Boston Children's Hospital, Boston, MA, USA.
Harvard Medical School, Boston, MA, USA.
J Appl Clin Med Phys. 2020 Mar;21(3):178-183. doi: 10.1002/acm2.12814. Epub 2020 Jan 28.
Size-specific dose estimates (SSDE) requires accurate estimates of patient size surrogates. AAPM Report 204 shows that the SSDE is the product of CTDIvol and a scaling factor, the normalized dose coefficient (NDC) which depends on patient size surrogates for CT axial images. However, SSDE can be determined from CT localizer prior to CT scanning. AAPM Report 220 charges that a magnification correction is needed for geometric patient size-surrogates. In this study, we demonstrate a novel "model-based" magnification correction on patient data.
573 patient scans obtained from a clinical CT system including 229 adult abdomen, 284 adult chest, 48 pediatric abdomen, and 12 pediatric chest exams. LAT and AP dimensions were extracted from CT localizers using a threshold extraction method (the ACR DIR). The model-based magnification correction was applied to the AP and LAT dimensions extracted using the ACR DIR. NDC was calculated using the effective diameter for the ACR DIR only, the model-based localizer-based and axial-based approaches. The LAT and AP dimensions were extracted from the "gold" standard CT axial scans. Outliers are defined as points outside the 95% confidence intervals and were analyzed.
NDC estimates for the localizer-based model-based approach had an excellent correlation (R = 0.92) with the gold standard approach. The effective diameter for ACR DIR and model-based approaches are 8.0% and 1.0% greater than the gold standard respectively. Outliers were determined to be primarily patient truncation, with arms down or with devices. ACR DIR size extraction method fails for bariatric patients where the threshold is too high and some of their anatomy was included in the CT couch, and small patients due to the CT couch being included in the size measurement.
The model-based magnification method gives an accurate estimate of patient size surrogates extracted from CT localizers that are needed for calculating NDC to achieve accurate SSDE.
基于大小的剂量估算（SSDE）需要准确估计患者大小的替代指标。AAPM 报告 204 表明，SSDE 是 CTDIvol 与归一化剂量系数（NDC）的乘积，NDC 取决于 CT 轴向图像的患者大小替代指标。然而，SSDE 可以从 CT 定位器在 CT 扫描之前确定。AAPM 报告 220 指出，需要对几何患者大小替代指标进行放大校正。在这项研究中，我们展示了一种新的基于模型的患者数据放大校正方法。
从临床 CT 系统中获得 573 例患者扫描，包括 229 例成人腹部、284 例成人胸部、48 例儿科腹部和 12 例儿科胸部检查。使用阈值提取方法（ACR DIR）从 CT 定位器中提取 LAT 和 AP 尺寸。将基于模型的放大校正应用于使用 ACR DIR 提取的 AP 和 LAT 尺寸。仅使用 ACR DIR 计算 NDC，使用基于定位器的模型和轴向方法计算 NDC。从“金标准”CT 轴向扫描中提取 LAT 和 AP 尺寸。定义离群值为超出 95%置信区间的点，并进行分析。
定位器基于模型的方法的 NDC 估计值与金标准方法具有极好的相关性（R=0.92）。ACR DIR 和基于模型的方法的有效直径分别比金标准大 8.0%和 1.0%。离群值主要是由于患者截断、手臂放下或带有器械所致。ACR DIR 尺寸提取方法在肥胖患者中失败，因为阈值过高，他们的一些解剖结构被包含在 CT 台架中，而在小患者中，由于 CT 台架被包含在尺寸测量中。
基于模型的放大方法可准确估计从 CT 定位器中提取的患者大小替代指标，这些指标是计算 NDC 以实现准确 SSDE 所必需的。
