Reply: Neuromelanin? MRI of catecholaminergic neurons

We express our gratitude to Dr Watanabe1 for the editorial comment regarding our review article2 and the thought-provoking discussion concerning the origins of neuromelanin (NM)-MRI contrast. Considering the increasing clinical interest in NM-MRI, gaining a profound understanding of the contrast mechanisms responsible for this signal is crucial, as it directly impacts the accurate interpretation of NM-MRI findings. The author has summarized the various mechanisms associated with this contrast, underscoring the importance of the high-water content within the dopaminergic neurons of the substantia nigra (SN) and the noradrenergic neurons of the locus coeruleus (LC). The existing body of evidence supports the notion that these two structures exhibit higher proton density and a lower macromolecular-to-free-water pool size ratio compared to other anatomical structures.3-5 Notably, the SN and LC are grey matter structures surrounded by white matter, and in the presence of magnetization transfer (MT) effects, whether arising from incidental MT effects in turbo spin echo (TSE) sequences or explicit MT preparation in TSE and gradient echo sequences, the contrast between SN/LC and the surrounding tissues is significantly enhanced. This enhancement occurs because white matter is more effectively suppressed than grey matter, analogous to the MT-based background suppression observed in MR angiography.

Source: Brain - November 18, 2023 Category: Neurology Source Type: research