profile from the beam profile yielding relative flux intensity variations. 
     The claimed subject matter describes a novel technique to measure the beam profile using an area detector. No special requirement is needed for the detector and without requiring prior calibration. In one embodiment, the technique comprises generating two images from an area detector, deriving one or more sets of one-dimensional beam profile measurements from the generated images, and decoupling measured gain effects and flux intensity measurements from the sets of one-dimensional beam profiles, thereby advantageously providing a simple and relatively quick process to perform beam calibration. The technique provides a method of generating a beam profile for the x-ray field and a gain profile for the detector, which can used to properly calibrate the detector such as if the field were flat. 
     In one embodiment, a set of one-dimensional beam profile measurements is performed by taking two images under the same source conditions but at two different positions of the detector, with each position of the detector shifted by a certain distance in the direction corresponding to the direction of the one-dimensional profile measurement. In further embodiments, a set of two-dimensional beam profile measurements is achieved by determining a second set of one-dimensional profiles from the same sampling points in a second direction and building a two-dimensional map of the beam profile by correlating the first one-dimensional profile measurement with the second one-dimensional profile measurement. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
         FIG. 1  depicts an illustration of an exemplary radiation therapy and imaging device, in accordance with embodiments of the present invention. 
         FIG. 2  depicts a flowchart of a method for measuring the beam profile of a photon beam, in accordance with embodiments of the present invention. 
         FIG. 3  depicts a flowchart of a method for minimizing trapped charge-induced errors during image acquisition, in accordance with one embodiment. 
         FIG. 4  is an illustration of an exemplary configuration of an area detector panel comprising a plurality of single point detectors, in accordance with embodiments of the present invention. 
         FIG. 5  depicts an illustration of an exemplary arrangement of single point detectors in an area detector panel, in accordance with embodiments of the present invention. 
         FIG. 6   a  depicts an illustration of an exemplary configuration of an area detector panel during a process for determining the gain profile of the area detector panel, in accordance with embodiments of the present invention. 
         FIG. 6   b  depicts an illustration of an alternate exemplary configuration of an area detector panel during a process for determining the gain profile of the area detector panel, in accordance with embodiments of the present invention. 
         FIG. 7   a  depicts an illustration of an exemplary configuration of regions of interest in an area detector corresponding to a set of one-dimensional beam profile measurements, in accordance with embodiments of the present invention. 
         FIG. 7   b  depicts an illustration of an exemplary configuration of regions of interest in an area detector corresponding to a set of one-dimensional beam profile measurements, in accordance with embodiments of the present invention. 
         FIG. 8  depicts an illustration of an exemplary two dimensional map of beam profile measurements, in accordance with embodiments of the present invention. 
         FIG. 9  depicts an exemplary computing environment, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to several embodiments. While the subject matter will be described in conjunction with the alternative embodiments, it will be understood that they are not intended to limit the claimed subject matter to these embodiments. On the contrary, the claimed subject matter is intended to cover alternative, modifications, and equivalents, which may be included within the spirit and scope of the claimed subject matter as defined by the appended claims. 
     Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. However, it will be recognized by one skilled in the art that embodiments may be practiced without these specific details or with equivalents thereof. In other instances, well-known methods, procedures, and components, have not been described in detail as not to unnecessarily obscure aspects and features of the subject matter. 
     Portions of the detailed description that follows are presented and discussed in terms of a method. Although steps and sequencing thereof are disclosed in figures herein (e.g.,  FIGS. 2 and 3 ) describing the operations of this method, such steps and sequencing are exemplary. Embodiments are well suited to performing various other steps or variations of the steps recited in the flowchart of the figure herein, and in a sequence other than that depicted and described herein. 
     Embodiments described herein may be discussed in the general context of computer-executable instructions residing on some form of computer-usable medium, such as program modules, executed by one or more computers or other computing devices. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments. 
     By way of example, and not limitation, computer-usable media may comprise computer storage media and communication media. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, random access memory (RAM), read only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory or other memory technology, compact disk ROM (CD-ROM), digital versatile disks (DVDs) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store the desired information. 
     Communication media can embody computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency (RF), infrared and other wireless media. Combinations of any of the above should also be included within the scope of computer-readable media. 
     Exemplary Radiation Treatment and Imaging Machine 
     With reference now to  FIG. 1 , an illustration of an exemplary radiation therapy and imaging device  100  is depicted, in accordance with one embodiment. In one configuration, radiation therapy and imaging device  100  includes a support structure (e.g., gantry  101 ), and a radiation source  103  (e.g., a medical or clinical linear accelerator) including a treatment head  103 . In one embodiment, the treatment head  103  receives a primary electron beam (e.g., from an electron gun), which will then hit a target material (commonly consisting of a high-Z metal), after which the electrons will produce what are referred to as “bremsstrahlung photons,” and thereby generating a photon beam, ultimately emanating from a treatment source  105 . 
     In further embodiments, a plurality of robotic arms (e.g., robotic arms  107 ,  108  and  109 ) may be attached to the gantry and coupled to a second (diagnostic) radiation source  111 , and an area detector panel  113  (typically comprising a grid of single point detectors). According to some embodiments, an additional arm may be attached to a portal area detector panel  118  for receiving radiation from the treatment source  105 . In still further embodiments, the device  100  may also include a patient couch  115 . The radiation therapy and imaging device  100  may further include a communicatively coupled computing device  117  for processing images and/or controlling and manipulating the device  100 . According to some embodiments, radiation emanating from the treatment source  105  and received in the portal area detector panel  118  may operating in the range of mV radiation (mega-voltage radiation), whereas radiation distributed from the second radiation source  111  and received in the area detector panel  113  may operate in the range of kV radiation (kilo-voltage). 
     In one embodiment, the end of gantry  101  positioned above patient couch  115  is attached to a radiation source  105  used to generate radiation (e.g., for therapeutic or imaging purposes). A patient is positioned (typically supine) on patient couch  115 . A target volume (generally disposed within or about the patient subject) is irradiated by transmitting a radiation beam (e.g., X-ray photon beam) through the target volume and receiving the beam in the area detector panel  113  from source  111  or detector panel  118  from source  105 . According to one embodiment, an image of the target volume can be acquired using either the diagnostic source  111  or therapeutic radiation source  105  in conjunction with either the area detector  113  or area detector  118 , respectively. In alternate embodiments, x-ray detectors and radiation sources may be used for diagnostic and other applications that are not related to the radiotherapy field. 
