Department of Imaging, Cedars-Sinai Medical Center, Los Angeles, California, USA.
J Appl Clin Med Phys. 2024 Apr;25(4):e14167. doi: 10.1002/acm2.14167. Epub 2023 Oct 9.
Optimizing CT protocols is challenging in the presence of automatic dose modulation because the CT dose index (CTDI) at different patient sizes is unknown to the operator. The task is more difficult when both the image quality index and iterative reconstruction prospectively affect the dose determination. It is of interest in practice to be informed of the CTDI during the protocol initialization and evaluation. It was our objective to obtain a predictive relationship between CTDI, the image quality index, and iterative reconstruction strength at various patient sizes.
Dose modulation data were collected on a GE Revolution 256-slice scanner utilizing a Mercury phantom and selections of the noise index (NI) from 8 to 17, the third generation iterative reconstruction (ASIR-V) from 0% to 80%, and phantom diameters from 16 to 36 cm. The fixed parameters were 120 kVp, a pitch of .984, and a collimation of 40 mm with a primary slice width of 2.5 mm. The CTDI per diameter was based on the average tube current over three adjacent slices (same or similar diameter) multiplied by a conversion factor between the average mA of the series and the reported CTDI. The relationship between CTDI, NI, and ASIR-V for each diameter was fitted with a 2nd order polynomial of ASIR-V multiplied by a power law of NI.
The ASIR-V fit parameters versus diameter followed a Lorentz function while the NI exponent versus diameter followed an exponential growth function. The CTDI predictions were accurate within 15% compared to phantom results on a separate GE Revolution. For clinical relevance, the phantom diameter was converted to an abdomen or chest equivalent diameter and was well matched to patient data.
The fitted relationship for CTDI. for given values of NI and ASIR-V blending for a range of phantom sizes was a good match to phantom and patient data. The results can be of direct help for selecting adequate parameters in CT protocol development.
在自动剂量调制存在的情况下，优化 CT 协议具有挑战性，因为操作人员不知道不同患者体型的 CT 剂量指数（CTDI）。当图像质量指数和迭代重建都对剂量确定产生前瞻性影响时，任务就更加困难。在协议初始化和评估过程中了解 CTDI 是很有意义的。我们的目标是获得各种患者体型下 CTDI、图像质量指数和迭代重建强度之间的预测关系。
在 GE Revolution 256 层扫描仪上使用 Mercury 体模收集剂量调制数据，并从 8 到 17 选择噪声指数（NI），从 0%到 80%选择第三代迭代重建（ASIR-V），从 16 到 36cm 选择体模直径。固定参数为 120kVp、0.984 的螺距和 40mm 的准直器，主切片宽度为 2.5mm。每个直径的 CTDI 基于三个相邻切片（相同或相似直径）的平均管电流乘以系列平均毫安与报告的 CTDI 之间的转换系数。每个直径的 CTDI 与 NI 和 ASIR-V 的关系用 ASIR-V 的二次多项式乘以 NI 的幂律拟合。
直径的 ASIR-V 拟合参数遵循洛伦兹函数，而直径的 NI 指数遵循指数增长函数。与单独的 GE Revolution 体模结果相比，CTDI 预测的准确性在 15%以内。对于临床相关性，将体模直径转换为腹部或胸部等效直径，并与患者数据很好地匹配。
对于给定的 NI 和 ASIR-V 值，为一系列体模尺寸混合的 CTDI 拟合关系与体模和患者数据很好地匹配。结果可以直接帮助在 CT 协议开发中选择适当的参数。
