When the appendix hides: the radiologist's guide to appendix variants and mimics

The identification of the appendix often presents a diagnostic challenge due to its significant anatomical variability, compounded by the frequency with which appendiceal pathologies manifest as atypical clinical presentations that mimic diverse abdominal and pelvic conditions. Beyond acute appendicitis, a wide spectrum of inflammatory, infiltrative, and neoplastic processes can involve the appendix, necessitating the rigorous recognition of their distinct imaging signatures. This pictorial essay reviews the normal anatomy of the appendix, its positional variations, and the spectrum of diseases that may affect it, including acute appendicitis, endometriosis, epiploic appendagitis, appendiceal diverticulitis, mucocele, and primary tumors. While computed tomography (CT) remains the gold standard for primary diagnostic evaluation, magnetic resonance imaging (MRI) serves as a critical complementary modality in specific clinical scenarios, providing superior soft-tissue characterization and improving diagnostic confidence in challenging cases. Recognition of anatomical variants and the primary mimics of the appendix is essential for the accurate interpretation of abdominal imaging studies.