     Certain parameters of the device  100  may be calibrated to produce or emphasize certain qualities of generated images. These parameters may include, for example, modifying the intensity or angle of the photon beam, and/or position of area detector  113  or area detector  118 . However, due to the inherent variances in flux intensity of the generated photon beam and gain variances among the single point detectors within the area detector, the image generated by such a device may suffer from undesirable effects which may reduce the image quality. According to one aspect of the present claimed subject matter, a method is provided for measuring the beam profile by using the area detector panel  113  in conjunction with a radiation imaging source  111  to generate two or more spatially correlated images, and mathematically decoupling the detector gain profile from the beam profile yielding relative flux intensity variations derived from the correlated images, thereby allowing a more efficient calibration of the imaging device  100  and enabling the production of artifact-free images. Similarly this could be done with source  105  and detector  118 . 
     In other embodiments, the system may not include a radiation therapy device. Instead, the system may consist of a radiation source and an imager. For example, in other embodiments, the system may include an x-ray source and a flat panel detector. In further embodiments, the source and imager may or may not be mounted on a rotational gantry. While embodiments are described herein to comprise the generation and usage of X-ray (photon) beams, it is to be understood that alternate embodiments are well suited for the generation and use of other particles and corresponding particle beams. These alternate embodiments may include, but are not limited to, electrons and electron beams; protons and proton beams; and ions and ion beams. 
     Beam Profile Measurement 
       FIG. 2  is a flowchart  200  of one embodiment of a method for measuring the beam profile of a photon beam, in accordance with one embodiment. In one embodiment, the method generates one dimensional sets of beam profile and gain measurements. In further embodiments, the method may be used to generate a two-dimensional map of a beam profile for a beam field and a gain map for a corresponding area detector, thereby enabling the efficient calibration of a photon beam generated in a radiation imaging device. This calibration advantageously provides a way to calibrate the imaging device to account for variances in the gain profile and flux intensity within the area detector and beam source, respectively, without requiring user-intensive and time consuming manipulation of individual single point detectors. Steps  201 - 209  describe exemplary steps comprising the process depicted in flowchart  200  in accordance with the various embodiments herein described. In one embodiment, the flowchart  200  is implemented as computer-executable instructions stored in a computer-readable medium. 
     At step  201 , a first image of a target volume is generated. Generating the first image may be performed by, for example, receiving photons of a photon beam emitted by a beam source of an imaging device in an area detector of the imaging device at a first position. In one embodiment, the photon beam is a beam of X-rays, and the image is acquired by directing the stream of radiation through the target volume and collecting the photon particles in the area detector positioned on the other side of the target volume from the x-ray source, such that the target volume is interposed between the source and detector. The data collected from the arrangement of the particles in the detector may be subsequently used to generate an image of the target volume. 
     The photon beam may be generated by, for example, applying an electron beam (e.g., generated by an electron gun and accelerated via an accelerating device in a medical linear accelerator) to a metal target (e.g., consisting of a high-Z metal) and generating a beam of bremsstrahlung photons. In further embodiments, once the photon beam has been generated, the beam may be modified by passing the beam through a configuration of modulating and/or particle monitoring devices. Typically, one or more flattening filters, wedges, collimators may be implemented to refine (e.g., attenuate) the photon beam before application to the subject volume (patient). 
     At step  203 , a second image of the target volume is generated with the area detector positioned at a second position which is not the equivalent of the first position. In still further embodiments, each of the first and second images may be spatially correlated. That is, each image may be displaced from other images by a specific, pre-determined distance and in a desired dimension (e.g., horizontally or vertically). According to some embodiments, the photon beam may also be generated according to a plurality of conditions (e.g., beam source conditions). These conditions may include, but are not limited to, generally customizable conditions such as the angle of the beam relative to the target volume and/or area detector, the area of the incident beam, distance from the beam source to the area detector, etc. According to one embodiment, the source conditions under which the beam source generates the photon beams used to acquire the first and second images may be consistent (i.e., equivalent). In still further embodiments, the area detector used to receive the photons of the photon beam may be implemented as a grid (matrix) or two dimensional array of single point detectors, each detector operable to generate at least one pixel in a generated image. 
     At step  205 , a first set of one dimensional beam profile measurements is derived from the first and second image and oriented in a first direction (dimension) which corresponds to the direction of the displacement of the second image from the first. In some embodiments, the set of one-dimensional beam profiles may be derived by determining a first set of regions of interest in the first image, determining a second and corresponding set of regions of interest in the second image and acquiring data measurements for the data points corresponding to each of the regions of interest. These regions of interest may correspond to one or more single point detectors (and therefore, one or more pixels in an image). In some embodiments, the regions of interest in each image may comprise a plurality of regularly spaced regions of interest oriented in a single direction (dimension). In these cases, at least some of the regions of interest are separated in the said single direction by the specified displacement between the first and the second images. Data measurements from these regions of interest for each image may be used to provide a data point as a reference with which a set of one dimensional beam profiles of the beam field and the gain of corresponding pixels in an area detector may be derived. 
     For example, for a detector operating within a typical linear response range, the non-gain-calibrated output of the detector is proportional to the incident x-ray beam intensity, and (for exemplary purposes) may be expressed as: 
                       ADU     i   ,   j       =       g     i   ,   j       ·       ∫     Pixel   ⁡     (     i   ,   j     )         ⁢         I   0     ⁡     (     x   ,   y     )       ⁢     ⅆ   x     ⁢     ⅆ   y             ,           (   1   )               
where I 0 (x,y) is the photon flux at the location (x,y) on the detector plane, which is then integrated over the area of the detector pixel (i,j); ADU i,j  is the detector output value (analog-to-digital units) at the detector pixel (i,j); and the conversion and proportionality factor g i,j , is the intrinsic gain of the pixel (i.e. detector element). The indices i and j range from 1 to the maximum size of the detector matrix in the x and y directions.
 
