Quantification of Neuromelanin as a Neuroimaging Biomarker for Parkinson's Disease.

Loss of signals from substantia nigra (SN) and locus coeruleus (LC) on neuromelanin (NM)-sensitive sequences of MRI is reported as a potential biomarker in patients with Parkinson's disease (PD) and related diseases. This scoping review aims to consolidate current knowledge on MRI techniques to visualize and quantify these signals and their clinical applications in PD. Publicly available databases were searched for original studies using MRI to quantify NM in PD and other related disorders. Different studies were compared based on MRI sequence, quantification techniques and correlations with clinical scores. Furthermore, studies on genetic forms of PD and prodromal PD were also evaluated and compared. The most common MRI sequences used were T1-weighted sequences and gradient echo sequences. Different studies used different quantitative measures such as signal-to-noise ratio, contrast-to-noise ratio and contrast ratio. Morphometric evaluations such as volume and area of the SN and LC signals were also used. Most studies showed evidence of significant difference in the signals in different stages of PD compared to controls both at the SN and LC. There were significant correlations between the SN and LC signals and clinical scores. Hence, quantification of these signals may be reliable in diagnosis and disease monitoring in PD. The relative ease of signal quantification and widespread availability of MRI may make it a quantitative surrogate biomarker.