Department of Medicine, Division of Cardiology, Columbia University Medical Center and New York-Presbyterian Hospital, New York, NY, 10032, USA.
Philips Research, Eindhoven, 5656AE, The Netherlands.
Med Phys. 2017 Dec;44(12):6589-6602. doi: 10.1002/mp.12592. Epub 2017 Oct 26.
Metal-oxide-semiconductor field-effect transistors (MOSFETs) serve as a helpful tool for organ radiation dosimetry and their use has grown in computed tomography (CT). While different approaches have been used for MOSFET calibration, those using the commonly available 100 mm pencil ionization chamber have not incorporated measurements performed throughout its length, and moreover, no previous work has rigorously evaluated the multiple sources of error involved in MOSFET calibration. In this paper, we propose a new MOSFET calibration approach to translate MOSFET voltage measurements into absorbed dose from CT, based on serial measurements performed throughout the length of a 100-mm ionization chamber, and perform an analysis of the errors of MOSFET voltage measurements and four sources of error in calibration.
MOSFET calibration was performed at two sites, to determine single calibration factors for tube potentials of 80, 100, and 120 kVp, using a 100-mm-long pencil ion chamber and a cylindrical computed tomography dose index (CTDI) phantom of 32 cm diameter. The dose profile along the 100-mm ion chamber axis was sampled in 5 mm intervals by nine MOSFETs in the nine holes of the CTDI phantom. Variance of the absorbed dose was modeled as a sum of the MOSFET voltage measurement variance and the calibration factor variance, the latter being comprised of three main subcomponents: ionization chamber reading variance, MOSFET-to-MOSFET variation and a contribution related to the fact that the average calibration factor of a few MOSFETs was used as an estimate for the average value of all MOSFETs. MOSFET voltage measurement error was estimated based on sets of repeated measurements. The calibration factor overall voltage measurement error was calculated from the above analysis.
Calibration factors determined were close to those reported in the literature and by the manufacturer (3 mV/mGy), ranging from 2.87 to 3.13 mV/mGy. The error σ of a MOSFET voltage measurement was shown to be proportional to the square root of the voltage V: σV=cV where c = 0.11 mV. A main contributor to the error in the calibration factor was the ionization chamber reading error with 5% error. The usage of a single calibration factor for all MOSFETs introduced an additional error of about 5-7%, depending on the number of MOSFETs that were used to determine the single calibration factor. The expected overall error in a high-dose region (30 mGy) was estimated to be about 8%, compared to 6% when an individual MOSFET calibration was performed. For a low-dose region (~3 mGy), these values were 13% and 12%.
A MOSFET calibration method was developed using a 100-mm pencil ion chamber and a CTDI phantom, accompanied by an absorbed dose error analysis reflecting multiple sources of measurement error. When using a single calibration factor, per tube potential, for different MOSFETs, only a small error was introduced into absorbed dose determinations, thus supporting the use of a single calibration factor for experiments involving many MOSFETs, such as those required to accurately estimate radiation effective dose.
金属氧化物半导体场效应晶体管（MOSFET）可作为器官辐射剂量测定的有用工具，并且在计算机断层扫描（CT）中得到了广泛应用。尽管已经采用了不同的方法对 MOSFET 进行校准，但那些使用常用的 100mm 铅笔电离室的方法并未纳入对其全长进行的测量，而且以前的工作也没有严格评估 MOSFET 校准中涉及的多种误差源。在本文中，我们提出了一种新的 MOSFET 校准方法，可将 MOSFET 电压测量值转换为 CT 中的吸收剂量，该方法基于在 100mm 长的电离室全长上进行的连续测量，并对 MOSFET 电压测量误差和校准中的四个误差源进行了分析。
在两个站点使用 100mm 长的铅笔电离室和 32cm 直径的圆柱形 CT 剂量指数（CTDI）体模对管电压为 80、100 和 120kVp 的 MOSFET 进行校准，以确定单个校准因子。在 CTDI 体模的九个孔中，九个 MOSFET 以 5mm 的间隔对 100mm 离子室轴上的剂量分布进行采样。吸收剂量的方差被建模为 MOSFET 电压测量方差和校准因子方差之和，后者由三个主要子分量组成：电离室读数方差、MOSFET 之间的差异以及与以下事实相关的贡献：使用少数 MOSFET 的平均校准因子作为所有 MOSFET 的平均值的估计值。MOSFET 电压测量误差是基于重复测量的集合来估计的。根据上述分析计算了校准因子的总电压测量误差。
确定的校准因子接近文献和制造商报告的值（约 3mV/mGy），范围为 2.87 至 3.13mV/mGy。MOSFET 电压测量的误差σ被证明与电压 V 的平方根成正比：σ V = cV，其中 c = 0.11mV。校准因子误差的主要来源是电离室读数误差，误差为 5%。使用单个校准因子对所有 MOSFET 进行校准会引入大约 5-7%的额外误差，具体取决于用于确定单个校准因子的 MOSFET 数量。在高剂量区域（约 30mGy）中，预计的总误差约为 8%，而单独进行 MOSFET 校准的总误差约为 6%。在低剂量区域（约 3mGy）中，这些值分别为 13%和 12%。
开发了一种使用 100mm 铅笔电离室和 CTDI 体模的 MOSFET 校准方法，并进行了吸收剂量误差分析，反映了多个测量误差源。当对不同的 MOSFET 使用单个管电压校准因子时，仅会对吸收剂量测定引入很小的误差，因此支持对涉及多个 MOSFET 的实验使用单个校准因子，例如准确估计辐射有效剂量所需的实验。
