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.