Institute of Medical Physics, University of Erlangen-Nürnberg, Erlangen, Germany.
Med Phys. 2012 Feb;39(2):658-70. doi: 10.1118/1.3675400.
At present, no established methods exist for dosimetry in micro computed tomography (micro-CT). The purpose of this study was therefore to investigate practical concepts for both dosimetric scanner quality assurance and tissue dose assessment for micro-CT.
The computed tomography dose index (CTDI) was adapted to micro-CT and measurements of the CTDI both free in air and in the center of cylindrical polymethyl methacrylate (PMMA) phantoms of 20 and 32 mm diameter were performed in a 6 month interval with a 100 mm pencil ionization chamber calibrated for low tube voltages. For tissue dose assessment, z-profile measurements using thermoluminescence dosimeters (TLDs) were performed and both profile and CTDI measurements were compared to Monte Carlo (MC) dose calculations to validate an existing MC tool for use in micro-CT. The consistency of MC calculations and TLD measurements was further investigated in two mice cadavers.
CTDI was found to be a reproducible quantity for constancy tests on the micro-CT system under study, showing a linear dependence on tube voltage and being by definition proportional to mAs setting and z-collimation. The CTDI measured free in air showed larger systematic deviations after the 6 month interval compared to the CTDI measured in PMMA phantoms. MC calculations were found to match CTDI measurements within 3% when using x-ray spectra measured at our micro-CT installation and better than 10% when using x-ray spectra calculated from semi-empirical models. Visual inspection revealed good agreement for all z-profiles. The consistency of MC calculations and TLD measurements in mice was found to be better than 10% with a mean deviation of 4.5%.
Our results show the CTDI implemented for micro-CT to be a promising candidate for dosimetric quality assurance measurements as it linearly reflects changes in tube voltage, mAs setting, and collimation used during the scan, encouraging further studies on a variety of systems. For tissue dose assessment, MC calculations offer an accurate and fast alternative to TLD measurements allowing for dose calculations specific to any geometry and scan protocol.
目前，微计算机断层扫描（micro-CT）中不存在既定的剂量学方法。因此，本研究旨在探讨适用于 micro-CT 的剂量学扫描仪质量保证和组织剂量评估的实用概念。
将计算机断层扫描剂量指数（CTDI）适用于 micro-CT，使用经低管电压校准的 100mm 铅笔电离室，在 6 个月的时间间隔内分别在空气中和 20mm 和 32mm 直径的圆柱形聚甲基丙烯酸甲酯（PMMA）体模中心进行 CTDI 自由空气和中心测量。为了进行组织剂量评估，使用热释光剂量计（TLD）进行 z 剖面测量，并将剖面和 CTDI 测量与蒙特卡罗（MC）剂量计算进行比较，以验证用于 micro-CT 的现有 MC 工具。在两个小鼠尸体中进一步研究了 MC 计算和 TLD 测量的一致性。
CTDI 被发现是研究中的 micro-CT 系统恒定性测试的一个可重复的量，其与管电压呈线性关系，并且根据定义与 mAs 设置和 z 准直成正比。与在 PMMA 体模中测量的 CTDI 相比，在 6 个月的时间间隔后，在空气中测量的 CTDI 显示出更大的系统偏差。当使用在我们的 micro-CT 安装处测量的 X 射线光谱时，MC 计算与 CTDI 测量值的匹配度在 3%以内，而当使用半经验模型计算的 X 射线光谱时，匹配度优于 10%。视觉检查显示所有 z 剖面都有很好的一致性。在小鼠中，MC 计算和 TLD 测量的一致性优于 10%，平均偏差为 4.5%。
我们的结果表明，为 micro-CT 实施的 CTDI 是一种有前途的剂量学质量保证测量候选方法，因为它线性地反映了扫描过程中管电压、mAs 设置和准直的变化，鼓励对各种系统进行进一步的研究。对于组织剂量评估，MC 计算为 TLD 测量提供了一种准确和快速的替代方法，允许针对任何几何形状和扫描协议进行剂量计算。
