The fluorescent dye 3,3'-diethylthiatricarbocyanine iodide is unsuitable for in vivo imaging of myelination in the mouse.

There is a growing interest to use non-invasive optical imaging methods to study central nervous system diseases. The application of a myelin-binding fluorescent dye, 3,3-diethylthiatricarbocyanine iodide (DBT) was recently described for in vivo optical imaging of demyelination in the mouse. In the present study we aimed at adapting the method to our optical imaging systems, the IVIS Lumina II to measure epifluorescence and the fluorescent molecular tomograph (FMT) for 3-dimensional quantification of the fluorophore. Epifluorescent imaging was performed 5-30 min after DBT injection which was followed by FMT imaging at 40 min. Two mice also underwent micro-CT imaging in the FMT cassette for the purpose of FMT-CT co-registration. Ex vivo imaging of the brain and other tissues of the head and neck was carried out 1 h after injection. Both the FMT-CT co-registration and the ex vivo imaging of organs proved that DBT poorly crossed the blood-brain barrier. The dye did not accumulate in the myelin sheath of the sciatic nerve. In contrast, there was an intense accumulation in the pituitary and salivary glands. The FMT-CT co-registration unequivocally demonstrated that the signal localized to the head did not originate from beyond the blood-brain barrier. No myelin binding was demonstrated by the ex vivo imaging either. In conclusion, DBT is unsuitable for in vivo imaging of myelination due to its poor BBB penetration, accumulation in other structures of the head and neck region and lack of selective binding towards myelin in vivo.