MR Imaging of the Temporal Bone

Magnetic resonance (MR) imaging is a useful modality for imaging soft tissue lesions of the temporal bone. This article provides an overview of the principles of MR imaging and details of its use for diagnosis in the practice of otology. ^{[1, 2, 3, 4]}

The physics of MR imaging are beyond the scope of this article, but a brief overview is provided. For further detail, see the reference section.

The production of an MR image begins with the alignment of tissue hydrogen nuclei by an external strong magnetic field. Hydrogen nuclei are targeted because they are abundant in tissue and produce a strong signal. Radiofrequency (RF) pulses are directed at the tissues, which excite the nuclei. As these nuclei relax, they emit RF signals. Delineation of tissue types and tissue boundaries is produced by differences in relaxation rates by different tissues. Further localization of the signal is determined by manipulation of the magnetic field. Emitted signals are measured at varying times after the initial RF pulse. A computer algorithm is used to reconstruct the data into 2-dimensional images.

The contrast in the MR signal has 2 primary determinants: spin density and relaxation. Density simply refers to the number of hydrogen nuclei per unit volume. Relaxation refers to the process and timing of the return of the excited nuclei to the unexcited state.

Two types of relaxation times exist: longitudinal, or spin lattice, called T1 and transverse, called T2. Each describes the way the nuclei return to the pre-excited state in the longitudinal and transverse planes. T1 relaxation refers to the time for the nuclei's spins to return to an equilibrium state. T2 relaxation refers to the time for loss of transverse magnetization that is affected by local field inhomogeneity. Differences in relaxation properties among tissues cause contrast between these tissues.

Fat and fluid have a high proton density and therefore can produce a bright signal depending on the repetition time (TR) and echo time (TE) lengths. Air and cortical bone have low proton density; they appear as dark signal with all parameters. Vessels containing flowing blood emit no or low signal because the excited protons move out of the area before detection of the emitted signal.

Various techniques can be used in MR imaging of temporal bone area lesions. Screening unenhanced T2-weighted fast spin-echo (FSE) MR imaging provides inexpensive high-resolution images. Recently, a newer modification of this technique, T2-weighted 3D FSE, has demonstrated clinical utility.

In the early 1990s, 3-dimensional constructive interference in steady state (3D CISS) was introduced for imaging temporal bone pathology. This method offers high resolution and contrast and can be extremely useful in showing small structures surrounded by fluid, including detailed delineation of the seventh-eighth nerve complex in the temporal bone as well as the membranous labyrinth. 3D CISS and 3D FSE offer submillimeter slices, which provide much higher resolution than conventional techniques and have become widely available.

Fat saturation is commonly used in MR imaging to suppress signal from adipose tissue. In temporal bone imaging, it is often used to remove high-signal fat found in the inner ear region. Fat suppression can also be used to detect adipose tissue (for example, in the diagnosis of lipomas). Two properties make this possible. Protons in lipid and water show a small difference in resonance frequency. Additionally, a difference in T1 exists between adipose and water. Fat suppression can be accomplished through several techniques, but only the techniques using the resonant frequency distinguish between fat and other tissues with a short T1 relaxation time. ^[5]

Lastly, intravenous contrast agents can be used to enhance MR imaging. Most commonly, gadolinium, a paramagnetic element, is used in conjunction with a chelator. Gadolinium increases the signal by shortening the T1 of tissues and thereby increasing signal on T1-weighted scans. Enhancement is commonly observed in neoplasms, infections, and inflammatory conditions.

The ideal MR scan would be short in duration and nonstrenuous for the patient and technician. It would provide images that demonstrate high spatial resolution, minimal noise, and consistent signal intensities throughout the scan. To these ends, hardware such as phased array and surface coils have been developed. More powerful scanners are coming into use, with current literature focusing on differences between 6- and 3-T machines. Three-dimensional Fourier transform imaging increases signal-to-noise ratios. Open MRI technology can make the examination tolerable for the claustrophobe and has been used intraoperatively to guide transsphenoidal approaches to the petrous apex.

