Wolfson Medical Vision Laboratory, Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK.
Phys Med Biol. 2012 Oct 21;57(20):6541-70. doi: 10.1088/0031-9155/57/20/6541. Epub 2012 Sep 25.
We present an efficient method to calculate the primary and scattered x-ray photon fluence component of a mammographic image. This can be used for a range of clinically important purposes, including estimation of breast density, personalized image display, and quantitative mammogram analysis. The method is based on models of: the x-ray tube; the digital detector; and a novel ray tracer which models the diverging beam emanating from the focal spot. The tube model includes consideration of the anode heel effect, and empirical corrections for wear and manufacturing tolerances. The detector model is empirical, being based on a family of transfer functions that cover the range of beam qualities and compressed breast thicknesses which are encountered clinically. The scatter estimation utilizes optimal information sampling and interpolation (to yield a clinical usable computation time) of scatter calculated using fundamental physics relations. A scatter kernel arising around each primary ray is calculated, and these are summed by superposition to form the scatter image. Beam quality, spatial position in the field (in particular that arising at the air-boundary due to the depletion of scatter contribution from the surroundings), and the possible presence of a grid, are considered, as is tissue composition using an iterative refinement procedure. We present numerous validation results that use a purpose designed tissue equivalent step wedge phantom. The average differences between actual acquisitions and modelled pixel intensities observed across the adipose to fibroglandular attenuation range vary between 5% and 7%, depending on beam quality and, for a single beam quality are 2.09% and 3.36% respectively with and without a grid.
我们提出了一种计算乳腺 X 射线摄影图像中初级和散射 X 射线光子注量分量的有效方法。这可用于一系列临床重要的用途，包括估计乳房密度、个性化图像显示和定量乳腺 X 线摄影分析。该方法基于以下模型：X 射线管、数字探测器和一种新的射线追踪器，该射线追踪器模拟从焦点发出的发散光束。管模型包括对阳极足跟效应的考虑，以及对磨损和制造公差的经验修正。探测器模型是经验性的，基于一系列传递函数，涵盖了临床上遇到的束质和压缩乳腺厚度范围。散射估计利用最佳信息采样和插值（以产生临床可用的计算时间），根据基本物理关系计算散射。为每条初级射线计算散射核，并通过叠加这些散射核来形成散射图像。考虑了束质、场中的空间位置（特别是由于周围散射贡献的耗尽而在空气边界处产生的位置）以及网格的存在，还使用迭代细化过程考虑了组织成分。我们展示了许多使用专门设计的组织等效阶跃楔形体模的验证结果。在脂肪到纤维腺体衰减范围内，实际采集和模型化像素强度之间的平均差异在 5%到 7%之间，具体取决于束质，对于单一束质，有和没有网格的情况下分别为 2.09%和 3.36%。
