[18F]ACI-15916 PET in α-synucleinopathies

The goal of this clinical trial is to test whether we can reliably and safely measure the accumulation of pathological protein α-synuclein \[involved in some diseases such as Parkinson's disease, Lewy body dementia and Multiple System Atrophy (MSA), collectively named α-synucleinopathies\] using a new positron emission tomography (PET) tracer called \[18F\]ACI-15916. Both healthy people and people with (suspected) α-synuclein pathology will participate to this trial. The main questions it aims to answer are: * whether \[18F\]ACI-15916 is safe and well tolerated when injected into participants * whether \[18F\]ACI-15916 reliably detects α-synuclein in the brain using PET technique. * whether there are differences in the amount of this protein between people with diseases related to α-synuclein accumulation in the brain and people without these diseases. Participants will: * Visit the clinic to consent to their participation and to ensure they are eligible \[physical and neurological examinations, questionnaires, blood and urine tests, ECG and in some cases a MRI and a PET scan with a licensed tracer (\[18F\]FE-PE2I) to confirm or not the disease\]. * Visit the clinic to receive the tracer \[18F\]ACI-15916 intravenously and be scanned in a PET scanner, during which blood will be collected (and optionally spinal fluid). * Receive a phone call from the clinic 1 week after the PET scan to report any symptoms and side-effects that they may be having. Some of the participants may be asked to come again to the clinic for a second PET scan with \[18F\]ACI-15916, allowing the researchers to determine if the measurements with the first PET scan are stable and reproducible. Some of the participants will participate in a specific part of the study to evaluate the distribution of the PET ligand in the whole body, with a similar visit schedule.