Positron emission tomography (PET) is a brain imaging technique used to study the metabolic activities of different brain cells associated with the processing of brain functions.
PET uses the radioactive oxygen isotope (15O) to measure the changes in uptake of oxygen and glucose by the brain cells. These oxygen isotopes are very unstable and are quickly eliminated after their breakdown. PET can also detect the neurochemical changes occurring in the brain.
Thus, the blanks can be filled with radioactive labelled molecules; neurochemical.