Design and calibration of a total-body counting system for measuring radioactivity in beagles

A positioner for holding dogs during total-body counting was designed. It was constructed so that when an animal is placed on his back, his midline is curved into a 33-cm radius arc, which approximates an isoresponse line for a 20 x 10 cm NaI(Tl) crystal positioned with its face at the center of curvature for this arc. Unlike other counting systems, the response is relatively independent of the location of an emitter at various points along the dog's midline within about +- 5%. The positioner molds the animal to a uniform 15 cm lateral thickness so that body attenuation of photons is similar among adult beagles. Effects on counting rate of differences in position of dogs within its confines are minor, and in the order of +- 2%. Calibration was accomplished by counting in this positioner a number of dogs containing various radionuclides. Their body burdens were then determined by the radioactivity of their individual bones and tissues following autopsy or from analysis of their total excretion from injection up to the time of counting. The calibration factor appeared to depend upon the photon energy of the emitter at about 100 keV and below, but was independent of energy above about 200 keV. This concept can be adapted readily to the total-body counting of other large animals including humans. (author)