     With gain calibration, the intrinsic proportionality factor g i,j  is multiplied by another quantity gc i,j  imposed by the gain calibration: 
                       ADU     i   ,   j       =       (       gc       i   ,   j     ⁢               ·     g     i   ,   j         )     ·       ∫     Pixel   ⁡     (     i   ,   j     )         ⁢         I   0     ⁡     (     x   ,   y     )       ⁢     ⅆ   x     ⁢     ⅆ   y             ,           (   2   )               
where gc i,j  represents an artificial scaling factor to scale the detector output, usually aimed to compensate for the non-uniformity in g i,j , so that when a uniform field irradiates the detector, a uniform image is output by the detector. Equation (1) can be considered a special case of Eq. (2), where gc i,j  is set to 1. The detector gain obtained by the measurement technique described here is a collective measure of the detector response including the intrinsic gain (g) and the gain calibration (gc), if any.
 
     Rewriting the above expression in a fully discrete format with renamed variables provides the following expression:
 
 I   m ( i,j )= g ( i,j )· I   0 ( i,j ),  (3)
 
where I 0 (i,j) is the flux intensity that an ideal detector with uniform gain would measure at the pixel (i,j), I m (i,j) is the measured intensity, and g(i,j) is the total gain, which includes the intrinsic detector gain plus any software-based calibrations that have previously been included. Accordingly, a means of finding the relative gain g of each detector element (i,j) and the relative flux intensity I 0  at all positions in the radiation field is thereby provided.
 
