CT and MRI for Pre-Operative Cochlear Imaging

Cochlear implant is the treatment of choice for deep sensorineural hearing loss, notably in those patients for whom conventional amplification devices do not provide significant clinical improvement. Imaging plays an important role in the workup of cochlear implant candidates not only to identify inner ear congenital and acquired abnormalities or cochlear nerve anomalies but also to detect temporal bone abnormalities that may be encountered during surgery and may alter surgical approach.

Some variations are potential surgical hazards that may lead to problems during the surgery and may alert the surgeon regarding potential surgical dangers and complications.

The radiologist and surgeon must be familiar with these imaging findings. Both computed tomography and magnetic resonance imaging should be used as they delineate, in different manners, cochlear and middle ear anatomy as well as other anatomical variants.

Mastoid pneumatization is important for planning the surgery. It is classified into pneumatic, diploic, sclerotic, and mixed. Effusion of the middle ear cleft should also be reported.

Korner's septum divides the mastoid process into a superficial squamous portion and a deep petrous portion. It may mislead the surgeon to the mastoid antrum during surgery. Mastoid emissary veins participate in extracranial venous drainage of the posterior fossa dural sinuses. Most of them disappear, however, some persist and enlarge.

Low lying dura represents difficulty to access the aditus, lateral semicircular canal, and posterior tympanotomy. It is often associated with sclerotic mastoid. Posterior tympanotomy is a well known otologic procedure that allows surgeons to access the middle ear cavity. The surgeon opens a window in the posterior wall of the middle ear in the angle between the chorda tympani and the mastoid part of the facial nerve. Laterally or anteriorly positioned mastoid part of the facial nerve may hinder the access to the facial recess or may even force the surgeon to change his approach.

The sigmoid sinus passes along the posteromedial border of the mastoid air cells. An anteriorly located sinus produces a deep bulge in the mastoid and may reach the posterior wall of the external auditory canal being separated from it only by a thin bony plate.

Jugular bulb variations are common, the roof of a normal jugular bulb lies either at or slightly below the level of the external auditory canal floor and is separated from the middle ear cavity by the thin bony sigmoid plate. The average width of the jugular bulb is 1 cm. A jugular bulb larger than 1 cm is called a giant or mega jugular bulb. A jugular bulb that extends over the basal turn of the cochlea or abuts the round window is called a high riding jugular bulb. Dehiscence of the sigmoid plate with upward protrusion of the bulb into the posterior hypotympanum is called a dehiscent jugular bulb, which may obliterate a round window niche.

The aberrant internal carotid artery is an enlarged inferior tympanic artery that occurs as a result of agenesis or underdevelopment of the cervical segment of the internal carotid artery. It runs along the medial aspect of the middle ear coursing anteriorly across the cochlear promontory to join the horizontal carotid canal through a dehiscence in the carotid plate.

Cochlear duct patency and axis, patency of the round window niche and the patent even caliber of the cochlea must be adequately evaluated by both computed tomography and magnetic resonance imaging. Otospongiotic foci compromise the insertion of the cochlear implant electrode if they occlude the round window niche or cochlear duct. Labyrinthitis ossificans may partially or completely obliterate cochlear lumen. Fibrosis may precede ossification and areas of fibrosis and ossification may coexist. Cochlear ossification with luminal obstruction is not a contraindication for implantation, however, it is important to be identified preoperatively.

Vestibular aqueduct (VA) is considered dilated if its width is greater than the width of the posterior SCC or if its midpoint width is greater than 1.5 mm. Computed tomography shows dilatation of the VA only whereas magnetic resonance imaging shows the dilatation of the VA and of the endolymphatic sac.