Cederström Björn, Fredenberg Erik
Department of Physics, Royal Institute of Technology (KTH), AlbaNova University Center, Stockholm 106 91, Sweden.
Med Phys. 2014 Dec;41(12):121903. doi: 10.1118/1.4900611.
Beam-quality optimization in digital mammography traditionally considers detection of a target obscured by quantum noise in a homogeneous background. This does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. The purpose of this paper is to assess the influence on optimal beam quality in mammography when anatomical noise is taken into account.
The detectability of microcalcifications and masses was quantified using a theoretical ideal-observer model that included quantum noise as well as anatomical noise and a simplified model of a photon-counting mammography system. The outcome was experimentally verified using two types of simulated tissue phantoms.
The theoretical model showed that the detectability of tumors and microcalcifications behaves differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods yield, which is to be expected because quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure noise has similar energy dependence as tumor contrast, the optimal x-ray energy was found to be higher and the useful energy region was wider than traditional methods suggest. A simplified scalar model was able to capture this behavior using a fitted noise mixing parameter. The phantom measurements confirmed these theoretical results.
It was shown that since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Furthermore, when anatomical noise is included, the tube voltage can be increased well beyond what is conventionally considered optimal and used clinically, without loss of image quality. However, no such conclusions can be drawn for the more complex mammographic imaging task as a whole.
传统上，数字乳腺摄影中的射束质量优化考虑的是在均匀背景中检测被量子噪声掩盖的目标。这与临床成像任务不太相符，因为实际的乳腺摄影图像包含解剖结构的复杂叠加，会产生可能比量子噪声更占主导的解剖噪声。本文的目的是评估在考虑解剖噪声时对乳腺摄影中最佳射束质量的影响。
使用一个理论理想观察者模型对微钙化和肿块的可检测性进行量化，该模型包括量子噪声以及解剖噪声和一个光子计数乳腺摄影系统的简化模型。使用两种类型的模拟组织体模对结果进行了实验验证。
理论模型表明，肿瘤和微钙化的可检测性在射束质量和剂量方面表现不同。小微钙化的结果与传统优化方法的结果相似，这是可以预期的，因为在高空间频率下量子噪声比解剖噪声更占主导。然而，对于较大的肿瘤，低频解剖噪声是限制因素。由于解剖结构噪声与肿瘤对比度具有相似的能量依赖性，发现最佳X射线能量更高，有用能量区域比传统方法所建议的更宽。一个简化的标量模型能够使用一个拟合的噪声混合参数来捕捉这种行为。体模测量证实了这些理论结果。
结果表明，由于量子噪声仅占噪声的一小部分，在不牺牲肿瘤可检测性的情况下可以大幅降低剂量。此外，当考虑解剖噪声时，管电压可以提高到远远超过传统上认为的最佳值并在临床上使用，而不会损失图像质量。然而，对于整个更复杂的乳腺摄影成像任务，无法得出这样的结论。