     For the sake of simplicity, the problem is presented in a single dimension. A first intensity measurement I m1  is made with the detector in the “home” or first position. For element i, the measured signal intensity is the product of that element&#39;s gain g(i) and the x-ray flux intensity at position I 0 (i)
 
 I   m1 ( i )= g ( i )· I   0  ( i ).  (4)
 
Note, while the elements are numbered i=1 to N where N is the total number of detector pixels in a row, other notations are suitable. Element  1  is the leftmost element and element N is the rightmost element.
 
A second measurement I m1  is subsequently performed with the detector shifted to the right. For the special case of the shift being exactly one element, the element i−1 will be at the location in what was position i for the first measurement. Thus, the second measurement may be expressed as:
 
 I   m2 ( i− 1)= g ( i− 1) ·I   0 ( i ),  (5)
 
Where I 0 (i) is the same flux intensity as in Eq. 4. Combining equations 4 and 5, the gain for element i can be written in terms of element i−1:
 
                     g   ⁡     (   i   )       =           I     m   ⁢           ⁢   1       ⁡     (   i   )           I     m   ⁢           ⁢   2       ⁡     (     i   -   1     )         ⁢       g   ⁡     (     i   -   1     )       .               (   6   )               
The recursive relationship depicted in Equation 6 shows that the gain values for all the detector pixels can be expressed in terms of the gain of a single (reference) detector pixel. For the case where the reference element is first (leftmost) detector pixel, the gain for element i is:
 
     
       
         
           
             
               
                 
                   
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     After the gain for each element is obtained, Equation 4 can be used to establish the spatially dependent beam profile I 0 (i): 
                       I   0     ⁡     (   i   )       =           I     m   ⁢           ⁢   1       ⁡     (   i   )         g   ⁡     (   i   )         .             (   8   )               
An alternative way of deriving the beam profile is from the perspective of a fixed detector pixel. The measured intensity for a given detector pixel (i) for measurement  1  and measurement  2 , where again the detector is shifted 1 pixel, may be expressed using the following two equations:
 
                   {                 I     m   ⁢           ⁢   1       ⁡     (   i   )       =       g   ⁡     (   i   )       ·       I   0     ⁡     (   i   )                         I     m   ⁢           ⁢   2       ⁡     (   i   )       =       g   ⁡     (   i   )       ·       I   0     ⁡     (     i   +   1     )                 .             (   9   )               
Note I 0 (i) and I 0 (i+1) are the flux intensities at the locations of elements i and i+1, respectively, before the detector is moved (i.e. for measurement  1 ). Following the same reasoning shown above for calculating detector gains, a recursive relation for the incident intensity for element i is determined:
 
                       I   0     ⁡     (     i   +   1     )       =           I     m   ⁢           ⁢   2       ⁡     (   i   )           I     m   ⁢           ⁢   1       ⁡     (   i   )         ⁢       I   0     ⁡     (   i   )                 (   10   )               
and the flux intensity for element i relative to that at a fixed location (e.g., I 0 (1)) may be expressed as:
 
                       I   0     ⁡     (   i   )       =       ∏     n   =   1       i   -   1       ⁢           I     m   ⁢           ⁢   2       ⁡     (   n   )           I     m   ⁢           ⁢   1       ⁡     (   n   )         ⁢         I   0     ⁡     (   1   )       .                 (   11   )               
The detector gain can be subsequently calculated from Eq. 8 to be:
 
                     g   ⁡     (   i   )       =           I     m   ⁢           ⁢   1       ⁡     (   i   )           I   0     ⁡     (   i   )         .             (   12   )               
In summary, two sets of inter-related formulas (Eqs. 7, 8 and Eqs. 11, 12) have been shown for deriving the beam profile and the detector gains by taking two measurements with the detector shifted from measurement  1  to measurement  2 . These formulas can be used separately, or together in an alternating order, pixel-by-pixel, to solve for the gains and flux intensities across the entire detector.
 
