THE TEMPORAL BONE

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HEAD AND NECK IMAGING

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THE TEMPORAL BONE Contemporary Diagnostic Dilemmas Joel D. Swartz, MD, H. Ric Harnsberger, MD, and Suresh K. Mukherji, MD

The entire topic of temporal bone imaging cannot be addressed in a single Radiologic Clinics of North America article. In consultation with the editor, We have decided to limit our discussion to topics about which there have been particularly important advances in recent years: inflammatory disease, sensorineural hearing deficit, pulsative tinnitus (PT), facial nerve dysfunction, and the postoperative temporal bone. The common thread linking those sections is an attempt to emphasize the following potential pitfalls: Inflammatory Disease Acute otomastoiditis Failure to recognize coalescent disease (erosion of mastoid septae) Failure to diagnose sigmoid sinus thrombosis Chronic otomastoiditis Failure to diagnose labyrinthine fistula Overdiagnosis of cholesteatoma (absent bony erosion) Failure to diagnose tegmen-sinus plate defects Sensorineural Hearing Deficit Failure to utilize precontrast T1-weighted images Intralabyrinthine hemorrhage Lipoma Intratumoral hemorrhage Failure to identify pneumolabyrinth (perilymphatic fistula) Failure to diagnose vestibular aqueduct syn-

drome (most common cause of congenital sensorineural hearing defect for which there is an imaging correlate) Overdiagnosis of Mondini's disease Failure to identify intra-axial causes (especially lesions involving the cochlear nuclear complex) of hearing loss Failure to identify intralabyrinthine disorders Schwannoma Labyrinthitis Hemorrhage Pulsatile Tinnitus Failure to understand that asymmetry of the jugular foramina is a normal variation Failure to understand that interruption of cortication of the foramen is abnormal regardless of foramen size Failure to diagnose vascular variants (especially aberrant internal carotid artery) Misdiagnosis of endolymphatic sac tumor for glomus jugulare Facial Nerve Dysfunction Failure to study entire course of facial nerve Failure to diagnose protruding tympanic segment of nerve into oval window Failure to recognize the normal enhancement pattern of the intratemporal facial nerve Postoperative Temporal Bone Failure to diagnose meningoencephalocele (lack of appreciation of tegmen defects) Failure to appreciate abnormal communication between IAC fundus and inner ear

From the Department of Imaging Services, The Germantown Hospital and Medical Center (JDS), Philadelphia, Pennsylvania; the Section of Neuroradiology, Department of Radiology, The University of Utah Medical Center (HRH), Salt Lake City, Utah; the Department of Radiology, University of North Carolina Medical Center; and the University of North Carolina School of Dentistry (SJM), Chapel Hill, North Carolina

RADIOLOGIC CLINICS OF NORTH AMERICA VOLUME 36 * NUMBER 5 * SEMEMBER 1998

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(leading to perilymphatic gusher upon stapes manipulation) Misdiagnosing incus interposition as traumatic dislocation Failure to identify an uncovered portion of the facial nerve canal prior to reoperation

ACUTE OTOMASTOl DITIS, CHRONIC OTOMASTOIDITIS, AND COMPLICATIONS The imaging evaluation of patients with middle ear and mastoid inflammatory disease is facilitated by considering acute otomastoiditis and chronic otomastoiditis as two different disease pr0cesses.5~, 55, 88 Acute otomastoiditis is the result of bacterial infection; complications include coalescent mastoiditis, subperiosteal abscess, dural sinus thrombosis, meningitis, brain abscess, labyrinthitis, and petrous apicitis. Chronic otomastoiditis is the result of eustachian tube dysfunction; complications include tympanic membrane retraction, acquired cholesteatoma, granulation tissue, cholesterol granuloma, postinflammatory ossicular fixation, and noncholesteatomatous ossicular erosions. The vast majority of patients with acute otomastoiditis are cured after a course of antibiotics and no imaging is required.55Imaging is performed when there is clinical suspicion of coalescent mastoiditis. This is diagnosed with CT by identification of thinning or erosion of the mastoid septae (Fig. 1). An intramastoid empyema is present in this context. Once coalescent disease is diagnosed,

Figure 1. Coalescent mastoiditis. Magnified axial CT image of the right ear. Diffuse debris throughout the entire mastoid as well as irregularity of numerous mastoid separation is present. Note clearly definable defects in the internal and external mastoid cortices (arrows).

the imaging specialist must examine the external mastoid cortex for defects that could result in a subperiosteal abscess. If this defect occurs at the mastoid tip, the phlegmonous debris may extend inferiorly into the soft tissues of the neck. An abscess developing under these circumstances is referred to as a Bezold’s abscess. Defects of the internal mastoid cortex have the potential to allow direct apposition of the inflammatory debris to the dura over the sigmoid sinus. Dural sinus thrombosis may be secondary to direct extension or result from retrograde thrombophlebitis. Retrograde thrombophlebitis may also be causative with regard to other complications, such as meningitis and abscess formation. Dural sinus thrombosis is extremely difficult to diagnose regardless of the imaging modality.98 Correlation with clinical symptomatology is essential. These patients are typically extremely ill with high spiking fevers and altered mental status. MR imaging findings may be direct or indire~t.~’ Indirect findings include absence of normal flow void on spin echo images and absence of flow-related enhancement on gradient echo images. These are, of course, quite nonspecific but should be viewed with suspicion in the appropriate clinical context. Direct evidence of dural sinus thrombosis includes the identification of thrombus within the sinus. This may be appreciated in the deoxyhemoglobin stage utilizing T2 and proton density weighted spin echo images. MR angiography with reversal of saturation pulse (MR venography [MRV]) is considered by most investigators to be the stateof-the-art in the evaluation of this very dangerous clinical condition (Fig. 2). False-positive MR imaging diagnosis of dural sinus thrombosis may result from the presence of aberrant arachnoid granulation that appears as filling defects within the sinus.56, Spread of infectious debris into the labyrinth is usually via the round window or oval window and results in labyrinthitis, which is diagnosed in the subacute phase by the presence of faint enhancement within the membranous labyrinth on enhanced T1-weighted MR images. Petrous apicitis theoretically occurs only in individuals with a pneumatized petrous apex; however, this is a matter of Patients with petrous apex infection may present with a classic Gradenigo’s syndrome consisting of abducens nerve palsy, pain in the distribution of the trigeminal nerve, and otitis media. Imaging studies typically demonstrate erosive change of the petrous apex and abnormal contrast enhancement of the adjacent meninges (Fig. 31.” As indicated previously, complications of chronic otomastoiditis are different from those of acute otoma~toiditis.~~ Granulation tissue is probably the most common cause of middle ear debris and is diagnosed at CT by its lack of bony erosion or ossicular di~placement.~’ The degree of conductive hearing deficit is variable. Granulation tissue enhances intensely with gadolinium on T1-

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Figure 2. Dural sinus thrombosis. A, Axial T2-weighted image. Diffuse debris is identified within the mastoid on the right. i3,Axial enhanced T1-weighted image. There is enhancement in the jugular vein (double arrows) and sigmoid sinus (single arrow) indicating slow flow within the structures. C, Coronal MRV sequence. MRV reveals complete occlusion at the level of the midtransverse sinus on the right with some evidence of adjacent collateralization (arrow). (Courtesy of J. Finizio, MD.) (from Swartz JD: Temporal bone II, inflammatoiy disease, sensorineural hearing deficit and the neurovascular compartments. ln Special Course in Head and Neck Imaging, 1996, pp 133-141 ; with permission from the Radiologic Society of North America.)

Figure 3. Petrous apex infection. A, Axial CT. Diffuse debris throughout the mastoid on the right with several fluid levels is identified. There is a well-marginated defect at the right petrous apex (arrows). B, Contrast enhanced axial T1-weighted image. The lesion is of low signal intensity; however, there is faint adjacent meningeal enhancement (arrow) near Meckel's cave.

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weighted MR images, thus differentiating it from cholesteatoma, which does not enhance with gadolinium (Fig. 4).62 Cholesterol granuloma is a specific subtype of granulation tissue that differs histopathologically from cholesteatoma because it is lined by fibrous connective tissue rather than keratinized stratified squamous epithelium.60Cholesterol granuloma is the result of a hemorrhagic foreign body response elicited by cholesterol crystals. Otoscopy reveals a vascular mass potentially mimicking paraganglioma or aberrant internal carotid artery if the tympanic membrane is healed and a history of chronic otitis media is not elicited. The CT appearance of tympanic cavity cholesterol granuloma is nonspecific and similar to typical granulation tissue. MR imaging is diagnostic because extracellular methemoglobin within the lesion results in bright signal on all spin echo pulse sequences (Fig. 5). Cholesterol granuloma may also occur at the petrous apex. In this location, there is a strong tendency for it to exhibit dramatic expansile characteristics (Fig. 6). It most often occurs in the context of chronic otitis, although this is not invariable and many have been described in patients with a wellpneumatized mastoid. A cholesterol granuloma is often referred to as giant cholesterol cyst in this context, although the imaging manifestations and the pathogenesis are Acquired cholesteatoma is the classic entity con-

sidered in the context of chronic otitis media and a retrotympanic mass.9I A cholesteatoma is a concentrically enlarging collection of exfoliated keratin within a sac of stratified squamous epithelium, or more simply stated “skin in the wrong place.” Congenital middle ear cholesteatomas occur behind an intact tympanic membrane presumably resulting from an epithelial rest. They may occur at any location within the middle ear but are most often found in either the posteromedial or anteromedial tympanic cavity (Fig. 7).51Acquired middle ear cholesteatomas are much more common (98%) and occur in association with a disrupted tympanic membrane or a history of chronic otomastoiditis. A number of theories have been advanced to explain these lesions. Most authors favor the invagination theory, which suggests that acquired cholesteatomas develop from retraction pockets in the tympanic membrane (eustachian tube dysfunction). Normally, the skin on the outer surface of the tympanic membrane continuously migrates outward with cerumen. Retraction pockets impede this normal physiologic process, allowing keratinized debris to accumulate and form a keratin plug. Moisture then allows the plug to expand, producing a cholesteatoma. Most acquired cholesteatomas develop from retraction pockets in the smaller more superior pars flaccida portion of the tympanic membrane. These arise within Prussak‘s space located between the mal-

Figure 4. Granulation tissue. A, Magnified coronal CT image of the right ear. Nonexpansile debris spares the ossicular chain (curved arrow) and scutum (outlined arrow). B and C, Pre- (6)and postcontrast (C) axial T1-weighted images. Diffusely enhancing debris throughout the middle ear and mastoid is compatible with granulation tissue.

