Determination of depth of a radionuclide source in a tissue equivalent phantom

Hydrophilic materials which refer to a group of cross-linked polymers originally developed in the 60s to produce soft contact lenses are characterised by an equilibrium water uptake in the range 15–95 % by wet weight when hydrated in water or normal saline solution at 25 °C. This ability to absorb controlled amounts of water makes them suitable human tissue substitutes. Four types of hydrophilic materials of different composition and thickness were placed between a ²²Na calibrated point source and a single LaBr₃: Ce (5 %) detector (Saint-Gobain) at a fixed distance between source and detector. The scatter-to-peak ratio was determined by selecting five scatter angle windows between 25 and 50^° (467–376 keV), at 5^° intervals, to study how the ratio varied with attenuating material, thickness and composition. From photon spectra recorded a ‘best’ estimate of the depth of the source in the hydrophilic material was obtained by finding the most sensitive scattering window. Theoretical and practical models of the source-to-detector geometries are included and discussed.