Photograph of the underside of a Varian EPID showing the support arm assembly. The white outline represents the approximate location of the imager panel.
Backscatter removal process. I M is the measured on-arm image and I 0, j is the estimated backscatter-free image at the jth iteration. I 0,est is the final estimated backscatter-free image. The function AddScatter includes the modeled backscatter contribution using Eq. (3) .
A comparison of cross-plane (left) and in-plane profiles (right) for a 10 × 10 cm2 field located at the center of the imager. The inset shows the off-arm image for visual clarity. Note that this field was used for model optimization and so was not included in the results of Table II .
A comparison of cross-plane (left) and in-plane profiles (right) for a 5 × 5 cm2 field located in the arm region of the imager. The effect of backscatter in this case is approximately 3%. The inset shows the off-arm image for visual clarity.
A comparison of cross-plane (left) and in-plane profiles (right) for an example sliding window prostate IMRT field located away from the arm region of the imager. The inset shows the off-arm image for visual clarity.
A summary of the gamma test results found comparing on-arm images (no backscatter removal) to off-arm images as well as comparing on-arm images (with backscatter removal) to off-arm images using 1%, 1 mm tolerances for 42 IMRT fields. The top panel shows the mean γ for each field while the bottom panel shows the percentage of pixels with γ < 1 in each field.
Optimized backscatter model parameters. The origin of the pixel coordinate system is in the top left corner of the imager, nearest the gantry. All dimensions are measured in the EPID detector plane.
RMS difference between the measured on-arm images with and without backscatter removal as compared to the measured off-arm images along in-plane profiles through the center of each rectangular, jaw-defined field studied. Profiles were calculated within the central 70% of the field to avoid the penumbra region. Uncertainties were determined from the standard deviation of 5 adjacent profiles. All field dimensions are defined at the isocenter. Positive in-plane field offset is toward the gantry. Positive cross-plane offsets are toward the right of the gantry, when facing the front.
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