Abstract:
A computer-implemented method for correcting artifacts in measured image data due to differential scatter rejection in a computed tomography system is provided. A system for correcting artifacts in measured image data due to differential scatter rejection in a computed tomography system is also provided. Additionally, a non-transitory computer readable storage medium storing computer-executable instructions thereon for correcting artifacts in measured image data due to differential scatter rejection in a computed tomography system is provided.

Description:
BACKGROUND OF THE INVENTION 
     The embodiments described herein relate generally to x-ray computed tomography and, more particularly, to computed tomography systems having compact geometry and highly uniform resolution throughout the field of view (“FOV”). Embodiments described herein also relate to processes for correcting artifacts in image data collected by a computed tomography system. 
     In at least some known computed tomography (“CT”) imaging systems, an x-ray source projects a fan-shaped or a cone-shaped beam towards an object to be imaged. The x-ray beam passes through the object, and, after being attenuated by the object, impinges upon an array of radiation detectors. Each radiation detector produces a separate electrical signal that is a measurement of the beam intensity at the detector location. During data acquisition, a gantry that includes the x-ray source and the radiation detectors rotates around the object. 
     Traditional designs for CT systems place the detectors on an arc that is centered on the focal spot. As a result, the ratio between the usable FOV and the outer diameter of the CT system is relatively small. A typical CT system capable of scanning an 85 centimeter opening is in excess of 200 centimeters in diameter. Additionally, CT systems of the prior art have a resolution that is highest at the center of the FOV and decreases toward the edges of the FOV. 
     Turning to the correction of artifacts in image data, it is known that ring artifacts due to detector errors affect CT systems. Methodologies for correcting those artifacts have been developed to correct slight non-linearities in the responses of neighboring, contiguous detector elements. However, such methodologies do not correct artifacts resulting from small, high density objects and edges as they transition through a differential scatter region in a CT system. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a computer-implemented method for correcting artifacts in measured image data due to differential scatter rejection in a computed tomography system is provided. The computed tomography system includes an array of adjacent x-ray detector modules. The array has a first end and a second end located opposite the first end. Each detector module has a grid of detector elements arranged in columns and rows. Each column has associated measured values representing image data. The array of adjacent x-ray detector modules has at least one erroneous column of detector elements causing artifacts due to differential scatter rejection. The method comprises the steps of, for each erroneous column, calculating predicted values for the image data associated with the erroneous column based on image data associated with a first column and a second column, the first column and the second column being on opposite sides of the erroneous column and the first end being closer to the first column than to the second column. The method further includes calculating adjusted values for the image data associated with the erroneous column by subtracting correction values associated with the erroneous column from the measured values associated with the erroneous column. The method further includes replacing the measured values associated with the erroneous column with the calculated adjusted values. The method further includes increasing or decreasing the correction values associated with the erroneous column based on the difference between the predicted values for the erroneous column and the adjusted values for the erroneous column. 
     In a further aspect, a system for correcting artifacts in measured image data due to differential scatter rejection in a computed tomography system is provided. The system includes a processor, a memory coupled to the processor and an array of adjacent x-ray detector modules. The array has a first end and a second end located opposite the first end. Each detector module has a grid of detector elements arranged in columns and rows. Each column has associated measured values representing image data. The array of adjacent x-ray detector modules has at least one erroneous column of detector elements causing artifacts due to differential scatter rejection. The system is configured to, for each erroneous column, calculate predicted values for the image data associated with the erroneous column based on image data associated with a first column and a second column, the first column and the second column being on opposite sides of the erroneous column and the first end being closer to the first column than to the second column. The system is further configured to calculate adjusted values for the image data associated with the erroneous column by subtracting correction values associated with the erroneous column from the measured values associated with the erroneous column. The system is further configured to replace the measured values associated with the erroneous column with the calculated adjusted values. The system is further configured to increase the correction values associated with the erroneous column based on the difference between the predicted values for the erroneous column and the correction values for the erroneous column. 
