Comparison between 153Gd and 241Am, 137Cs for dual-photon absorptiometry of the spine.

The bone mineral content (BMC) of the spine can be measured from the attenuation of two photon beams. An equation was derived for the theoretical precision of BMC measurement and validated experimentally. Though 153Gd was found to possess the better energy combination, the precision due to the photon energy levels using 241Am, 137Cs was only 1.5 times the value obtained from 153Gd. The available photon outputs from 241Am and 153Gd were investigated and with a 12 mm diameter disc source a theoretical precision of 1% could be obtained with source strengths 20 GBq 241Am or 5GBq 153Gd in a 40 min and 20 min patient scan respectively. There was no great advantage in either energy combination when the problems of fat or patient dose were considered. Given the disadvantages of 153Gd, its cost and availability, 241Am, 137Cs can be used as a practical alternative to measure spine BMC from a reconstructed bone mineral image.