Radiology, Vol 166, 773-776, Copyright © 1988 by Radiological Society of North America
A circuit modification that improves mammographic phototimer performance
R LaFrance, DE Gelskey and GT Barnes
DISC Corporation, St. Malo, Manitoba, Canada.
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 Senographe
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.
