The advent of high-resolution computed tomography (CT) scanning in the 1980s has revolutionized diagnostic imaging of the temporal bone. CT scanning offers the greatest structural definition of any currently available imaging modality. [1, 2] The purpose of this article is to familiarize the reader with the normal anatomy of the temporal bone depicted by CT scanning. The article reviews the anatomy of the middle ear space and surrounding bone and presents radiographic imaging in both axial and coronal views, with labeled salient features and relevant text.
An axial view through the superior portion of the temporal bone can be seen below.
A study by Visvanathan and Morrissey used high-resolution CT scanning to determine that temporal bone variations are not uncommon. Evaluating 339 temporal bones, the investigators found that the incidences of deep sinus tympani, anteriorly located sigmoid sinus, high dehiscent jugular bulb, enlarged internal auditory meatus, and enlarged cochlear aqueduct were 5.01%, 2.94%, 2.76%, 1.76%, and 0.58%, respectively. 
Normal Anatomy of the Middle Ear
The temporal bone houses and is surrounded by many vital structures. The temporal bone is actually composed of four bones, consisting of the squamous, petrous, tympanic, and mastoid segments.
The bony framework of the temporal bone contains multiple air spaces. The most complex of these spaces is the middle ear cavity, or tympanum. The middle ear space is shaped somewhat in the form of a red blood cell stood on end. This space is narrow in a medial-lateral direction and more elongated anteroposteriorly and superoinferiorly.
Laterally, the tympanic membrane, annulus, and the handle of the malleus make up the wall of the tympanum. The inner ear forms the medial wall of the middle ear. The largest anatomic structure of the medial wall is the promontory of the cochlea.
The roof of the middle ear space is formed by the tegmen tympani. This structure separates the middle ear space from the middle cranial fossa. The tegmen slopes inferiorly as it courses laterally along the temporal bone; remember this point during mastoidectomy to avoid violating the tegmen with the drill. In addition, the tegmen is located higher than the superior border of the tympanic membrane, forming a space, the epitympanum. The epitympanum houses the head of the malleus and the body and short process of the incus.
The floor of the middle ear is primarily composed of the bone covering the jugular bulb. The bone over the bulb may be dehiscent, rendering it more susceptible to injury. The floor of the middle ear cavity is located further inferiorly than the lowest extent of the tympanic membrane, creating a space, the hypotympanum.
Anteriorly and inferiorly, the carotid artery limits the tympanum. More superiorly, the eustachian tube, tensor tympani, and cochleariform process can be observed along the anterior margin of the middle ear.
The mastoid air cells lie immediately posterior to the middle ear space. The entrance to the mastoid air cells is the aditus ad antrum. The pyramidal eminence (giving rise to the stapedius tendon) and the incudal fossa can also be observed immediately posterior to the middle ear space.
The middle ear space contains several spaces of clinical significance. The sinus tympani is located between the labyrinthine wall and the pyramidal eminence. This area is a common site for recurrence of cholesteatoma. The facial recess is found between the tympanic annulus and pyramidal eminence. This recess provides improved access to the middle ear space during a tympanomastoidectomy.
Normal Anatomy of the Inner Ear
The inner ear is housed in the bony labyrinth, which is well demonstrated on CT scans. The cochlea lies anteriorly. The cochlea is a conical structure, with its apex pointed anteriorly, inferiorly, and laterally; its base rests near the internal auditory canal and extends outward for 2.5-2.75 turns. Immediately anterior to the cochlea is the carotid artery. The round window is located in the scala tympani of the basal turn of the cochlea. The round window niche houses the round window. This window is the termination of the scala tympani of the cochlea. The niche protects the round window from direct exposure to sound waves in the event of a tympanic membrane perforation.
The vestibule lies posterior to the cochlea, abutting the internal auditory canal medially. The stapes footplate transmits vibrations to the vestibule at the oval window. The 3 semicircular canals emanate from the vestibule. The lateral canal lies 30° from horizontal. The 3 canals lie at right angles to each other.