     Note that the expressions in Eqs. (7) and (11) give the gain and intensity, respectively, at a given location (i) relative to that at the leftmost pixel, and thus the solution can be considered as going from left to right. Alternatively, these equations also can be expressed in terms of the gain and intensity at the rightmost pixel, in which case they are solved from right to left. 
     In addition, the shift may not be limited to just one pixel, nor necessarily an integer number of multiples of the pixel size. More importantly, random noise associated with the measurements from single pixels is also a consideration. Therefore, in some embodiments, the average from a group of pixels may be used to evaluate the quantities of I m , I 0 , and g. The detector shift may be chosen empirically, and ideally should be meaningfully large enough so the distance is sufficiently accurate and the averaging areas do not overlap, but not too large to leave only a very few sampling points and a large separation in between. One possible selection for the shift distance could be the ASIC width if there are a number of detector ASICs along the measuring direction. This way, the group of pixels that are averaged are within each ASIC thus avoiding averaging measurements that potentially have large differences. 
     According to these embodiments, the calculation may be performed exactly the same as for the exemplary 1-pixel shift case and the same set of equations may be applied, with the caveat that pixel i and pixel i+1 are no longer two physically neighboring pixels ( 405 ), but two groups of pixels separated by the detector shift distance ( 407 ) (see  FIG. 4 ). In still further embodiments, the distance of the first such group of pixels from the imager edge is another free variable and may be used to globally shift the sampling points on the detector plane. In the case that the x-ray field is larger than the detector in the measuring direction, the measurements at different locations may be repeated and the resultant gain profiles and the beam profiles may be subsequently combined together, to form a result covering the entire field. The combination can be done in the resultant gain and beam profile space, but also can be performed in the original detector image space. The latter case being effectively the equivalent of using a larger detector. 
     Step  205  thus provides a method for determining the gain at each pixel of an area detector as well as the beam profile at each space in a beam field corresponding to a set of one dimensional arrays of regularly spaced regions of interest. According to some embodiments, a two-dimensional beam profile and gain map may be subsequently achieved for the same two spatially correlated images by determining a second set of one-dimensional beam profiles at step  207  for the images in a separate direction (e.g., dimension) from the first set of one-dimensional beam profiles determined in step  205 . The two sets of single-dimensional beam profiles can be correlated by selecting the same sampling points within the two images, thereby obtaining a full two dimensional map of the beam profile and detector gain at step  209 . 
     The technique described above with reference to  FIG. 2  can be used for various applications including improving scatter estimates for both radiographic and tomographic imaging, improving the accuracy of dose measurements during radiotherapy using an EPID, to characterize x-ray tube output, to facilitate flattening filter design, and for real time alignment of the electron beam in clinical linear accelerator (CLINAC) assembly. An accurate beam profile can also be used to aid beam hardening corrections where the heel effect is a factor by determining a map of the amount of target material being traversed by the radiation beam from the intensity drop across the detector, thereby providing accurate spectral information over the entire detection area. Being able to generate a detector gain map can also enable the study of detector gain variations by using the measured gain profile to perturb the gain calibration so that the output of the detector reflects the true incident flux intensity distribution. When a detector&#39;s gain is modified in this way, it frees up its dependence on the beam flux. The detector then measures the real time flux map and can be used in applications where a real time flux map is needed. An example is to help align the treatment beam in LINACs during assembly, by locating its peak output on the detector. 
     Decoupling Charge Trapping and Lag Effects 
     If not properly accounted for, the gain and beam profile measurements may be skewed by charge trapping effects in an imager made from amorphous silicon photo-diodes and transistors. Although gain changes may occur from frame to frame due to these effects, they can be substantially reduced by filling the traps as much as possible before the procedure starts. Alternatively, charge trapping effects can be mitigated by the following processing steps. 
     Starting with Eq. (9) above, the two measured intensities are: 
                   {                 I     m   ⁢           ⁢   1       ⁡     (   i   )       =       g   ⁡     (   i   )       ·       I   0     ⁡     (   i   )                         I     m   ⁢           ⁢   2       ⁡     (   i   )       =       g   ⁡     (   i   )       ·     (     1   +     α   ⁢           ⁢     I     m   ⁢           ⁢   1           )     ·       I   0     ⁡     (     i   +   1     )                 .             (   13   )               
For the second measurement, the gain g(i) is modified by factor given by 1+ a constant of proportionality (α) multiplied by the first measurement I m1 . The constant α can be determined from a separate calibration step such as by taking two readings without moving the detector.
 
     Following the same reasoning outlined in Eqs. (6) to (8), a recursive relation for the incident intensity may be expressed by the equation: 
                       I   0     ⁡     (     i   +   1     )       =           I     m   ⁢           ⁢   2       ⁡     (   i   )           I     m   ⁢           ⁢   1       ⁡     (   i   )         ·     1     1   +     α   ⁢           ⁢       I     m   ⁢           ⁢   1       ⁡     (   i   )             ·       I   0     ⁡     (   i   )                 (   14   )               
and the intensity relative to that at any location (e.g., to I 0 (1)):
 
                       I   0     ⁡     (   i   )       =       ∏     n   =   1       i   -   1       ⁢           I     m   ⁢           ⁢   2       ⁡     (   n   )             I     m   ⁢           ⁢   1       ⁡     (   n   )       ·     (     1   +     α   ⁢           ⁢       I     m   ⁢           ⁢   1       ⁡     (   n   )           )         ⁢       I   0     ⁡     (   1   )                   (   15   )               
The detector gain is determined as shown in Eq. 12:
 
