A statistical alternative to current measures of image quality in digital mammography

Abstract 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) an d (ii) A statistical estimate of contrast based on image noise levels ( C STAT ). Approach . The d ′ 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, C STAT , is 3.29 σ at a 95% level of confidence, where σ is the standard deviation of the background pixels. Theoretical analysis predicts a simple relationships between C STAT , T and d ′. Measured values of C STAT were compared to T and d ′ as a function of air kerma at the detector for ten digital mammography systems from three different manufacturers. Main Results . Theoretical relationships between C STAT , d ′ and T were demonstrated. Minimum acceptable image quality performance for 0.10 and 0.25 mm diameter discs, defined by the European Guidelines in terms of T , are equivalent to d ′ values of 0.85 and 5.36 and threshold C STAT values of 0.055 and 0.022. Significance . Strong correlations between log( T ), log( d ′) and log( C STAT ) suggest that either alternative approach produces information corresponding to that obtained using the CDMAM. C STAT should be considered as a simple, objective and cost-effective alternative to routine image quality assessment in mammography.