Noise power spectra of images from digital mammography detectors

Noise characterization through estimation of the noise power spectrum (NPS) is a central component of the evaluation of digital x-ray systems. We begi n with a brief review of the fundamentals of NPS theory and measurement, de rive explicit expressions for calculation of the one- and two-dimensional ( 1D and 2D) NPS, and discuss some of the considerations and tradeoffs when t hese concepts are applied to digital systems. Measurements of the NPS of tw o detectors for digital mammography are presented to illustrate some of the implications of the choices available. For both systems, two-dimensional n oise power spectra obtained over a range of input fluence exhibit pronounce d asymmetry between the orthogonal frequency dimensions. The 2D spectra of both systems also demonstrate dominant structures both on and off the prima ry frequency axes indicative of periodic noise components. Although the two systems share many common noise characteristics, there are significant dif ferences, including markedly different dark-noise magnitudes, differences i n NPS shape as a function of both spatial frequency and exposure, and diffe rences in the natures of the residual fixed pattern noise following flat fi elding corrections. For low x-ray exposures, quantum noise-limited operatio n 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 fr om the 2D NPS), on-axis periodic structures can be misleadingly smoothed or missed entirely. Our measurements indicate that for these systems, 1D spec tra 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 a n asymptotic value with increasing slit length when 1D spectra are generate d using the scanned synthetic slit method. Aliasing can contribute signific antly to the digital NPS, especially near the Nyquist frequency. Calculatio n of the theoretical presampling NPS and explicit inclusion of aliased nois e power shows good agreement with measured values. (C) 1999 American Associ ation of Physicists in Medicine. [S0094-2405(99)00707-5].