Figure 2.

Confounding physical effects that cause bias in positron emission tomography (PET) coincidence imaging. (Left to right) Attenuation: One of the annihilation photons does not reach the detector either because it has undergone absorption or Compton scattering that deflects it away from the detectors or because it does not interact in a measureable way with a scanner detector. Scattered-coincidence event: One of the two photons undergoes Compton scattering and changes its trajectory, but both photons are still detected, thus defining an incorrect line of response (LOR). Random-coincidence event: Two simultaneous annihilations produce two pairs of photons; if for any reason two of the photons belonging to different annihilations are detected in a time coincidence, they will define a randomly oriented (i.e., incorrect) LOR.