Johns P C, Yaffe M J
Med Phys. 1985 May-Jun;12(3):289-96. doi: 10.1118/1.595766.
Detection of a target object in a radiological image is often impeded by an obscuring background "clutter" resulting from the contrast between various materials in the neighborhood of the target. Dual-energy techniques can reduce or remove this clutter. In order for the target to be detectable in the image after dual-energy processing, the signal-to-noise ratio (SNR), defined as the difference between the target and the background divided by the photon noise in the difference, must exceed some threshold. A given SNR may be obtained for a wide range of the energies of the two x-ray beams and the ratio of their fluences. A theoretical model is developed which permits the choice of beams to be optimized with respect to some critical parameter--in this case, patient dose. The analysis is applied to the detection of calcifications in mammography. For an ideal imaging system, we predict that the optimum beam energies are 19 and 68 keV. A dose of 0.42 cGy is required to obtain an SNR of 5 for detection of a 0.02-cm cubic calcification in the resulting clutter-free image. This can be reduced to 0.16 cGy if the higher energy image is smoothed, prior to dual-energy processing, such that its variance is reduced to one-fourth of its unsmoothed value.
在放射图像中检测目标物体时，往往会受到目标周围各种物质之间对比度所导致的模糊背景“杂波”的阻碍。双能技术可以减少或消除这种杂波。为了使目标在双能处理后的图像中可被检测到，定义为目标与背景之差除以差值中的光子噪声的信噪比（SNR）必须超过某个阈值。对于两束X射线的各种能量及其注量比，可以获得给定的信噪比。开发了一个理论模型，该模型允许根据某个关键参数（在这种情况下是患者剂量）来优化束的选择。该分析应用于乳腺摄影中钙化的检测。对于理想的成像系统，我们预测最佳束能量为19 keV和68 keV。在最终无杂波的图像中检测边长为0.02 cm的立方钙化，获得信噪比为5需要0.42 cGy的剂量。如果在双能处理之前对较高能量的图像进行平滑处理，使其方差减小到未平滑值的四分之一，则该剂量可降低至0.16 cGy。
