[Quantitative scintigraphic detection of 111In sources].

This paper reports on a quantitative scintigraphic method for 111In sources using counts from isoboundary regions of interest of source images. Accordingly, total counts from regions of the image excluding the counts below a given percentage of the maximum are recorded and attenuation correction is implemented using the Depth Independent Buildup Factor method which corrects the effects of Compton scattering on the transmission function of the photon flux leaving the source. The transmission function of both the 172 and the 247 keV photons of 111In was determined for a set of circular flat sources of varying diameter (33 to 111 mm), using counts recorded in isoboundary regions of interest of images of the sources placed in air and at various depths in water. The images were acquired using a gamma camera equipped with a parallel hole medium energy collimator. The influence of the isoboundary value on the shape of the transmission function was investigated, and thus the chosen isoboundary value was set to no more than 20% of the maximum count on the image. Two parallel and dimetrically opposed views were acquired to determine the unknown activity of the sources placed in a water phantom. This counting geometry was chosen to operate in conditions of depth independent of the source in the phantom. Our results show a high degree of accuracy, the experimental estimate of the activity obtained by using the counts in the regions of interest with the lowest isoboundary value exhibiting 4% error rate. The error rate increases with the increase in the isoboundary values. The method proved to be simple, practical and accurate. Moreover, it overcomes the problem of accurate estimation of sources size, a variable usually not known with sufficient accuracy in clinical scintigraphic studies.