Comparing scans of the nerves that supply the heart

Inclusion Criteria: 1. Healthy Controls: * Voluntary written informed consent. * Use of highly effective methods of birth control. * Age between 18 and 85 years. * Good health based on medical history, physical examination, clinical laboratory tests, and urinalysis. * No history or evidence of major neurological, internal, or psychiatric disorders. * Normal structural MRI scan for subjects \< 60 years or minor lesions for subjects \>= 60 years. 2. Parkinson's Disease: * Age 45-85 years. * Clinically established PD based on Movements Disorder Society diagnostic criteria. * Disease duration since onset of motor symptoms: 5 years or longer for one group and less than 5 years for another. * Previous abnormal \[18F\]-FE-PE2I PET or \[123I\]-FP-CIT SPECT scan. * Ability to understand the patient information brochure and provide written informed consent. 3. Multiple System Atrophy - Parkinsonian Variant: * Age 45-85 years. * Clinically established or clinically probable MSA-P based on MDS diagnostic criteria. * Previous abnormal \[18F\]-FE-PE2I PET or \[123I\]-FP-CIT SPECT scan. * Ability to understand the patient information brochure and provide written informed consent. 4. Dementia Due to Alzheimer's Disease: * Age 50-85 years. * Diagnosis of probable AD with evidence of the AD pathophysiological process. * Ability to understand the patient information brochure and provide written informed consent. 5. Dementia with Lewy Bodies: * Age 50-85 years. * Diagnosis of probable DLB. * Previous abnormal \[18F\]-FE-PE2I PET or \[123I\]-FP-CIT SPECT scan. * Ability to understand the patient information brochure and provide written informed consent. Exclusion Criteria: 1. Healthy controls: * Major diseases that may interfere with the investigations. * Evidence of cognitive impairment. * History or evidence of psychiatric disease. * Use of illicit drugs or history of drug or alcohol abuse. * Chronic medication interfering with cardiac neuronal norepinephrine transporter (NET) or \[18F\]-FE-PE2I imaging. * Exposure to ionizing radiation \> 1 mSv in other research studies within the last 12 months. * Contraindication for MRI scanning. * Claustrophobia or inability to tolerate confinement during PET-MRI scanning. * Unwillingness to avoid strenuous physical activity. * Lack of understanding of the study procedures. * Pregnancy or breastfeeding. * Lack of agreement to communicate incidental findings to the general practitioner. * Abnormal Allen test or lidocaine hypersensitivity/allergy for subjects willing to undergo arterial sampling. 2. Parkinson's Disease: * Neuropsychiatric diseases other than PD. * Major internal medical comorbidity, especially diabetes or heart disease. * White matter lesion load on FLAIR Fazekas score 2 or higher or other relevant MRI abnormalities. * History of alcohol or drug abuse. * Previous participation in research studies involving ionizing radiation. * Contraindications for MR. * Claustrophobia or inability to tolerate confinement during PET scanning. * Unwillingness to avoid strenuous physical activity. * Lack of understanding of the study procedures. * Pregnancy or breastfeeding. * Lack of agreement to communicate incidental findings to the general practitioner. * Anticoagulant therapy. 3. Multiple System Atrophy - Parkinsonian Variant: * Same as for Parkinson's disease. 4. Dementia Due to Alzheimer's Disease: * Same as for Parkinson's disease. 5. Dementia with Lewy Bodies: * Same as for Parkinson's disease.