Akeed Habeeb, summarizing discussion from Jim Hoffman's course, Cognition (PSYC 340)

Oxygen is made radioactive (through the use of a cyclotron) and incorporated into a sugar solution which is injected into the patient's blood via the arm. This radioactively-oxygenated-sugar-solution then can make its way throught the blood/brain barrier. Brain areas that are active uptake the radioactive solution quickly since they need more blood and nutrients. Thus the more active areas of the brain uptake more radioactivity. Numerous extremely sensitive 'Geiger-counters' are mounted in a ring around the patient's head and a computer converts their readings into a colorized map of the brain. The radiation, according to the government, is not in high enough quantities or around for long enough to do considerable damage and is approximately equivalent to a trip across the United States in an airplane (a person receives more radiation in an airplane (as compared to the ground) due to the fact that they are at a higher elevation and the shielding of the atmosphere to space radiation is reduced)