     
       
         
           
             
               
                 
                   
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                         ⁡ 
                         
                           ( 
                           i 
                           ) 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     The charge trapping in the photo diode is responsible for the lag and the gain effects. The lag effect—sometimes also referred to as ghosting—is caused by trapped charges that are released with a very slow time constant. The gain effect is primarily a result of the missing charge—the charges that get trapped—at the beginning of an exposure sequence. Some fraction of the charges gets trapped and cannot be read immediately, this effect appears as a reduced gain in the acquired image for earlier frames of a time sequence. As a result, the associated time constant may be disproportionately long compared to the frame read out time. 
     The behavior of the gain and the lag effects are very similar (but not identical), since they are caused by the same underlying physical effect, that is, the charge trapping. In order to minimize the errors caused by the trapped charge, and to reduce the reliance on Eqs. 13-16, an alternative image acquisition method is presented in  FIG. 3 .  FIG. 3  depicts a flowchart  300  of one embodiment of a method for minimizing trapped charge-induced errors during image processing, in accordance with one embodiment. In one embodiment, the method enables the efficient correction of gain-defects due to the trapped charge effect in images. Steps  301 - 305  describe exemplary steps comprising the process depicted in flowchart  300  in accordance with the various embodiments herein described. In one embodiment, the flowchart  300  is implemented as computer-executable instructions stored in a computer-readable medium. 
     As presented in  FIG. 3 , at step  301 , a sequence of frames comprising a target volume is generated by an imaging device. According to one embodiment, the sequence of frames comprises corresponding pairs of frames, wherein each pair of frames in the sequence of frames comprises a frame of the target volume with a generated radiation beam and a frame of the target volume without a generated radiation beam. According to some embodiments, the method described in  FIG. 3  may be implemented with as few as two pairs of frames, and the sequence of frames may number as few as three frames. That is, a single exposure may be used as the first frame. The radiation beam may be implemented as a diagnostic radiation beam or a therapeutic radiation beam, according to various embodiments. 
     At step  305 , the gain is normalized for each pair of frames. Normalizing the gain for each pair of frames may be achieved by, in one embodiment, deriving one-dimensional gain profiles or two-dimensional gain maps for both frames of each frame pair and subtracting the gain of the frame acquired without the radiation beam from the gain of the frame acquired with the radiation beam. The gain itself may be derived according to the methodology provided above with respect to  FIG. 2 . In another embodiment, the frame acquired without the radiation beam is subtracted from the frame acquired with the radiation beam. The resulting frame forms a first image. The imager is then shifted by a known distance, and the process is repeated to form a second image. The beam profile may be derived according to  FIG. 2 . 
     At step  307 , the gain measurements from the gain-normalized frames obtained in step  305  are averaged to form a single image having as a set of gain measurements the average gain measurements of the sequence of frames. This is expressed by the following formula: 
     
       
         
           
             
               
                 
                   Image 
                   = 
                   
                     
                       ∑ 
                       
                         j 
                         = 
                         
                           n 
                           - 
                           m 
                           - 
                           1 
                         
                       
                       n 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               Frame 
                               WithBeam 
                             
                             ⁡ 
                             
                               ( 
                               j 
                               ) 
                             
                           
                           - 
                           
                             
                               Frame 
                               WithoutBeam 
                             
                             ⁡ 
                             
                               ( 
                               j 
                               ) 
                             
                           
                         
                         ) 
                       
