Contrast mechanisms associated with neuromelanin-MRI.

Purpose To investigate the physical mechanisms associated with the contrast observed in neuromelanin-MRI. Methods Phantoms having different concentrations of synthetic melanins with different degrees of iron loading were examined on a 3T scanner using relaxometry and quantitative magnetization transfer (MT). Results Concentration-dependent T1- and T2-shortening was most pronounced for the melanin pigment when combined with iron. Metal-free melanin had a negligible effect on the magnetization transfer spectra. On the contrary, the presence of iron-laden melanins resulted in a decreased magnetization transfer ratio. The presence of melanin or iron (or both) did not have a significant effect on the macromolecular content, represented by the pool size ratio. Conclusion The primary mechanism underlying contrast in neuromelanin-MRI seems to be the T1 reduction associated with melanin-iron complexes. The macromolecular content is not significantly influenced by the presence of melanin with or without iron, and thus the MT is not directly affected. However, as T1 plays a role in determining the MT-weighted signal, the magnetization transfer ratio is reduced in the presence of melanin-iron complexes.