TH-C-18A-05: Characteristics and Accuracy of CT Radiation Profile Width Measurements Using Radiochromic Films

Purpose: To investigate the impact of several acquisition and image analysis factors on the accuracy of CT beam width measurement using Gafchromic XR-QA2 film and an off the shelf document scanner. Methods: The film was carefully placed at the iso-center and exposed at various x-ray beam energies (80–140 kVp), exposure levels (50–400 mAs), and nominal beam widths (1.25, 5, and 10 mm). Films were scanned on a RICOH MP3501 flat-bed document scanner using reflection mode and various spatial resolutions (100 to 600 dpi) and locations within the scanner bed. Radiation dose profiles were derived by converting the net reflection measurements to dose values (in mGy) through film calibration (which is also described here). The beam widths were characterized by the FWHM and FWTM of the dose profiles and the deviations from the hybrid reference standards were quantified. The impact of x-ray beam energy, exposure intensity, film calibration, and the uniformity and spatial resolution of the document scanner were investigated. Results: The uncertainties in both FWHM and FWTM caused by varying beam energy, exposure level, and scanner uniformity were all within approximately 5%. Increasing scanner spatial resolution increased the uncertainty in both FWHM and FWTM (R2 = 0.35–0.71), with the FWTM affected by more than five times than the FWHM (28% vs. 4.9%). When uncalibrated dose profiles were used, the FWHM and FWTM were over-estimated by as much as 11.6% and 7.6%, respectively. Narrower beam width was more sensitive to the film calibration than the wider one (R2 = 0.68 and 0.85 for the FWHM and FWTM, respectively). Conclusion: Measurement of CT beam width using Gafchromic XR-QA2 films is robust against x-ray energy, exposure level, and scanner uniformity. It can provide adequate accuracy for all practical purposes with appropriate film calibration and scanner spatial resolution (100 dpi or less)