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Figure 5. Cholesterol granuloma of the middle ear. A, Magnified axial CT image of the left ear. Nonerosive debris throughout the middle ear cavity. 6,Noncontrast magnified axial T1-weighted image of the left ear. Hyperintense debris on T1-weighted images is compatible with the hemorrhagic nature of this process. (From Swartz JD: Temporal bone 11, inflammatory disease, sensorineural hearing deficit and the neurovascular compartments. In Special Course in Head and Neck Imaging, 1996, pp 133-141; with permission from the Radiologic Society of North America.)

Figure 6. Cholesterol granuloma of the petrous apex. A, Axial CT. There is a large erosive, expansile mass involving the right petrous apex/clivus/adjacentcranial base. 6,Nonenhanced axial T1-weighted image. Lesion is hyperintense on all spin echo pulse sequences indicating the presence of hemorrhagic byproducts. (Courtesy of P. S. Yussen, MD.)

Figure 7. Congenital cholesteatoma. Axial CT image of the tiny mass in the anteromedial aspect of the left tympanic cavity (arrow).

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9C). In our experience, this occurs more commonly

Figure 8. Prussak (pars flacida) cholesteatoma. Magnified coronal CT image of the right ear. Nondependent soft tissue mass within Prussak's space is identified (wavy arrow). Note erosion and blunting of the scutum (arrow).

leus neck and the lateral attic wall and are best appreciated on coronal CT images (Fig. 8). As these cholesteatomas enlarge, they extend posteriorly into the mastoid antrum. Cholesteatomas may result in erosion of the tegmen tympani (roof of middle ear) and allow for the cholesteatoma to be in direct apposition to the dura subjacent to the temporal lobe. Aggressive cholesteatomas may also result in labyrinthine fistula or petrous apex extension. Fistulae usually occur at the level of the lateral semicircular canal (Fig. 9A). Of course, ossicular erosion is extremely common with cholesteatoma." Pars flaccida cholesteatomas result in disruption of the "ice cream cone" within the attic (malleus head and incus body). These lesions may also extend to the petrous apex (Fig.

in individuals who have undergone prior mastoid surgery. Acquired cholesteatomas arising from posterosuperior pars tensa retraction pockets begin in the facial recess and are best seen on axial CT images. As these enlarge they erode the more distal ossicular chain. Regardless of site of origin or etiology, cholesteatomas do not enhance with gadolinium unless infiltrated with granulation This is a helpful differential diagnostic point that distinguishes cholesteatoma from other retrotympanic masses, such as paraganglioma or schwannoma2l Postinflammatory noncholesteatomatous-conductive hearing deficit may result from ossicular fixation (tympanosclerosis) or ossicular erosion. Ossicular fixation most commonly occurs in the attic or oval window niche. Ossicular erosions most commonly involve the distal incus, presumably due to its tenuous vascular supply.

THE IAC AND CPA

As imaging specialists, we commonly examine patients with sensorineural hearing deficit, dizziness, or vertigo. A wide experience has been gained over the past few years in the evaluation of these patients both with respect to differential diagnosis and technical advancement. Evaluation of the patient with sensorineural hearing deficit requires a thorough understanding of the entire auditory pathway, not just the IAC.99 Findings at audiometric examination allow patients with sensorineural deficit to be divided into two categories: (1) those with sensory loss and (2) those with neural loss. Sensory loss implies

Figure 9. Erosive atticoantral cholesteatoma. A, Magnified axial CT image of the right ear. There is an expansile mass in the attic with absence of ossicular chain. There is a localized defect within the cortex of the lateral semicircular canal (arrow) consistent with the diagnosis of labyrinthine fistula. 6, Magnified axial T2-weighted image. Relative hypointensity of the cholesteatoma (arrow) compared with surrounding highly intense mastoid debris. C, Different patient, petrous apex extension, axial image of the right ear. There is a mastoidectomy cavity. The mass has invaded the first genu of the facial nerve canal (arrows) as the cholesteatoma extends to the petrous apex (arrowheads).

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Figure 10. Cochlear otospongiosis. Magnified coronal CT image of the left ear. There is diffuse dernineralization involving the bone around the cochlea, especially the promontory. (Courtesy of R. A. Holliday, MD, New York, NY.)

involvement of the cochlea. Imaging studies in these patients require evaluation of both the bony 59, 87 CT labyrinth and the membranous lab~rinth.~, is the preferred method for evaluation of the bony labyrinth. Thin-section axial and coronal CT images are necessary. The imaging specialist should search for regions of demineralization, which may be caused by cochlear otosclerosis, otosyphilis, Paget’s disease, or osteogenesis imperfecta (Fig. 10). Both CT and MR imaging are used to evaluate the membranous labyrinth. Labyrinthitis may be classified by its causative agent (bacterial, viral, leutic, autoimmune) or by its etiology.5sTympanogenic labyrinthitis occurs secondary to middle ear disease. Meningogenic labyrinthitis occurs secondary to meningitis and as such is often Hematogenic and posttraumatic labyrinthitis are much less common. Regardless of its cause, sub-

acute labyrinthitis characteristically results in faint, diffuse, but often segmental enhancement of the membranous labyrinth on contrast-enhanced T1weighted images (Fig. 11)? Facial nerve enhancement may be associated. Intralabyrinthine schwannoma is an additional cause for pathologic enhancement within the membranous labyrinth (Fig. 12).52Such enhancement is characteristically much more intense and localized in contradistinction to that which occurs with labyrinthitis. If labyrinthitis progresses to the chronic stage (treatment failure), the membranous labyrinth is replaced by fibrous tissue that later may ossify (Figs. 13 and 14). This stage of chronic labyrinthitis (labyrinthitis ossificans) is readily identified at CT as ossific replacement of the membranous labyrinth. The fibrous stage of chronic labyrinthitis is more difficult to diagnose and is probably best seen as relative hy-

Figure 11. Subacute labyrinthitis, segmental. Contrast enhanced axial T1-weighted image. Pathologic enhancement within the cochlea of the left (arrow).

Figure 12. lntralabyrinthine schwannoma. Axial contrast enhanced T1-weighted image. Intense focal enhancement within the vestibule on the right compatible with the diagnosis of schwannoma.

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Figure 13. Fibrous replacement, cochlear apex. A, Magnified axial T2-weighted fast spin echo image of right ear. Pathologic hypointensity is seen involving the modiolus/osseous spiral lamina (arrow). 6,Magnified axial TP-weighted fast spin echo image, right ear, different patient. Normal findings for comparison. Note the modiolus/osseous spiral lamina within the apex of the cochlea (arrow). Vestibule (outlined arrowhead). Cochlear nerve anteriorly (double arrows) and inferior vestibular nerve posteriorly are also demonstrated within the internal auditoty canal.

pointense signal within the membranous labyrinth on high-resolution gradient echo or T2-weighted fast spin echo images." Intralabyrinthine hemorrhage is rare and may be due to coagulopathy, trauma, or tumor. This is diagnosed when hyperintense signal is identified on unenhanced T1-weighted images (Fig. 15)?9,loo Congenital inner ear deformities and temporal bone trauma are additional causes for sensory (cochlear) hearing loss. Congenital sensorineural deficit is not considered in detail in this article. Perhaps only 20% of individuals with such a deficit have imaging manifestations. Because these are primarily osseous, CT is the initial examination at most centers. A wide variety of cochlear deformities have been described and classified by embryogenesis.= Of these, Mondini's disease is the most common and is diagnosed when the apical

and middle turns are not fully developed (incomplete partition) but the basilar turn is normal. Individuals with complicated Mondini's disease may have abnormality of the semicircular canals and vestibular aqueduct. The vestibular aqueduct syndrome is the most common cause of congenital sensorineural hearing deficit for which there is an imaging manifestation: the large vestibular aqueduct at CT and the large endolymphatic duct or sac at MR imaging (Fig. 16).17,54 Vestibular aqueduct syndrome is four times as common as Mondini's disease. Virtually all patients with vestibular aqueduct syndrome have a deformity of the cochlear modiol~s.4~ The reader is referred elsewhere for a more thorough discussion of congenital inner ear deformity.36,91 Posttraumatic sensorineural hearing deficit may result from fracture, perilymphatic fistula, or cochlear concussion. Trans-

Figure 14. Labyrinthine ossification. Axial CT scan. Ossification is present within the cochlear apex on the right (arrow) when compared with the left is present. Note that the patient has

undergone mastoidectomy on the left.

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Figure 15. lntralabyrinthine hemorrhage. Magnified noncontrast T1-weighted image of the right ear. Note the pathologic hyperintensity within the vestibule and semicircular canals (arrow).

verse fractures are more common than the longitudinal variety in this context.29 Neural (retrocochlear) hearing loss implies involvement of the auditory pathway exclusive of the cochlea (Fig. 17)? Neural impulses pass from the cochlea to the brain stem along the cochlear nerve located within the anteroinferior quadrant of the IAC.%After exiting the IAC the nerve passes through the CPA to synapse at the dorsal and ventral cochlear nuclei in the medulla adjacent to the inferior cerebellar peduncle.57,87 Unilateral retrocochlear loss is caused only by a lesion of the cochlear nerve or cochlear nuclei. Gebarski et alzs have coined the term cochlear nuclear complex to describe an 8 x 3-mm tubular structure that results in a convexity along the posterolateral surface of the upper medulla. This region is bordered by the foramen of Luschka with its accompanying choroid plexus (Fig. 18). Importantly, lesions in

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this location may result in unilateral retrocochlear hearing loss identical to that caused by acoustic tumor (Fig. 19). Ischemic and demyelinating lesions predominate in this location. From the cochlear nuclei, fibers ascend crossed and uncrossed via the lateral lemniscus to the inferior colliculi in the midbrain. From the midbrain, fibers continue to the medial geniculate body in the thalamus and subsequently via auditory radiations to the superior temporal gyrus. Insults in the remainder of the auditory pathway result in bilateral sensorineural hearing loss, which is paradoxically more noticeable on the contralatera1 side.4 Insults to the superior temporal gyrus result in an auditory agnosia, which is an impaired interpretation of sound rather than a true loss of hearing. High-resolution thin-section T2-weighted fast spin echo sequences in the evaluation of the IAC, CPA, and membranous labyrinth are advocated by some authors.” 17, ST Although numerous gradient echo techniques have also been developed, fast spin echo sequences have been demonstrated to have numerous advantages, including less tendency for magnetic susceptibility artifact. Utilizing both techniques, the high signal of fluid in the subarachnoid cisterns and inner ear provides excellent contrast with small structures, such as the nerve bundles of the seventh and eighth cranial nerves (Fig. 20A). The fluid signal of the membranous labyrinth defines its configuration and allows characterization of numerous pathologic processes. Dual phased-array coils, in order to improve the signal-to-noise ratio, and a long repetition time (TR of 4000 to 5000 millisecond msec), which is practical due to decreased acquisition time resulting from long echo trains inherent to this technique, are recommended. Both axial and coronal images are included in the protocol. Importantly, these techniques have great value in the screening of individuals with suspected acoustic schwan-

Figure 16. Vestibular aqueduct syndrome. A, Magnified axial CT image of the left ear. There is a large vestibular aqueduct (arrow) as well as deformity of the posterior petrous surface (double arrowheads). 6, Corresponding magnified axial T2-weighted fast spin echo image. Unusually large endolymphatic duct (arrow) and sac (double arrowheads).

