Institute of Medical Physics, Friedrich-Alexander-University Erlangen-Nürnberg, 91052 Erlangen, Germany.
Phys Med Biol. 2013 Jun 21;58(12):4205-23. doi: 10.1088/0031-9155/58/12/4205. Epub 2013 May 28.
The purpose of this study was to investigate the effect of shaped filters specifically designed for dedicated breast computed tomography (CT) scanners on dose and image quality. Optimization of filter shape and material in fan direction was performed using two different design methods, one aiming at homogeneous noise distributions in the CT images and the other aiming at a uniform dose distribution in the breast. The optimal filter thickness as a function of fan angle was determined iteratively to fulfil the above mentioned criteria for each breast diameter. Different filter materials (aluminium, copper, carbon, polytetrafluoroethylene) and breast phantoms with diameters between 80-180 mm were investigated. Noise uniformity in the reconstructed images, obtained from CT simulations based on ray-tracing methods, and dose in the breast, calculated with a Monte Carlo software tool, were used as figure of merit. Furthermore, CT-value homogeneity, the distribution of noise in cone direction, spatial resolution from centre to periphery and the contrast-to-noise ratio weighted by dose (CNRD) were evaluated. In addition, the decrease of scatter due to shaped filters was investigated. Since only few or one filter are practical in clinical CT systems, the effects of one shaped filter for different breast diameters were also investigated. In this case the filter, designed for the largest breast diameter, was simulated at variable source-to-filter distances depending on breast diameter. With the filter design method aiming at uniform noise distribution best results were obtained for aluminium as the filter material. Noise uniformity improved from 20} down to 5} and dose was reduced by about 30-40} for all breast diameters. No decrease of noise uniformity in cone direction, CT-value homogeneity, spatial resolution and the CNRD was detected with the shaped filter. However, a small improvement of CNRD was observed. Furthermore, a scatter reduction of about 20-30} and a more homogeneous scatter distribution were reached which led to reduced cupping artefacts. The simulations with one shaped filter at variable source-to-filter distance resulted in nearly homogeneous noise distributions and comparable dose reduction for all breast diameters. In conclusion, by means of shaped filters designed for breast CT, significant dose reduction can be achieved at unimpaired image quality. One shaped filter designed for the largest breast diameter used with variable source-to-filter distance appears to be the best solution for breast CT.
这项研究的目的是探讨专门为专用乳腺计算机断层扫描（CT）扫描仪设计的成形滤波器对剂量和图像质量的影响。使用两种不同的设计方法优化滤波器的形状和材料，一种方法旨在使 CT 图像中的噪声分布均匀，另一种方法旨在使乳房中的剂量分布均匀。为了满足每个乳房直径的上述标准，通过迭代确定最佳的滤波器厚度与扇形角度的关系。研究了不同的滤波器材料（铝、铜、碳、聚四氟乙烯）和直径为 80-180mm 的乳房体模。使用基于射线追踪方法的 CT 模拟获得的重建图像中的噪声均匀性和使用蒙特卡罗软件工具计算的乳房中的剂量作为衡量标准。此外，还评估了 CT 值均匀性、锥形方向的噪声分布、从中心到外围的空间分辨率以及加权剂量的对比度噪声比（CNRD）。此外，还研究了成形滤波器对散射的降低作用。由于在临床 CT 系统中只能使用少数或一个滤波器，因此还研究了不同乳房直径下一个成形滤波器的效果。在这种情况下，根据乳房直径，在不同的源滤器距离下模拟设计用于最大乳房直径的滤波器。采用旨在均匀分布噪声的滤波器设计方法，对于铝作为滤波器材料，可以获得最佳结果。对于所有乳房直径，噪声均匀性从 20%降低到 5%，剂量降低了约 30-40%。在锥形方向上，噪声均匀性、CT 值均匀性、空间分辨率和 CNRD 没有检测到成形滤波器的降低。然而，观察到 CNRD 的轻微改善。此外，还达到了约 20-30%的散射减少和更均匀的散射分布，从而减少了杯状伪影。在不同的源滤器距离下使用一个成形滤波器的模拟导致所有乳房直径的噪声分布几乎均匀，并实现了可比的剂量减少。总之，通过为乳腺 CT 设计的成形滤波器，可以在不影响图像质量的情况下显著降低剂量。使用可变源滤器距离的最大乳房直径设计的一个成形滤波器似乎是乳腺 CT 的最佳解决方案。
