Li Xinhua, Zhang Da, Yang Jie, Liu Bob
Division of Diagnostic Imaging Physics, Department of Radiology, Massachusetts General Hospital, Boston, Massachusetts 02114 and Webster Center for Advanced Research and Education in Radiation, Department of Radiology, Massachusetts General Hospital, Boston, Massachusetts 02114.
Pinnacle Health - Fox Chase Regional Cancer Center, Harrisburg, Pennsylvania 17109.
Med Phys. 2014 Sep;41(9):091912. doi: 10.1118/1.4893753.
For CT scanning in the stationary-table modes, AAPM Task Group 111 proposed to measure the midpoint dose on the central and peripheral axes of sufficiently long phantoms. Currently, a long cylindrical phantom is usually not available in many clinical facilities. The use of a long phantom is also challenging because of the heavy weight. In order to shed light on assessing the midpoint dose in CT scanning without table movement, the authors present a study of the short- to long-phantom dose ratios, and perform a cross-comparison of CT dose ratios on different scanner models.
The authors performed Geant4-based Monte Carlo simulations with a clinical CT scanner (Somatom Definition dual source CT, Siemens Healthcare), and modeled dosimetry measurements using a 0.6 cm3 Farmer type chamber and a 10-cm long pencil ion chamber. The short (15 cm) to long (90 cm) phantom dose ratios were computed for two PMMA diameters (16 and 32 cm), two phantom axes (the center and the periphery), and a range of beam apertures (3-25 cm). The results were compared with the published data of previous studies with other multiple detector CT (MDCT) scanners and cone beam CT (CBCT) scanners.
The short- to long-phantom dose ratios changed with beam apertures but were insensitive to beam qualities (80-140 kV, the head and body bowtie filters) and MDCT and CBCT scanner models.
The short- to long-phantom dose ratios enable medical physicists to make dosimetry measurements using the standard CT dosimetry phantoms and a Farmer chamber or a 10 cm long pencil chamber, and to assess the midpoint dose in long phantoms. This method provides an effective approach for the dosimetry of CBCT scanning in the stationary-table modes, and is useful for perfusion and interventional CT.
对于固定扫描台模式下的CT扫描，美国医学物理师协会任务组111建议测量足够长的模体中心轴和外周轴上的中点剂量。目前，许多临床机构通常没有长圆柱形模体。由于重量较大，使用长模体也具有挑战性。为了阐明在不移动扫描台的CT扫描中评估中点剂量的方法，作者开展了一项关于短模体与长模体剂量比的研究，并对不同扫描仪型号的CT剂量比进行了交叉比较。
作者使用临床CT扫描仪（西门子医疗的Somatom Definition双源CT）进行了基于Geant4的蒙特卡罗模拟，并使用0.6 cm³的 Farmer型电离室和10 cm长的笔形电离室对剂量测定测量进行建模。针对两种聚甲基丙烯酸甲酯（PMMA）直径（16 cm和32 cm）、两个模体轴（中心和外周）以及一系列射束孔径（3 - 25 cm），计算了短（15 cm）模体与长（90 cm）模体的剂量比。将结果与之前其他多排探测器CT（MDCT）扫描仪和锥束CT（CBCT）扫描仪研究的已发表数据进行了比较。
短模体与长模体的剂量比随射束孔径变化，但对射束质（80 - 140 kV，头部和体部弓形滤过器）以及MDCT和CBCT扫描仪型号不敏感。
短模体与长模体的剂量比使医学物理师能够使用标准CT剂量测定模体和Farmer电离室或10 cm长的笔形电离室进行剂量测定测量，并评估长模体中的中点剂量。该方法为固定扫描台模式下的CBCT扫描剂量测定提供了一种有效方法，对灌注CT和介入CT很有用。
