When a high activity of a radionuclide is used for a PET scan, image quality is degraded because of which reason?

In the context of PET scans, when a high activity of a radionuclide is used, the quality of the images can be compromised primarily due to an increase in random events. This phenomenon occurs because, with higher levels of radioactivity, there is a greater likelihood of detecting photons that are not directly related to the annihilation events of interest.

In PET imaging, two photons are emitted in opposite directions when a positron encounters an electron and annihilates. When the system is subjected to high activity levels, the number of coincident detection events (those coming from actual positron-electron annihilation) can become overwhelmed by an increase in random coincidences. Random events result from the detection of two photons that were not emitted simultaneously to each other, which can generate false signals and decreased image clarity.

This degradation of image quality due to random events is a critical consideration in optimizing PET imaging protocols, as it can affect quantitative analysis and diagnostic accuracy. Balancing the radionuclide activity is essential to ensure that the benefits of increased signal strength do not lead to an unacceptable level of noise and false readings in the images obtained.