Medical Physics in Radiology, German Cancer Research Center (DKFZ), Im Neuenheimer Feld, Heidelberg, Germany.
Med Phys. 2022 Jan;49(1):93-106. doi: 10.1002/mp.15355. Epub 2021 Dec 10.
Various studies have demonstrated that additional prefilters and/or reduced tube voltages have the potential to significantly increase the contrast-to-noise ratios at unit dose (CNRDs) and thereby to significantly reduce patient dose in clinical CT. An exhaustive analysis, accounting for a wide range of filter thicknesses and a wide range of tube voltages extending beyond the 70 to 150 kV range of today's CT systems, including their specific choice depending on the patient size, is, however, missing. Therefore, this work analyzes the dose reduction potential for patient-specific selectable prefilters combined with a wider range of tube voltages. We do so for soft tissue and iodine contrast in single energy CT. The findings may be helpful to guide further developments of x-ray tubes and automatic filter changers.
CT acquisitions were simulated for different patient sizes (semianthropomorphic phantoms for child, adult, and obese patients), tube voltages (35-150 kV), prefilter materials (tin and copper), and prefilter thicknesses (up to 5 mm). For each acquisition soft tissue and iodine CNRDs were determined. Dose was calculated using Monte Carlo simulations of a computed tomography dose index (CTDI) phantom. CNRD values of acquisitions with different parameters were used to evaluate dose reduction.
Dose reduction through patient-specific prefilters depends on patient size and available tube current among others. With an available tube current time product of 1000 mAs dose reductions of 17% for the child, 32% for the adult and 29% for the obese phantom were achieved for soft tissue contrast. For iodine contrast dose reductions were 57%, 49%, and 39% for child, adult, and obese phantoms, respectively. Here, a tube voltage range extended to lower kV is important.
Substantial dose reduction can be achieved by utilizing patient-specific prefilters. Tube voltages lower than 70 kV are beneficial for dose reduction with iodine contrast, especially for small patients. The optimal implementation of patient-specific prefilters benefits from higher tube power. Tin prefilters should be available in 0.1 mm steps or lower, copper prefilter in 0.3 mm steps or lower. At least 10 different prefilter thicknesses should be used to cover the dose optima of all investigated patient sizes and contrast mechanisms. In many cases it would be advantageous to adapt the prefilter thickness rather than the tube current to the patient size, that is, to always use the maximum available tube current and to control the exposure by adjusting the thickness of the prefilter.
多项研究表明，附加预滤波器和/或降低管电压有可能显著提高单位剂量下的对比噪声比（CNRDs），从而显著降低临床 CT 中的患者剂量。然而，目前还缺乏对各种滤波器厚度和管电压范围（包括根据患者体型选择特定的管电压范围，超出了当今 CT 系统的 70-150kV 范围）的详尽分析。因此，这项工作分析了结合更广泛的管电压选择患者专用可选择预滤波器的剂量降低潜力。我们在单能量 CT 中对软组织和碘对比进行了研究。研究结果可能有助于指导 X 射线管和自动滤波器更换器的进一步发展。
对不同体型（半模拟人体模型的儿童、成人和肥胖患者）、管电压（35-150kV）、预滤波器材料（锡和铜）和预滤波器厚度（最高 5mm）的患者进行了 CT 采集模拟。对于每种采集，确定软组织和碘 CNRD。使用 CT 剂量指数（CTDI）体模的蒙特卡罗模拟计算剂量。使用不同参数采集的 CNRD 值评估剂量降低。
通过患者专用预滤波器的剂量降低取决于患者体型和可用管电流等因素。对于可用的管电流时间乘积为 1000mAs 的情况下，对于软组织对比，儿童、成人和肥胖患者的剂量降低分别为 17%、32%和 29%。对于碘对比，儿童、成人和肥胖患者的剂量降低分别为 57%、49%和 39%。在这里，扩展到较低千伏的管电压范围很重要。
通过利用患者专用预滤波器可以实现大量剂量降低。对于碘对比，管电压低于 70kV 有利于降低剂量，特别是对于小患者。通过利用更高的管功率，可以优化实施患者专用预滤波器。锡预滤波器应提供 0.1mm 或更小的步长，铜预滤波器应提供 0.3mm 或更小的步长。至少应使用 10 种不同的预滤波器厚度来覆盖所有研究患者体型和对比机制的剂量最佳值。在许多情况下，通过调整预滤波器厚度而不是管电流来适应患者体型会更有利，即始终使用最大可用管电流，并通过调整预滤波器厚度来控制曝光。