MR imaging does not delineate bony anatomy as well as CT scanning; therefore, the complex anatomy of the temporal bone is discussed extensively in other sections of this journal. MR imaging is most useful in delineating soft tissue relationships in this area.

Cerebellopontine angle tumors represent up to 10% of intracranial tumors. Vestibular schwannomas (VS) account for approximately 75% of these lesions, with the remainder including meningiomas, arachnoid cysts, epidermoids, lipomas, metastatic tumors, or vascular lesions.

The cerebellopontine angle is a region of the subarachnoid space in the posterior fossa. Its borders include anteriorly, the posterior surface of the temporal bone; posteriorly, the cerebellum; medially, the brain stem; and inferiorly, the cerebellar tonsil. The trigeminal nerve courses above the space, while the glossopharyngeal, vagus, and hypoglossal nerves travel below it. The vestibular, cochlear, and facial nerves travel through this space in a superoanterolateral course from the brain stem toward the temporal bone, where they enter the internal auditory canal. Remaining structures in the space include the flocculus, the foramen of Luschka, and the anterior inferior cerebellar artery. The vertical diameter of the internal auditory canal may vary from 2-12 mm, with a mean diameter of 5 mm; usually, variance is less than a 1 mm from side to side. The length of the internal auditory canal ranges from 4-15 mm, with a mean of 8 mm; this measurement usually varies less than 2 mm from side to side.

Advances in MR techniques now allow diagnosis of tumors as small as 2 mm in size. Generally, images are obtained in coronal and axial format as well as pre- and post-gadolinium infusion. MR is used with both T1- and T2-weighted imaging for cerebellopontine angle tumors. ^[6] Pre-infusion images can be used to diagnose lesions such as lipomas, which are bright on both T1 and T2 weighting. Gadolinium must always be used to evaluate for tumor enhancement.

Screening protocols exist using surface magnetic coils and special algorithms with a technique called T2-weighted T2 FSE. T2-weighted FSE does not require contrast, reducing its cost. Also, these limited scans focus on the internal auditory canal and brain stem, which shortens imaging time and cost. Using this technique, the tumor is hypointense relative to CSF.

Key information to be gleaned from the MR image includes tumor size and extension into the cerebellopontine angle. These and the audiometric findings are used to determine the approach for surgical removal. Less morbidity from resection is associated with size less than 2 cm, intracanicular lesions confined to posterior superior quadrant of the internal auditory canal, and lesions without extension to the internal auditory canal fundus.

Vestibular schwannomas

VS typically arise from the eighth cranial nerve at the entrance to the internal auditory canal. These tumors are usually unilateral, and the 5% of VS that are bilateral are associated with neurofibromatosis type II. Patients with these tumors usually present in middle age with progressive unilateral sensorineural hearing loss. Symptoms are variable, although they usually correlate with the size of the tumor. As the tumor enlarges, it can compress other adjacent structures such as the cerebellum and trigeminal nerve. Optimal treatment is resection, although consideration must be made of hearing status, rate of growth, location and size, age, and symptoms.

Typical findings of VS on MR imaging include a pear-shaped mass centered on the internal auditory canal. Small tumors (up to 1.5 cm) appear as tubular masses in the internal auditory canal. Medium-sized tumors (3 cm) resemble ice cream cones, with the "ice cream" in the cistern and the "cone" in the internal auditory canal. Some tumors are entirely within the cerebellopontine angle with no extension into the internal auditory canal. These findings can be used to distinguish VS from other cerebellopontine angle tumors such as meningiomas, which usually show more flattening against the dura. VS appear isointense on T1-weighted images and hyperintense on T2-weighted images. These tumors enhance intensely with contrast. The internal auditory canal may be eroded and enlarged by tumor expansion. The tumor may have nonhomogenous areas with central necrosis. Cystic changes are noted in 10-15% of cases, and 5% have associated arachnoid cysts.

