OVER the past four years a steadily increasing interest has been evident in investigating the cerebrospinal fluid (CSF) cavities by means of radioisotope scanning techniques (scintiphotography). As a possible indication of this, contributions1-7from seven different centers were presented on this subject alone at the Eighth Symposium Neuroradiologicum held in September 1967 in Paris. The reports dealt with the clinical applications of these techniques in a number of conditions including intraspinal pathology, hydrocephalus, CSF, rhinorrhea and otorrhea, subarachnoidal cysts, and porencephaly. Choice of Tracer Numerous radioactive tracers have been used for studying the spinal and endocranial cerebrospinal fluid pathways by external detection (Table). Among these agents99mTc-pertechnetate stands out for its almost ideal physical characteristics: 6-hour half-life, lack of β-emission, and single 140 Kev γ-photon. Therefore, sodium pertechnetate (Na-99mTcO4) was tried as agent for isotope cisternography by one of us.12,19It became apparent,