Optical CT scanner for in-air readout of gels for external radiation beam 3D dosimetry

Abstract

Optical CT scanners for a 3D readout of externally irradiated radiosensitive hydrogels currently require the use of a refractive index (RI) matching liquid bath to obtain suitable optical ray paths through the gel sample to the detector. The requirement for a RI matching liquid bath has been negated by the design of a plastic cylindrical gel container that provides parallel beam geometry through the gel sample for the majority of the projection. The design method can be used for various hydrogels. Preliminary test results for the prototype laser beam scanner with ferrous xylenol-orange gel show geometric distortion of 0.2 mm maximum, spatial resolution limited to beam spot size of about 0.4 mm and 0.8% noise (1 SD) for a uniform irradiation. Reconstruction of a star pattern irradiated through the cylinder walls demonstrates the suitability for external beam applications. The extremely simple and cost-effective construction of this optical CT scanner, together with the simplicity of scanning gel samples without RI matching fluid increases the feasibility of using 3D gel dosimetry for clinical external beam dose verifications.

Optical CT imaging of solid radiochromic dosimeters in mismatched refractive index solutions using a scanning laser and large area detector

“…These ray paths are used to sort CT data into parallelbeam geometry. Rebinning to correct for refraction effects is a known technique previously applied in gel dosimetry 30 as well as biological imaging. 31,32 Unlike these previous studies, here we are not assuming knowledge of the refractive indices.…”

Optical and X-ray computed tomography scanning of 3D dosimeters

“…Examples of ray position correction have been reported by several authors [10,12,13,20]. Scattered light in hydrogels is a common problem especially with broad beam geometries and small fields [17,[21][22][23][24].…”

“…This approach should minimize stray light. Simpler systems may have more optical aberrations but computations can sort actual ray paths through dosimeters [9][10][11][12][13]. Lowering acceptance angles is an effective method to reduce stray light.…”

Hydrogels for Three-Dimensional Ionizing-Radiation Dosimetry

“…However, this scanner is not ideal for external beam dosimetry as the projection data are limited to the middle third of the gel sample and this can lead to inaccurate reconstruction when significant optical attenuation occurs in the missing data regions. A more recent design utilizes a a plastic cylindrical container that provides parallel beam geometry through the gel sample for the majority of the projections and no longer requires immersion in a tank with matching refractive index fluid [ 194 ]. New algorithms were developed for 3D dose reconstruction with these scanners [ 195 ].…”