Jansen Jan T M, Shrimpton Paul C
Centre for Radiation, Chemical and Environmental Hazards, Public Health England, Chilton, Didcot, Oxfordshire, OX11 0RQ, UK.
Phys Med Biol. 2016 Jul 21;61(14):5356-77. doi: 10.1088/0031-9155/61/14/5356. Epub 2016 Jun 30.
The ImPACT (imaging performance assessment of CT scanners) CT patient dosimetry calculator is still used world-wide to estimate organ and effective doses (E) for computed tomography (CT) examinations, although the tool is based on Monte Carlo calculations reflecting practice in the early 1990's. Subsequent developments in CT scanners, definitions of E, anthropomorphic phantoms, computers and radiation transport codes, have all fuelled an urgent need for updated organ dose conversion factors for contemporary CT. A new system for such simulations has been developed and satisfactorily tested. Benchmark comparisons of normalised organ doses presently derived for three old scanners (General Electric 9800, Philips Tomoscan LX and Siemens Somatom DRH) are within 5% of published values. Moreover, calculated normalised values of CT Dose Index for these scanners are in reasonable agreement (within measurement and computational uncertainties of ±6% and ±1%, respectively) with reported standard measurements. Organ dose coefficients calculated for a contemporary CT scanner (Siemens Somatom Sensation 16) demonstrate potential deviations by up to around 30% from the surrogate values presently assumed (through a scanner matching process) when using the ImPACT CT Dosimetry tool for newer scanners. Also, illustrative estimates of E for some typical examinations and a range of anthropomorphic phantoms demonstrate the significant differences (by some 10's of percent) that can arise when changing from the previously adopted stylised mathematical phantom to the voxel phantoms presently recommended by the International Commission on Radiological Protection (ICRP), and when following the 2007 ICRP recommendations (updated from 1990) concerning tissue weighting factors. Further simulations with the validated dosimetry system will provide updated series of dose coefficients for a wide range of contemporary scanners.
尽管ImPACT（CT扫描仪成像性能评估）CT患者剂量计算器基于反映20世纪90年代初实际情况的蒙特卡罗计算，但至今仍在全球范围内用于估算计算机断层扫描（CT）检查的器官剂量和有效剂量（E）。CT扫描仪、E的定义、人体模型、计算机和辐射传输代码的后续发展，都迫切需要更新当代CT的器官剂量转换因子。现已开发出一种用于此类模拟的新系统，并进行了令人满意的测试。目前针对三台旧扫描仪（通用电气9800、飞利浦Tomoscan LX和西门子Somatom DRH）得出的归一化器官剂量的基准比较结果，与已发表值相差在5%以内。此外，这些扫描仪的CT剂量指数计算归一化值与报告的标准测量值合理一致（分别在测量和计算不确定度±6%和±1%范围内）。当使用ImPACT CT剂量测定工具对新型扫描仪进行测量时，针对一台当代CT扫描仪（西门子Somatom Sensation 16）计算得出的器官剂量系数显示，与目前假定的替代值（通过扫描仪匹配过程）相比，潜在偏差高达约30%。此外，针对一些典型检查和一系列人体模型的E的说明性估计表明，当从先前采用的程式化数学模型转换为国际放射防护委员会（ICRP）目前推荐的体素模型时，以及遵循ICRP 2007年建议（从1990年更新）中有关组织权重因子的规定时，可能会出现显著差异（约百分之几十）。使用经过验证的剂量测定系统进行的进一步模拟，将为广泛的当代扫描仪提供更新的剂量系数系列。