     In a further aspect, a non-transitory computer readable storage medium storing computer-executable instructions thereon for correcting artifacts in measured image data due to differential scatter rejection in a computed tomography system is provided. The computed tomography system includes a processor, a memory coupled to the processor, and an array of adjacent x-ray detector modules. The array has a first end and a second end located opposite the first end. Each detector module has a grid of detector elements arranged in columns and rows. Each column has associated measured values representing image data. The array of adjacent x-ray detector modules has at least one erroneous column of detector elements causing artifacts due to differential scatter rejection. When executed by the processor, the computer-executable instructions cause the processor to, for each erroneous column, calculate predicted values for the image data associated with the erroneous column based on image data associated with a first column and a second column, the first column and the second column being on opposite sides of the erroneous column and the first end being closer to the first column than to the second column. The computer-executable instructions further cause the processor to calculate adjusted values for the image data associated with the erroneous column by subtracting correction values associated with the erroneous column from the measured values associated with the erroneous column. The computer-executable instructions further cause the processor to replace the measured values associated with the erroneous column with the calculated adjusted values. The computer-executable instructions further cause the processor to increase the correction values associated with the erroneous column based on the difference between the predicted values for the erroneous column and the correction values for the erroneous column. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an imaging system in accordance with an exemplary embodiment of the present invention. 
         FIG. 2  is a perspective view of a gantry assembly in accordance with the exemplary embodiment of present invention. 
         FIG. 3  is another perspective view of the gantry assembly in accordance with the exemplary embodiment. 
         FIG. 4  is a perspective view of the gantry frame in accordance with the exemplary embodiment of the present invention. 
         FIG. 5  is a perspective view showing an array of detector modules in accordance with the exemplary embodiment of the present invention. 
         FIG. 6  is a diagram showing the paths of x-rays emitted by the radiation source to detector modules in accordance with the exemplary embodiment of the present invention. 
         FIG. 7  is a partially exploded view of an exemplary detector module in accordance with the present invention. 
         FIG. 8  is a perspective view of an exemplary detector module in accordance with the present invention. 
         FIG. 9  is a simplified perspective view of an exemplary detector module in accordance with the present invention. 
         FIG. 10  is a perspective view of an exemplary gantry frame in accordance with the present invention, showing where detector modules are inserted into place. 
         FIG. 11  is a perspective view of a detector module being aligned for insertion into the gantry frame, in accordance with an exemplary embodiment of the present invention. 
         FIG. 12  is a perspective view of the detector module being inserted into the gantry frame, in accordance with the exemplary embodiment of the present invention. 
         FIG. 13  is a perspective view showing the detector module secured in the gantry frame, in accordance with the exemplary embodiment of the present invention. 
         FIG. 14  is a side view showing detector modules located in the center of the array of detector modules, in accordance with the exemplary embodiment of the present invention. 
         FIG. 15  is a side view showing detector modules located on one side of the array of detector modules, in accordance with the exemplary embodiment of the present invention. 
         FIG. 16  is a side view showing a gap between two detector modules and a gap-shield located in the gap, in accordance with the exemplary embodiment of the present invention. 
         FIG. 17  is a perspective view showing a portion of the array of detector modules within the gantry frame, in accordance with the exemplary embodiment. 
         FIG. 18  is a simplified diagram showing detector elements on two adjacent detector modules, in accordance with an embodiment of the present invention. 
         FIG. 19  is a flowchart of an exemplary embodiment of a process for correcting image artifacts in accordance with the present invention. 
         FIG. 20  is a block diagram of an exemplary embodiment of a computer communicatively coupled to detector modules to correct image artifacts in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective view of an imaging system  100  in accordance with an exemplary embodiment of the present invention. The imaging system  100  in this embodiment is a baggage scanning system, for viewing items in baggage passing through imaging system  100 . For example, imaging system  100  may be used to detect contraband (e.g., explosives, drugs, weapons, etc.) located in the baggage. Imaging system  100  includes a tunnel  106  and a conveyor  104  extending through tunnel  106 . Also included in imaging system  100  is a gantry assembly  108 , shown in more detail in  FIG. 2 . In this embodiment, imaging system  100  has a housing  102  having a width of approximately 150 centimeters and a height of approximately 147 centimeters. 
