Partial body calcium determination in bone by proton activation analysis.

The feasibility of a new method for in vivo regional bone calcium measurement has been studied in phantoms using the 160 MeV Harvard cyclotron, Advantages include the capability of measuring bone calcium directly directly in a well defined anatomical region (such as one or several vertebrae) and restriction of the dose to the immediate region of interest. Proton activation of 40Ca (97% natural abundance) produces radionuclide 38K. Its 2-17 MeV gamma ray (T 1/2 = 7-71 min) is detected by a NaI counter. Separation of this activity from room background and interfering nuclides, 14O and 34Clm, proceeds by decay curve analysis. Phantom studies showed the dependence of 38K activity to be highly linear with calcium content (r= 0-998). Non-linearities with dose did not appear below the 20 rad level. The precision of measurements on a phantom with two calf vertebrae exposed to 2-4 rad was 3% and was mostly limited by counting statistics. System reproducibility on phantoms given a high dose proved better than 0-5%.