What non-collinearity spacing degrades spatial resolution of the PET scanner?

In positron emission tomography (PET), non-collinearity refers to the phenomenon where the annihilation photons emitted from a positron-electron pair do not travel in exactly opposite directions due to factors like the finite timing resolution of the detectors and the physics of photon emission. This non-collinearity can introduce errors in the localization of the event, which directly affects the spatial resolution of the PET scanner.

The spacing of 1.8-2 mm is significant because it represents a range where the degradation in spatial resolution becomes pronounced. As this spacing increases, the non-collinearity effects can lead to an approximate increase in the effective parallax error, which can produce blurred images and reduce the overall resolution of the reconstructed images. At this spacing, the scatter and random coincidences also tend to increase, further complicating the resolution of the image.

While other spacings may affect resolution as well, the specific range of 1.8-2 mm is critical because it is at this spacing that the sharpness and contrast of the PET images start to be noticeably compromised, impacting the diagnostic quality. This makes the understanding of non-collinearity and its relationship to spatial resolution essential for optimizing PET imaging performance.