       FIG. 2  is a perspective view of gantry assembly  108  in accordance with the exemplary embodiment of the present invention. A radiation source  112 , which emits x-rays, is mounted to a gantry frame  114  by an x-ray mount  110 . In this embodiment, gantry frame  114  is a steel bolted structure with a bore of approximately 85 centimeters in diameter. The interior surface of gantry frame  114  is lined with lead. In this embodiment, x-ray mount  110  is cast steel with a lead cast window. X-ray mount  110  is configured to allow for position adjustment along an axis that is parallel to a length of tunnel  106  (the Z-axis). 
     On a first side  107  of gantry assembly  108 , as shown in  FIG. 2 , are a bearing  128  and a slip ring  130 . Bearing  128  allows gantry assembly  108  to rotate around an object to be imaged. In this exemplary embodiment, gantry assembly  108  is capable of rotating continuously, at approximately 150 rotations per minute. Slip ring  130  allows data signals and power to be transmitted between gantry assembly  108  and a remainder of imaging system  100 , as will be appreciated by those skilled in the art. Attached to a second side  109  of gantry frame  114 , opposite first side  107 , is a plenum  120 , which operates as a heat sink. Mounted to plenum  120  are global back planes  126 , which contain electronics and circuitry for proper operation of gantry assembly  108 , power management converter  124 , for powering the components of gantry assembly  108 , and fans  118  to transfer heat away from gantry assembly  108 . 
     A plurality of detector modules  122  are arranged in an array  123 , inside gantry frame  114 . Detector modules  122  receive x-ray beams emitted from radiation source  112  and convert the x-ray beams to electrical signals representing image data. Detector modules  122  are positioned in the gantry assembly  108  with an axis of symmetry running from radiation source  112  to the center of central detector module  122 . In alternative embodiments, there is an even number of detector modules, and an axis of symmetry runs from the radiation source to a point between two central detector modules. As explained below, detector modules  122  are arranged to increase an inner diameter of gantry assembly  108  relative to an outer diameter of gantry assembly  108 , when compared to prior CT imaging systems. The benefit is that imaging system  100  is given a smaller footprint while maintaining or increasing the size of objects, such as baggage, that can be scanned. 
       FIG. 3  is another perspective view of gantry assembly  108  in accordance with the exemplary embodiment. An opening  132  in gantry frame  114  allows x-ray beams from radiation source  112  to be emitted into gantry assembly  108 . The x-rays are emitted in a cone beam that intersects the entire tunnel  106 . An x-ray seal with a pre-collimator  164  of x-ray attenuating material is located between radiation source  112  and opening  132 . As shown in  FIG. 3 , slip ring  130  is attached to one side of gantry assembly  108  opposite plenum  120  and two global back planes  126  are mounted to plenum  120 . Power management converter  124  is connected to global back planes  126 . Fans  118  mounted to plenum  118  help transfer heat away from plenum  120  and gantry assembly  108  in general. Detector modules  122  are positioned such that during cone-to-parallel rebinning, resolution loss is minimized As shown in  FIG. 3 , some detector modules  122  are removed to expose a portion of underlying gantry frame  114 . In  FIG. 4 , discussed below, the gantry frame  114  is shown without any other components attached. 
       FIG. 4  is a perspective view of gantry frame  114  in accordance with the exemplary embodiment. Opening  132  allows x-rays from radiation source  112  to be emitted into gantry assembly  108  in a cone beam. Included on opposite interior sides of gantry frame  114  are positioning rails  138  that provide a mounting point for each detector module  122  in gantry assembly  108 . Included along opposite outer sides of gantry frame  114  are cooling holes  134 , to facilitate heat transfer away from gantry frame  114 . Also included in gantry frame  114  are torsion force stiffeners  136 , which provide structural support for gantry frame  114 . 
