Williams M B, Mangiafico P A, Simoni P U
Department of Radiology, University of Virginia, Charlottesville 22908, USA.
Med Phys. 1999 Jul;26(7):1279-93. doi: 10.1118/1.598623.
Noise characterization through estimation of the noise power spectrum (NPS) is a central component of the evaluation of digital x-ray systems. We begin with a brief review of the fundamentals of NPS theory and measurement, derive explicit expressions for calculation of the one- and two-dimensional (1D and 2D) NPS, and discuss some of the considerations and tradeoffs when these concepts are applied to digital systems. Measurements of the NPS of two detectors for digital mammography are presented to illustrate some of the implications of the choices available. For both systems, two-dimensional noise power spectra obtained over a range of input fluence exhibit pronounced asymmetry between the orthogonal frequency dimensions. The 2D spectra of both systems also demonstrate dominant structures both on and off the primary frequency axes indicative of periodic noise components. Although the two systems share many common noise characteristics, there are significant differences, including markedly different dark-noise magnitudes, differences in NPS shape as a function of both spatial frequency and exposure, and differences in the natures of the residual fixed pattern noise following flat fielding corrections. For low x-ray exposures, quantum noise-limited operation may be possible only at low spatial frequency. Depending on the method of obtaining the 1D NPS (i.e., synthetic slit scanning or slice extraction from the 2D NPS), on-axis periodic structures can be misleadingly smoothed or missed entirely. Our measurements indicate that for these systems, 1D spectra useful for the purpose of detective quantum efficiency calculation may be obtained from thin cuts through the central portion of the calculated 2D NPS. On the other hand, low-frequency spectral values do not converge to an asymptotic value with increasing slit length when 1D spectra are generated using the scanned synthetic slit method. Aliasing can contribute significantly to the digital NPS, especially near the Nyquist frequency. Calculation of the theoretical presampling NPS and explicit inclusion of aliased noise power shows good agreement with measured values.
通过估计噪声功率谱（NPS）来进行噪声特性描述是数字X射线系统评估的核心组成部分。我们首先简要回顾NPS理论和测量的基本原理，推导用于计算一维和二维（1D和2D）NPS的显式表达式，并讨论将这些概念应用于数字系统时的一些注意事项和权衡。给出了两种用于数字乳腺摄影探测器的NPS测量结果，以说明可用选择的一些影响。对于这两个系统，在一系列输入注量范围内获得的二维噪声功率谱在正交频率维度之间表现出明显的不对称性。两个系统的二维谱还在主频率轴上和轴外显示出主导结构，表明存在周期性噪声成分。尽管这两个系统具有许多共同的噪声特性，但也存在显著差异，包括明显不同的暗噪声幅度、NPS形状随空间频率和曝光的变化差异，以及在平场校正后的残留固定图案噪声性质的差异。对于低X射线曝光，仅在低空间频率下才可能实现量子噪声限制操作。根据获取一维NPS的方法（即合成狭缝扫描或从二维NPS中提取切片），轴上周期性结构可能会被误导性地平滑或完全遗漏。我们的测量表明，对于这些系统，可通过对计算出的二维NPS中心部分进行薄切片来获得用于探测量子效率计算的一维谱。另一方面，当使用扫描合成狭缝方法生成一维谱时，低频谱值不会随着狭缝长度的增加而收敛到渐近值。混叠对数字NPS有显著贡献，尤其是在奈奎斯特频率附近。理论预采样NPS的计算以及对混叠噪声功率的明确纳入与测量值显示出良好的一致性。
