Analysis of quantum noise fraction in different frequency bands for direct and indirect digital mammography detectors

Objective.To quantify differences in quantum noise fraction (QNF) between direct and indirect mammography detectors across spatial frequency bands, assessing the ability of QNF to reveal performance degradation at high frequencies.Approach.QNF of various mammography detectors were analyzed by selectively restricting the analysis to medium and high spatial frequency bands. The study included detectors from different manufacturers, both direct and indirect types. Noise components were determined from the sub-bands of a wavelet packet decomposition applied to uniform images acquired at different entrance kerma levels. The noise components were obtained from the terms of a second-degree polynomial fitted to the total noise variance of the sub-bands as a function of entrance kerma to the detector. The results obtained were compared with detective quantum efficiency calculations and contrast-detail curves of the detectors studied.Main results.The QNF results obtained in the different frequency bands studied differ significantly from those calculated in the image domain. In all cases, the QNF decreases with increasing frequency across the entire range of kerma values. Moreover, this decrease is much more pronounced for indirect detectors than for direct ones. Despite showing similar QNF values in the spatial domain, the differences between direct and indirect detectors become substantial in the mid-frequency bands and increase even further in the high-frequency bands. Among the image quality metrics evaluated, the largest differences between detectors were observed in the frequency-dependent evolution of the QNF.Significance.Decreasing values of the QNF reflect the loss in signal-to-noise ratio transfer from input to output in digital detectors, which is directly associated with a loss in detectability. The greater reduction in quantum noise observed in the mid- and high-frequency components of indirect detectors may indicate a decline in their performance for tasks involving the detectability of small objects. QNF exhibited the greatest sensitivity to scintillator-induced signal blurring, making it the metric that best distinguishes between direct and indirect detector designs.