Department of Radiology, University Hospitals Leuven, Herestraat 49, Leuven 3000, Belgium.
Phys Med Biol. 2011 Sep 21;56(18):5823-43. doi: 10.1088/0031-9155/56/18/004. Epub 2011 Aug 16.
This paper proposes a hybrid technique to simulate the complete chain of an oral cone beam computed tomography (CBCT) system for the study of both radiation dose and image quality. The model was developed around a 3D Accuitomo 170 unit (J Morita, Japan) with a tube potential range of 60-90 kV. The Monte Carlo technique was adopted to simulate the x-ray generation, filtration and collimation. Exact dimensions of the bow-tie filter were estimated iteratively using experimentally acquired flood images. Non-flat radiation fields for different exposure settings were mediated via 'phase spaces'. Primary projection images were obtained by ray tracing at discrete energies and were fused according to the two-dimensional energy modulation templates derived from the phase space. Coarse Monte Carlo simulations were performed for scatter projections and the resulting noisy images were smoothed by Richardson-Lucy fitting. Resolution and noise characteristics of the flat panel detector were included using the measured modulation transfer function (MTF) and the noise power spectrum (NPS), respectively. The Monte Carlo dose calculation was calibrated in terms of kerma free-in-air about the isocenter, using an ionization chamber, and was subsequently validated by comparison against the measured air kerma in water at various positions of a cylindrical water phantom. The resulting dose discrepancies were found <10% for most cases. Intensity profiles of the experimentally acquired and simulated projection images of the water phantom showed comparable fractional increase over the common area as changing from a small to a large field of view, suggesting that the scatter was accurately accounted. Image validation was conducted using two small phantoms and the built-in quality assurance protocol of the system. The reconstructed simulated images showed high resemblance on contrast resolution, noise appearance and artifact pattern in comparison to experimentally acquired images, with <5% difference for voxel values of the aluminum and air insert regions and <3% difference for voxel uniformity across the homogeneous PMMA region. The detector simulation by use of the MTF and NPS data exhibited a big influence on noise and the sharpness of the resulting images. The hybrid simulation technique is flexible and has wide applicability to CBCT systems.
本文提出了一种混合技术，用于模拟完整的口腔锥形束 CT（CBCT）系统链，以研究辐射剂量和图像质量。该模型是围绕一个 3D Accuitomo 170 单元（日本 J Morita）开发的，管电压范围为 60-90kV。采用蒙特卡罗技术模拟 X 射线的产生、过滤和准直。使用实验获得的洪水图像迭代地估计蝴蝶结滤波器的精确尺寸。通过“相空间”调节不同曝光设置的非平面辐射场。通过射线追踪在离散能量下获得原始投影图像，并根据从相空间导出的二维能量调制模板进行融合。通过粗蒙特卡罗模拟进行散射投影，并用 Richardson-Lucy 拟合对生成的噪声图像进行平滑处理。使用测量的调制传递函数（MTF）和噪声功率谱（NPS）分别包括平板探测器的分辨率和噪声特性。蒙特卡罗剂量计算以空气中自由空气比释动能为基准，在等中心处使用电离室进行校准，然后通过与圆柱形水模中不同位置的水中空气比释动能的测量值进行比较来验证。大多数情况下，得到的剂量差异<10%。水模的实验获得和模拟投影图像的强度曲线显示，随着视场从小到大变化，在公共区域的分数增加相当，表明散射被准确地考虑在内。使用两个小体模和系统内置的质量保证协议进行图像验证。与实验获得的图像相比，重建的模拟图像在对比度分辨率、噪声出现和伪影模式上表现出高度相似性，铝和空气插入区域的体素值差异<5%，均匀 PMMA 区域的体素均匀性差异<3%。使用 MTF 和 NPS 数据进行的探测器模拟对噪声和图像的锐度有很大影响。混合模拟技术具有灵活性，适用于广泛的 CBCT 系统。
