Proton relaxation times of liposomal lipid bilayers

Liposomes are small lipid vesicles consisting of a multilamellar lipid bilayer surrounding an aqueous compartment. They have found many uses, particularly as in vivo delivery systems for drugs, enzymes, and radiotracers. The authors synthesized liposomes using deuterium oxide for the aqueous component to permit measurement of the proton relaxation times of the lipid bilayer. It is their hope that these vesicles may prove useful as a model for myelin and facilitate the development of imaging strategies for myelin in the central nervous system. Formation of multilamellar liposomes was verified by electron microscopy; the periodicity of the bilayer membrane was about 60 Angstroms (the minor periodicity of myelin is about 50 Angstroms). The relaxation time measurements for liposomes are not surprising given the organized nature of the lipid bilayer. These results imply that if liposomes are a reasonable model for myelin, direct imaging of myelin with conventional pulse sequences appears unlikely