What limits spatial resolution in PET?
Spatial resolution in PET is fundamentally limited by three physical factors: positron range, non-collinearity of annihilation photons, and detector size. Even with perfect reconstruction, these factors introduce unavoidable uncertainty in localising the annihilation event.
PET spatial resolution is limited by positron range, photon non-collinearity, and detector size. these are fundamental physical constraints that cannot be fully eliminated.
Positron range causes displacement between the site of radioactive decay and the annihilation point. Non-collinearity causes slight angular deviation of the two annihilation photons. Detector crystal size determines how precisely photon interactions can be localised within the scanner.
Together, these effects define the intrinsic resolution limit of PET systems.
Understanding the physics
When a positron-emitting radionuclide decays, the emitted positron travels through tissue before annihilating. The distance travelled depends on its initial kinetic energy. For example, F-18 emits relatively low-energy positrons and has a short mean range in tissue, while Rb-82 emits higher-energy positrons with a longer range. The longer the positron range, the greater the uncertainty between the original decay location and the annihilation point.
After annihilation, the two 511 keV photons are emitted in nearly opposite directions. However, due to conservation of momentum, they are not emitted at exactly 180°. This slight deviation (approximately 0.5°) produces increasing spatial uncertainty as scanner diameter increases. This effect is independent of detector quality and represents a fundamental physical limit.
Detector resolution further contributes to localisation uncertainty. Each detector element has finite size, and the interaction location within the crystal cannot be determined with infinite precision. Smaller detector crystals improve spatial resolution but increase system complexity and cost.
Unlike SPECT, PET does not suffer from collimator-induced blurring, but these intrinsic physical effects impose limits that cannot be eliminated entirely.
Where this matters clinically
Resolution limits influence the detection of small lesions, particularly sub-centimetre nodules. Radionuclide choice affects resolution, which is why F-18 provides better spatial resolution than many other PET isotopes. Understanding these limits prevents over-interpretation of very small foci of uptake.