By 2030 the number of patients with Parkinson's Disease (PD) would increase by 56% affecting 1 out of 120 people older than 45 years-old. It is known that 10-15 years before the onset motor symptoms such as tremor, rigidity and akinesia, patients often experience a specific sleep trouble called REM sleep behaviour disorder (RBD). Follow-up of those subjects showed there was a conversion rate to PD and related disorders (called synucleinopathies) over 80%. The pathophysiology of RBD is poorly understood. The development of cutting-edge technologies such as 7 Tesla MRI and the optimisation of image processing methods made it possible to non-invasively explore in vivo small brain structures involved in sleep and movement disorders. The investigators hypothesize that brain and brainstem microstructure, composition, sodium homeostasis and connectivity may change in 15 isolated RBD (iRBD) subjects compared with 15 healthy controls and that these changes may be correlated with clinical scores. This study would help fill the gap in early diagnosis of synucleinopathies, by contributing to better targeting patients who could be included in therapeutic trials with a neuroprotective effect. Besides, the exploration of original pathophysiological pathways such as sodium homeostatis could provide the necessary arguments for the development of new target therapeutics.