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Superior temporal gyrus (Brodmans area 41,42)

teral lemniscus Superior olivary nucleus

Ventral cochlear cleus Dorsal cochlear nucleus

Vestibulocochlearnerve

Figure 17. Auditory pathway. (From Armington WH, Harnsberger HR, Smoker WRK, Osborne AG: Normal

and diseased acoustic pathway: Evaluation with MR imaging. Radiology 167:506-515, 1998;with permission.)

noma because they often eliminate the need for gadolinium-enhanced T1-weighted images.2 The IACs are oriented perpendicular (or nearly perpendicular) to the sagittal plane of the skull (Fig. 20B and C). Often, the mediolateral orientation is such that the meatus (porus) is more superior as seen on coronal images. We have observed a wide variation in the width and length of the canal. There is also significant normal variation with respect to the degree of flaring of the meatus (porus). The vestibulocochlear nerve enters the IAC and divides into vestibular and cochlear components at approximately the midportion of the 74 The most lateral aspect of the IAC is

referred to as thefundus. It is at this location that the IAC is subdivided by the crista falciformis, a horizontally oriented bony crest, into superior and inferior portions. The superior one half of the canal is further subdivided by a thin, variably ossified, vertically oriented structure referred to as Bill’s bar (after the world-renowned otologic surgeon William F. House) (Fig. 20D).M The superior and inferior vestibular nerves are located posteriorly. The cochlear nerve lies within the anteroinferior quadrant and the facial nerve is anteros~perior.~~ The superior vestibular nerve innervates the superior and lateral semicircular canals as well as the utricle. The saccule and posterior semicircular canal are innervated by the inferior vestibular nerve. The singular branch of the inferior vestibular nerve is often identified separate from the main trunk within the posterior inferior quadrant. This nerve supplies the posterior semicircular canal and occupies a separate and distinct bony orifice, the singular foramen, which is consistently appreciated coursing parallel to the fundus of the IAC on both axial and coronal CT images (see Fig. 20C)?9,91 Schwannomas of the eighth cranial nerve (acoustic neuromas) are noncalcifying, slow-growing, well-encapsulated lesions that account for 6% to 10% of all intracranial tumors and 60% to 90% of CPA tumors2l They are somewhat more common and tend to be larger in women. They most commonly present as a combined intracanalicularCPA lesion; however, they may be entirely intracanalicular in nature (Fig. 21). Schwannomas arising from the extracanalicular portion of the nerve involve only the CPA cistern and masquerade as meningioma or metastasis. Intralabyrinthine schwannomas may occur within the cochlea or vestibule (see Fig. 12).23, 52, 75 Eighth nerve schwannomas typically present with progressive unilateral high-frequency retrocochlear sensorineural hearing loss. Sudden worsening of the hearing deficit occurs commonly and may be due to occlusive changes involving the internal auditory branch of the anterior inferior cerebellar artery (AICA).MThis hearing loss is often associated with high-pitched tinnitus and vertigo. Vertigo, an episodic illusion of motion, is clinically differentiated from dysequilibrium, a continuous sense of instability. As indicated elsewhere in this article, secondary facial nerve symptomatology is unusual regardless of the size of the CPA component because this cranial nerve may undergo substantial torsion without losing its functional integrity. The reader should be aware that despite the progressive nature of the hearing deficit, eight nerve schwannomas often arise from the vestibular rather than the cochlear portion of the nerve (85%). As these schwannomas enlarge, regions of internal necrosis and cyst formation may result in a heterogeneous a p p e a r a n ~ eThese . ~ ~ are manifest as regions of nonenhancement on contrast-enhanced T1-weighted images. These larger lesions are often

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Figure 18. Normal appearance of cochlear nuclear complex. A, Contrast-enhanced axial T1 -weighted image. Normal enhancement of choroid plexus within foramina of Lushka extending to cerebellopontine angle outlines normal cochlear nuclear complex on each side (arrows). 6, Axial T2-weighted fast spin echo image demonstrates CSF intensity within the lateral recesses surrounding cochlear nuclear complex on each side (arrows).

associated with the development of extramural (arachnoid) cysts presumably secondary to elevation and deformation of the leptomeninges, which results in formation of peritumoral adhesions creating a pseudoduplication of the arachnoid and subsequent fluid trapping. The reader should be aware that false-positive gadolinium-enhanced T1-weighted examinations have been documented most commonly in the presence of relatively faint fundal enhancement." Nonneoplastic conditions encountered during surgical exploration of these lesions have lead to speculation that perhaps a "wait and see" approach should be advocated in this circumstance, with reimaging in 6 months. The disappearance of this enhancement has been documented in several pa-

Figure 19. Demyelinating disease, multiple sclerosis. Contrast-enhanced axial TI -weighted image demonstrated an intensely enhancing mass in the vicinity of the cochlear nuclear complex (arrow) on the right. There was unilateral retrocochlear hearing loss.

tients. Importantly, eighth nerve schwannomas are far more common at the meatus ( p o r ~ s )All . ~ fun~ dal lesions should therefore be viewed with skepticism. There are numerous causes for pathologic contrast enhancement within the IAC other than eighth nerve schwannoma. Recall that the leptomeninges follow the seventh and eighth cranial nerves into the IAC.13As such, any leptomeningeal disease process may involve the IAC and cause related clinical symptomatology. Neoplastic entities involving leptomeninges, including meningioma and meningeal metastases, may manifest within or adjacent to the IAC.48The MR imaging signal characteristics of meningiomas in this location are similar to those described elsewhere (relative T2 hypointensity). Meningiomas arising within the CPA are characterized by eccentricity from the porus and an obtuse angle with the petrous bone. Meningeal metastases (leptomeningeal carcinomatosis, carcinomatous meningitis) may result from solid tumors or lymphoproliferative mal i g n a n c i e ~ .These ~ ~ metastases may be diffuse (plaque-like) or discrete (nodular). Nodular disease may masquerade as a primary neoplasm, such as schwannoma or meningioma, and clinical correlation is obviously critical in this regard. Meningitis may result in pathologic meningeal enhancement identical to that caused by neoplasm. Bacterial and fungal disease are more predisposed to this manifestation than viral etiologies. Meningeal disease is an extremely common manifestation of neurosarcoidosis.96This granulomatous process may also result in both diffuse and nodular enhancement patterns (Fig. 22). Findings may be indistinguishable from meningeal metastases and granulomatous meningitis. Involvement of other cranial nerves is quite common. Other lesions occurring within the CPA and IAC include arachnoid cyst and e p i d e r m ~ i d .41~ Both ~,

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Figure 20. Normal anatomy, internal auditory canal. A, Magnified sagittal TPweighted fast spin echo image. Superior vestibular nerve (S), inferior vestibular nerve (I), facial nerve (F), and cochlear nerve (C). B, Magnified coronal CT image of the left ear. Normal development of the anterior, middle, and proximal basilar turns of the cochlea (small arrows). Vertical portion of carotid canal (CC), malleus head (M), and scutum (arrow). C,Magnified axial CT image of the right ear. Normal internal auditoly canal (I), cochlear apex (C), and vestibule (V). Note narrow channel (small arrow) representing the singular canal. 0,F = Facial nerve; I = intermediate nerve (Wrisberg); Gg = geniculate ganglion; Sv = superior vestibular nerve; C = cochlear nerve; IV = inferior vestibular nerve. (Part D from Swartz JD, Harnsberger HR (eds): Imaging of the Temporal Bone, ed 3. Thieme Medical Publishers, 1998;with permission.)

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Figure 21. Eighth nerve schwannoma. Axial T2-weighted fast spin echo image. Hypointense mass (arrow) within the porus of the internal auditory canal is classic for intracanalicular acoustic tumor. Note the normal cerebrospinal fluid signal within the fundus of the internal auditory canal.

are of CSF density at CT and cerebrospinal fluid (CSF)-like intensity at MR imaging. They typically manifest no contrast enhancement. Arachnoid cyst is more localized and well marginated; epidermoids may encase vascular structures and insinuate along adjacent CSF spaces. CPA and IAC lipomas are rare. They are easy to diagnose provided that a precontrast T1-weighted sequence is obtained. These images demonstrate the predictable T1 hyperintense signal (Fig. 23). Fat-suppressed sequences may also be useful. Choristomas, containing smooth muscle fibers and fibrous tissues, have been described within the fundus of the IAC. They enhance with contrast and may be indistinguishable from acoustic tumor.-

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As indicated elsewhere in this article, facial nerve schwannomas arising from the cistemal or intracanalicular segments of the nerve appear identical to eighth nerve lesions unless extension into the intratemporal facial nerve canal can be demonstrated (labyrinthine segment).**Trigeminal schwannomas have epicenters anteromedial to the IAC. Posterior fossa choroid plexus papillomas usually arise within the fourth ventricle; however, they may also develop within the lateral recess and result in CPA mass. Exophytic brain stem tumors are an additional cause of CPA tumor (Fig. 24). Multiplanar MR imaging usually facilitates diagnosis. Vascular loops and aneurysms occur in the CPA anywhere from the root entry zone to the porus and result in hearing loss and vertigo. A dolichoectatic basilar artery is often the culprit under these circumstances. IAC and CPA lesions include the following: Schwannoma Eighth nerve Seventh nerve Fifth nerve Developmental Lipoma Epidermoid Arachnoid cyst Vascular AICA aneurysm Hemangioma (cavernous) Arteriovenous malformation Vestibulobasilar dolichoectasia Meningeal based Meningitis Meningioma Lymphoma Metastasis Siderosis

Figure 22. Sarcoidosis. Coronal contrast-enhanced T1weighted image. Diffuse right cerebellopontine angle mass extending into the porus of the right internal auditory canal. The lesion extends inferior to the level of the foramen magnum. Note nodular areas of adjacent leptomeningeal enhancement (small arrows).

