Objective. Mammogram image quality in European breast screening systems is defined by threshold gold thickness (T) assessment of the CDMAM contrast-detail phantom. Previous studies have outlined several limitations of the phantom including expense, number of images required and inter-phantom manufacturing variability. Two alternative approaches to image quality assessment for routine quality control are examined and compared to the CDMAM technique: (i) A detectability index (d') based on a non-prewhitened model observer with an eye filter (NPWE) and(ii) A statistical estimate of contrast based on image noise levels (CSTAT).Approach. Thed' calculation follows a previously published methodology based on the NNPS and contrast, both measured from an image of 5 cm of PMMA containing a 0.2 mm Al target, as well as the MTF measured under standard conditions. For the proposed statistical method, pixels in the centre of the same NNPS image were re-binned into a range of equivalent CDMAM target areas. For any area, the minimum contrast necessary to distinguish a signal from the background,CSTAT, is 3.29σat a 95% level of confidence, whereσis the standard deviation of the background pixels. Theoretical analysis predicts a simple relationships betweenCSTAT,Tandd'. Measured values ofCSTATwere compared toTandd' as a function of air kerma at the detector for ten digital mammography systems from three different manufacturers.Main Results. Theoretical relationships betweenCSTAT,d' andTwere demonstrated. Minimum acceptable image quality performance for 0.10 and 0.25 mm diameter discs, defined by the European Guidelines in terms ofT, are equivalent tod' values of 0.85 and 5.36 and thresholdCSTATvalues of 0.055 and 0.022.Significance. Strong correlations between log(T), log(d') and log(CSTAT) suggest that either alternative approach produces information corresponding to that obtained using the CDMAM.CSTATshould be considered as a simple, objective and cost-effective alternative to routine image quality assessment in mammography.
Keywords: NPWE model observer; detectability index; image quality; low contrast detectability; mammography physics.
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