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Investigations into the feasibility of optical-CT 3D dosimetry with
minimal use of refractively matched fluids
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In optical-CT, the use of a refractively matched polyurethane solid-tank in place of
a fluid bath has the potential to greatly increase practical convenience, reduce
cost, and possibly improve the efficacy of flood corrections. This work
investigates the feasibility of solid-tank optical-CT imaging for 3D
dosimetry through computer simulation.
A matlab ray-tracing simulation platform, ScanSim, was used to model a
parallel-source telecentric optical-CT imaging system through a polyurethane
solid-tank containing a central cylindrical hollow into which
PRESAGE radiochromic dosimeters can be placed. A small amount
of fluid fills the 1–5 mm gap between the dosimeter and the walls of the tank. The
use of the solid-tank reduces the required amount of fluid by
approximately 97%. To characterize the efficacy of solid-tank, optical-CT
scanning simulations investigated sensitivity to refractive index (RI)
mismatches between dosimeter, solid-tank, and fluid, for a variety of dosimeter
(RI = 1.5–1.47) and fluid (RI = 1.55–1.0) combinations. Efficacy was evaluated
through the usable radius (ru
) metric, defined as the
fraction of the radius of the dosimeter where measured dose is predicted to be
within 2% of the ground truth entered into the simulation. Additional simulations
examined the effect of increasing gap size (1–5 mm) between the dosimeter and
solid-tank well. The effects of changing the lens tolerance
(0.5°–5.0°) were also investigated.
As the RI mismatch between the dosimeter and solid-tank increased
from 0 to 0.02, the usable radius decreased from 97.6% to 50.2%. The optimal fluid
RI decreased nonlinearly from 1.5 to 1.34 as the mismatch increased and was up to
9% lower than the tank. Media mismatches between the dosimeter and solid-tank also
exacerbate the effects of changing the gap size, with no easily quantifiable
relationship with usable radius. Generally, the optimal fluid RI value increases
as gap size increases and is closely matched to the dosimeter at large gap sizes
(>3 mm). Increasing the telecentric lens tolerance increases the usable radius
for all refractive media combinations and improves the maximum usable radius of
mismatched media to that of perfectly matched media for tolerances >5.0°. The
maximum usable radius can be improved up to a factor of 2 when lens tolerances are
Dry solid-tank optical-CT imaging in a
telecentric system is feasible if the dosimeter RI is a close match with the
solid-tank (<0.01 difference), providing accurate dose
measurements within ±2% of true dose to over 80% of the dosimeter volume. In order
to achieve accurate measurements over 96% of the dosimeter volume (representing
out to 2 mm from the dosimeter edge), the dosimeter-tank RI mismatch must be less
than 0.005. Optimal results occur when the RI of the dosimeter and tank is the
same, in which case the fluid will have the same RI. If mismatches between the
tank and dosimeter RI occur, the RI of the matching fluid needs to be fine tuned
to achieve the highest usable radius.
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