An empirical approach to calibration of a mammography system for volumetric measurement of density. (a) "Staircase" calibration tool. It is composed of a range of thicknesses of breast tissue equivalent plastics. On each step, the composition mimics fat, fibroglandular tissue and 30:70, 50:50, and 70:30 combinations of the two. (b) Radiograph of the calibration tool. (c) Calibration surface created from the radiograph in (b).