Figure 4.

Energy of signals reaching the detector from activity in the tumor remnant are depth dependent. This figure demonstrates the potential for each radiation signal to generate charge carriers in a detector. Gamma photons, the most energetic, least attenuated signal, provide the greatest energy to be detected at all depths. Electrons and positrons provide a relatively energetic signal, but are rapidly attenuated with increasing depth. Optical photons provide the least energy but are less depth sensitive than the e^+/− signal.