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Figure 23. lntracanalicular lipoma. A, Magnified axial T1-weighted image of the left ear. Facial area of hyperintensity (arrow) at the fundus of the left internal auditory canal in the approximate location of the cochlear newe. B, Magnified axial Tl -weighted fat-suppressed image confirms the lipomatous

nature of the lesion. Sarcoidosis Postcraniotomy enhancement Systemic hypotension Other tumors Choroid plexus papilloma Glioma (exophytic) EVALUATION OF PT Tinnitus may be classified as pulsatile or nonpulsatile. PT is rhythmic and presumably pulse71, 82 PT is further subclassified as synchrono~s.~~, arterial or venous. Arterial PT rises in systole and does not disappear with pressure upon the internal jugular vein. Venous PT is more continuous and may be abolished by light pressure on the vein. Tinnitus may be referred to as subjective, perceived only by the patient, or objective, audible by the exam-

iner as well. PT is an extremely common problem and most imaging specialists are accustomed to receiving requests for imaging these patients. There is disagreement among imaging specialists as to the manner of initial evaluation for these patients. A reasonable approach is as follows: if there is an otoscopically demonstrable vascular lesion, CT should be the initial method. Otherwise, MR imaging and MR angiography should be performed. CT examination utilizes thin sections, preferably 1mm, obtained in the axial and coronal projection. Examination must be extended posteriorly and inferiorly in order to evaluate the vascular compartments at the skull base. MR imaging examination should include both precontrast and postcontrast T1-weighted images as well as axial T2-weighted fast spin echo images. Two-dimensional time of flight MR angiography with MRV is an excellent screening sequence for evaluation of

Figure 24. Exophytic brain stem glioma. A, Contrast enhanced axial Ti-weighted image. Intensely enhancing mass is identified within the cerebellopontine angle and internal auditory canal strongly suggestive for eighth nerve schwannoma. Note the faint enhancement within the dorsal aspect of the pons just ventral to the deformed and displaced fourth ventricle (arrow). f3, Axial T2-weighted image. Note the diffusely infiltrative lesion involving the brain stem.

THE TEMPORAL BONE

Figure 25. Asymmetrically large right jugular foramen, normal variant. Note smooth cortication (arrows). F = Facial nerve, mastoid segment.

the venous structures. Importantly, objective PT should be viewed with great suspicion even if CT and MR imaging and MR angiography examinations are negative. Subselective catheter angiography may ultimately be needed. Imaging evaluation of these patients often focuses upon the jugular foramen (Fig. 25). The key to the imaging of the foramen is CT evaluation of cortication. Regardless of foramen size, the cortex should be smooth and ~ninterrupted.~~ The reader should keep in mind that the asymmetrically large jugular foramen is more common on the right. This is predictable because the superior sagittal sinus often drains preferentially into the right transverse sinus. These individuals also have asymmetric prominence of the ipsilateral internal jugular vein in the neck. Signal characteristics on conventional spin echo examination are highly variable and primarily relate to flow characteristics. When venous flow is slow, bright signal is possible (flow-related enhancement) and enhancement may occur with gadolinium on T1-weighted images. These flow vagaries create problems in diagnosing neoplasm and even more difficulty when occlusive disease is suspected. Gradient echo sequences are sensitive to flow-related enhancement but are probably of limited value in the average case. Many classic anatomy texts subdivide the jugular foramen into a smaller anteromedial pars nervosa containing the glossopharyngeal (ninth) cranial nerve and the inferior petrosal sinus and a larger posterolateral pars vascularis containing the vagus (10th) cranial nerve, the accessory (11th) cranial nerve, and the internal jugular vein.18 These are separated by a fibrous or bony septum. More recently performed anatomic studies have indicated that this classic subdivision is much less common than once thought, although some form of subdivision at the endocranial aspect of the

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foramen is relatively common.', 73 The inferior petrosal sinus represents the primary drainage from the cavernous This vessel courses inferiorly within a bony sulcus just lateral to the clivus and petro-occipital fissure (Fig. 26). This vessel can be appreciated in cross-section on gadoliniumenhanced T1-weighted images. Below Dorello's canal at the level of the jugular tubercle the inferior petrosal sinus abruptly turns laterally to enter the anteromedial compartment of the jugular foramen (pars nervosa). The glossopharyngeal (ninth) cranial nerve invariably occupies the most anterior, medial, and superior portions of the foramen. The glossopharyngeal meatus, which receives the nerve from the brain stem and premedullary cistern, should not be confused with the petrous portion of the cochlear aqueduct.73The vagus (10th) and accessory (11th) cranial nerves course within the posterolateral aspect of the jugular foramen (pars vascularis). The internal jugular vein is a continuation of the sigmoid sinus, which itself is a continuation of the transverse sinus. The internal jugular vein lies within the jugular foramen (pars vascularis) posterolateral to all three cranial nerves. On occasion, there may be a dehiscence at the floor of the middle ear and the dome of the jugular vein may protrude into the hypotympanum resulting in a vascular-appearing retrotympanic mass. This is referred to as the dehiscenf infernal jugular vein.91Regardless of the degree of dehiscence, the margins of the large jugular foramen are smooth and well corticated, allowing accurate diagnosis (Fig. 27). Medial internal jugular vein diverticula have been described, which interrupt the posterior cortex of the IAC and may masquerade as a significant lesion. The petrous carotid artery is described as having vertical and horizontal segments. The vertical portion of the carotid canal is best identified on coronal CT images obtained at the level of the anterior turns of the cochlea. This segment of the canal is appreciated en face on axial CT images (see Fig. 20B). The horizontal portion of the carotid canal is well visualized on axial CT images oriented anteromedial to posterolateral. Normally, a clearly definable bony septum separates this portion of the carotid canal from the middle ear cavity proper (hypotympanum) (Fig. 28). At this juncture it is useful to discuss an extraordinarily important although uncommon entity, the aberrant internal carotid a~tery.4~~ In these patients, the vertical portion of the carotid canal does not develop. Instead, the internal carotid artery arises as an extension of the inferior tympanic branch of the external carotid artery. This vessel enters the temporal bone via the inferior tympanic canaliculus rather than the vertical portion of the carotid canal. The aberrant vessel courses through the middle ear cavity proper and continues along the horizontal portion of the carotid canal toward the cavernous sinus. In addition to absence of the vertical portion of the carotid canal, the aberrant internal carotid artery

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Inferior Pel Sinus :h, IX

Arnold’s Nei-ve, X Branch

0 Figure 26. Jugular foramen anatomy. (From Swartz JD, Harnsberger HR (eds): Imaging of the Temporal Bone, ed 3. Thieme Medical Publishers, 1998; with permission.)

THE TEMPORAL BONE

is associated with absence of the normal plate of bone separating the horizontal portion of the carotid canal from the middle ear cavity proper. Contiguity of the otoscopically demonstrable vascular mass with this segment of the carotid canal is clearly visualized on axial CT scanning (see Fig. 28). MR imaging and MR angiography are almost never needed in this context, but may be useful under exceptional circumstances (Fig. 29).19 The persistent stapedial artery is a rare vascular anomaly. This vessel arises from the primitive second aortic arch and eventually becomes the middle meningeal arte1y.4~.5o Normally, the stapedial artery involutes and the middle meningeal artery develops originating from the maxillary artery entering the cranial cavity via the foramen spinosum. The persistent stapedial artery courses through the obturator foramen between the stapes crura and continues along the tympanic segment of the facial nerve canal, anomalously enlarging this structure. Also, the foramen spinosum (which usually transmits the middle meningeal artery from its maxillary artery origin into the cranial cavity) is absent (Fig. 30). There is a strong association of the persistent stapedial artery and the aberrant internal carotid artery. Importantly, the internal carotid artery may be anomalously laterally placed rather than truly Close inspection reveals that the artery does actually enter the carotid canal rather than the inferior tympanic canaliculus. Petrous aneurysms most commonly arise at the junction between the vertical and horizontal segments of this vessel. Agenesis of the internal carotid artery has also been reported. The carotid canal is absent in these cases. Total agenesis of the internal carotid artery has a left-sided predilection and there is an association with intracranial aneur y s m ~ ?Fenestration ~ (splitting) of the petrous internal carotid artery may occur. This is extraordinarily rare.4o Numerous neoplasms may result in PT. The most important of these is paragangli~ma.~', 91

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Jugulotympanic paragangliomas may arise within the middle ear cavity proper (glomus tympanicum) or from the adventitia of the jugular bulb (glomus jugulare). The glomus tympanicum tumor typically arises from glomus formations associated with the nerve of Jacobson, the inferior tympanic branch of the ninth cranial nerve (see Fig. 26).49 This structure enters the middle ear via a normal inferior tympanic canaliculus. The glomus tympanicum tumor is typically isolated to the middle ear and has a predilection for origination along the surface of the promontory. These are usually easily diagnosed with CT scanning (Fig. 31). Confusion may arise when these lesions are holotympanic. Importantly, in our experience they have a tendency to encase rather than destroy the ossicular chain, best differentiating them from cholesteatoma. These lesions enhance intensely with gadolinium allowing further differentiation (Fig. 32). Glomus jugulare tumors result in enlargement of the jugular foramen; however, as indicated previously, such enlargement is not inherently abnormal. CT is diagnostic in that the glomus jugulare tumor results in numerous focal defects within the cortex of the foramen (Fig. 33).9' Gross destruction is also possible. As these lesions enlarge they have a tendency to extend superiorly into the middle ear cavity proper, becoming visible otoscopically. They typically extend along pathways of least resistance, such as pre-existing air cell tracts, fissures, and Inferior growth along the internal jugular vein with eventual occlusion of this vessel is characteristic (see Fig. 3 3 0 ) . The examining physician is often unable to distinguish a glomus tympanicum tumor from a glomus jugulare tumor. Detailed imaging examination in this context is critical. Lateral extension of the glomus jugulare tumor may disrupt the mastoid segment of the facial nerve canal (see Fig. 33B). Further enlargement posteriorly may result in encroachment upon the posterior fossa. Classically, they result in a "salt and pepper" appearance second-

Figure 29. Aberrant internal carotid artery. A, Source image. 6, MR angiography of circle of Willis, axial projection. There is an aberrant internal carotid artery extending into the middle ear on the left (arrows).

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Figure 30. Persistent stapedial artery. A, Coronal CT. Large facial nerve canal (tympanic I, CT. Poor definition of stapes (artery courses between crura) (arrow). segment) (arrow). €Axial C,Axial CT, more inferior. Absent foramen spinosum (arrow). FO = Foramen ovale. (Courtesy of J. I. Weiner, MD, Miami, FL.)

Figure 31. Glomus tympanicum paraganglioma. Magnified axial CT image of the lefi ear. Well-marginated sofi tissue mass (arrow) adjacent to the promontoty. Note the well-pneumatized middle ear and mastoid.

