Establishing minimum performance standards, calibration intervals, and optimal exposure values for a whole breast digital mammography unit

Methods are developed to establish minimum performance standards, calibrati on intervals, and criteria for exposure control for a whole breast digital mammography system. A prototype phantom was designed, and an automatic meth od programmed, to analyze CNR, resolution, and dynamic range between CCD co mponents in the image receptor and over time. The phantom was imaged over a 5 month period and the results are analyzed to predict future performance. White field recalibration was analyzed by subtracting white fields obtaine d at different intervals. Exposure effects were compared by imaging the pro totype phantom at different kVp, filtration (Mo vs Rh) and mAs. Calcificati on detection tests showed that phantom images, obtained at 28 kVp with a Mo /Mo anode/filter and low mAs technique, often could not depict Al2O3 specks 0.24 mm in diameter, while a 28 kVp Mo/Rh, higher mAs technique usually co uld. Stability of the system tested suggests that monthly phantom imaging m ay suffice. Differences in CCD performance are greater (12%) than differenc es in a single CCD over time (6%). White field recalibration is needed week ly because of pixel variations in sensitivity which occur if longer interva ls between recalibration occur. When mean glandular dose is matched, Rh fil tration gives better phantom performance at 28 kVp than Mo filtration at 26 kVp and is recommended for clinical exposures. An aluminum step wedge show s markedly increased dynamic range when exit exposure is increased by using a higher energy spectrum beam. Phantoms for digital mammography units shou ld cover the entire image receptor, should test intersections between compo nents of the receptor, and should be automatically analyzed. (C) 1998 Ameri can Association of Physicists in Medicine. [S0094-2405(98)00312-5].