The purpose of this study is to use transcanial doppler (TCD) ultrasound to look for possible risk factors for ischemic brain injury. We will perform contrast TCD with agitated saline contrast to identify and contrast right-to-left shunt (RLS) and breath holding to assess cerebrovascular reactivity (CVR) in the elderly.

Inclusion Criteria:

1. Males and females ≥ 60 years of age.
2. Fluent in English.
3. Part of the NACC longitudinal cohort (Protocol #: 068200) of the PMC/ADCC with consensus conference designation of cognitively normal, Mild Cognitive Impairment, or probable/possible Alzheimer's disease.
4. Has consented to participate in the ADCC high-resolution MRI sub-study.
5. NACC longitudinal visit must be within 6 months of the TCD study and brain MRI study.
6. Women: post-menopausal or surgically sterile.
7. Willing and able to complete all required study procedures.
8. Completed 6 grades of education.
9. Geriatric Depression scale less than 6 (assessed within 6 months at the NACC longitudinal visit).

Exclusion Criteria:

1. Prior clinically symptomatic ischemic or hemorrhagic stroke.
2. Inability to tolerate or contraindication to TCD study in the opinion of an investigator or treating physician.
3. Any medical or psychological conditions that, in the opinion of the investigator, would compromise the subject's safety or successful participation in the study.

Only subjects that are enrolled in the NACC longitudinal cohort (Protocol 068200) will be eligible for this study. Subjects must also agree to participate in the ADCC high-resolution MRI sub-study to be eligible.