Neuroinflammation in Asymptomatic Carotid Artery Disease - Imaging Substudy

Inclusion Criteria: Cohort 1 - Critical Asymptomatic Carotid Stenosis Group Inclusion Criteria: 1. Critical Stenosis Group: Diagnosis of Asymptomatic Carotid Artery Disease (aCAD) with \>70% stenosis or peak systolic velocity on DUS ≥ 230 cm/s plus CTA or MRA confirmation. 2. Participation in University of Alabama at Birmingham (UAB) IRB protocol "Neuroinflammation in Asymptomatic Carotid Artery Disease" (IRB-300007806, PI Lazar). 3. Male or female age \>18 years 4. English speaking with at least 8th grade education 5. High affinity binder for TSPO ligands based on genotyping for single nucleotide polymorphism (SNP) rs6971 6. Planned revascularization for aCAD in \>14 days from time of consent Cohort 2 - Non-Critical Asymptomatic Carotid Stenosis Group Inclusion Criteria: 1. Participation in UAB IRB protocol "Neuroinflammation in Asymptomatic Carotid Artery Disease" (IRB-300007806, PI Lazar). 2. \<40% carotid stenosis 3. No planned revascularization 4. Male or female age 18 or older 5. English speaking with at least 8th grade education 6. High affinity binder for TSPO ligands based on genotyping SNP rs6971 Exclusion Criteria: 1. Contraindication to MRI 2. Pregnancy or lactation 3. Previous revascularization for treatment of aCAD 4. Prior stroke 5. Severe anemia (blood hemoglobin ≤ 8 mg/dL) 6. Previously obtained MRI scan with evidence of clinically significant abnormality 7. History of traumatic head injury defined by loss of consciousness \>30 minutes or seizure at the time of the injury 8. Major depression 9. Known history of dementia 10. Serious medical comorbidity that, based on the judgement of the principal investigator, may interfere with study participation 11. Low or mixed-affinity binding for TSPO ligands based on genotyping for SNP rs6971.