Omar Artur, Andreo Pedro, Poludniowski Gavin
Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, Stockholm, 17176, Sweden.
Department of Oncology-Pathology, Karolinska Institutet, Stockholm, 17176, Sweden.
Med Phys. 2020 Oct;47(10):4763-4774. doi: 10.1002/mp.14359. Epub 2020 Aug 8.
To develop an analytical model for bremsstrahlung production in a thick x-ray target (i.e., the x-ray tube anode) that takes into account the intrinsic bremsstrahlung angular distribution.
X-ray spectrum models developed from theoretical principles have traditionally treated the angular distribution of the bremsstrahlung production as spherically uniform. This assumption stems from the rationale that electrons promptly attain a diffuse directional distribution in an x-ray target due to multiple scattering, thereby effectively masking the intrinsic bremsstrahlung angular distribution. In this work, a model that explicitly accounts for the angular distribution of the bremsstrahlung production is presented. The model combines Monte Carlo-calculated depth, energy, and angular distributions of electrons penetrating the x-ray target, and incorporates theoretical results for the differential bremsstrahlung cross section. The effects of using different simplified model assumptions for the electron penetration and the intrinsic bremsstrahlung angular distribution are analyzed for tungsten and molybdenum targets in the energy range 20-300 keV.
Typical assumptions of previous models are shown to introduce errors in calculated spectra. Particularly, it is shown that predictions of fluence and air kerma free-in-air can be overestimated by 15-30% (2-3% in aluminum half-value layer thickness) for clinically relevant beam qualities. The present model is able to reproduce comprehensive Monte Carlo calculations of the bremsstrahlung production generally to within 1%.
The bremsstrahlung model developed in this work is an improvement over previous models in that the main features of the electron penetration and the resulting bremsstrahlung are considered in detail. The model can be used for more accurate predictions of the energy and angular distribution of x rays emitted from an x-ray tube.
建立一个用于厚X射线靶（即X射线管阳极）中轫致辐射产生的分析模型，该模型考虑了本征轫致辐射角分布。
从理论原理发展而来的X射线光谱模型传统上把轫致辐射产生的角分布视为球对称均匀分布。这一假设源于这样的理论依据：由于多次散射，电子在X射线靶中迅速获得漫射方向分布，从而有效地掩盖了本征轫致辐射角分布。在这项工作中，提出了一个明确考虑轫致辐射产生角分布的模型。该模型结合了蒙特卡罗计算的穿透X射线靶的电子的深度、能量和角分布，并纳入了微分轫致辐射截面的理论结果。分析了在20 - 300 keV能量范围内，对钨和钼靶采用不同简化模型假设处理电子穿透和本征轫致辐射角分布所产生的影响。
结果表明，先前模型的典型假设会在计算光谱中引入误差。特别是，对于临床相关的束流质量，注量和空气中空气比释动能的预测可能会被高估15% - 30%（在铝半价层厚度中为2% - 3%）。当前模型通常能够将轫致辐射产生的综合蒙特卡罗计算结果再现到1%以内。
本工作中开发的轫致辐射模型是对先前模型的改进，因为详细考虑了电子穿透的主要特征以及由此产生的轫致辐射。该模型可用于更准确地预测从X射线管发射的X射线的能量和角分布。
