What determines spatial resolution in PET?

Spatial resolution in PET is determined by a combination of detector resolution, positron range, and non-collinearity of annihilation photons. Unlike SPECT, PET resolution is not limited by mechanical collimation but by fundamental physical processes and detector design.

Spatial resolution in PET is limited by positron range, photon non-collinearity, and detector size, rather than by mechanical collimation.

Detector size and reconstruction parameters influence resolution, but even with ideal detectors, intrinsic physical limits such as positron travel distance and slight deviation from perfect 180° photon emission restrict achievable resolution.

Modern clinical PET systems typically achieve spatial resolution of approximately 4–6 mm.

Understanding the physics

When a positron-emitting radionuclide decays, the emitted positron travels a short distance in tissue before annihilating with an electron. This positron range introduces spatial uncertainty because the annihilation point is displaced from the original decay site. Radionuclides that emit higher-energy positrons (such as Rb-82) have longer positron ranges and therefore poorer intrinsic resolution compared with F-18.

After annihilation, two 511 keV photons are emitted in nearly opposite directions. However, they are not perfectly collinear. Due to residual momentum of the positron–electron pair, the photons deviate slightly from 180° by about 0.5°. This non-collinearity introduces positional uncertainty that increases with scanner diameter.

Detector characteristics also influence resolution. Smaller detector crystals improve localisation of interaction position. Advanced systems use smaller crystals and improved electronics to enhance intrinsic detector resolution.

Reconstruction methods, including iterative algorithms and point spread function modelling, further affect the apparent resolution in the final image.

Unlike SPECT, PET does not suffer from the resolution–sensitivity trade-off imposed by collimators. Instead, its limitations arise from fundamental physical constraints and detector design.

Where this matters clinically

PET spatial resolution influences small lesion detectability. Radionuclide choice affects resolution due to differences in positron range. Understanding resolution limits is essential when interpreting sub-centimetre lesions in oncological imaging.