Cone beam optical CT scanner for 3D dosimetry

Purpose: To evaluate the performance of a cone beam optical CT scanner and to evaluate use of the scanner with a 3D solid radiochromic dosimeter. Method and Materials: The operating characteristics of a cone beam optical CT scanner were investigated using phantoms and solid dosimeters. The scanner acquires projection images (12 degree fan angle, 9 degree cone angle) during a single 360 degree rotation of a sample. 3D Images are reconstructed using Feldkamp backprojection with a Hamming filter. Background attenuation coefficients are removed by subtracting reference (pre‐irradiation) images from data (post‐irradiation) images prior to reconstruction. Reconstructed images have isotropic 2 mm, 1 mm, or 0.5 mm voxel dimensions in a 10 cm cube. Spatial resolution and image distortion were evaluated with a wire phantom and holes drilled in a solid dosimeter. Solid dosimeters were irradiated with a rectangular field and a complex Tomotherapy field. Profiles from scanned images were compared to profiles in calculated dose maps. Results: Reconstructed images of the geometry phantoms showed no spatial distortions. Artifacts are similar in appearance to sampling and beam hardening artifacts common in X‐ray CT. Attenuation coefficients in neutral density liquids are linear with respect to the spectrophotometer readings. Line profiles through calculated and measured dosemaps show qualitative agreement. Scanning times varied from 3 to 9 minutes. Reconstruction times varied from 2 to 20 minutes. The total time to scan a dosimeter was less than 1 hour. Conclusion: The optical CT scanner and solid dosimeter form a promising 3D dosimetry tool. Geometric accuracy, spatial resolution and imaging times are within clinical requirements Conflict of Interest: This research is funded by Modus Medical Devices Inc. and Heuris Pharma LLC. The authors have a commercial interest in the scanner and/or the dosimeter. © 2005, American Association of Physicists in Medicine. All rights reserved.