       FIG. 5  is a perspective view showing an array  123  of detector modules  122  in accordance with the exemplary embodiment of the present invention. Detector modules  122  are positioned along positioning rails  138 . In this exemplary embodiment, 17 detector modules are included in the array  123 . Array  123  includes a first end  194  and an opposite, second end  196 . Additionally, array  123  is divided into a first half  195 , extending from a center  198  of array  123  to first end  194 , and a second half  197 , extending from center  198  of array  123  to second end  196 . Other embodiments may include fewer or more detector modules and the total number of detector modules may be odd or even. In the exemplary embodiment, one detector module  122  is located at center  198  such that it is directly opposite radiation source  112 . Mirrored pairs of identical detector modules  122  extend outwards on either side. Detector modules  122  are gapped to allow for manufacturing tolerances in gantry assembly  108 . 
       FIG. 6  is a diagram showing x-ray beams  166  emitted by radiation source  112  to detector modules  122  in accordance with the exemplary embodiment. As can be seen, each detector module  122  is positioned so that the center of its collimator is normal to incident radiation bisecting detector module  122 . Adjacent edges of adjacent detector modules  122  are angularly spaced from each other. The angular spacing of the centerlines of beams  166  bisecting adjacent detector modules  122  decreases moving from ends  194  and  196  of array  123  of detector modules  122  to the center  198 . 
     Starting from detector module  122  at center  198 , shown in  FIG. 6 , and moving outwards, each detector module  122  is a different distance from radiation source  112 . That is, detector module  122  at center  198  is the furthest away from radiation source  112  and detector modules  122  along the first half  195  are closer to radiation source  112 . Moving from center  198  towards first end  194 , each successive detector module  122  is closer to radiation source  112  than the previous detector module  122 . Each detector module  122  along first half  195  has a corresponding detector module  122  on second half  197 , located at the same distance from radiation source  112 . That is, each detector module  122 , except detector module  122  located at center  198 , is part of a mirrored pair. The result of this arrangement is a smaller outer diameter of gantry assembly  108  as compared to prior CT imaging systems which have a constant radiation source to detector distance (SDD). As a result of this arrangement of separate detector modules  122 , the inner diameter of gantry assembly  108  is maximized relative to the outer diameter of gantry assembly  108 . 
     Data from the x-ray beams must be mapped to a different geometry (“rebinned”) once it is received by the detector modules, according to the following equation:
 
 R   mα ( n α)= P   (m+n)α ( D  sin  n α)
 
     In the above equation, a represents the pitch of the data after rebinning in the column direction of the data. Further, in the above equation, n represents the rebinned data column index. In the equation above, D represents the isocenter distance and m represents the view angle of the data. Rmα represents parallel beam data, P represents fan beam data, (m+n)α represents angular interpolation, and D sin nα represents detector interpolation. CT imaging systems of the prior art rebin from fan-to-parallel geometry. Such systems rebin in angle, first, for non-equispaced rays and rebin in detectors, second, for equispaced rays. However, the arrangement of detector modules described above produces beams that are nearly equispaced. Minor modifications to the source-to-detector distances through mimimax optimization result in beams that are even closer to being perfectly equispaced. As a result, the resolution loss from the second interpolation step can be minimized This arrangement of detector modules  122  provides for a highly uniform resolution across the entire field of view of the imaging system. 
       FIG. 7  is a partially exploded view of an exemplary detector module  122 . In the exemplary embodiment, every detector module  122  is identical and interchangeable with every other detector module  122 . Each detector module  122  includes a one-dimensional collimator  156  that includes radiation-attenuating material, such as tungsten. In certain embodiments, collimator  156  also includes antimony and tin. Collimator  156  includes an array of fins, which in the exemplary embodiment, each have a minimum thickness of about 0.5 millimeters. Collimator  156  has a fixed focal length, and is about 20 millimeters in height. Collimator  156  mounts to substrate  150  on detector module  122 . Also mounted to substrate  150  is a grid  147  of detector elements  146  that include scintillators which convert ionizing radiation into light, and photodiodes, which convert light into electrical signals. Interposed between collimator  156  and grid  147  are two layers. One layer is a diode protection grid  158 . A second layer  160  includes a checkerboard pattern. In one embodiment, layer  160  includes copper. Alternative embodiments do not include diode protection grid  158 . Other alternative embodiments do not include second layer  160 . Yet other embodiments do not include either of diode protection grid  158  or second layer  160 . Since collimator  156  is externally mounted to detector module  122 , collimator  156  can be easily removed and replaced, should it become damaged. 
