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Simulations to Investigate the Feasibility of 'dry' Optical-CT Imaging for 3D Dosimetry

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K Chisholm

K Chisholm1*, L Rankine2 , M Oldham3 , (1) Duke University, Durham, NC, (2) Washington University , Saint Louis, MO, (3) Duke University Medical Center, Durham, NC


SU-E-T-294 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To perform simulations investigating the feasibility of “dry” optical-CT, and determine optimal design and scanning parameters for a novel dry tank telecentric optical-CT 3D dosimetry system. Such a system would have important advantages in terms of practical convenience and reduced cost.

Methods: A Matlab based ray-tracing simulation platform, ScanSim, was used to model a telecentric system with a polyurethane dry tank, cylindrical dosimeter, and surrounding fluid. This program’s capabilities were expanded for the geometry and physics of dry scanning. To categorize the effects of refractive index (RI) mismatches, simulations were run for several dosimeter (RI = 1.5-1.48) and fluid (RI = 1.55-1.33) combinations. Additional simulations examined the effect of increasing gap size (1-5mm) between the dosimeter and tank wall, and of changing the telecentric lens tolerance (0.5°-5°). The evaluation metric is the usable radius; the distance from the dosimeter center where the measured and true doses differ by less than 2%.

Results: As the tank/dosimeter RI mismatch increases from 0-0.02, the usable radius decreases from 97.6% to 50.2%. The fluid RI for matching is lower than either the tank or dosimeter RI. Changing gap sizes has drastic effects on the usable radius, requiring more closely matched fluid at large gap sizes. Increasing the telecentric tolerance through a range from 0.5°-5.0° improved the usable radius for every combination of media.

Conclusion: Dry optical-CT with telecentric lenses is feasible when the dosimeter and tank RIs are closely matched (<0.01 difference), or when data in the periphery is not required. The ScanSim tool proved very useful in situations when the tank and dosimeter have slight differences in RI by enabling estimation of the optimal choice of RI of the small amount of fluid still required. Some spoiling of the telecentric beam and increasing the tolerance helps recover the usable radius.

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