State-of-the-art imaging of the temporal bone includes three modalities: CT, MRI, and digital subtraction angiography. The natural contrast between air in the middle ear and mastoid and the surrounding bony framework makes CT the imaging modality of choice for temporal bone imaging. Imaging planes include both axial and coronal. Evaluation of the middle and inner ear requires use of sections of 1 to 1.5 mm in thickness. The axial scan should extend from the base of the skull inferiorly to the apex of the superior semicircular canal superiorly. Coronal sections should include the cochlea anteriorly to the posterior extent of the semicircular canal. In most cases, a noncontrast study is adequate. Intravenous contrast material is used to evaluate adjacent vascular structures and mass lesions.

MRI can be performed in three planes (axial, coronal, and sagittal) without changing the patient’s position in the gantry. It is used primarily for the diagnosis of tumors in the internal auditory canal, cerebellopontine angle (Fig. 48-1), and to assess the integrity of the semicircular canal in the preoperative evaluation of patients for cochlear implants. MRI can complement the CT in displaying soft tissue structures in and around the temporal bones, including brain, and cerebral spinal fluid (CSF). MR angiography (MRA) and venography (MRV) can highlight normal blood vessels and vascular lesions.

Temporal bone injuries occur with approximately 22% of all skull fractures. The three major complications of temporal bone trauma are hearing loss, facial nerve paralysis, and cerebrospinal fluid leak. A temporal bone fracture should be excluded when clinically there is evidence of any of the following: hemorrhage (hemotympanum), cerebrospinal fluid leak (otorrhea), facial nerve injury, hearing loss, or evidence of vestibular injuries (vertigo). A CT scan of the temporal bone in the axial and coronal planes is the most effective means of locating a fracture.

Tumors of the cerebellopontine angle are best assessed with MRI. The main advantages of MRI are that it provides images in
multiple planes, does not require ionizing radiation, and precisely depicts the intra- and extra-canalicular portions of the seventh and eighth cranial nerves. Evaluation of temporal bone tumors may include both MRI and CT, which can be supplemented with angiography or MRA. A combination of MRI and CT is useful in both diagnosis and follow-up evaluation of glomus tumors (glomus jugulare, glomus tympanicum).

MRA is useful to evaluate vascular compression and vessel patency and to characterize vascular masses and malformations. It is often helpful in delineating the vascular anatomy before resection of a glomus tumor. Embolization of feeding vessels by means of interventional angiography may be performed to minimize intraoperative blood loss (Figs. 48-2, 48-3).

CT allows excellent evaluation of congenital anomalies of the outer, middle, and inner ear. Evaluation of congenital sensorineural hearing loss includes CT because approximately 20% of these patients have radiographic anomalies of the inner ear.

Computed tomographic (CT) scanning is essential before cochlear implantation. Assessment of the size and anatomic features of the hypotympanum and round window niche is necessary
to perform the procedure successfully. MRI is useful in determining whether there is fibrous tissue in the semicircular canals. This diagnosis cannot be made on CT.

The initial radiographic evaluation of necrotizing (malignant) otitis externa should include CT and a radionuclide examination. CT is used to evaluate the temporal bone for the presence of bone destruction and inflammatory changes in the adjacent soft tissues. A radionuclide bone scan may show inflammatory changes not detected at CT. If an intracranial abnormality is suspected, MRI is the study of choice.