This application note explains how positron emission tomography (PET) imaging systems generate 3-D medical images. The article details how the PET system detects gamma-rays produced when injected doped sugars react differently to affected tissue. The article also discusses how competing electrical noise in the environment affects imaging, and why it is important to accurately detect photon timing and movement so the signal generated within the patient can be localized. A functional block diagram shows the IC components typically found in a PET system.