Eliminating the need for refractive index matching in optical CT scanners for radiotherapy dosimetry: I. Concept and simulations.

Optical computed tomography has now become a well-established method for making empirical measurements of 3D dose distributions in radiotherapy treatment verification. The requirement for effective refractive index matching as part of the scanning process has long been an inconvenience for users, limiting the speed of sample throughput. We propose a new method for reconstructing data that takes explicit account of the refracted path of the light rays and demonstrate theoretically the conditions under which there are sufficient data to create a good reconstruction. Examples of the performance of the algorithm are given. For smoothly varying data, reconstructed images of very high quality are obtained, with RMS deviation of under 1% from the original, provided that the irradiated region lies entirely within a critical radius. For the dosimeter material PRESAGE, this critical value is approximately 0.65 of the sample radius. Regions outside this are not reconstructed successfully, but we argue that there are many cases where this disadvantage is outweighed by the benefits of the technique.