     Detector module  122  includes multiple shields that include material such as lead for attenuating or blocking radiation from radiation source  112 . Below substrate  150  is a top shield  148 . Perpendicular to top shield  148  is a side shield  154 . In the exemplary embodiment, there is one side shield  154  on each side of detector module  122 . In addition, extending laterally from opposite sides of detector module  122  are wing shields, such as wing shield  152 . Extending longitudinally from detector module  122  are mounting extensions  140 , which facilitate aligning and mounting detector module  122  to gantry frame  114 . Each mounting extension includes a groove  172  which allows detector module  122  to slide along a guide rail, as explained with reference to  FIG. 11 . Mounting pins  168  also facilitate aligning and securing detector module  122  in gantry frame  114 . 
       FIG. 8  is another perspective view of detector module  122 . In  FIG. 8 , collimator  156  is mounted to substrate  150 , obstructing the view of diode protection grid  158  and checkerboard layer  160 . Side shield  154  for attenuating or blocking radiation is visible. In some embodiments, one or more side shields are secured externally to an outer cover of detector module  122 , while in other embodiments, one or more side shields are located within detector module  122 . Yet other embodiments do not include a side shield at all. Mounting extensions  140  can be seen extending longitudinally from detector module  122 . 
       FIG. 9  is a simplified perspective view of detector module  122 . As seen in  FIG. 9 , collimator  156  includes a plurality of fins, such as fin  170 . Detector elements  146 , which receive x-rays after the x-rays pass through collimator  156 , include scintillators which convert ionizing radiation into light, and photodiodes, which convert light into electrical signals. Detector elements  146  are mounted to substrate  150 . Top shield  148 , side shields  154 , and wing shields  152  block radiation and wing shields  152  block radiation that would otherwise pass between adjacent detector modules  122 . However, between some detector modules  122  are gaps that cannot be completely filled by wing shields  152 . In such cases, a gap-shield is located in the gap to block any radiation that would otherwise escape through the gap. 
       FIG. 10  is a perspective view of exemplary gantry frame  114 , illustrating attachment surfaces  144  where detector modules  122  are to be positioned along gantry frame  114 . Positioning rail  138  includes multiple attachment surfaces  144 , each corresponding to a location where detector element  122  is to be positioned. Extending from each attachment surface  144  is a guide rail  142 . 
       FIG. 11  is a perspective view of detector module  122  being aligned for insertion into gantry frame  114 . Mounting extension  140  includes groove  172 , which slides over guide rail  142 . By sliding groove  172  along guide rail  142 , detector module  122  can be brought into alignment with corresponding attachment surface  144 . Mounting pins  168  extend into corresponding holes in attachment surface  144  to assist in properly aligning detector module  122 . 
       FIG. 12  is a perspective view of detector module  122  being inserted into gantry frame  114 . Shown in  FIG. 12 , both the bottom and top mounting extensions  140  have been lined up with corresponding guide rails  142  as detector module  122  is being slid along guide rails  142  to engage corresponding attachment surfaces  144 . 
       FIG. 13  is a perspective view showing detector module  122  secured in gantry frame  114 . More specifically, each mounting extension  140  has been slid along a corresponding guide rail  142 , and mounting pins  168  have engaged attachment surfaces  144 . Detector module  122  can now be further secured to gantry frame  114  using screws or other fasteners. 