THE TEMPORAL BONE

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Figure 32. Glomus tympanicum paraganglioma, attic block. A, Magnified coronal contrastenhanced T1-weighted image of the left ear. There is an intensely enhancing mass (curved arrow) identified within the middle ear cavity. B, Magnified coronal CT image of the left ear. C, Magnified coronal T2-weighted fast spin echo image of the left ear. CT scan demonstrates slight bulging of the tympanic membrane but no other evidence of erosive characteristics. Signal voids (small arrowheads) are identified in C,confirming the diagnosis of paraganglioma. Note diffuse attic and mastoid debris secondary to attic block.

ary to the presence of signal voids (vascular channels) (see Fig. 33C). Glomus jugulare lesions are characterized angiographically by an intense blush. Numerous arteries may contribute to the vascular supply. Of these the inferior tympanic branch of the ascending pharyngeal artery is most commonly involved. Vascular metastatic lesions, such as those arising from renal cell or thyroid carcinoma, may have an appearance identical to glomus jugulare tumor. Schwannomas (especially ninth cranial nerve) may also arise within or adjacent to the jugular foramen and result in PT. Importantly, these expand the jugular foramen and cause diffuse thinning of the cortex rather than gross destruction or focal defects more typical of glomus jugulare tumor (Fig. 34). Signal voids at MR imaging are extraordinarily uncommon. Endolymphatic sac tumors are aggressive papillary lesions characterized by intratumoral bony spiculation best seen at CT and intense enhancement (Fig. 35).45,&, 65 Flow voids (vascular channels) and regions of T1 hyperintense signal are

appreciated with MR imaging. Initial review of these lesions may suggest resemblance to glomus jugulare. Careful study, however, reveals that this destructive lesion is at the level of the vestibular aqueduct, not the jugular foramen. Acquired vascular lesions are an important cause of PT. Numerous entities have been described. Dural arteriovenous fistula (AVF), atherosclerotic disease, fibromuscular dysplasia, dissection, and aneurysm should be con~idered.'~, 82 MR imaging and MRV are crucial to the diagnosis of these entities. Conventional angiography may be needed, particularly if embolization i s contemplated. There is disagreement regarding the pathogenesis of the dural AVF. Most likely, this is an acquired lesion related to recanalization of a thrombosed transverse or sigmoid sinus with anomalous venous drainage.I4,31 Thrombosis may be spontaneous or occur secondary to trauma, surgery, or dehydration. Conventional MR pulse sequences may reveal an abnormal transverse or sigmoid sinus.

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Figure 33. Glomus jugulare paraganglioma. A, Magnified coronal CT image of the left ear at level of vestibule. Large erosive mass with epicenter in region of jugular foramen (arrowheads) is identified extending superiorly into the middle ear cavity (arrow). 13,Magnified coronal CT image, left ear more posteriorly. Large erosive mass is demonstrated. There is extension to the mastoid segment of the facial nerve canal (arrow). C,Contrast enhanced axial T1-weighted image. Intensely enhancing mass extends to lie in direct apposition to anterolateral aspect of left cerebellar hemisphere. Numerous signal voids are noted. 0,Magnetic resonance angiography with reversal of saturation pulse demonstrates normal jugular vein/sigmoid sinus on right (arrows). These structures were completely occluded on the side of the mass.

The source images for the MR angiography often delineate the transosseous collaterals present with this lesion to best ad~antage.'~ Of course, radiologic intervention requires catheter angiography. Direct arteriovenous shunts may arise from the vertebral artery, internal carotid artery, or external carotid artery. Penetrating injuries are usually the cause of the vertebral AVE PT associated with atherosclerotic cardiovascular disease results from arterial jetting and turbulence transmitted via the temporal bone. The pathogenesis of PT in individuals with fibromuscular dysplasia is similar. Petrous internal carotid artery aneurysms are associated with gross expansion of the carotid canal. Diagnosis usually requires both CT and MR imaging and MR angiography. FACIAL NERVE

The facial nerve, the nerve of the second branchial arch, is subtended by three brain stem

nuclei.91The motor nucleus is the largest and is located in the ventral aspect of the lower pons. This nucleus is responsible for innervation of the muscles of facial expression, as well as the posterior belly of the digastric, stapedius, and stylohyoid muscles. There is cortical input to the motor nucleus from the corticobulbar tract, via the internal capsule and brain stem. These are crossed fibers, which explains the distinction between central facial nerve palsy, which results in contralateral facial paralysis sparing the forehead, and peripheral facial nerve palsy, which causes ipsilatera1 paralysis and involves both the upper and lower face. The superior salivatory nucleus is located in the lower pons just dorsal to the motor nucleus, and the solitary tract nucleus is located in the upper medulla. The superior salivatory nucleus contributes preganglionic parasympathetic fibers destined for the lacrimal, submandibular, and sublingual glands. Lacrimal gland fibers travel with the

THE TEMPORAL BONE

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Figure 34. Glossopharyngealschwannoma. A, Magnified axial CT image at level of jugular foramen. Note the remarkably expanded jugular foramen that is smoothly marginated and expansile and does not demonstrate the destructive characteristics typical of paraganglioma. B, Coronal contrastenhanced T1-weighted image. The lesion enhances homogeneously other than a small central area of necrosis. There were no signal voids to suggest the presence of paraganglioma.

greater superficial petrosal nerve (GSPN) and the submandibular and sublingual gland fibers travel with the chorda tympani nerve. The anatomy of these neural structures is discussed later. The solitary tract nucleus receives afferent sensory fibers of taste (anterior two thirds of tongue) and sensory fibers for proprioception and sensation from the external auditory canal via the chorda tympani nerve. The abducens (sixth cranial) nerve nucleus is located in the dorsal aspect of the lower pons just ventral to the fourth ventricle. Motor fibers from the facial nerve loop dorsally around the abducens nerve nucleus creating the facial colliculus. Lesions

affecting the facial nerve in this location are commonly associated with sixth nerve palsy. Facial nerve fibers course through the CPA cistern to enter the anterosuperior quadrant of the IAC (see Fig. 20).74Motor fibers travel separately from the sensory fibers. Fibers associated with the superior salivatory nucleus and the solitary tract nucleus convey information via the intermediate nerve (Wrisberg). As one would expect, lesions affecting the facial nerve within the CPA or IAC are often associated with eighth nerve symptomatology due to the proximity of the vestibulocochlear nerve. Within the CPA cistern, the motor portion of the facial nerve is anteriorly located and

Figure 35. Endolymphatic sac tumor. A, Axial CT. Permeative lesion with epicenter along posterior petrous surface. Note multiple bone fragments. 6,T1-weighted MR image. Heterogeneous mass with high intensity enhancing regions.

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Figure 36. Normal facial nerve CT. A, Axial CT image demonstrates labyrinthine segment (single arrow), proximal tympanic segment (double arrows), as well as facial hiatus (triple arrows). Note

bony septum (cog). Please see text. €3, Coronal CT image of the right ear (different patient) demonstrates en face the distal labyrinthine (medial arrow) and proximal tympanic (lateral arrow) segments of facial nerve canal.

the vestibulocochlear nerve is posterior (see Fig. 26A). The intermediate nerve, as its name suggests, is located between these two larger nerves. Thus far, we have discussed the cisternal (CPA cistern), and intracanalicular (IAC)segments of the facial nerve. Within the temporal bone, the intratemporal facial nerve canal contains three segments: (1) labyrinthine, ( 2 ) tympanic, and (3) mastoid. The labyrinthine segment is a short, narrow channel coursing anterolaterally from the fundus of the IAC to its termination at the level of the geniculate ganglion (first genu). The first major branch of the facial nerve, the GSPN, arises in this location and travels anteromedially via the facial hiatus.30The nerve and hiatus can often be appreciated on axial MR images and CT (Fig. 36). The GSPN contains preganglionic parasympathetic fibers that travel as the vidian nerve to synapse in the pterygopalatine ganglion within the pterygopalatine fossa. These fibers are ultimately destined for the lacrimal gland. Interruption of the facial nerve prior to the exit of this nerve therefore results in deficiencies of lacrimation. The first (anterior) genu forms an inverted V with angulation of approximately 30 degrees and initiates the posterior tympanic segment, which courses posterolaterally through the middle ear cavity proper (Fig. 36B). The proximal tympanic segment lies along the medial wall of the anterior epitympanic recess, a structure often formed by a single air cell that is derived from the eustachian tube rather than the mastoid air cell system and as such is also known as the supratubal recess.68The anterior epitympanic recess is separated from the anterior portion of the epitympanum (attic) by a vertically oriented, often ossified bony septum referred to as the cog (see Fig. 36A). Epitympanic

cholesteatomas eroding the cog and extending into the anterior epitympanic recess are therefore in direct apposition to the proximal tympanic segment of the facial nerve. The midtympanic segment of the facial nerve courses along the undersurface of the lateral semicircular canal, and as such is immediately superior to the oval window. Dehiscences (gaps) within the cortical margin of the tympanic segment of the facial nerve are especially common in this location. At these dehiscences, the facial nerve is especially vulnerable. On occasion, the nerve may actually protrude through this dehiscence and lie within the oval window niche (Fig. 37)" These protruding facial nerves may rarely result in conductive

Figure 37. Dehiscent tympanic segment. Magnified coronal CT image of right ear. Prolapse of the mid tympanic segment of the facial nerve into the oval window niche (arrow).

