When using a high activity of a radionuclide for a PET scan, why is image quality degraded?

Using a high activity of a radionuclide in a PET scan can lead to degraded image quality primarily due to an increase in random events. In PET imaging, the detection of photons emitted from positron annihilation is crucial for generating accurate images. When the activity of the radionuclide is high, there is an increase in the number of positron emissions, which subsequently leads to a greater number of coincidences detected by the PET scanner.

However, not all detected events are true coincidences resulting from the annihilation of positrons; some are random coincidences occurring when two photons from different sources are detected simultaneously. This phenomenon is enhanced at high activity levels and introduces noise to the image, which reduces the overall image contrast and sharpness. As a result, the clarity of the clinical information obtained from the scan diminishes, making it difficult to accurately identify and assess areas of interest.

While deadtime, attenuation, and noise do influence image quality, they are not the primary factors at high radionuclide activities in this context. The predominant issue is the substantial increase in random events leading to a significant reduction in the quality of the PET images.