Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India.
J Radiol Prot. 2020 Jun;40(2):477-486. doi: 10.1088/1361-6498/ab752e. Epub 2020 Feb 11.
The detection of internal contamination may be carried out by direct or indirect methods. The lung counting technique using an array of High Purity Germanium (HPGe) detectors is one of such direct detection methods. It is known from the literature that the estimation of activity by organ counting can lead to erroneous results if an amount of activity is possessed by an adjacent organ. In the case of HPGe-based lung monitoring, the estimation could be misleading if the activity is possessed by the liver, which is a proximal organ. In such cases the measured activity should be modified using cross-talk coefficients which account for the contribution from adjacent organs. The determination of cross-talk coefficients for Am was carried out by placing the detectors over inactive lungs of an Lawrence Livermore National Laboratory phantom when the source activity was contained in the liver and vice versa. A calibration matrix was formulated with calibration coefficients as diagonal elements and cross-talk coefficients as off-diagonal elements. The measured activities may be modified by matrix multiplication with the inverse of the calibration matrix to nullify the contribution from adjacent organs. The current work has empirically determined the fitting equations which relate calibration and cross-talk coefficients for lungs and liver measurement geometries with muscle-equivalent chest wall thickness (MEQ-CWT) values. The values of these coefficients were determined for an average MEQ-CWT of 1.77 cm for lungs and 1.33 cm for liver. The calculations showed that the activity contribution from liver to lungs was 29% higher than that of lungs to liver. A verification exercise was conducted to demonstrate this method. For the given calibration source, the percent overestimation was reduced for lung activity, while the liver activity was slightly underestimated. In the case of old exposure follow-up monitoring cases, the Am activity built up in liver could interfere with the lung monitoring results and this method using the calibration matrix may be used for estimation of more accurate results.
内部污染的检测可以通过直接或间接的方法进行。使用高纯锗（HPGe）探测器阵列的肺部计数技术就是这样的一种直接检测方法。文献中已知，如果活性物质存在于相邻器官中，器官计数法估计活性物质的结果可能会出现错误。在基于 HPGe 的肺部监测中，如果活性物质存在于肝脏等邻近器官中，那么这种估计可能会产生误导。在这种情况下，应该使用交叉对话系数来修改测量的活性，以考虑到来自相邻器官的贡献。通过将探测器放置在劳伦斯利弗莫尔国家实验室（Lawrence Livermore National Laboratory）的无活性肺部上，同时将源活性置于肝脏中或反之，来确定 Am 的交叉对话系数。制定了一个校准矩阵，其中校准系数为对角元素，交叉对话系数为非对角元素。通过用校准矩阵的逆矩阵对测量的活性进行矩阵乘法，可以修改测量的活性，以消除来自相邻器官的贡献。目前的工作已经通过实验确定了与肺部和肝脏测量几何形状相关的校准和交叉对话系数与肌肉等效胸壁厚度（MEQ-CWT）值的拟合方程。这些系数的值是针对平均 MEQ-CWT 为 1.77 厘米的肺部和 1.33 厘米的肝脏确定的。计算表明，肝脏对肺部的活性贡献比肺部对肝脏的活性贡献高 29%。进行了验证实验以证明这种方法。对于给定的校准源，肺部活性的高估百分比降低，而肝脏活性则略有低估。在旧的暴露后监测病例中，肝脏中积累的 Am 活性可能会干扰肺部监测结果，并且可以使用校准矩阵来估计更准确的结果。