Following resection of vestibular schwannomas, MRI scans often show linear enhancement in the internal auditory canal. This does not suggest residual tumor, but nodular enhancement or progressive enhancements in serial scans indicates residual disease.

Meningiomas

Meningiomas derive from the arachnoid villi and usually present in patients aged 30-60 years. Clinical symptoms depend on the precise location, although compression on cranial nerves leads to presentation. These symptoms may overlap significantly with those of VS. These tumors are benign and locally invasive. Treatment is surgical removal, with radiation for residual tumor.

Cerebellopontine angle meningiomas typically appear as sessile masses, which are broadly based against a dural surface. They are not typically centered on the internal auditory canal, nor do they cause widening or erosion of it. A characteristic, although not diagnostic, feature of these masses is an extension of the dura, or a dural tail, present in up to 75% of cases. On T2-weighted scans, the tumors appear less bright than VS. They may be isointense because of cellularity or even hypointense because of calcification. They are typically hemispherical with obtuse angles and form half-moon or mushroom shapes. Signal is variable but usually appears isointense to parenchyma on T1, although hyperintense or hypointense signal can be observed. Twenty-five percent of tumors show a calcified internal matrix, which appears as small focal signal voids. A thin hypointense covering is usually observed, which represents a thin layer of CSF called the CSF cleft. They enhance brightly withgadolinium. ^{[7, 8]}

Glomus tumors

People with glomus tumors most commonly present with symptoms of pulsatile tinnitus and hearing loss and a middle ear mass on examination. ^[9] Also called paragangliomas, these tumors arise from neural crest elements near the nerves along the promontory, the jugular foramen, and the carotid sheath. Almost 95% benign, these tumors are usually solitary, although in hereditary cases they can be multifocal. Two anatomic classifications exist to describe these tumors in the ear: Fisch (by extension) and Glasscock-Jackson (by category, ie, tympanicum or jugulare, and by extension). Treatment is palliative (typically radiation) or curative (resection).

The extent of tumor and anatomic location are initially defined using CT scanning. Glomus tympanicum refers to tumors that arise on the cochlear promontory and medial wall of the middle ear. Glomus jugulare arises near the superolateral jugular foramen and extends into the hypotympanum and the mesotympanum. An intact jugular bulb plate defines a glomus tympanicum.

MR imaging is used to assess intracranial extension and anatomic relationships with neural and vascular structures. T1-weighted images with and without contrast in axial and coronal planes show tumor extent effectively; small tumors are often hyperintense. A classic salt and pepper appearance on unenhanced T1-weighted scans can be observed, with hyperintensity representing small tumor hemorrhages and signal voids representing feeding vessels.

Postcontrast fat saturated T1-weighted imaging is useful to better define tumors from surrounding marrow and soft tissue fat. Fat saturation techniques used with contrast aids in the differentiation between recurrence and postsurgical change.

Because these tumors are highly vascular, flow voids may be visible within the tumor mass. Angiography can also be a complementary imaging study to assess tumor blood supply, to evaluate collateral circulation, and to embolize preoperatively. MR venography is an adjunctive examination to measure jugular vein invasion, occlusion, and collateral venous sinus drainage.

Epidermoids

Epidermoids are congenital tumors arising from ectodermal inclusions. Young adults present with symptoms due to compression of nearby structures. Histologically these tumors are identical to acquired cholesteatomas. They contain keratin debris and cholesterol crystals. Indeed, these tumors are also called congenital cholesteatomas, epidermoid cysts, or epidermoid inclusion cysts.

Epidermoids can develop in a number of sites in the ear (ie, the tympanic cavity, the petrous apex, near the geniculate ganglion), although the cerebellopontine angle is the most common location. Cerebellopontine angle epidermoids cause trigeminal nerve and facial nerve symptoms when they enlarge symptomatically. Epidermoids account for about 9% of cerebellopontine angle tumors. Tumors usually fill the cistern. Treatment is surgical, with the approach determined by tumor extent and hearing status.

