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Background
High Resolution Computed Tomography (HRCT) is the imaging modality of choice in evaluating pathologies of the temporal bone. This guide provides a practical approach to understanding temporal bone anatomy and assessing frequently encountered but pathologies that may easily be missed of the external, middle, and inner ear with an emphasis on congenital, traumatic, inflammatory, and neoplastic processes. This guide will also highlight anatomical variants that may mimic pathology and fractures.

Educational Goals / Teaching Points
To describe subtle imaging features of various congenital, inflammatory, traumatic, and neoplastic pathologies of ear. To describe key landmarks for easy identification of ear pathologies. To emphasize anatomical variants that mimic pathology and distinguishing features.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Anatomical variants and various congenital, inflammatory, traumatic, and neoplastic processes are organized by the location of the pathology within external, middle, and inner ear. Within the external ear, we will discuss inflammatory: cholesteatoma; post-inflammatory medial canal fibrosis, and neoplastic (benign, exostoses, osteoma). Within the middle ear, we will discuss congenital (ossification of the tendons; persistent stapedial artery; jugular bulb, high-riding, dehiscent, and diverticulum; facial nerve venous malformation - formerly known as a hemangioma. Inflammatory (otospongiosis/otosclerosis, fenestral type; tympanosclerosis - myringosclerosis and intratympanic; chronic post-inflammatory ossicular erosion; ossicular chain fixation). Traumatic (ossicular chain dislocation - incudostapedial and incudomalleolar; temporal lobe cephalocele). Neoplastic (benign, glomus tympanicum; facial nerve schwannoma; middle ear adenoma). Miscellaneous (cholesteatoma, Prussak space and mural). Within the inner ear, we will discuss congenital (cochlear hypoplasia; incomplete partition -type II aka Mondini deformity; semi-circular canal dehiscence, commonly superior canal, rarely lateral and posterior canals). Inflammatory (otospongiosis/otosclerosis -retro-fenestral; labyrinthitis ossificans). Neoplastic – benign (intralabyrinthine schwannoma). Miscellaneous (labyrinthine hemorrhage, Meniere disease). We will describe relevant anatomical landmarks for easy identification of ear pathologies, e.g., fissula antefenestram (the site of otospongiosis) is found immediately anterior to the vestibular/oval window and posteromedial to the processus cochleariformis. We will also outline normal anatomical variants that mimic pathology, e.g., cochlear cleft may mimic otospongiosis. Mimics of fractures include foramen of Huschke, singular and subarcuate canals, temporal bone sutures (petrosquamous, petrotympanic, tympanosquamous and tympanomastoid), and inferior tympanic and mastoid canaliculi.

Conclusion
This guide reviews major anatomical landmarks in the temporal bone along with normal anatomy and its variants. Knowledge of pertinent positives and negatives is key to accurately describing pathologic findings and providing an interpretation for subtle findings that may easily be missed.