Which of the following is a determinant of random fraction in PET imaging?

The correct answer is the radioactivity within the field of view (FOV) and adjacent to the scanner, as it directly affects the random fraction in PET imaging. Random fraction refers to the proportion of coincidence events that are classified as random rather than true coincidences, and it is influenced by the amount of radioactive decay occurring in the patient and surrounding areas.

When the levels of radioactivity are high within the FOV, the likelihood of random coincidences increases because there are more positron emissions occurring that may be detected by the scanner but are not part of the same event. Additionally, radioactivity adjacent to the scanner can contribute to background noise that can also lead to random coincidental events being detected.

In contrast, while factors such as patient size, energy window selection, and detector sensitivity are important in shaping overall image quality and system performance, they do not specifically determine the random fraction as closely as the radioactivity within and around the FOV does. These other factors may influence detection efficiency or image noise but do not impact the statistical occurrence of random events in the same direct way.