       FIG. 14  is a side view showing detector modules  122  located in the center of array  123  of detector modules  122  attached to gantry frame  114 . As shown in  FIG. 14 , wing shields  152  of adjacent detector modules  122  are in close proximity to each other.  FIG. 15  is a side view showing detector modules  122  located on first half  195  of array  123  of detector modules  122 . Wing shields  152  are relatively far apart from each other, as compared to their proximity in  FIG. 14 . This illustrates the difference in angular spacing between detector modules  122  from the center of array  123  to the ends of array  123 . Shield  172  is included on gantry frame  114  for blocking radiation from radiation source  114 . Shield  172  includes a radiation-attenuating material, such as lead. 
       FIG. 16  is a side view showing a gap  188  between two adjacent detector modules  122  and a gap-shield  162  located in gap  188 . As explained above, the angular spacing between adjacent detector modules increases moving towards either end of array  123  from the center. Accordingly, detectors closer to the ends of array  123  may have gaps between them that are not completely blocked by wing shields  152 . As shown in  FIG. 16 , a gap  188  exists between wing shield  152  and a side  189  of detector module  122 . Gap-shield  162  is located between two detector modules  122 , thereby blocking radiation that might have passed through gap  188 . Gap-shield  162  includes a radiation-attenuating material, such as lead. 
       FIG. 17  is a perspective view showing a portion of array  123  of detector modules  122  within gantry frame  114 . As shown in  FIG. 17 , detector module  122  is missing. However, since detector modules  122  are identical and interchangeable, a replacement detector module  122  can be readily inserted into position using guiding rails  138 , as detailed above with reference to  FIGS. 10 through 13 . 
       FIG. 18  is a simplified diagram showing detector elements  146  on two adjacent detector modules  122 . As shown in  FIG. 18 , adjacent detector modules  122  each include a top row  184  and a bottom row  186  of detector elements  146 . Multiple rows exist between top row  184  and bottom row  186  as well. Detector elements  146  are also arranged in columns on each detector module  122 . Columns  176  and  178  are located on the same detector module and are contiguous with each other. Likewise, columns  180  and  182  are located on the same detector module and are contiguous with each other. Column  178  is located on a first side  190  of left detector module  122 , and is considered an “edge column.” Likewise, column  180  exists on a second side  192  of adjacent detector module  122  and is also considered an “edge column.” 
     Due to the compact geometry enabled by the configuration of detector modules  122  in gantry assembly  108 , detector elements  146  arranged in columns along the edges of each detector module  122  have different effective collimation than their neighboring detector elements  146 . This causes a differential scatter rejection in the measurements. The differential scatter signal present in these columns of detector elements  146  appears as an additive signal, in the intensity domain, that varies slowly as a function of time. The signal is predictable from the spatially-adjacent columns of detector elements  146 , as the signal is also spatially low-frequency, with respect to the spatial pitch of detector elements  146  themselves. The differential scatter rejection results in strong image artifacts, if uncorrected. However, the location of these artifacts is known a priori within the measurement data, and the data associated with these columns of detector elements  146  is not missing. Rather, the data is only corrupted. 
     A predictor-corrector algorithm, which acts as a temporal recursive filter to estimate a correction based on neighboring columns of detector elements, can be used to correct the measured data from detector elements  146  located in columns along the edges of detector modules  122 , for example columns  178  and  180 . That is, the scatter signal in a given view is estimated using an adaptive filter that attempts to predict the differential scatter signal as a function of time using the past estimates and the current signals from spatially-neighboring columns of detector elements  146 . Once the differential scatter is estimated, it is subtracted from the measurements to correct them. One exemplary embodiment of a process for correcting the artifacts is as follows: 
     1. A linear interpolation is used to calculate a predicted signal for an edge column. For example, if columns n, n+1 are edge columns on two adjacent detector modules, the prediction signal is made via a linear combination of columns n−1 and n+2. In some embodiments, columns n−1 and n+2 may be weighted differently from each other. In other embodiments, the two columns are given equal weight. 
     2. A scatter prediction for the current view and columns is used to correct the edge columns n, n+1, without using the predicted signal calculated above, in step 1. 
