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How Good Is Your Ruler - to Verify the Pixel to Mm Conversion Factor of EPID On Varian Edge Using An Instrument and An Algorithm with Sub-Pixel Accuracy

T Qu*, M Malin , NYU Langone Medical Center, New York, NY


(Wednesday, 7/17/2019) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 6

Purpose: To introduce an instrument and an algorithm with sub-pixel accuracy and use it to verify the pixel-to-mm conversion factor of the MV EPID on a Varian Edge Linac.

Methods: Three “bare� ball bearings (BBBs) are attached to form the BBB Triangle Phantom. The distances between the BBB centers are the diameter of a BBB. EPID images are taken with 6 MV at different Source-Imager-Distances (SIDs) and with the phantom positioned either at isocenter or approximately 10 cm away from iso on the treatment table. The centers of the nine BBBs are detected using a Matlab algorithm with subpixel accuracy. The ratio of the diameter of a BBB relative to the distance between the center of two BBBs is the pixel-to-mm conversion factor (pixel2mm). The pixel2mm at different SIDs is calculated and compared with nominal values and the values calculated by the Edge’s “Detect Ball� tool.

Results: The diameter of all BBBs are measured using three certified digital calipers with an average value of 12.70 mm and zero variation. The measured pixel2mm are 0.224, 0.211, 0.200, and 0.185 mm/pixel for SIDs of 150 cm, 160 cm, 170 cm, and 180 cm, respectively. They differ from the nominal values by 0.3, 0.5, 2.4, 0.9 micrometer, respectively. The pixel2mm vs SID distance is linear (R^2=0.997). The measured pixel2mm at an SID of 150 cm was within 0.001 mm of that computed with the “Detect Ball� tool.

Conclusion: An instrument is introduced to calculate the distance at micron level of accuracy. Use the instrument, the pixel to mm conversion factor (pixel2mm) is accurately calculated. The nominal pixel2mm value from the DICOM header is verified.


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