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Otosclerosis is a primitive osteodystrophia of the labyrinthine bone. Its diagnosis must be confirmed by a CT scan in order to eliminate the other causes that may lead to conductive hearing loss with an absence of stapedial reflex: fixation of the head of the malleus to the lateral wall of the tympanic cavity, absence of the long process of the incus or stapes, gusher syndrome, primary cholesteatoma, or tympanic facial nerve neuroma blocking the stapes. Particular problems in otosclerosis must be clarified: an extension to the round window (poor postoperative results), and extension to the tympanic cavity blocking the malleus and/or the incus, the labyrinthine lumen, or the internal auditory meatus (very rare). The position of the tympanic facial nerve canal and associated abnormalities must be assessed: stapedial artery, malformations of the ossicles and/or the labyrinth, and chronic otitis media. MRI is indicated in extension to the labyrinthine lumen, the internal auditory meatus, and in postoperative complications with labyrinthitis. MRI can also evaluate the active otosclerotic focus (gadolinium enhancement).