Normalized image noise as an absolute quantity for performance assessment in 2D mammography

Abstract

A method is proposed to equate the measured noise to a thickness of Aluminium by exposing a simple test object which includes an Aluminium step-wedge sandwiched between PMMA slabs. The scaling process turns the Image Noise expressed in mmAl into an absolute quantity which reflects exposure conditions. A second quantity, the dose independent Normalized Image Noise is defined. It is a characteristic value for every mammography unit/model and phantom setup and represents a measure of a system’s overall detection efficiency in clinical conditions. 8 mammography units of different manufacturers and detector technologies have been evaluated for PMMA thickness 3, 5 and 7 cm over a wide average glandular dose (AGD) range. The calculated Normalised Image Noise values were reproducible (uncertainty 3-5%) and coherent with known physical characteristics of the detector-grid combinations. Image Noise resulted sensitive to radiation spectra and scatter amount. Starting from threshold contrast detection with the CDMAM ver. 3.4 phantom, it was possible to identify Image Noiseacceptable/achievable thresholds which correspond to adequate image quality. Signal difference to noise (SDNR) analysis based on the proposed test object was in good agreement with SDNR evaluation according to the EUREF guideline (difference 1- 7%). Conversion coefficient, Image Noise and Normalised Image Noise could all be derived from a single exposure without having to determine the detector's response curve beforehand which is particularly advantageous for non-linear response. Given the sensitivity of Image Noise to radiation quality and dose, it is a suitable metric for optimization.