Department of Biomedical Physics, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90024, USA.
Med Phys. 2013 May;40(5):051903. doi: 10.1118/1.4798561.
In AAPM Task Group 204, the size-specific dose estimate (SSDE) was developed by providing size adjustment factors which are applied to the Computed Tomography (CT) standardized dose metric, CTDI(vol). However, that work focused on fixed tube current scans and did not specifically address tube current modulation (TCM) scans, which are currently the majority of clinical scans performed. The purpose of this study was to extend the SSDE concept to account for TCM by investigating the feasibility of using anatomic and organ specific regions of scanner output to improve accuracy of dose estimates.
Thirty-nine adult abdomen/pelvis and 32 chest scans from clinically indicated CT exams acquired on a multidetector CT using TCM were obtained with Institutional Review Board approval for generating voxelized models. Along with image data, raw projection data were obtained to extract TCM functions for use in Monte Carlo simulations. Patient size was calculated using the effective diameter described in TG 204. In addition, the scanner-reported CTDI(vo)l (CTDI(vol),global) was obtained for each patient, which is based on the average tube current across the entire scan. For the abdomen/pelvis scans, liver, spleen, and kidneys were manually segmented from the patient datasets; for the chest scans, lungs and for female models only, glandular breast tissue were segmented. For each patient organ doses were estimated using Monte Carlo Methods. To investigate the utility of regional measures of scanner output, regional and organ anatomic boundaries were identified from image data and used to calculate regional and organ-specific average tube current values. From these regional and organ-specific averages, CTDI(vol) values, referred to as regional and organ-specific CTDI(vol), were calculated for each patient. Using an approach similar to TG 204, all CTDI(vol) values were used to normalize simulated organ doses; and the ability of each normalized dose to correlate with patient size was investigated.
For all five organs, the correlations with patient size increased when organ doses were normalized by regional and organ-specific CTDI(vol) values. For example, when estimating dose to the liver, CTDI(vol),global yielded a R(2) value of 0.26, which improved to 0.77 and 0.86, when using the regional and organ-specific CTDI(vol) for abdomen and liver, respectively. For breast dose, the global CTDI(vol) yielded a R(2) value of 0.08, which improved to 0.58 and 0.83, when using the regional and organ-specific CTDI(vol) for chest and breasts, respectively. The R(2) values also increased once the thoracic models were separated for the analysis into females and males, indicating differences between genders in this region not explained by a simple measure of effective diameter.
This work demonstrated the utility of regional and organ-specific CTDI(vol) as normalization factors when using TCM. It was demonstrated that CTDI(vol),global is not an effective normalization factor in TCM exams where attenuation (and therefore tube current) varies considerably throughout the scan, such as abdomen/pelvis and even thorax. These exams can be more accurately assessed for dose using regional CTDI(vol) descriptors that account for local variations in scanner output present when TCM is employed.
在 AAPM 工作组 204 中，通过提供适用于 CT 标准化剂量指标 CTDI（vol）的大小调整因子，开发了剂量特异性剂量估计（SSDE）。然而，该研究侧重于固定管电流扫描，并未特别针对当前大多数临床扫描所采用的管电流调制（TCM）扫描。本研究的目的是通过研究使用扫描器输出的解剖和器官特异性区域来提高剂量估计准确性的可行性，将 SSDE 概念扩展到 TCM。
使用机构审查委员会批准的多排 CT 机对 39 例成人腹部/骨盆和 32 例胸部临床 CT 检查进行了 TCM 扫描，生成了体素化模型。除了图像数据外，还获得了原始投影数据，以提取 TCM 函数，用于蒙特卡罗模拟。使用 TG 204 中描述的有效直径计算患者的大小。此外，还为每个患者获取了基于整个扫描中平均管电流的扫描器报告的 CTDI（vol）（CTDI（vol），global）。对于腹部/骨盆扫描，从患者数据集中手动分割出肝脏、脾脏和肾脏；对于胸部扫描，从肺部和女性模型中仅分割出乳腺组织。使用蒙特卡罗方法估计每个患者的器官剂量。为了研究扫描器输出区域测量的效用，从图像数据中识别出区域和器官解剖边界，并用于计算区域和器官特异性平均管电流值。从这些区域和器官特异性平均值中，为每个患者计算了 CTDI（vol）值，称为区域和器官特异性 CTDI（vol）。使用类似于 TG 204 的方法，使用所有 CTDI（vol）值对模拟器官剂量进行归一化；并研究了每个归一化剂量与患者大小的相关性。
对于所有五个器官，当使用区域和器官特异性 CTDI（vol）值对器官剂量进行归一化时，与患者大小的相关性增加。例如，当估计肝脏剂量时，CTDI（vol），global 产生的 R（2）值为 0.26，当分别使用腹部和肝脏的区域和器官特异性 CTDI（vol）时，该值提高到 0.77 和 0.86。对于乳腺剂量，CTDI（vol），global 产生的 R（2）值为 0.08，当分别使用胸部和乳腺的区域和器官特异性 CTDI（vol）时，该值提高到 0.58 和 0.83。当将胸部模型分为女性和男性进行分析时，R（2）值也有所提高，这表明该区域的性别差异不能用有效直径的简单测量来解释。
这项工作证明了在使用 TCM 时，区域和器官特异性 CTDI（vol）作为归一化因子的效用。证明了 CTDI（vol），global 不是腹部/骨盆甚至胸部等扫描中衰减（因此管电流）变化很大的 TCM 检查的有效归一化因子。使用 TCM 时会出现局部扫描器输出变化，可以使用区域 CTDI（vol）描述符更准确地评估这些检查的剂量，这些描述符可以考虑到局部变化。
