This study compares two methods for assessing brain blood flow. PET scan will be use with a drug called Acetazolamide and compare its effectiveness to the standard scintigraphy method. The goal is to see if the PET scan is just as good as the traditional method in measuring how well the brain's blood vessels respond to the drug.

Inclusion Criteria:

• Chronic stenosing cerebral vascular pathology (stenosis > 70% on Doppler examination or angiography).
• Patient aged over 18 years.
• Patient who has voluntarily agreed to participate in the study and has signed the written informed consent.
• Patient affiliated with a social security system.

Exclusion Criteria:

• Patient with stenosis less than 70% on Doppler examination or angiography.
• Patient with a contraindication to 18F-FDG PET examinations: severe claustrophobia, poorly controlled diabetes during 18F-FDG PET examinations (fasting capillary blood glucose ≥ 11 mmol/L).
• Patient with a contraindication to scintigraphy.
• Patient with an allergy to sulfonamides (Acetazolamide DIAMOX®).
• Patient enrolled in another clinical study with a specified exclusion period.
• Minor patient.
• Patient unable to give informed consent.
• Vulnerable individuals