Medical Physics Graduate Program, Duke University, Durham, North Carolina 27710, USA.
Med Phys. 2013 May;40(5):051701. doi: 10.1118/1.4798980.
Achieving accurate optical-CT 3D dosimetry without the use of viscous refractive index (RI) matching fluids would greatly increase convenience.
Software has been developed to simulate optical-CT 3D dosimetry for a range of scanning configurations including parallel-beam, point, and converging light sources. For each configuration the efficacy of three refractive media was investigated: air, water, a fluid closely matched to PRESAGE(®), and perfect matching (RI = 1.00, 1.33, 1.49, and 1.501 respectively). Reconstructions were performed using both filtered backprojection (FBP) and algebraic reconstruction technique (ART). The efficacy of the three configurations and the two algorithms was evaluated by calculating the usable radius (i.e., the outermost radius where data were accurate to within 2%), and gamma (Γ) analysis. This definition recognizes that for optical-CT imaging, errors are greatest near the edge of the dosimeter, where refraction can be most pronounced. Simulations were performed on three types of dose distribution: uniform, volumetric modulated arc therapy (VMAT), and brachytherapy (Cs-137).
For a uniformly irradiated dosimeter the usable radius achieved with filtered backprojection was 68% for water-matching and 31% for dry-scanning in air. Algebraic reconstruction gave usable radii of 99% for both water and air (dry-scanning), indicating greater recovery of useful data for the uniform distribution. FBP and ART performed equally well for a VMAT dose distribution where less dose is delivered near the edge of the dosimeter. In this case, the usable radius was 86% and 53% for scanning in water and air, respectively. For brachytherapy, the usable radius was 99% and 98% for scanning in water and air, respectively using FBP, and a major decrease was seen with ART. Point source geometry provided 1%-2% larger usable radii than parallel geometry. Converging geometry recovered less usable dosimetry data (up to 10% reduced usable radii) than point and parallel geometries. A further disadvantage of converging geometry was an increased requirement on detector size by up to 18°.
For applications where dose information is not required in the periphery of the dosimeter, some dry and low-viscous matching configurations may be feasible. For all three dose distributions (uniform, VMAT, brachytherapy) the point source geometry produced slightly more favorable results (an extra 1%-2% usable radii) than parallel and converging. When dosimetry is required on the periphery, best results were obtained using close refractive matching and ART. A concern for water or dry-scanning is the increase in required detector size, introducing potential cost penalties for manufacturing.
在不使用粘性折射率（RI）匹配液的情况下实现精确的光学 CT 三维剂量测定，将极大地提高便利性。
开发了一种软件，用于模拟各种扫描配置下的光学 CT 三维剂量测定，包括平行束、点和会聚光源。对于每种配置，研究了三种折射介质的效果：空气、水、与 PRESAGE（®）紧密匹配的流体以及完美匹配（RI 分别为 1.00、1.33、1.49 和 1.501）。使用滤波反投影（FBP）和代数重建技术（ART）进行重建。通过计算可用半径（即数据在 2%以内准确的最外半径）和伽马（Γ）分析，评估了三种配置和两种算法的效果。这个定义认识到，对于光学 CT 成像，误差在剂量计的边缘最大，那里的折射可能最为明显。在三种类型的剂量分布上进行了模拟：均匀、容积调强弧形治疗（VMAT）和近距离放射治疗（Cs-137）。
对于均匀照射的剂量计，使用滤波反投影的可用半径在水匹配时为 68%，在空气干燥扫描时为 31%。代数重建术对于水和空气（干燥扫描）的可用半径分别为 99%，表明对于均匀分布，有用数据的恢复更好。对于 VMAT 剂量分布，FBP 和 ART 的性能相同，在剂量计边缘附近的剂量较低。在这种情况下，水和空气扫描的可用半径分别为 86%和 53%。对于近距离放射治疗，FBP 下水和空气扫描的可用半径分别为 99%和 98%，而 ART 则明显降低。点源几何形状比平行几何形状提供 1%-2%更大的可用半径。会聚几何形状恢复的可用剂量数据较少（可达 10%的可用半径减少），比点和平行几何形状都要少。会聚几何形状的另一个缺点是探测器尺寸增加了 18%。
对于不需要剂量计周边剂量信息的应用，某些干燥和低粘性匹配配置可能是可行的。对于所有三种剂量分布（均匀、VMAT、近距离放射治疗），点源几何形状产生的结果略好（可用半径增加 1%-2%），优于平行和会聚。当需要在周边进行剂量测定时，最好的结果是使用紧密的折射率匹配和 ART 获得。对水或干燥扫描的一个担忧是所需探测器尺寸的增加，这可能会对制造产生潜在的成本影响。