     3. The scatter prediction is updated using the difference between the corrected signal and the predicted signal from step 1. In some embodiments, the difference between the measurements is weighted by multiplying it by a coefficient. 
     The process will correctly restore, for example, the view of wires that pass through the scatter region, as the predictor has a limited slew rate. That is, only a fraction of the prediction error is fed back into the predictor. Thus, although a wire appears as a very large prediction error, it is very brief. Accordingly, the prediction filter does not react to the wire itself. On the other hand, general Compton scatter is a spatially low-frequency phenomenon, and is well-predicted from the spatially-neighboring columns of detector elements. Convergence of the filter is guaranteed in the steady state of a homogenous object. 
     The process uses a recursive filter to estimate the scatter contribution to an edge column, for example column  178 , of detector elements  146 , and updates that filter using the error between the corrected signal and the prediction based on the neighboring columns. The result is that small, high frequency perturbations, such as a metal wire, crossing the columns of detector elements located along edges of detector modules  122  are reconstructed correctly, and the correction based on the linear interpolation of data from neighboring detector elements  146  is only used to estimate the slowly varying scatter signal. While the above discussion provides an overview of the process, a more detailed description of an exemplary artifact-correcting process in accordance with the present invention is presented below. 
       FIG. 20  is a block diagram  300  of a computer  310  communicatively coupled to detector modules  122 . Computer  310  includes a processor  312 , which is communicatively coupled to a memory  314  and a display  316 . Stored in memory  314  is data received from detector modules  122  and instructions for carrying out a process of correcting artifacts in the data received from detector modules  122 . Processor  312  is capable of executing the instructions stored in memory  314 . Images may be displayed on display  316 . Computer  310  may be physically separate from a baggage scanning system or integrated therein. 
     As can be seen from  FIGS. 5 and 6 , for example, array  123  of detector modules  122  includes first end  194  and second end  196  and is divided into first half  195  and second half  197 . With reference to  FIG. 18  and flowchart  200  shown in  FIG. 19 , if two detector modules  122  of  FIG. 18  are located on first half  195  of array  123 , the algorithm works as follows. First, data values from detector elements  146  in column  176  and data values from detector elements  146  in column  180  are added together and the sum is multiplied by 0.5. In other words, the values for columns  176  and  180  are averaged. These averaged values are considered predicted values for the data from detector elements  146  in column  178 . This is shown as step  210  in  FIG. 19 . However, due to the above-discussed differential scatter rejection, the predicted values do not match the data that is actually measured by the detector elements  146  in column  178 . A set of correction values for each detector element in column  178  exists in memory  314  of computer  310 . 
     In the exemplary embodiment of the process, the correction values are initially zero. An adjusted value for the data corresponding to each detector element  146  in column  178  is calculated by subtracting a corresponding correction value from the measured value. This is shown as step  212  in  FIG. 19 . The measured value is then replaced by the adjusted value, as shown in step  214  in  FIG. 19 . Next, the difference between the predicted value and the adjusted value is added to the correction value for each detector element in column  178 . This is shown as step  216  in  FIG. 19 . In some embodiments, the difference between the predicted value and the measured value is multiplied by a coefficient, shown as “WEIGHT” in step  216 . The steps are performed on a row by row basis, as indicated by the determination of whether more rows exist at step  218 . If there are more rows in the current detector module, then the process proceeds to step  220 , wherein row index R is incremented and the process loops back to step  210  as shown in  FIG. 19 . If there are no more rows on the current detector module, the process proceeds to the next detector module in first half  195  of array  123  and repeats, as shown at step  222  in  FIG. 19 . As the steps discussed above are repeated, the correction values reach steady state. The coefficient “WEIGHT” shown in step  216  may be increased or decreased to adjust how quickly the correction values reach steady state. 
