Development of a tomographic myelin scan.

The principle that myelin can be imaged noninvasively using the emission tomographic distribution of a lipophilic radioactive tracer was investigated. Properties of agents suitable for noninvasive myelin scanning are discussed with specific reference to blood-brain barrier permeability, metabolism, and tracer lipophilicity. The brain distributions of inert tracers are correlated with their partitioning between octanol and saline. A test probe, iodobenzene, was labeled with iodine 125 for preliminary invasive studies in the rabbit. The equilibrium brain distribution, determined either autoradiographically or by regional dissection, corresponded closely to that of myelin. 123I-labeled iodobenzene, a gamma-emitting analog, was then administered to a monkey, and tomographic reconstruction revealed a pattern of brain uptake corresponding to white matter.