Regional perfusion scanning of the central nervous system requires lipophilic radionuclides that rapidly penetrate the intact blood brain barrier. The first compound of this type to be developed for human use is 123I-iodoantipyrine. 123I-iodoantipyrine rapidly crosses the blood brain barrier and is flow-limited in its delivery to the brain. This makes possible the CNS perfusion scanning of the human brain, thereby showing regional distribution of microcirculatory blood flow.