Baseline cerebral blood flow through the middle cerebral artery (MCA) and cerebral vasodilator response of the MCA to inhaled carbon dioxide (CO2) will be measured in young women, young men, older women, and older men using Magnetic Resonance (MR) imaging and transcranial Doppler ultrasound (TCD). Data collection techniques will be compared.

The investigators have recently shown that cerebral blood flow responses to chemical stimuli are reduced in healthy older adults. The investigators also have preliminary data suggesting that sex differences in cerebrovascular regulation exist. The overall goal of this application is to extend on the investigators previous studies by employing advanced neuroimaging techniques to enhance the investigators understanding of neurovascular coupling in healthy aging. This study will collect clinically significant data with implications for the future risk of cognitive decline and Alzheimer's disease.

The research aims are:

1. To determine the effect of age and sex on baseline cerebral blood flow through the middle cerebral artery (MCA).
2. To determine the effect of age and sex on cerebral vasodilator responses.
3. To determine if the cerebrovascular responses measured by transcranial Doppler ultrasound (TCD) are associated with the cerebrovascular responses measured by Magnetic Resonance (MR) imaging.

Inclusion Criteria:

• Between 20-34 or 50-69 years
• Body mass index <34 kg/m2

Exclusion Criteria:

• Current smoker
• History or evidence of: hepatic disease, renal disease, hematological disease, peripheral vascular disease, stroke/neurovascular disease, diabetes, hypertension
• Part of a vulnerable population (e.g. pregnant women, prisoner, individuals lacking capacity to consent, etc.)
• Older females only: subject is not post-menopausal

Subjects for this study will include healthy young adults (20-34 years old) and healthy older adults (50-69 years old). Both men and women will be eligible to participate in this study.