The trouble with CTDI100
The trouble with CTDI100
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DOI：
10.1118/1.2713240
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发表时间：
2007-04-01
期刊：
影响因子：
3.8
通讯作者：
Boone, John M.
中科院分区：
文献类型：
--
作者：
Boone, John M.
计算机断层扫描剂量指数（CTDI100）通常使用带有圆柱形聚甲基丙烯酸甲酯（PMMA）剂量学模型的100毫米长的铅笔离子室来测量。虽然该指标在准直层厚度为10mm或更小的单层CT扫描仪时代很有用，但在宽（40mm）准直x射线束的多层CT扫描仪中，该指标的效率尚不清楚。采用蒙特卡罗模拟方法对CTDI100参数在宽光束准直中的效率进行了评估。模拟利用了商用CT扫描仪的几何形状，并模拟了多能x射线光谱。单独计算剂量扩散函数（dsf）， 12.4 mm直径棒的长度放置在无限长160 mm直径（头部）和320 mm直径（身体）PMMA幻影的几个半径上。dsf用于计算1 ~ 400mm切片厚度的辐射剂量分布。CTDI100效率定义为沿CT切片位置为中心的100mm长度的PMMA棒收集的剂量除以沿无限长PMMA棒沉积的总剂量的分数。在10 mm的切片厚度、120 kVp的x射线光谱和PMMA头幻象下，CTDI100对中心孔和周围孔的效率分别为82%和90%。体幻影对应的效率值分别为63%和88%。当研究40毫米的切片厚度时，这些值仅降低了1%，因此CTDI100用于40分钟宽的x射线束的有效性不亚于用于10毫米宽的x射线束。然而，这些数据表明，即使在10分钟光束宽度下，CTDI100测量的效率也很低，因此，从该度量中得出的剂量计算可能不像期望的那样准确。(c) 2007年美国医学物理学家协会。
The computed tomography dose index (CTDI100) is typically measured using a 100 mm long pencil ion chamber with cylindrical polymethyl methacrylate (PMMA) dosimetry phantoms. While this metric was useful in the era of single slice CT scanners with collimated slice thicknesses of 10 mm or less, the efficiency of this metric in multi-slice CT scanners with wide (40 mm) collimated x-ray beams is unknown. Monte Carlo simulations were used to assess the efficiency of the CTDI100 parameter for wider beam collimations. The simulations utilized the geometry of a commercially available CT scanner, with modeled polyenergetic x-ray spectra. Dose spread functions (DSFs) were computed alone, the length of 12.4 mm diam rods placed at several radii in infinitely long 160 mm diam (head) and 320 mm diam (body) PMMA phantoms. The DSFs were used to compute radiation dose profiles for slice thicknesses from 1 to 400 mm. CTDI100 efficiency was defined as the fraction of the dose along a PMMA rod collected in a 100 mm length centered on the CT slice position, divided by the total dose deposited along an infinitely long PMMA rod. For a 10 mm slice thickness, a 120 kVp x-ray spectrum, and the PMMA head phantom, the efficiency of the CTDI100 was 82% and 90% for the center and peripheral holes, respectively. The corresponding efficiency values for the body phantom were 63% and 88%. These values are reduced by only 1% when a 40 mm slice thickness was studied, so the use of CTDI100 for 40 min. wide x-ray beams is no less valid than its use for 10 mm beam widths. However, these data illustrate that the efficiency of the CTDI100 measurement even with 10 min beam widths is low and, consequently, dose computations which are derived from this metric may not be as accurate as desirable. (c) 2007 American Association of Physicists in Medicine.