A New Method Correcting the Parabola Effect of Flat-Bed Scanners Used in Radiochromic Film Dosimetry
D Poppinga1,2*, A Schoenfeld1,2, O Blanck3, D Harder4, B Poppe1,2, (1) University of Oldenburg, Oldenburg, Germany (2) Pius Hospital, Oldenburg, Germany (3) CyberKnife Center Northern Germany, Guestrow, Mecklenburg / Vorpommern, (4) University of Goettingen, Goettingen, GermanySU-E-T-24 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: In flatbed scanners used to digitize radiochromic films, a system of mirrors provides a reduced optical image of the scanned film in the entrance plane of a small CCD array. In this optical system, artifacts of the optical density arise from anisotropic scattering and polarization of the scanner light. We quantified this perturbation effect and eliminated it by a software correction.
Methods: In order to measure the nonuniformity of an Epson 10000XL scanner and an Epson V700 scanner, seven EBT3 films of 4x4 cm², exposed to doses ranging from 0 to 3 Gy were scanned at different positions x on the axis parallel to the light source. An area of interest of 30x30 pixels at the center of each film was chosen for analysis. The corresponding red channel pixel values were averaged and converted to optical density OD. These optical density values were then fitted by a parabolic function OD(x), which fully characterizes the nonuniformity of the scanner. The correction algorithm transforms the OD value measured at position x into the corrected OD value, i.e. the value that would be measured at the apex position. The calibration films are scanned at the apex position and the calibration curve is related to the apex OD value.
Results: To test the correction algorithm a 20x20 cm² 6 MV photon field was measured by film and ionization chamber and the dose difference after correction was less than 2%. This result can be achieved all over the scanner bed, so it is possible to correct wide dose distributions measured by radiochromic films.
Conclusion: This new parabola correction algorithm reduces the scanner artifacts over the whole width of the scanner bed and presents an alternative approach or an extension to the three-channel analysis [Micke et al Med Phys 38, 2012].