Breast Density Measurement in Full-Field Digital Mammography: System Calibration and Stability

Breast density calibrations and stability measurements were undertaken on digital mammography systems to investigate whether a single calibration could be used for extended periods. The results indicated that the calibration did not change over time and was the same for two units investigated. The daily mean pixel value per mAs (MPV/mAs) for five systems was recorded over 22 months and showed varying periods of stability up to more than a year. However, step changes in MPV/mAs were noted and resulted from, for example, detector re-calibration or replacement. During stable periods the MPV/mAs for the static units varied mostly by <±5%, excluding outliers. This corresponds to <±2mm error on dense tissue thickness. The stability was worse for the mobile units. These results indicate that it should be possible to use a single calibration over extended periods of time, provided allowance is made for changes in stability exceeding 5%.