THE TEMPORAL BONE

Figure 38. Normal facial nerve, infraforamenal segment. A, Axial T1-weighted image demonstrates the normal ap-

pearance of the facial nerves (arrows) outlined by fat immediately at and inferior to the stylomastoid foramen. B, Magnified sagittal T1 -weighted image demonstrates the normal appearance of the infraforamenal segment of the nerve (arrows) prior to its entry into t h e parotid gland. hearing deficit. They are of interest to the otologic surgeon because they complicate prosthetic stapedectomy. The posterior tympanic segment lies in the posterior tympanic cavity beneath the short process of the incus within the pyramidal eminence. The facial recess is immediately lateral to this segment of the facial canal and the stapedius muscle is immediately medial to it. The second (posterior) genu of the facial nerve forms an angle of between 95 degrees and 125 degrees and forms the mastoid segment, which courses inferiorly lateral to the jugular fossa (see Fig. 25). When the jugular fossa is large, the facial nerve is in very close apposition or possibly even dehiscent. Lesions originating within the jugular foramen, such as glomus jugulare tumor (paraganglioma), commonly involve the facial nerve at this point. Importantly, the mastoid segment of the facial nerve is also in very close proximity to the posterior tympanic annulus (site of attachment of the tympanic membrane). The mastoid segment of the facial nerve can usually be appreciated in cross-section on both axial CT and MR imaging. This segment is readily appreciated in the diploic or sclerotic mastoid but is somewhat more difficult to identify in the well-pneumatized mastoid. The other two major intratympanic branches of

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the facial nerve arise from the mastoid segment. They are the stapedius nerve (proximally), which innervates the stapedius muscle, and the chorda tympani nerve (distally). Compromise of the facial nerve prior to exit of the stapedius nerve results in hyperacusis. The chorda tympani nerve represents the terminal branch of the intermediate nerve and courses anteriorly via the canaliculus chordae tympani to enter the middle ear and continues between the incus and the malleus to exit anteriorly through the petrotympanic (glaserian) fissure. This nerve carries afferent fibers from the anterior two thirds of the tongue and efferent parasympathetic fibers to the submandibular and sublingual glands. The stylomastoid foramen is located between the mastoid process posterolaterally and the styloid process anterolaterally. Fat is consistently located at the level of this foramen and allows easy identification of the infraforaminal segment of the facial nerve in cross-section on axial images (Fig. 38). This was originally reported with CT and is appreciated on MR imaging examinations as well. The infraforaminal segment of the facial nerve delivers branches to the posterior belly of the digastric and the stylohyoid muscle prior to entry into the substance of the parotid gland. The branches of the facial nerve that terminate in the muscles of facial expression diverge within the parotid gland.5 The reader should be aware of the circumneural venous plexus surrounding the intratemporal facial nerve, which is especially noticeable in the region of the geniculate gang1i0n.z~Enhancement within the facial nerve canal in this location as seen on gadolinium-enhanced axial T1-weighted images is a normal finding (Fig. 39). This enhancement may be intense and is often a~ymrnetric.~~ In the absence' of nodularity or obvious expansion,

Figure 39. Normal facial nerve enhancement. Contrastenhanced axial T1 -weighted image. Normal, although asymmetric, enhancement in the vicinity of the first genus of the facial nerve bilaterally (arrows). Note enhancement within the facial hiatus along the greater superficial petrosal nerve (arrowhead).

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MR imaging diagnosis of the facial nerve abnormality in this location is probably not possible. The reader should also be aware, however, that enhancement of the intracanalicular, labyrinthine or cisternal segments of the facial nerve is never normal (Fig. 40). There is no clear correlation with the degree of enhancement and the extent of sympt~matology.~~ Imaging specialists differ on the most appropriate approach to the evaluation of individuals with peripheral facial nerve palsy. In our view, individuals with an otoscopically demonstrable mass or those with a history of chronic otitis or previous mastoid surgery should probably undergo high-resolution CT in the axial and coronal planes as the initial examination. Contrast is not needed for this study. Otherwise, precontrast and postcontrast thin section T1-weighted images in the axial and coronal planes are the initial recommended diagnostic method. T2-weighted fast spin echo images utilizing thin sections are also a part of this protocol in most centers. The imaging evaluation of patients with peripheral facial nerve palsy must include not only the temporal bone but also the brain stem (pontomedullary junction); CPA (IAC), and the parotid gland. Lesions within the brain stem (nuclear level) associated with peripheral facial nerve palsy include infarct; vascular malformations; demyelinating disease (multiple sclerosis); and much less commonly, neoplasm including both primary and secondary malignancies. As indicated previously, facial palsy resulting from brain stem lesions often has concurrent abducens (sixth cranial nerve) palsy. They are also associated with deficits in lacrimation (GSPN); loudness recruitment (stapedius); and taste (chorda tympani). Facial palsy accompanied by these three additional symptoms is described as supvugeniculate (Fig. 41). Lesions within the CPA and IAC are a cause of facial nerve dysfunction and are discussed elsewhere in this article. The facial nerve may undergo considerable stretching without losing its functional integrity and therefore facial weakness is

Figure 40. Abnormal facial nerve enhancement: Bell's palsy. Axial contrast-enhancedT l -weighted image. There is pathologic enhancement of the intracanalicular portion of the facial nerve (arrow).

Figure 41. Cavernous hemangioma: Sixth, seventh nerve palsy. Axial T2-weighted image. There is a heterogeneous lesion compatible with the diagnosis of cavernous hemangioma located in the left paramedian aspect of the posterior aspect of the lower pons which involved the motor nucleus of the seventh nerve as well as the abducent nucleus.

relatively uncommon even with large lesions.34 Meningitis (especially bacterial and fungal) and sarcoidosis may also result in facial nerve paralysis via involvement of these segments of the facial nerve. Vascular lesions including vertebrobasilar dolichoectasia, aneurysm, and vascular loops, are an additional important cause of facial nerve symptomatology. Often, vascular loops result in hemifacial spasm as the predominant symptom rather than facial nerve paralysis. Hemifacial spasm is a slowly progressive condition more common in women and is characterized by intermittent involuntary spasmodic tonic and clonic contraction, which usually begins with spasm of the orbicularis oculi.hThe frontalis muscle is usually uninvolved. Pain is uncommonly associated; however, there are reports of coexistence of trigeminal neuralgia and hemifacial spasm. The most common etiology of hemifacial spasm is a tortuous and redundant vascular loop, usually the AICA, posterior inferior cerebellar artery, or the vertebral artery. A dolichoectatic basilar artery may also be the offending vessel. The observer should carefully evaluate the root entry zone of the facial nerve at the level of the upper medulla. MR angiography is the imaging study of choice. Often, valuable information is obtained from the source images. High-resolution T2weighted fast spin echo sequences are also recommended as are routine precontrast and postcontrast T1-weighted sequences. Surgical intervention is highly successful and therefore the role of the imaging specialist cannot be underestimated. Neoplasm is a relatively uncommon cause of peripheral facial palsy2' Facial nerve schwannoma

THE TEMPORAL BONE

is the most common of these neoplasms. Schwannomas may involve the facial nerve at any location from its brain stem origin to the parotid gland. Geniculate ganglion (first genu) origination is the most common.63 As with eighth nerve schwannomas, they arise from the outer layer (nerve sheath) and expand eccentrically. Importantly, fewer than one half of these lesions present with facial paralysis. Hearing loss, both sensorineural (cistemal or intracanalicular segments) and conductive (intratympanic segment) is commonly associated. Intratympanic masses may erode or mechanically impede ossicular movement, thus producing conductive hearing deficit. Facial nerve schwannomas are homogeneous lesions that intensely enhance with gadolinium (Fig. 42). As such, intracanalicular and cisternal facial nerve schwannomas appear identical to those originating from the eighth nerve. The imaging diagnosis of facial nerve schwannomas in this location can only be secured if extension along the intratemporal portion of the facial nerve can be demon~ t r a t e d Of . ~ ~course, careful review of the labyrinthine segment of the facial nerve is necessary to

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make this diagnosis. CT and MR imaging are highly complimentary in the diagnosis of these lesions. Facial nerve hemangiomas (ossifying hemangioma) are common primary facial nerve neoplasms that usually begin at the level of the geniculate ganglion.I5,47 They vary in size but are usually approximately 1 cm in diameter at presentation. They typically have a distinctive CT appearance secondary to the presence of internal bony spicules. If these bony spicules are absent, the lesion is indistinguishable from a schwannoma. At MR imaging, these lesions are somewhat heterogeneous but otherwise nonspecific. Metastatic disease may involve the intratemporal facial nerve canal. Importantly, many of these reflect perineural spread of tumor, usually from the parotid gland via the stylomastoid foramen, and begin in the mastoid segment. Long segments of the nerve can be involved and extension of the tumor from this foramen to the level of the IAC has been reported. The most common parotid tumor to result in such perineural spread is the adenoid cystic carcinoma. Careful review of the paro-

Figure 42. Seventh nerve schwannoma. A, Magnified axial C T image of the left ear. Obvious expansion of mastoid segment of facial nerve canal (arrow). 6, Magnified coronal CT image of the left ear and large soft tissue mass within tympanic cavity (arrow). C, Magnified coronal CT image of the left ear, more posterior. There is obvious expansion of the mastoid segment of the facial nerve canal extending to the stylomastoid foramen (arrows). 0,Contrast-enhanced axial T1-weighted image. €, Contrast-enhanced sagittal T1-weighted image. Intense enhancement of the seventh nerve schwannoma is demonstrated (arrows).

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tid gland is important in all patients with facial palsy of unknown origin, particularly when diffuse expansion of the intratemporal facial nerve canal is diagnosed. Rarely, involvement of the facial nerve is spread via the GSPN from lesions that have extended to the pterygopalatine fossa.30 Numerous primary sites of origin need to be considered in this circumstance. Other neoplasms to consider include paraganglioma, lymphoma, and endolymphatic sac tumors. These are discussed elsewhere in this article. Glomus formations may occur along the course of the facial nerve, especially in the region of the first genu. Paragangliomas arising from these specific glomus formations are referred to as glornus faciale. This is an extraordinarily rare lesion. Peripheral facial nerve palsy may also result from trauma. Facial nerve involvement has been reported with both longitudinal and transverse fractures.29The incidence and morbidity is greater with the transverse ~ a r i e t yMechanisms .~ of injury include displaced bone fragment, nerve transection, and intraneural hematoma or edema.77The latter is believed to result from traction and stretching of the GSPN, which is accentuated by the relative immobility of the nerve within the labyrinthine segment. The most severe damage to the nerve is at the meatal foramen (entrance of the distal intracanalicular segment into the labyrinthine segment). Intense pathologic enhancement of the nerve, especially within the distal intracanalicular and labyrinthine segments, may be prolonged due to breakdown of the blood-nerve barrier as the nerve regenerates. CT and MR imaging are thus highly complimentary in cases of posttraumatic facial palsy. Delayed onset and incomplete facial palsy have a superior prognosis for recovery compared with those that are immediate onset and complete. Facial nerve palsy may also be associated with congenital and acquired cholesteatoma. As indicated previously, facial nerve palsy occurring in the context of long-standing chronic otitis media and cholesteatoma mandates careful CT study as the initial diagnostic examination. Inflammatory disease, specifically Bell's palsy, is a far more common cause of facial paralysis than any of the previously discussed conditions. There is recent evidence of a strong association between the herpes simplex virus and Bell's palsy, implying viral reacti~ation.~~ Some have replaced the term Bell's palsy with herpetic facial paralysis. Neural entrapment due to interstitial edema and swelling is the presumed etiology of the facial nerve dysfunction. Interruption of vascular supply also plays a role in the pathophysiology. Individuals with Bell's palsy typically present with an acute onset of peripheral facial nerve paralysis but have no concurrent evidence of central nervous system, temporal bone, or parotid disease. The facial paralysis is often preceded by a viral prodrome. Alterations in taste or ipsilateral pain or facial numbness may precede the onset of facial paralysis.

