In PET, how many opposite detectors are needed?

• A. One detector pair
• B. Two opposite detectors
• C. Three detectors
• D. Four detectors

Answer: (B)

Two opposite detectors are needed because PET relies on detecting two 511 keV photons emitted in nearly opposite directions from a single annihilation event. When both photons are detected within a short coincidence window, the origin of that event must lie somewhere along the straight line connecting the two detectors—this is the line of response. A single detector can’t tell where along space the annihilation happened, but a coincident pair defines a definite line. Using more detectors can improve sensitivity and image reconstruction, but the fundamental requirement for localizing each event is a pair of opposite detectors detecting the coincidence photons.