Diagnosis is confirmed by demonstration of nonenhancing lesions with irregular margins. The shape of epidermoids typically conforms to the surrounding structures. Their appearance is dependent on the composition of the tumor (amounts of keratin/cholesterol). Commonly, keratin predominates, with lesions appearing hypointense on T1 scans and hyperintense on T2 scans. They may appear similar to CSF. High-resolution imaging may demonstrate septa, which distinguishes them from CSF-filled arachnoid cysts. These tumors do not enhance, distinguishing them from schwannomas and meningiomas.

Arachnoid cysts

Arachnoid cysts develop from congenital splits in the arachnoid membrane, which allow CSF to collect in between. Again, patients present after these lesions cause compression on local nervous structures or elevation of intracranial pressure. They are associated with hearing loss only when they are near the vestibulocochlear nerve. The size of the cyst can determine the likelihood of expansion; smaller tumors usually remain stable. Treatment is controversial.

Findings on MR imaging include smooth margins with well-defined round borders that do not enhance and a homogenous signal intensity that is similar to CSF on all sequences (ie, hypointense on T1-weighted images, hyperintense on T2-weighted images). Arachnoid cysts can be distinguished from epidermoids by several other findings. First, these lesions displace surrounding structures, whereas epidermoids encompass and surround them. Second, arachnoid cysts can cause scalloping of the bony table. Third, arachnoid cysts do not have multiple septae.

Dural sinus thrombosis

Thrombosis of the dural sinuses, particularly the transverse and sigmoid sinuses, can be imaged by MRI and MR venography (MRV). Trauma, coagulopathies, systemic inflammatory diseases, hormonal changes, otitic infection, and other sources cause these thromboses. They can progress to neurologic deficit and death. T1-weighted MRI shows hyperintense signal in the sinus. T1-weighted MRI with contrast shows the empty delta sign—enhancement of the dural leaves without signal in the sinus itself. MRV shows absent filling of the sinus.

Other lesions of the cerebellopontine angle

Several other rare lesions may affect the cerebellopontine angle, for example, facial nerve schwannomas, vascular tumors, and lipomas. See the Reference section for descriptions of these tumors.

Lesions of the membranous labyrinth are rare and may cause vertigo, tinnitus, and profound sensorineural hearing loss. MR imaging has recently emerged as a useful imaging modality for diagnosis of membranous labyrinth pathology. Specialized techniques such as 3D FSE and 3D CISS allow for accurate imaging of these delicate structures. Enhanced T1-weighted sequences are also useful in the diagnosis of these lesions.

Labyrinthitis

Labyrinthitis is inflammation of the membranous labyrinth. Associated symptoms include sensorineural hearing loss and vestibular dysfunction. This process has a variety of causes, including infection (the most common), trauma (causing fistulae), or autoimmune disease. Treatment is supportive, with use of antibiotics and antivirals for clear infectious cases.

Cases of labyrinthitis can show faint enhancement of the fluid-filled spaces of the membranous labyrinth on contrast T1-weighted images. This is thought to be due to gadolinium accumulation due to inflammation-induced vessel wall breakdown.

Labyrinthitis appears in several portions of the labyrinth (ie, cochlea, vestibule, semicircular canals). This enhancement subsides after time, typically 6 months or more.

Endolymphatic sac tumors

Tumors of the endolymphatic sac are rare, low-grade, locally invasive masses. They arise from papillary cystadenomatous tissue and can cause hearing loss, tinnitus, fleeting vertigo, or facial nerve dysfunction. Late diagnosis may lead to intracranial involvement. These tumors can be associated (both unilateral and bilateral) with von Hippel-Landau disease, an autosomal dominant disorder. Treatment is excision with radiation therapy for residual tumor.

