A circuit modification that improves mammographic phototimer performance

The typical mammographic phototimer does not track with breast thickness. For four common, relatively new mammographic units, phototimed density decreases markedly as breast thickness increases. This trend is attributed to three factors: beam hardening, film reciprocity law failure (RFL), and photosensor dark or leakage current. The contributions of these three factors were experimentally quantitated for the phototimer of a Senographé 500T mammography unit. For a phototimed 28-kVp nongrid technique, the density varied from 2.0 for a 2.5-cm-thick phantom to 0.3 for one 7.6 cm thick. Of the 1.7 difference in film density, 1.1 was attributed to beam hardening, 0.2 to RFL, and 0.4 to photomultiplier tube dark current. A circuit modification was installed in the phototimer that offsets the photomultiplier dark current and has a nonlinear response to compensate for beam hardening and RFL effects. The modified phototimer tracked to within +/- 0.06 density for a 28-kVp grid technique as phantom thickness was varied from 2.0 to 6.0 cm. Similar results were obtained for nongrid techniques.