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Evaluation of automatic exposure control performance in full-field digital mammography systems using contrast-detail analysis

Full Field Digital Mammography (FFDM) is increasingly replacing screen-film systems for screening and diagnosis of breast abnormalities. All FFDM systems are equipped with an Automatic Exposure Control (AEC) which automatically selects technique factors to optimize dose and image quality. It is therefore crucial that AEC performance is properly adjusted and optimized to different breast thicknesses. In this work, we studied the AEC performance of three widely used FFDM systems using the CDMAM and QUART mam/digi phantoms. We used the CDMAM phantom to generate Contrast-Detail (C-D) curves for each AEC mode available in the FFDM systems under study for phantoms with equivalent X-Ray attenuation properties as 3.2 cm, 6 cm and 7.5 cm thick breasts. Generated C-D curves were compared with ideal C-D curves constructed using a metric referred to as the k-factor which is the product of the thickness and the diameter of the smallest correctly identified disks in the CDMAM phantom. Previous observer studies have indicated that k-factor values of 60 to 80 μm2 are particularly useful in demonstrating the threshold for object detectability for detectors used in digital mammography systems. The QUART mam/digi phantom was used to calculate contrast-to-noise ratio (CNR) values at different phantom thicknesses. The results of the C-D analysis and CNR measurements were used to determine limiting CNR values intended to provide a threshold for proper image quality assessment. The results of the Contrast-Detail analysis show that for two of the three evaluated FFDM systems, at higher phantom thicknesses, low contrast signal detectability gets worse. This agrees with the results obtained with the QUART mam/digi phantom, where CNR decreases below determined limiting CNR values.