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 3 Tesla scanner using relaxometry and quantitative magnetization transfer (MT). Results Concentration‐dependent T 1 and T 2 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 appears to be the T 1 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 T 1 plays a role in determining the MT‐weighted signal, the magnetization transfer ratio is reduced in the presence of melanin‐iron complexes. Magn Reson Med 78:1790–1800, 2017.