The underlying goal of this study is to assess 123-I IBVM SPECT imaging as a tool to assess cholinergic transporter binding in the brain of AD and PD research participants and age- and gender-matched healthy subjects.

Approximately 25 patients with mild to moderate Alzheimer's disease (AD) and 25 patients Parkinson's disease and 20 healthy controls will be recruited to participate in this study.

AD patients will be eligible to participate if they have a diagnosis of AD based on NINCDS-ADRDA criteria.

PD patients will be eligible to participate if they have a clinical diagnosis of Parkinson's disease.

Healthy controls will be evaluated to ensure that there is no evidence of neurodegenerative changes including cognitive decline.

All subjects will undergo written informed consent and a screening evaluation including baseline clinical laboratory testing, a physical and neurological evaluation and cognitive assessment.

Subjects will be asked to undergo an injection of 123-I IBVM followed by up to three approximately thirty minute SPECT scans during the next 24 hours.

Subjects may be asked to undergo a second imaging visit two-weeks to six months following the initial imaging visit to assess the reproducibility of the imaging outcome and/or the progressive change from baseline in 123-I IBVM binding.

The imaging analyses will be performed by an image-processing specialist who will remain masked to clinical diagnosis. The primary imaging outcome measure will be the brain regional distribution volumes expressed as a target region to cerebellar ratio.

Alzheimer's Inclusion Criteria:

• The participant is 50 years or older.
• Written informed consent is obtained.
• Participants have a clinical diagnosis of Alzheimer's disease
• Mini-Mental Status Exam score >10 and < 25.
• Modified Hachinski Ischemia Scale score of ≤ 4.
• Geriatric Depression Scales (GDS) ≤ 10.
• For females, non-child bearing potential or negative urine pregnancy test on day of 123-I IBVM injection.

Alzheimer's Exclusion Criteria:

• The subject has signs or symptoms of another neurodegenerative disease
• Subjects with an iodine allergy.
• The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable medical or psychiatric illness
• The subject has any disorder that may interfere with drug absorption distribution, metabolism, or excretion (including gastrointestinal surgery).
• The subject has received an investigational drug within 60 days before the screening visit.
• Pregnancy

Parkinson's Inclusion Criteria:

• The participant is 30 years or older.
• Written informed consent is obtained.
• Participants have a clinical diagnosis of Parkinson's disease
• Geriatric Depression Scales (GDS) ≤ 10.
• For females, non-child bearing potential or negative urine pregnancy test on day of 123-I IBVM injection.

Parkinson's Exclusion Criteria:

• Subjects with an iodine allergy.
• The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable medical or psychiatric illness
• The subject has any disorder that may interfere with drug absorption distribution, metabolism, or excretion (including gastrointestinal surgery).
• Pregnancy