Chisholm Kelsey, Miles Devin, Rankine Leith, Oldham Mark
Medical Physics Graduate Program, Duke University, Durham, North Carolina 27710.
Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
Med Phys. 2015 May;42(5):2607-14. doi: 10.1118/1.4915530.
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 small (< 1.0°).
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.
在光学CT中，使用折射匹配的聚氨酯固体水箱代替液体浴有潜力极大地提高实际便利性、降低成本，并可能提高泛光校正的效果。这项工作通过计算机模拟研究了固体水箱光学CT成像用于三维剂量测定的可行性。
使用一个Matlab光线追踪模拟平台ScanSim，对一个平行源远心光学CT成像系统进行建模，该系统通过一个聚氨酯固体水箱，水箱中有一个中心圆柱形空洞，可在其中放置PRESAGE放射变色剂量计。少量液体填充剂量计与水箱壁之间1 - 5毫米的间隙。使用固体水箱可使所需液体量减少约97%。为了表征固体水箱的效果，光学CT扫描模拟研究了剂量计、固体水箱和液体之间折射率（RI）不匹配的敏感性，针对各种剂量计（RI = 1.5 - 1.47）和液体（RI = 1.55 - 1.0）组合。通过可用半径（ru）指标评估效果，可用半径定义为剂量计半径中预测测量剂量在输入模拟的真实值的2%以内的部分。额外的模拟研究了增加剂量计与固体水箱之间间隙尺寸（1 - 5毫米）的影响。还研究了改变透镜公差（0.5° - 5.0°）的影响。
随着剂量计与固体水箱之间的RI不匹配从0增加到0.02，可用半径从97.6%降至50.2%。随着不匹配增加，最佳液体RI从1.5非线性降至1.34，比水箱低达9%。剂量计与固体水箱之间的介质不匹配也会加剧间隙尺寸变化的影响，与可用半径没有容易量化的关系。一般来说，最佳液体RI值随着间隙尺寸增加而增加，在大间隙尺寸（> 3毫米）时与剂量计紧密匹配。增加远心透镜公差会增加所有折射介质组合的可用半径，并将不匹配介质的最大可用半径提高到完美匹配介质在公差> 5.0°时的水平。当透镜公差较小时（< 1.0°），最大可用半径可提高至2倍。
如果剂量计RI与固体水箱紧密匹配（差值< 0.01），远心系统中的干式固体水箱光学CT成像是可行的，能在真实剂量的± 2%范围内对超过80%的剂量计体积进行准确剂量测量。为了在超过96%的剂量计体积（从剂量计边缘向外2毫米）上实现准确测量，剂量计 - 水箱RI不匹配必须小于0.005。当剂量计和水箱的RI相同时会出现最佳结果，此时液体将具有相同的RI。如果水箱和剂量计RI之间出现不匹配，需要微调匹配液体的RI以实现最高可用半径。
