Patients must meet ALL criteria listed below for entry: - Biochemically diagnosed ACTH-dependent Cushing disease. - No previous pituitary surgery. - Planned to undergo pituitary surgery for localization/resection of adenoma. - No contraindications for PET/CT. - Recent (within 3 months) pituitary MR, or pending pituitary MR to be performed as close as possible to FET PET/CT. - Age ≥ 18 years. - Ability to provide informed consent.

• Pregnant as determined by a pregnancy test as per institutional guidelines for individuals of child-bearing potential - Declining to use effective contraceptive methods during the study (for individuals of child-bearing potential) - Need for emergent surgery that would be delayed by participation - Bacterial, viral, or fungal infections requiring systemic therapy - Serious co-morbidities and serious nonmalignant disease (e.g., hydronephrosis, kidney failure, liver failure, systemic or local inflammatory or autoimmune diseases or other conditions) that in the opinion of the investigator could compromise patient safety and/or protocol objectives - Known diagnosis of autoimmune disorders - Patients receiving any other investigational agent within the past 28 days - Breastfeeding. Note: nursing parents are allowed if the potential participant commits to pumping breast milk and discarding it from injection to ≥ 24 hours from the time of the FET injection. - Known hypersensitivity to any excipients used in FET