     A similar set of process steps are applied to detector modules  146  on second half  197  of array  123 . Referring again to  FIGS. 18 and 19 , and assuming that two detector modules  122  shown in  FIG. 18  are instead located on second half  197  of array  123 , the process works as follows. First, data values from detector elements  146  in column  178  and data values from detector elements  146  in column  182  are added together and the sum is multiplied by 0.5. In other words, the values for the columns  178  and  182  are averaged. These averaged values are considered predicted values for the data from detector elements  146  in column  180 . This is shown as step  210  in  FIG. 19 . However, due to the above-discussed differential scatter rejection, the predicted values do not match the data that is actually measured by detector elements  146  in column  180 . 
     A set of correction values for each detector element  146  in column  180  exists in memory  314  of computer  310 . In the exemplary embodiment, the correction values are initially zero. An adjusted value for the data corresponding to each detector element  146  in column  180  is calculated by subtracting the correction value from the measured value. This is shown as step  212  in  FIG. 19 . The measured value is then replaced by the adjusted value, as shown in step  214  in  FIG. 19 . Next, the difference between the predicted value and the adjusted value is added to the correction value for each detector element  146  in column  180 . This is shown as step  216  in  FIG. 19 . Again, in some embodiments, the difference between the predicted value and the measured value is multiplied by a coefficient, shown as “WEIGHT” in step  216 . The steps are performed on a row by row basis, as indicated by the determination of whether more rows exist at step  218 . If more rows exist on the current detector module, the process proceeds to step  220 , where the row index R is incremented and the process loops back to step  210  as shown in  FIG. 19 . If there are no more rows, the process proceeds to the next detector module and repeats, as shown at step  222  in  FIG. 19 . As the steps discussed above are repeated, the correction values reach steady state. As mentioned above, the coefficient “WEIGHT” shown in step  216  may be increased or decreased to adjust how quickly the correction values reach steady state. 
     The above-discussed process steps are performed on a row by row, detector module by detector module basis, in multiple passes. There is an initial period where each pass causes the correction values to be adjusted. Ultimately, however, the correction values reach steady state and, when subtracted from the measured data as discussed above, the image artifacts are removed. 
     It should be understood that processor as used herein means one or more processing units (e.g., in a multi-core configuration). The term processing unit, as used herein, refers to microprocessors, microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), logic circuits, and any other circuit or device capable of executing instructions to perform functions described herein. 
     It should be understood that references to memory mean one or more devices operable to enable information such as processor-executable instructions and/or other data to be stored and/or retrieved. Memory may include one or more computer readable media, such as, without limitation, hard disk storage, optical drive/disk storage, removable disk storage, flash memory, non-volatile memory, ROM, EEPROM, random access memory (RAM), and the like. 
     Additionally, it should be understood that communicatively coupled components may be in communication through being integrated on the same printed circuit board (PCB), in communication through a bus, through shared memory, through a wired or wireless data communication network, and/or other means of data communication. Additionally, it should be understood that data communication networks referred to herein may be implemented using Transport Control Protocol/Internet Protocol (TCP/IP), User Datagram Protocol (UDP), or the like, and the underlying connections may comprise wired connections and corresponding protocols, for example, Institute of Electrical and Electronics Engineers (IEEE) 802.3 and/or wireless connections and associated protocols, for example, an IEEE 802.11 protocol, an IEEE 802.15 protocol, and/or an IEEE 802.16 protocol. 
     A technical effect of systems and methods described herein includes at least one of: (a) calculating predicted values for image data associated with an edge column of a detector module based on image data associated with a first column and a second column, the first column and the second column being on opposite sides of the edge column; (b) calculating adjusted values for the image data associated with the edge column by subtracting correction values associated with the edge column from the measured values associated with the edge column; (c) replacing the measured values associated with the edge column with the calculated adjusted values; (d) increasing the correction values associated with the edge column based on the difference between the predicted values for the edge column and the correction values for the edge column. 
     Exemplary embodiments of the compact geometry CT system and methods for correcting image artifacts are described above in detail. The methods and systems are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with other imaging systems and methods, and are not limited to practice with only the compact geometry CT systems as described herein. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.