Investigations into the Feasibility of Optical-CT 3D Dosimetry with Minimal Use of Refractively Matched Fluids

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2014

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Oldham, Mark

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Abstract

Purpose: Optical-CT imaging with radiochromic dosimeters is a powerful method of evaluating 3D dose distributions at high resolution and sensitivity. Current optical-CT systems require large quantities of refractively matched fluid surrounding the dosimeter in order to minimize refraction artifacts. The use of a refractively matched solid polyurethane solid-tank, in place of a fluid bath, has the potential to greatly increase practical convenience, reduce cost, and improve the efficacy of flood corrections. This thesis aims to investigate the feasibility of solid-tank optical-CT imaging for 3D dosimetry, and to use computer simulation to investigate optimal design and scanning parameters.

Methods: A Matlab based ray-tracing simulation platform, ScanSim, was used to model a parallel-source imaging system through a cubic polyurethane solid-tank containing a central cylindrical hollow into which cylindrical PRESAGE® radiochromic dosimeters can be placed. A small amount of fluid surrounds the dosimeter in the tank. ScanSim's capabilities were expanded from previous work to include the geometry and physics of dry scanning. Two imaging methods were investigated, representing a telecentric detector and an ideal detector: in the latter, all light rays are collected and used in reconstruction. In order to characterize the efficacy of these systems, and dependence on refractive index (RI) mismatches between dosimeter, solid-tank, and fluid, simulations were run for a variety of dosimeter (RI = 1.5-1.47), and fluid (RI = 1.55-1.0) combinations. Additional simulations examined the effect of increasing gap size (1-5mm) between the dosimeter and solid-tank well. For the telecentric setup, the effects of changing the lens tolerance (0.5-5.0 degrees) were also investigated. The metric for evaluation of efficacy is the usable radius, which is defined as the distance from the dosimeter center where the measured and true (known) dose differs by less than 2%.

Results: As the refractive index mismatch between the dosimeter and tank increases from 0-0.02, the telecentric system showed a significant decrease in the usable radius from 97.6% to 50.2% compared to a decrease from 97.6% to 96.4% for the ideal system. When the three media are perfectly matched, the telecentric system and ideal system perform identically. For mismatched dosimeter and solid-tank in a telecentric system, the optimal fluid match has a refractive index lower than either the tank or dosimeter, decreasing non-linearly from 1.5-1.34 as the dosimeter-tank refractive mismatch increases from 0 to 0.02. Media mismatches between the dosimeter and solid-tank also exacerbate the effects of changing the gap size, with no apparent quantifiable relationship. Generally, the optimal fluid match is closer to the dosimeter RI when the gap size is large (>3mm). Increasing the telecentric lens tolerance improves the usable radius for all refractive media combinations, and approaches the behavior of the ideal system for tolerances >5.0°.

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Chisholm, Kelsey (2014). Investigations into the Feasibility of Optical-CT 3D Dosimetry with Minimal Use of Refractively Matched Fluids. Master's thesis, Duke University. Retrieved from https://hdl.handle.net/10161/8853.

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