CT and MR imaging are both used in the diagnosis of these tumors. MR imaging reveals heterogeneity of signal intensity on both T1- and T2-weighted images and with contrast.

A recent series of these tumors revealed that most of these tumors have areas of increased signal on T1-weighted images, which corresponds to subacute hemorrhage in these highly vascular lesions. A sizable fraction of these tumors, though, were isointense to white matter. T1- and T2-weighted images with and without contrast are used to characterize tumors of the membranous labyrinth. Three-dimensional Fourier transformation constructive interference in steady state (3DFT-CISS) images provides high spatial and contrast resolution to detect these and other small lesions in this area. These include fistulas or masses (eg, granulomas) in the labyrinthine space and malformations and congential defects of the membranous and bony labyrinth (eg, large endolymphatic sac, abnormalities of the lateral canal).

Intralabyrinthine schwannomas

MR imaging is useful to distinguish inflammatory conditions and tumor in the labyrinth. The enhancement of tumors persists. Tumors occupy only a portion of the labyrinth and have sharp borders. They also displace endolymphatic fluid. Comparison with unenhanced images eliminates the possibility of spontaneous hyperintensity.

Similar to the more common schwannomas of the cerebellopontine angle, these tumors derive from Schwann cells of the eighth nerve. They are similar histopathologically to VS and also cause symptoms by causing pressure on the eighth nerve complex or by disrupting cochlear or vestibular labyrinthine function.

Intralabyrinthine schwannomas appear most clearly on T1-weighted images as signal with intermediate intensity and enhance well with contrast. Their morphology is clearly defined inside any portion of the otic capsule.

Congenital abnormalities

A wide spectrum of developmental and other congenital abnormalities can affect the temporal bone area. A full discussion of these lesions is beyond the scope of this article. These lesions can be divided categorically into those that affect the outer ear (eg, aural atresia, bony abnormalities), the middle ear (eg, ossicular abnormalities, facial nerve anomalies), and the inner ear (eg, cochlear or canal aplasia/dysplasia, membranous labyrinth defects). For many abnormalities, CT scanning is an initial choice in imaging modalities because of its excellence in delineating bony anatomy.

MR imaging is being increasingly used in the diagnostic workup of children with congenital hearing disorders. This information can be important in diagnosing lesions affecting hearing in these locations.

• A common cause of sensorineural hearing loss in infancy and childhood is the large vestibular aqueduct (LVA) syndrome. LVA can occur with other malformations. CT imaging demonstrates a vestibular aqueduct that is greater than 1.5 mm at the midway between the common crus and the external aperture. An associated finding is observed on T2-weighted FSE: a dilated endolymphatic sac and duct of variable size. Notably, a large endolymphatic sac/duct can cause sensorineural hearing loss without a large vestibular aqueduct. Here, one sees that MR imaging can provide complimentary information on soft tissue structures in workup of congenital lesions.

• Dysplasia of the membranous labyrinth can be delineated using high-resolution MR imaging. Varying labyrinth abnormalities can be observed in Waardenburg syndrome and branchiootorenal syndrome, as well as other syndromes. Hearing with these syndromes typically varies with the extent of the membranous abnormality. For example, lateral squamous cell carcinoma dysplasia, an asymptomatic condition, is a common incidental finding on MR imaging.

In summary, advanced MR techniques can provide high-resolution images of such areas as the membranous labyrinth and nervous structures of the temporal bone.

Cholesterol granuloma

Cholesterol granuloma is thought to arise in the mastoid air cells as a result of inadequate ventilation in the setting of otitis media and eustachian tube dysfunction. Hemorrhage in these cells without the ability to drain leads to an inflammatory response and erosion. Cholesterol granuloma contains brownish fluid with cholesterol crystals, which cause an inflammatory response and giant cell reaction. These lesions may be present in the middle ear cleft as well as within the mastoid air cells and petrous apex. Treatment is surgical.