The endolymphatic sac is posteromedial to the semicircular canals on the posterior margin of the petrous bone. The seventh and eighth cranial nerves (CN VII and CN VIII) course through the internal auditory canal. CN VIII enters the structures of the inner ear to innervate them and CN VII passes laterally and anteriorly to the geniculate ganglion, then posteriorly along the medial wall of the tympanum before heading inferiorly to the stylomastoid foramen.
A retrospective CT-scan study by Saxby et al found the rate of semicircular canal dehiscence in pediatric patients to be significantly lower than rates found in previous studies. The study involved 334 children (649 temporal bones), with temporal bone imaging revealing superior canal dehiscence in 3.3% of patients (1.7% of temporal bones) and posterior canal dehiscence in 2.1% of patients (1.2% of temporal bones). 
Axial CT Images of the Temporal Bone
The images of the temporal bone are examined in 2 planes, axial and coronal, because both views are crucial to an adequate study. The images below are axial sections through a left temporal bone. The images are ordered from the most superior (the first image) to the most inferior (the ninth image).
Coronal CT Imaging of the Temporal Bone
The images below are coronal sections of the same left temporal bone studied in the axial sections. They are ordered from anteriorly (the first image) to posteriorly (the fifth image).
The first image below provides a view of the facial nerve (a) coursing just inferior to the lateral semicircular canal (b). Another key point in otologic surgery for chronic ear disease is to examine the bone of the lateral canal. Cholesteatoma may erode this bone, increasing the patients' risk of a postoperative fistula. In addition, if the bone of the semicircular canal is eroded, be careful in exposing the facial nerve because the bone over the nerve may be dehiscent as well.
In a study comparing the preoperative findings on high-resolution CT scans (axial and coronal) of the temporal bone with intraoperative findings, Rogha et al concluded that such scans can accurately demonstrate the extent of damage from cholesteatoma. In the report, on 36 patients with cholesteatoma, the investigators found excellent correlation between scan and intraoperative findings for sigmoid plate and scutum erosion and for widening of the aditus, and good correlation for malleus and tegmen erosion. However, scan results correlated poorly with intraoperative findings for erosion of the incus and stapes and for facial nerve dehiscence. 
Recently, angiographic techniques have been combined with CT imaging to create CT angiography. The resulting images show vascular structures in the context of soft tissue and bony structures. In the temporal bone this is particularly useful for imaging the carotid artery, sigmoid sinus, and internal jugular vein as they course through and along the otic capsule. These images are particularly useful in the diagnosis of venous sinus thrombosis.
Future and Controversies
Flat-panel detectors in CT scanning have been developed and used in order to obtain higher-resolution images. A 2009 study by Majdani et al compared the imaging of a flat-panel volume CT (fpVCT) with that of a flat-panel digital volume tomography scanner (fpDVT) in temporal bone scanning. The authors found an improvement in image quality, as well as other advantages, among flat-panel detector-equipped scanners; however, when scanning whole cadaveric heads, the differences between these scanners and multisection CT scanners were not significant. 
A study by Locketz et al indicated that the fusion of temporal bone CT scanning with PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) diffusion-weighted magnetic resonance imaging (MRI) is superior to PROPELLER diffusion-weighted MRI alone for the preoperative identification and localization of cholesteatomas. The diagnostic sensitivity, positive predictive value, and negative predictive value of the CT/MRI-scan fusion were 0.88, 0.88, and 0.75, respectively, compared with 0.75, 0.86, and 0.60, respectively, for PROPELLER diffusion-weighted MRI by itself. 
Similarly, a study by Campos et al found that fusion imaging using high-resolution temporal bone CT scanning without intravenous contrast and PROPELLER diffusion-weighted MRI provided a successful preoperative evaluation of cholesteatoma. In 31 of 33 patients with the disorder, the fused-image results coincided with intraoperative findings, with one false positive and one false negative occurring.