Fluorine-18-N-methylspiroperidol: radiolytic decomposition as a consequence of high specific activity and high dose levels.

High specific activity [18F]N-methylspiroperidol(8-[4-(4-[18F]fluorophenyl)-4-oxobutyl]-3-me thyl l-1-phenyl-1,3,8-triazaspiro[4.5]decan-4-one, 5-10 mCi/ml, 4-8 Ci/mumol at EOB) in saline solution undergoes significant radiolytic decomposition resulting in a decrease in radiochemical purity of 10-25% during the first hour. The rate of decomposition is affected by the specific activity, total dose to and chemical composition of the solution. That radiolysis is responsible for the observed decomposition was verified by the observation that unlabeled N-methylspiroperidol is decomposed in the presence of [18F]fluoride.