Long-Term Stability of Image Quality Measurements for Two Digital Mammography Systems

Contrast-detail measurements were made at approximately weekly intervals for three months, for two full-field mammography systems with different types of detector. The measured threshold contrast values were found to be reasonably stable but with some random variation. The coefficient of variance was 8-10% for detail sizes 0.1 and 1.0mm, and 3-5% for detail sizes 0.25 and 0.5mm. The output of both X-ray sets was also monitored, and found to vary within ±1% of the mean. The variation in threshold contrast is likely to be mainly due to variation of noise in the CDMAM images. Care should be taken when setting baselines and acceptable limits, so that measured changes in threshold contrast that are of the order of ±10% of the mean are not wrongly interpreted as significant changes in performance of a digital mammography system.