                       / 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     Where n is the total number of acquired “frame pairs” and m is the number of frame pairs that are used for the calculation. 
     Once exposed to targeted radiation from a radiation source and after a period of idleness or disuse (e.g., overnight), an area detector may experience exponential increases in the effects of trapped charges that affect the measurements of detected gains. Over time and consistent (continuous) usage and exposure to radiation, the effects of the trapped charges level off. Accordingly, in some embodiments, the frames acquired during an initial period for a sequence of frames may be purged from consideration prior to the performance of step  303 . Candidate frames for removal may comprise, for example, the frames acquired below a certain threshold of frames (e.g., 120 frames) or during an initial duration of time. In still further embodiments, instead of using a previous frame acquired without the radiation beam, a subsequent frame acquired without the radiation beam may be used to process the image. Also a single frame acquired without the beam could be used to process multiple frames acquired with the beam and multiple frames acquired without beam could be used to process a single frame acquired with beam. This method also corrects for other time variant errors such as, for instance, temperature related offset drifts or other slow changing image parameters. 
     Errors in measurements can propagate to the resulting beam profile and gain map. In particular, the overall pixel response and detector offset between acquired images may affect the accuracy of measurements. An equation which represents the relationship between two measurements with an offset vector Δ between measurements may be expressed as: 
                       I   0     ⁡     (       x   →     +     Δ   →       )       =           I     m   ⁢           ⁢   2       ⁡     (     x   →     )           I     m   ⁢           ⁢   1       ⁡     (     x   →     )         ⁢       I   0     ⁡     (     x   →     )                 (   18   )               
In one embodiment, two measurements without spatial offset but with different source to imager distances are used to derive any exponential contribution due to an overall pixel response error. In another embodiment, two measurements without spatial offset but with rotations are used to find pixel response error. Any incorrect assumption with respect to the position change between the first and the second measurements leads to distorted beam profiles and detector gains. Thus, in still further embodiments, a generic correction function approximation by a bivariate power series is used with a plurality of measurements to account for the unknown overall pixel response and to correct the calculated gain.
 