There are numerous misconceptions regarding the ability of MR imaging to enable diagnosis of Bell's palsy. Early reports indicated that pathologic enhancement of a normal or only mildly enlarged facial nerve was diagnostic; however, as indicated previously, significant enhancement within the facial nerve canal, especially in the region of the first genu and proximal tympanic segment, is normal. Therefore, the diagnosis of Bell's palsy on the basis of enhancement of these segments of the nerve is 78 The reader should be probably not possible.Z7, aware that enhancement of the intracanalicular or cisternal segments of the nerve is never normal and enhancement of the labyrinthine, distal tympanic, and mastoid segments is unusual (see Fig. 40). These all should be viewed with suspicion in the appropriate clinical context. The readen should also be aware that most patients with suspected Bell's palsy do not require radiologic evaluation. If the facial palsy is prolonged or of subacute onset (atypical Bell's palsy), MR imaging is clearly indicated. Lyme disease, a tick-borne-spirochetal illness, commonly produces facial paralysis and should be considered in the differential diagnosis in endemic areas.85When skin rash, arthralgias, or meningitis are associated, the diagnosis is secure. Otherwise, the facial paralysis may be indistinguishable from Bell's palsy. Serologic tests are less specific than CSF analysis. An additional important inflammatory entity involving the facial nerve is Ramsay Hunt syndrome (herpes zoster o t i c ~ s )This . ~ is associated with acute onset of facial palsy and painful vesicles within the external auditory canal. Eighth cranial nerve involvement (vertigo, hearing loss) is common. Reactivation of latent herpetic infection is the suspected etiology in Ramsay Hunt syndrome as well as in typical Bell's palsy. MR imaging findings are identical to that of Bell's palsy except that pathologic enhancement of the eighth nerve (linear, nonbulky) and membranous labyrinth are associated. THE POSTOPERATIVE TEMPORAL BONE The type and extent of middle ear and mastoid surgery performed for chronic inflammatory disease depends on the amount of disease encountered. Each case is therefore necessarily individualized. The object is to remove as much diseased tissue as possible while preserving as many normal structures as possible. Specifically, an attempt is made to preserve the integrity of the bony wall of the external auditory canal and the ossicular chain?' Mastoidectomy is subdivided into closed cavity (external auditory canal wall maintained) and open cavity types.67Closed cavity procedures include simple (cortical) mastoidectomy and intact canal wall (canal wall up) mastoidectomy. Simple mastoidectomy was performed extensively in the past but has fallen into disrepute in recent years

THE TEMPORAL BONE

except under certain specific circumstances. Intact canal wall mastoidectomy includes inspection of the middle ear with removal of Koerner’s septum. The cavity created communicates with both the attic and antrum. This procedure is useful for cholesteatoma in children with well-pneumatized mastoids. The incidence of recurrence varies among observers. Open cavity (canal wall down) mastoidectomy mandates removal of the external auditory canal, thereby allowing communication between the mastoid cavity an$ tKe external auditory canal (Fig. 43). Resection of the scutum results in exposure of the attic. More detailed inspection of the ossicular chain requires careful dissection (skeletonization) of the mastoid segment of the facial nerve canal. Recurrence of cholesteatoma is much less common with this greater degree of exposure. Radical mastoidectomy is necessary for holotympanic disease. The external auditory canal and mastoid air cells are removed and most of the ossicular chain with the exception of the stapes is also sacrificed. The examination of the mastoidectomy cavity is best performed with CT due to the striking contrast between residual or recurrent debris and the air-containing cavity. CT also provides important information regarding the margins of the mastoid cavity as well as the current status of the inner ear. CT is usually unable to distinguish recurrent cholesteatoma from granulation tissue in the mastoidectomy cavity. Fortunately, the type of debris within the cavity matters less than the extent and careful CT evaluation reveals this information easily. MR imaging may provide more histopathologic information than CT because granulation tissue,

Figure 43. Mastoidectomy, open cavity. Coronal CT image. The mastoid air cells and external auditory canal have been removed. The ossicular chain in this case has been preserved (arrows). Also referred to as modified radical mastoidectomy. (From Swartz JD, Harnsberger HR (eds): Imaging of the Temporal Bone, ed. 3. Thieme, 1998; with permission.)

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common in the mastoid bowl, enhances intensely with contrast but recurrent or residual cholesteatoma does not. As in the unoperated middle ear cavity, cholesterol granuloma may occur that is of bright signal on all spin echo MR image pulse sequences due to its hemorrhagic nature. Within the mastoid bowl this is commonly referred to as chocolate CT examination of the margins of the mastoidectomy cavity, especially the tegmen tympani and roof of the middle ear, is of extreme importance. A soft tissue mass identified within the mastoid bowl immediately adjacent to such a defect may represent meningocele or meningoencephalocele.3” MR imaging confirms contiguity of the mass with adjacent brain. Such information is obviously crucial prior to the reoperation. On occasion, meningocele and meningoencephalocele may be congenital, posttraumatic, or occur in the context of chronic otitis in the absence of surgical history (Fig. 44).* Often, past medical records are unavailable to the treating otologic surgeon and otoscopic inspection of the cavity may provide suboptimal information regarding the type and extent of the previous surgery. Properly performed high-resolution CT provides this information in an exquisite fashion. The surgeon requests knowledge regarding the residual ossicular chain. The status of the stapes is of particular importance and overlapping thin section images obtained through the oval window usually allow for identification of this structure. The surgeon also wants to know if previous ossiculoplasty has been attempted. If bone has been aggressively removed such that further intervention might compromise the facial nerve canal, particularly the mastoid segment, the surgeon must be warned. This evaluation may have medicolegal implications as well if facial palsy is present. Rarely, the central portion of a large middle ear cholesteatoma may drain externally, leaving only the aggressive peripheral membrane. This may remodel the middle ear and mastoid to such a degree that it becomes indistinguishable from a postoperative cavity (automastoidectomy)(Fig. 45). Numerous varieties of ossicular reconstruction have been developed over the past few decades. Of these, prosthetic stapedectomy is by far the most commonly encountered. This procedure involves complete removal of the stapes with attachment of the prosthesis to the distal incus and insertion into the oval window. Many different devices are in use, each of which have a characteristic CT a p ~ e a r a n c eThe . ~ ~observer should be aware that a recent study revealed that virtually all of these prostheses are nonferromagnetic and therefore MR imaging is generally considered safe for these patients?O The exception may be those patients who underwent surgery many years before with a device that was not tested in this study. These prostheses vary greatly in bulk. On one end of the spectrum are the stainless steel piston devices,

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Figure 44. Postoperative rneningoencephalocele. A, Magnified coronal CT image, right ear. The patient has undergone canalwall-up (intact canal wall) rnastoidectomy. There is dehiscence of the tegmen (straight arrows). Curved arrow demonstrates bony margin of external auditory canal. B, Coronal T2-weighted image. There is downward herniation of the right temporal lobe (arrow) via the tegmen defect identified in A. These findings are characteristic of a postoperative encephalocele.

Figure 45. Automastoidectomy. Magnified coronal CT image of the left ear. Expansion of the middle ear and mastoid with some residual bulky cholesteatorna within the attic. There has been surgery.

THE TEMPORAL BONE

Figure 46. Wire stapes prosthesis. Magnified axial CT image of the right ear. Normal appearance of wire stapes prosthesis inserting into anterior portion of oval window (arrow).

which are readily identified on both axial and coronal CT sections. At the other end of the spectrum is the 35-gauge wire, which is particularly difficult to image in the coronal and axial planes; often only the tip of the prosthesis can be seen (Fig. 46). Importantly, the prosthesis need not be centrally located within the oval window to be effective. Audiometric information in this regard is far more valuable than CT information. Although the anteroposterior location of the prosthesis is not of great importance, identification of the tip of the prosthesis superficial to the oval window membrane should be viewed with great suspicion because this may imply graft lateralization (Fig. 47). Numerous complications of this surgical procedure have been reported including recurrent conductive deficit, vertigo, and sensorineural hearing

847

loss?l Recurrent conductive hearing loss may result from incus necrosis (at the attachment of the prosthesis); prosthesis subluxation; granuloma development; or postoperative tympanic fibrosis or regrowth of otosclerosis. We have identified a few cases of malleoincudal dislocation secondary to regrowth of otosclerotic bone within the oval window. This is easy to diagnose by CT criteria and is likely secondary to torsional stresses. CT study is useful. Postprocedural vertigo may result from a poorly positioned prosthesis, postoperative labyrinthitis, or perilymphatic fistula. Prolapse of the prosthesis into the vestibule is commot~Iyassociated with this type of symptomatology. Such a subluxation is ordinarily well appreciated in both axial and coronal CT planes. Labyrinthitis may result in disabling symptomatology. Imaging manifestations are identical to the primary variety and consist of enhancement of the membranous labyrinth with gadolinium on T1weighted images during the subacute stage and labyrinthine ossification at a later stage.58This disease process is discussed elsewhere in this article. Perilymphatic fistula is a very serious potential complication of prosthetic stapedectomy. In this circumstance there is an abnormal communication between the inner ear (vestibule) and the middle ear. This may result from contraction of aging fibrous tissue or barotrauma. The hearing loss may be mixed or predominantly sensorineural. Vertigo and dizziness are associated. Symptoms typically fluctuate. CT diagnosis is difficult. An air-fluid level (perilymph) may be present within the middle ear depending upon the pressure dynamics within the vestibule. Such a fluid level is obviously nonspecific, although it should be viewed with suspicion in the appropriate clinical circumstance. Alternatively, a pneumolabyrinth may result (Fig. 48).”

Figure 47. Malpositioned stapes prosthesis. A, Stapes prosthesis, grafl lateralization. Magnified axial CT image of the right ear. Patient with recurrent conductive deficit demonstrates an abnormal location of the tip of the prosthesis (arrow) superficial to the oval window membrane (small arrowhead). 6, Stapes prosthesis, prolapse in vestibule. Coronal CT, lefl ear. Tip of metallic prosthesis (arrow) is identified within the vestibule.

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Figure 48. Perilymphatic fistula. Magnified axial CT image of the right ear. Air within the vestibule (arrow). There is a history of trauma but no fracture is seen. At surgery an oval window defect was identified and repaired. (From JD, Harnsberger HR (eds): Imaging Of the Temporal Bone, ed. 3. Thieme, 1998; with permission.)

Figure 49. X-linked mixed hearing loss, Magnified axial CT image of the right ear, Abnormal cochlear apex with contiguity of the fundus of the internal auditory canal with a deformed (arrows,, These patients are predisposed to the perilymphatic gusher. (From Tang A, Parnes LS: X-linked progressive mixed hearing loss: Computed tomography findings. Ann Otol Rhino1 Laryngol 193:655, 1994; with permission.)

