FIGURE 1. Phantom measurements to quantify positron penetration into tissue-equivalent medium using MD-55 Gafchromic film. (A) Exposure of film by balloon catheter filled with liquid radioactivity. (B) Exposed film showing concentric blue halo around hole with superimposed 2 rectangular axes for laser scanner read-out. (C) Typical result of evaluation of optical density of film (660 nm) indicating dose-over-depth function.