Exemplary Area Detector Panel
 
       FIG. 4  is an illustration of an exemplary configuration  400  of an area detector panel  401  comprising a plurality of single point detectors  405 . As depicted, the single point detectors  405  have been arranged as a single one dimensional array of detectors. However, according to alternate embodiments, the area detector panel  401  may be implemented as a two dimensional array (e.g., grid) of single point detectors spanning all or a significant portion of the area of the area detector panel  401 . In some embodiments, a plurality of regions of interest  403  may be generated (e.g., by a computing device executing a process for deriving beam profile data) to correspond to one or more detectors  405  of the area detector panel  401 . In further embodiments, the plurality of regions of interest  403  may be regularly spaced and separated by the detector shift distance. Note the distance of the first such group of pixels (e.g., leftmost region of interest  403 ) from the imager edge is another free variable and it can be used to globally shift the sampling points on the detector plane 
     Exemplary Area Detector Panel Configuration 
       FIG. 5  depicts an illustration of an exemplary arrangement of single point detectors in an area detector panel. For the sake of simplicity, an area detector panel ( 501   a ,  501   b ) is depicted in  FIG. 5  as a single row or one dimensional array of single point detectors (e.g.,  503   a - 511   a  and  503   b - 511   b ). Data measurements obtained by a row of single point detectors may be used in conjunction with a radiation beam field to obtain a set of beam profile measurements corresponding to a single dimension (in this case, the horizontal dimension) of a beam field as captured by an area detector. Alternate embodiments may be implemented as a multi-dimensional array or grid of detectors as described above. In one embodiment, an image may be generated with the area detector at a first position  501   a  in space. A subsequent or second image may be generated with the area detector at a second relative position  501   b , offset in a corresponding orientation (e.g., horizontally) by a pre-determined distance. 
     As depicted in  FIG. 5 , the positions in space of one or more single point detectors of the area detector panel at the first position  501   a  may correspond to the positions in space of different single point detectors at the second position  501   b . For example, as depicted, point detector  507   a  corresponds to point detector  505   b ,  509   a  corresponds to  507   b , etc. According to such a configuration, the flux from the photons received at point detector  507   a  will be equivalent to the flux received at point detector  505   b . Any changes from the total derived beam profile generated at that point detector, then, may be attributable to changes in the gain profile for the respective point detectors ( 507   a ,  505   b ). 
       FIGS. 6   a  and  6   b  depict illustrations of exemplary configurations of an area detector panel  601 , and sets of regions of interest  603   a ,  603   b  in a consistent beam field  600  during a process for determining the gain profile of the area detector panel. As depicted in  FIGS. 6   a  and  6   b , one or more regions of interest ( 605   a ,  605   b ) may correspond to the same position in space (e.g., the same position within the beam field  600 ) despite an incongruity in the relative position within the area detector panel  601 . 
     In one embodiment, as presented in  FIG. 6   a , one or more regions of interest  605   a  in the set of regions of interest  603   a  may correspond to the same position in space as a region of interest  605   b  in the set of regions of interest  603   b  of  FIG. 6   b , despite an incongruity in the relative position within the respective area detector panel  601 . Therefore, the pixels corresponding to the region of interest  605   a  in a first image generated with an area detector panel  601  at the indicated position of  FIG. 6   a  will have a flux consistent with pixels corresponding to the region of interest  605   b  in a second image generated with the area detector panel  601  at the indicated position of  FIG. 6   b . And although the gain profile at the particular single point detectors corresponding in the regions of interest may vary as a result of inherent gain variance between the detectors comprising the area detector panel, since the only difference in the generated beam profile (e.g., image) data under these circumstances would be due to changes in the flux of the emitted photon beam, the gain profile for the respective single point detectors may be derived by factoring the derived constant flux from the acquired beam profile data by applying the methodology described above with respect to  FIG. 2 . 
       FIGS. 7   a , and  7   b , depict illustrations of exemplary configurations of regions of interest in an area detector corresponding to a set of one-dimensional beam profile measurements, in accordance with embodiments of the present invention. As depicted in  FIGS. 7   a  and  7   b , an area detector panel  700  may comprise a plurality of regions of interest  701   a ,  701   b  oriented in a given direction/dimension. As presented, the regions of interest ( 701   a ,  701   b ) are oriented horizontally in  FIG. 7   a , and vertically in  FIG. 7   b.    
     In one embodiment, a set of single-dimensional beam profile measurements as described above with respect to  FIG. 2  may be acquired for the regions of interest in either the horizontal or vertical dimensions ( FIGS. 7   a ,  7   b , respectively). Each region of interest thus represents the combination of a pixel or group of pixels of an area detector with the area of a beam field in an incident beam, and the data acquired at each region of interest (as per step  205 ) comprises both the flux intensity of the beam as well as the gain. 
       FIG. 8  depicts an illustration of an exemplary two dimensional map of beam profile measurements, in accordance with embodiments of the present invention. As depicted in  FIG. 8 , the individual sets of one-dimensional beam profile measurements of  FIGS. 7   a  and  7   b  described above have been merged to form a two dimensional map. Merging the individual sets of one-dimensional beam profile measurements may be achieved by, for example, selecting the same sample points as regions of interest in the individual sets of one-dimensional beam profile measurements and correlating the derived data. 
     In another embodiment, the spatially correlated images may be taken by rotating the imager. In this case, the beam profile and gain map are sampled in a set of circularly arranged regions of interest. According to a further embodiment, the spatially correlated images may be taken by moving the imager closer to or further from the radiation source. In still further embodiments, the spatially correlated images may be taken under any arbitrary but known position changes. 
     Exemplary Computing Device 
     As presented in  FIG. 9 , an exemplary system upon which embodiments of the present invention may be implemented includes a general purpose computing system environment, such as computing system  900 . In its most basic configuration, computing system  900  typically includes at least one processing unit  901  and memory, and an address/data bus  909  (or other interface) for communicating information. Depending on the exact configuration and type of computing system environment, memory may be volatile (such as RAM  902 ), non-volatile (such as ROM  903 , flash memory, etc.) or some combination of the two. 
     Computer system  900  may also comprise an optional graphics subsystem  905  for presenting information to the computer user, e.g., by displaying information on an attached display device  910 , connected by a video cable  911 . According to embodiments of the present claimed invention, the graphics subsystem  905  may be coupled directly to the display device  910  through the video cable  911 . A graphical user interface of an application for controlling a medical linear accelerator executing in the computer system  900  may be generated in the graphics subsystem  905 , for example, and displayed to the user in the display device  910 . In alternate embodiments, display device  910  may be integrated into the computing system (e.g., a laptop or netbook display panel) and will not require a video cable  911 . In one embodiment, the processes  200  and  300  may be performed, in whole or in part, by graphics subsystem  905  in conjunction with the processor  901  and memory  902 , with any resulting output displayed in attached display device  910 . 
     Additionally, computing system  900  may also have additional features/functionality. For example, computing system  900  may also include additional storage (removable and/or non-removable) including, but not limited to, magnetic or optical disks or tape. Such additional storage is illustrated in  FIG. 9  by data storage device  907 . Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. RAM  902 , ROM  903 , and data storage device  907  are all examples of computer storage media. 
     Computer system  900  also comprises an optional alphanumeric input device  906 , an optional cursor control or directing device  907 , and one or more signal communication interfaces (input/output devices, e.g., a network interface card)  909 . Optional alphanumeric input device  906  can communicate information and command selections to central processor  901 . Optional cursor control or directing device  907  is coupled to bus  909  for communicating user input information and command selections to central processor  901 . Signal communication interface (input/output device)  909 , also coupled to bus  909 , can be a serial port. Communication interface  909  may also include wireless communication mechanisms. Using communication interface  909 , computer system  900  can be communicatively coupled to other computer systems over a communication network such as the Internet or an intranet (e.g., a local area network), or can receive data (e.g., a digital television signal). 
     According to some embodiments, the computer system  900  may be communicatively coupled with an imaging device to derive beam profile information and enable the accurate and efficient calibration of an area detector panel and/or beam source. In still further embodiments, the computer system  900  may receive as data beam profile information collected from an imaging device. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.