In PET, what leads to axial blurring of images?

In the context of Positron Emission Tomography (PET), axial blurring is primarily caused by detector parallax. This phenomenon occurs because the geometry of emission from a point source in the body can create multiple possible paths for the detected photons to reach the detectors. When these photons are detected at angles rather than directly on-axis, it can lead to inaccuracies in the location where the event is registered, thus resulting in a blurring effect in the axial direction of the images obtained.

This blurring is exacerbated by the arrangement of the detectors in the PET scanner and their angular response. If the detectors are not perfectly aligned or if the emitted gamma rays do not travel in a straight line due to their emission angles, the accurate position of the radiopharmaceutical distribution becomes less certain. The net effect is a spreading of the detected signals, which manifests as blurring in the resultant images.

Understanding the nature of detector parallax is essential for improving PET image quality. Efforts to minimize this effect can involve optimizing detector design, using advanced algorithms for image reconstruction, and ensuring proper calibration of the system to account for any geometric inaccuracies.