Several additional terms describe these lesions. A cholesterol granuloma at the petrous apex is also called a giant cholesterol cyst. A cholesterol granuloma present in the mastoid cavity is also called a chocolate cyst or blue-dome cyst.

Cholesterol granulomas are bright on all spin-echo sequences, which differentiates them from cholesteatoma. Special techniques can be used to distinguish fat present in normal marrow due to unique signaling properties. A ring of low signal intensity may confirm the presence of hemosiderin-laden macrophages. The center may produce a cystic appearance consistent with lipid and cholesterol crystals.

Cholesteatoma

The pathology of cholesteatoma is poorly understood. Cholesteatoma can deposit keratinizing squamous epithelium in the middle ear, epitympanum, mastoid air cells, and even petrous apex. It causes an inflammatory response associated with bony erosion. Concomitant infection with Pseudomonas aeruginosa is common. Congenital cholesteatoma presents typically in children younger than 5 years and is thought to be caused by failure of involution of the epidermoid formation. Treatment of these lesions is surgical.

CT scanning is the study of choice for imaging of the temporal bone in cholesteatoma. However, MR imaging can be a complimentary modality when intracranial extension and bony defects are observed or suspected. Additionally, MR imaging is indicated for cases of facial nerve involvement and unexplained sensorineural hearing loss.

Cholesteatomas have variable signal intensities in contrast-enhanced MR imaging. Generally, signals are isointense on T1-weighted imaging and become moderately hyperintense on lengthening of the TR. Debris is usually present. Cholesteatomas do not typically enhance with gadolinium, except rarely at the edges of the lesion. MR imaging is useful to delineate intracranial complications, including abscesses, lateral sinus thrombosis, and meningitis. These lesions may occur in the middle ear, mastoid cavity, or the petrous apex.

Facial nerve lesions

The facial nerve is of paramount importance to the otolaryngologist because of its critical motor function in the head and neck. It can be traced through its 6 segments, namely the cisternal or intracranial segment, the intracanalicular segment (in the internal auditory canal), the labyrinthine segment, the tympanic or horizontal segment, the mastoid or vertical segment, and the extracranial or parotid segment. A variety of diseases may affect this nerve, including neoplastic, inflammatory, and vascular processes.

Bell palsy is the most common inflammatory disease of the facial nerve. Some evidence suggests that this disorder is caused by herpes simplex virus infection. The facial paralysis occurs secondary to compression of the facial nerve due to edema in the bony canal. Symptoms include sudden-onset facial paralysis without other findings. Treatment includes antivirals and is supportive.

MR imaging is used in cases of facial palsy to search for any pathology that may be causing symptoms. In Bell palsy, postcontrast MR imaging may reveal enhancement of the facial nerve without any evidence of the thickening observed in other inflammatory processes, such as granulomatous disease (tuberculosis or sarcoidosis). These findings, which are often more severe, can also be observed in Ramsey Hunt syndrome. Resolution of imaging findings may, although often does not, parallel the clinical course. Thickening, focal enlargement, or nodularity is a sign of tumor.

A relatively new MR technique known as magnetization-prepared rapid gradient echo (MP-RAGE) is well suited for revealing the perineural spread of tumor along the facial nerve. The technique provides thin slices, excellent T1 contrast enhancement, and fat suppression. Spreading tumor is marked by enhancing tissue that traverses a widened stylomastoid foramen.

MR imaging can be used to evaluate facial and to delineate vascular abnormalities affecting the nerve. Useful images can be obtained with T1- and T2-weighted protocols with gadolinium enhancement in the coronal, axial, and oblique sagittal planes. The nerve is traced from brain stem to parotid. Fat saturation, a technique used to remove high-signal fat from the images, is useful in following the nerve.

MR imaging provides important information for the otologist. It allows multiplanar and noninvasive visualization with excellent resolution without ionizing radiation. This article has detailed the basic principles of some of the most common areas for its use.