Practical dose point-based methods to characterize dose distribution in a stationary elliptical body phantom for a cone-beam C-arm CT system.
Practical dose point-based methods to characterize dose distribution in a stationary elliptical body phantom for a cone-beam C-arm CT system.
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DOI：
10.1118/1.4927257
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发表时间：
2015-08
期刊：
影响因子：
3.8
通讯作者：
Jang-Hwan Choi;D. Constantin;A. Ganguly;E. Girard;R. Morin;R. Dixon;R. Fahrig
中科院分区：
文献类型：
--
作者：
Jang-Hwan Choi;D. Constantin;A. Ganguly;E. Girard;R. Morin;R. Dixon;R. Fahrig
目的提出新的基于剂量点测量的度量，以表征在固定的大型体型体模上启用自动曝光控制的基于C形臂的宽锥角计算机断层扫描系统的单次部分旋转的剂量分布和平均剂量。方法采用0.6cm ~ 3小型电离室（IC），在由组织等效材料制成的椭圆体模中测量辐射剂量。将IC放置在体模中心和外围区域的23个均匀分布的孔处，并记录最小kVp（109和125 kVp）和z准直器孔径（全：22.2 cm;中：14.0 cm;小：8.4 cm）不同组合的6种采集方案的剂量。进行蒙特卡罗（MC）模拟以在中心平面（z = 0）中生成完整的2D剂量分布。MC模型在23个剂量点对IC实验数据进行了验证。然后使用两种建议的方法使用点剂量测量的子集估计平面剂量分布：（1）基于邻近度的加权方法（方法1）和（2）剂量点表面拟合方法（方法2）。对28个不同的剂量点分布和6个不同的剂量点数量（4、5、6、7、14和23个剂量点）进行了评价，以确定剂量点的最佳数量及其在体模中的位置。的方法的性能进行了确定，通过比较他们的结果与验证MC模拟。在测量不确定度的存在下的方法的性能进行了评估。结果5点、6点和7点情况下的差异低于2%，两种方法的范围为1.0%至1.7%，这与具有相对大量点的方法的性能相当，即，14和23点的情况下。然而，在4点的情况下，这两种方法的性能急剧下降。在4分、5分、6分和7分病例中，方法1和方法2的7分病例（差异1.0% [±0.6%]）和6分病例（差异0.7% [±0.6%]）表现最佳。此外，方法2证明了仅用5个点的高保真表面重建，显示出3.80 mGy（±0.32 mGy）的像素绝对差异。尽管性能对体模从等中心点的位移敏感，但在中心体模平面的x轴和y轴上位移高达2 cm时，性能变化小于2%。结论：方法1和方法2能够以合理的准确度计算平均剂量，仅需5个点，差异分别为1.7%（±1.2%）和1.3%（±1.0%）。大量的点并不一定保证更好的性能的方法，最佳的选择点的位置是必要的。该方法的性能对体模中心相对于等中心的对准敏感。在剂量分布很重要的身体应用中，方法2是比方法1更好的选择，因为它使用少至五个点以高保真度重建剂量表面。
PURPOSE To propose new dose point measurement-based metrics to characterize the dose distributions and the mean dose from a single partial rotation of an automatic exposure control-enabled, C-arm-based, wide cone angle computed tomography system over a stationary, large, body-shaped phantom. METHODS A small 0.6 cm(3) ion chamber (IC) was used to measure the radiation dose in an elliptical body-shaped phantom made of tissue-equivalent material. The IC was placed at 23 well-distributed holes in the central and peripheral regions of the phantom and dose was recorded for six acquisition protocols with different combinations of minimum kVp (109 and 125 kVp) and z-collimator aperture (full: 22.2 cm; medium: 14.0 cm; small: 8.4 cm). Monte Carlo (MC) simulations were carried out to generate complete 2D dose distributions in the central plane (z = 0). The MC model was validated at the 23 dose points against IC experimental data. The planar dose distributions were then estimated using subsets of the point dose measurements using two proposed methods: (1) the proximity-based weighting method (method 1) and (2) the dose point surface fitting method (method 2). Twenty-eight different dose point distributions with six different point number cases (4, 5, 6, 7, 14, and 23 dose points) were evaluated to determine the optimal number of dose points and their placement in the phantom. The performances of the methods were determined by comparing their results with those of the validated MC simulations. The performances of the methods in the presence of measurement uncertainties were evaluated. RESULTS The 5-, 6-, and 7-point cases had differences below 2%, ranging from 1.0% to 1.7% for both methods, which is a performance comparable to that of the methods with a relatively large number of points, i.e., the 14- and 23-point cases. However, with the 4-point case, the performances of the two methods decreased sharply. Among the 4-, 5-, 6-, and 7-point cases, the 7-point case (1.0% [±0.6%] difference) and the 6-point case (0.7% [±0.6%] difference) performed best for method 1 and method 2, respectively. Moreover, method 2 demonstrated high-fidelity surface reconstruction with as few as 5 points, showing pixelwise absolute differences of 3.80 mGy (±0.32 mGy). Although the performance was shown to be sensitive to the phantom displacement from the isocenter, the performance changed by less than 2% for shifts up to 2 cm in the x- and y-axes in the central phantom plane. CONCLUSIONS With as few as five points, method 1 and method 2 were able to compute the mean dose with reasonable accuracy, demonstrating differences of 1.7% (±1.2%) and 1.3% (±1.0%), respectively. A larger number of points do not necessarily guarantee better performance of the methods; optimal choice of point placement is necessary. The performance of the methods is sensitive to the alignment of the center of the body phantom relative to the isocenter. In body applications where dose distributions are important, method 2 is a better choice than method 1, as it reconstructs the dose surface with high fidelity, using as few as five points.