In this radiology lecture, we discuss the ultrasound and CT appearance of retroperitoneal fibrosis. Key points include: * Most cases (70%) are idiopathic = Ormond disease. * Nonspecific symptoms depending on involved structures: Malaise, weight loss, low-grade fever. * Ureteral entrapment: Obstructive uropathy or renal failure, may see medial deviation of middle third of ureters with hydronephrosis. * Venous entrapment: Lower extremity edema, deep venous thrombosis. * CT: Soft tissue mass anterolateral to aorta with posterior sparing. * DDx: Retroperitoneal Lymphoma will Lift the aorta. * MRI: Low T1/T2 signal when inactive, T2 bright with early enhancement when active inflammation. * PET/CT: Avid when metabolically active, may aid in identifying appropriate biopsy sites. Click the YouTube Community tab or follow on social media for bonus teaching material posted throughout the week! Instagram: https://www.instagram.com/radiologistHQ/ Facebook: https://www.facebook.com/radiologistHeadQuarters/ Twitter: https://twitter.com/radiologistHQ