Radiation dose to upper airways from inhaled oxygen-15 carbon dioxide

According to Powell et al., significant retention of 15O in the tracheal mucosa results in a radiation absorbed dose of 75 to 200 rad upon breathing of 15O-CO2 for one hour at 1 mCi/liter. Such a high dose would seriously compromise the 15O-CO2 inhalation method for positron emission tomographic (PET) measurement of cerebral blood flow (CBF). In order to verify these results, we have assayed 15O activity in the tracheal region of three volunteers by PET during inhalation of 15O-CO2 and 15O-O2. Using methods similar to those of the above authors for estimating absorbed dose in the tracheal mucosa, we have obtained a value of 14-38 rad, which is more in keeping with 3-5 rad found by Bigler et al. from direct assay of mucus and saliva. We conclude that the 15O-CO2 inhalation method is a safe and practical means of measuring CBF by PET.