Figure 50. Incus interposition. A, Magnified axial CT image of the left ear. Normal position of the malleus head (arrow).The incus body is absent. 5, Magnified axial CT image of the left ear, more inferior. There is an incus interposition graft (arrow) in normal position. C, Magnified coronal CT image of the left ear. The graft is demonstrated. Note that this could be mistaken for a traumatic incus dislocation.

THE TEMPORAL BONE

Although fortunately rare, the otologic surgeon may encounter profuse perilymph flow upon stapes manipulation. This is referred to as perilymphatic gusher. Historically, this was associated with anomalous size of the cochlear aqueduct, but this concept has fallen into disrepute in recent years.35 In this context, congenital x-linked hearing loss must be considered. CT examination of these patients demonstrates deformity of the IAC fundus indicating direct transmission of CSF pressure into the inner ear.93Such a deformity is highly predictive of gusher (Fig. 49). These patients have mixed (both sensorineural and conductive) hearing deficit. Long-standing middle ear inflammatory disease with or without cholesteatoma may result in extensive compromise or destruction of the ossicular chain. Ossiculoplasty is often performed in this circumstance. Provided that the majority of the incus is preserved in an individual with an undamaged stapes, the incus may be disarticulated from the malleus and surgically altered by drilling a depression in the undersurface of the body in such a way that the stapes capitulum fits into this depression?', 95 The modified and repositioned incus transmits sound directly from the manubrium of the malleus to the stapes. This is referred to as incus interposition (Fig. 50). Because the incus is entirely removed from the attic and placed within the middle ear cavity, findings in incus interposition may be identical to posttraumatic incus dislocation and result in considerable confusion in those who do not have this surgical history. Total ossicular replacement prosthesis (TOW) and partial ossicular replacement prosthesis (POW) all have been developed and used regu-

Figure 51. Normal appearance and alignment of Black TORP. Magnified coronal CT image of the right ear. The head of the prosthesis (curved arrow) and its shaft (straight white arrow) are aligned toward the oval window. There was recurrent conductive hearing deficit because of the soft tissue mass (black arrow) in the oval window niche. The patient has undergone previous canal-walldown mastoidectorny (M).

849

Figure 52. Slight anterior subluxation of TORP. Magnified axial CT image of the right ear. Goldenberg TORP (curved arrow) with slight anterior subluxation of the shaft (arrowhead) on the oval window (black arrow).

larly by experienced surgeon^.^' The TOW conducts sound directly from the newly formed tympanic membrane to the oval window (Fig. 51). The PORP is placed between the tympanic membrane and the intact stapes capitulum. The appearance of the Black and Goldenberg prosthesis is well described. The Black TORP has a semicircular head and a radiopaque polyethylene shaft. The lateral end of the shaft fits into the central core of the prosthesis; the medial end abuts the oval window. The Black POW has a similar head, which is difficult to distinguish from the TOW. The POW has a hollow shaft, which appears to be radiolucent and attaches to the capitulum of the stapes. The alignment of the process should be toward the

Figure 53. Subluxation of PORP. Magnified coronal image of the right ear (at level of vestibule). Inferior subluxation of the Black PORP (curved arrow). Note that the central canal of the PORP does not surround the stapes, thereby leaving the stapes uncovered (straight arrow) within the oval window niche.

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Figure 54. Postoperative, acoustic tumor removal. Plaque-like enhancement (arrows) lines the internal auditory canal. This is an expected appearance and does not necessarily reflect residual tumor.

oval window. The Goldenberg TORP/PORP has a different imaging appearance; however, the basic concepts regarding positioning are similar to those of the Black prosthesis. The head of the Goldenberg device is composed of hydroxylapatite; the shaft is a homogeneous material consisting of 40% hydroxylapatite and 60% p ~ l y e t h y l e n e .Because ~~ these grafts are obliquely placed, visualization in a single CT section is often not possible and overlapping sections are usually required. Frank dislocations are usually easy to diagnose, particularly those involving the TOW (Figs. 52 and 53). Fibrous adhesions are also well visualized with CT but may paradoxically impede identification of the device itself. The collaboration of the imaging specialist and the surgical team is required in the choice of a definitive surgical procedure when an eighth nerve tumor is diagnosed. Three major approaches are currently used: (1) translabyrinthine (TL), (2) retrosigmoid (RS), and (3) middle fossa (MF).= Factors influencing the decision include the prognosis for hearing conservation; depth of tumor extension into the IAC (fundal involvement); and the presence and size of a CPA component. If there is no serviceable hearing or the prognosis for acceptable hearing is dim, the TL approach is most desirable because surgical morbidity is greatly reduced due to a much lower incidence of headache and postoperative CSF leak. The TL approach is also required when an intralabyrinthine component is present. Despite common misconception, the surgical exposure necessary for removal of large CPA components can be accomplished with the TL approach in experienced hands. Hearing conservation requires either the MF or Rs approach. In this context, the imaging findings become especially crucial. The presence of a fundal component precludes the RS technique because a portion of the labyrinth has to be removed in order to surgically expose the lateral third of the IAC. There are strong indications that fast spin echo MR imaging sequences utilizing T2-weighting are more sensitive to the presence of fundal tumor because bright

CSF provides better contrast than dark CSF visualized on gadolinium-enhanced MR imaging (T1 ~ e i g h t i n g ) ?The ~ presence of a significant CPA component greater than 1.5 cm precludes an MF technique. The MF technique affords the greatest likelihood of hearing conservation but does so at the expense of the need for greater facial nerve manipulation because the anterosuperior location of the nerve within the IAC and CPA places the nerve between the surgeon and the tumor.34This is especially true when the neoplasm arises from the inferior instead of the superior vestibular nerve. The likelihood of accomplishing satisfactory hearing conservation is also reduced when the inferior vestibular nerve is the source of the tumor due to its proximity to the cochlear nerve. Inferior vestibular nerve tumors are also in closer proximity to the internal auditory artery. Compromise of this vessel is an important contributor to postoperative hearing dysfunction. Evaluation of the postoperative CPA and IAC is often a substantial challenge. Thin, linear enhancement within and immediately adjacent to the IAC is generally considered a normal postoperative finding presumably reflecting dural (pachymeningeal) irritation (Fig. 54). This may persist over many months.64,lol Follow-up examination at a specified interval is recommended under these circumstances. The TL approach may present a special problem with respect to postoperative evaluation because these surgical defects are commonly packed with fat. T1 hyperintensity of this fat may be impossible to distinguish from enhancing debris and tumor unless fat suppression or a precontrast T1-weighted sequence is used. References 1. Adetokunboh-Ayeni S, Ohata K, Tanaka K, Hakuba A. The Microsurgical anatomy of the jugular foramen. J Neurosurg 83:903-909, 1995 2. Allen RW, Harnsberger HR, Shelton C, et a1 Low cost high resolution fast spin echo MR of acoustic

THE TEMPORAL BONE schwannoma: An alternative to enhanced conventional spin echo MR. AJNR Am J Neuroradiol 171205-1210, 1996 3. Anderson LE, Laskoff J M Ramsay-Hunt syndrome mimicking intracanalicular acoustic neuroma on contrast-enhanced MR. AJNR Am J Neuroradiol 11:409410, 1990 4. Armington WH, Hamsberger HR, Smoker WRK, Osbome AG: Normal and diseased acoustic pathway: Evaluation with MR imaging. Radiology 167506-515, 1998 5. Balle VH, Greisen 0: Neurilemmomas of the facial nerve presenting as parotid tumors. Ann Otol Rhino1 Laryngol93:70-72, 1984 6. Bemardi B, Zimmerman RA, Savino PJ, Adler C: Magnetic resonance tomographic angiography investigation of hemifacial spasm. Neuroradiology 35:606-611, 1993 7. Bourgouin PM, Tampieri D, Melancon D, et al: Superficial siderosis of the brain following unexplained subarachnoid hemorrhage: MRI diagnosis and clinical significance in neurology. Neuroradiology 34407-410, 1992 8. Bowes AK, Wiet PJ, Monsell SM, et al: Brain hemiation and space-occupying lesions eroding the tegmen tympani. Laryngoscope 971172-1175,1987 9. Brodie HA, Thompson TC: Management of complications from 820 temporal bone fractures. Am J Otol 18:188-197, 1997 10. Burke JW, Podrasky AE, Bradley WG: Meninges: Benign postoperative enhancement on MR images. Radiology 174:99-102, 1990 11. Casselm-& JW, Kuhweid ER, Ampe W, et al: Pathology of the membranous labyrinth Comparison of T1 and T2 weighted and gadolinium-enhanced spinecho and 3D FT-CISS imaging. AJNR Am J Neuroradiol 14:5949, 1993 12. Casselman TW, Offeciers EF. Govaerts PI. et al: Aplasia a n d hypoplasia of the vestibulocochlear nerve: Diagnosis with MR imaging. Radiology 202:773-781, 1997 13. Cinnamon J, Sharma M, Gray D, et al: Neuroimaging of meningeal disease. Semin Ultrasound CT MR 15:46&498, 1994 14. Cognard C, Gobany P, Pierot L, et al: Cerebro-dural arterial venous fistulous: Clinical and angiographic correlation with a revised classification of venous drainage. Neuroradiology 194:671-680, 1995 15. Curtin HD, Jensen JE, Barnes L, May M. “Ossifying” hemangiomas of the temporal bone: Evaluation with CT radiology 164:831-835, 1987 16. Curtin HD, Wolfe EP, Synderman N: Facial nerve between the stvlomastoid foramen and the parotid: Computed tomographic imaging. Radiology 149:6568, 1983 17. Dahlen RT, Hamsberger HR, Gray SD, et al: Overlapping thin-section fast spin-echo MR of the large vestibular aqueduct syndrome. AJNR Am J Neuroradiol 18:67-75, 1987 18. DiChiro DJ, Fisher RL, Nelson KB: The jugular foramen. J Neurosurg 21:447460, 1964 19. Dietz RR, David WL, Hamsberger HR, et al: MR imaging and MR angiography in the evaluation of pulsatile tinnitus, AJNR Am J Neuroradiol 15:879889, 1994 20. Donnelly MJ, Cass AD, Ryan L: False-positive MRI in the diagnosis of small intracanalicular vestibular schwannomas. J Laryngol Otol986-988,1994 21. Elster AD, DiPersio DA: Cranial postoperative site:

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Address reprint requests to Joel D. Swartz, MD Department of Radiology The Germantown Hospital and Medical Center One Perm Boulevard Philadelphia, PA 19144