Chen-Mayer H Heather, Tosh Ronald E, Bateman Fred B, Bergstrom Paul M, Zimmerman Brian E
National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
J Res Natl Inst Stand Technol. 2022 Mar 10;126:126054. doi: 10.6028/jres.126.054. eCollection 2021.
A portable calorimeter for direct realization of absorbed dose in medical computed tomography (CT) procedures was constructed and tested in a positron emission tomography (PET) CT scanner. The calorimeter consists of two small thermistors embedded in a polystyrene (PS) cylindrical "core" (1.5 cm diameter) that can be inserted into a cylindrical high-density polyethylene (HDPE) phantom (30 cm diameter). The cylindrical design of core and phantom allows coaxial alignment of the system with the scanner rotation axis, which is necessary to minimize variations in dose that would otherwise occur as the X-ray source is rotated during scanning operations. The core can be replaced by a cylindrical ionization chamber for comparing dose measurement results. Measurements using the core and a calibrated thimble ionization chamber were carried out in a beam of 6 MV X-rays from a clinical accelerator and in 120 kV X-rays from a CT scanner. Doses obtained from the calorimeter and chamber in the 6 MV beam exhibited good agreement over a range of dose rates from 0.8 Gy/min to 4 Gy/min, with negligible excess heat. For the CT beam, as anticipated for these X-ray energies, the calorimeter response was complicated by excess heat from device components. Analyses done in the frequency domain and time domain indicated that excess heat increased calorimetric temperature rise by a factor of about 15. The calorimeter's response was dominated by dose to the thermistor, which contains high-atomic-number elements. Therefore, for future construction of calorimeters for CT beams, lower-atomic-number temperature sensors will be needed. These results serve as a guide for future alternative design of calorimeters toward a calorimetry absorbed dose standard for diagnostic CT.
构建了一种用于在医学计算机断层扫描（CT）程序中直接实现吸收剂量的便携式量热计，并在正电子发射断层扫描（PET）CT扫描仪中进行了测试。该量热计由两个嵌入聚苯乙烯（PS）圆柱形“芯体”（直径1.5厘米）中的小型热敏电阻组成，该芯体可插入圆柱形高密度聚乙烯（HDPE）体模（直径30厘米）中。芯体和体模的圆柱形设计允许系统与扫描仪旋转轴同轴对齐，这对于在扫描操作期间X射线源旋转时最小化否则会出现的剂量变化是必要的。芯体可以用圆柱形电离室代替，以比较剂量测量结果。使用芯体和校准的指形电离室在来自临床加速器的6 MV X射线束和来自CT扫描仪的120 kV X射线中进行了测量。在0.8 Gy/min至4 Gy/min的剂量率范围内，量热计和电离室在6 MV束中获得的剂量显示出良好的一致性，多余热量可忽略不计。对于CT束，正如这些X射线能量所预期的那样，量热计的响应因设备部件产生的多余热量而变得复杂。在频域和时域进行分析表明，多余热量使量热温度升高了约15倍。量热计的响应主要由对包含高原子序数元素的热敏电阻的剂量决定。因此，对于未来用于CT束的量热计的构建，将需要低原子序数的温度传感器。这些结果为未来量热计的替代设计提供了指导，以实现诊断CT的量热吸收剂量标准。
