Precision measurement of annihilation point spread distributions for medically important positron emitters

Abstract

An experiment using very thin sources in low density polyurethane foam to expand the annihilation point spread distributions by linear factors of 20 and 50 was designed. By placing the same sources in an aluminum cylinder, range effects are effectively suppressed and broadening from all other sources may be measured. Three positron sources were used: C-11, Ga-68 and Sr-82, having beta end-points of 0.96, 1.90, and 3.35 MeV respectively. The sources were evaporated from solutions of small volume and high specific activity and sealed between two 50 ..mu..m sheets of plastic 10 mm in diam. The sources were placed at the center of 50 cm diam cylinders of polyurethane foam of density 0.020 gm/cm/sup 3/ and 0.05 gm/cm/sup 3/ (including the density of trapped air). A typical foam cell had a volume of 1 mm/sup 3/ and 8 ..mu..m walls. Thus the absorber was homogeneous on the scale of the positron range and the observed distributions were 50-fold or 20-fold enlargements of those that occur in water or tissue. The projected distribution of annihilation points was measured using the Donner 280-Crystal Circular Positron Coincidence Tomograph. This instrument measures the integral of positron annihilations between each of 14,700 detector pairs. These data are reorganized into 140 angles (1.29/sup 0/ spacing) of 105 parallel rays (5 mm spacing). The data for the sources in metal were used to estimate the effect of all sources of spatial broadening other than positron range. These include source size, detector size, deviations from 180/sup 0/ emission, and scattering in the foam or aluminum and lead shielding.