4 Radiological Studies for the Vestibular Patient

Imaging plays an important role in the diagnosis and treatment of patients with vestibular abnormalities. There are multiple imaging tools available for evaluation of patients with vestibular and temporal bone abnormalities. Among the imaging modalities available today, computed tomography (CT), magnetic resonance imaging (MRI), magnetic resonance angiography (MRA), magnetic resonance venography (MRV), and digital subtraction angiography (DSA) are the most commonly used. Each imaging modality has inherent strengths and weaknesses that will depend on the anatomic location and pathology studied. Often, multiple imaging modalities are complementary and add information not available from a single technique. Communication with the radiologist prior to testing ensures that the most appropriate modality is employed for the clinical situation. Certain implanted devices, such as cardiac pacemakers and cochlear implants, are relative contraindications to MRI, but some patients with certain types of these devices can be scanned safely under carefully controlled circumstances1,2 Older intracranial aneurysm clips and other metallic devices may also preclude MRI examination.

Imaging Modalities

Computed Tomography

Computed tomography is especially well suited for evaluation of the osseous structures of the middle and inner ear. Computed tomography provides excellent anatomic detail in areas where bone and air are closely apposed. A complete temporal bone CT study consists of one axial acquisition with the patient supine and an additional coronal acquisition with the patient prone and the neck extended. The advent of slip-ring technology and multidetector CT (MDCT) scanners has dramatically improved the utility of CT in vestibular disease. Advancements in detector technology have resulted in excellent multiplanar reformatting from a single axial acquisition, reducing imaging times and motion artifacts. Temporal bone imaging on current MDCT scanners uses a helical acquisition and 0.3 to 0.6 mm collimation. The data are then reformatted in the axial, coronal, and potentially other planes if needed (Fig. 4.1).

Using such a protocol, high-resolution images of both temporal bones can be obtained in 8 to 10 seconds. Small structures, such as the ossicles, cochlea, semicircular canals, facial nerve canal, and vestibular aqueduct, can be confidently identified on modern CT studies. With the current technology, structures as small as 0.4 mm can be depicted accurately. Normal CT anatomy of the temporal bone is illustrated in Fig. 4.2, Fig. 4.3, and Fig. 4.4.

Magnetic Resonance Imaging

The improved soft tissue contrast of MRI complements the bone detail of CT when imaging the temporal bone. Magnetic resonance imaging provides exquisite imaging of fluid-filled structures, fat, cranial nerves, and the central nervous system. Labyrinthine anatomy is best depicted on T2-weighted sequences, which accentuate bright signal from perilymphatic, endolymphatic, and cerebrospinal fluid. Commonly employed three-dimensional (3D) T2-weighted MR cisternogram sequences include fast spin-echo (FSE), constructive interference in the steady state (CISS), and true fast imaging with steady-state precession (FISP) (Fig. 4.5).

Each has its inherent advantages. T1-weighted sequences with gadolinium contrast agents are used for detecting enhancement due to infection, inflammation, or tumors. In certain situations, fat-saturation techniques can be useful in nulling inherent bright signal from fat or bone marrow and improving the conspicuity of abnormal contrast enhancement. Magnetic resonance imaging also provides the ability to image in any plane without repositioning the patient. By changing the scan parameters, coronal, axial, and sagittal images in any obliquity can be easily obtained.

Fig. 4.1 (a) Volumetric CT with isotropic voxels (0.6 cubic mm) permits reconstructions in any plane without loss of resolution. (b) Orthogonal coronal and sagittal reconstructions from axial acquisition demonstrate degrees of obliquity (white lines) required to produce (c) double oblique axial reconstruction profiling of the stapes in its entirety (arrow).

Fig. 4.2 Normal anatomy depicted on axial high-resolution CT. TT, tensor tympani; Ma, malleus; Co, cochlea; In, incus; PSC, posterior semicircular canal; ST, sinus tympani; KS, Koerner septum.

Fig. 4.3 Axial CT demonstrates normal anatomy. Co, basal turn cochlea; Ma, head of malleus; In, short process incus; KS, Koerner septum; HSC, horizontal semicircular canal; Ve, vestibule; IAC, internal auditory canal.

Fig. 4.4 High-resolution coronal CT demonstrates normal anatomy. CF, crista falciformis; Ve, vestibule; Co, basal turn cochlea; Sc, scutum; PS, Prussak space.

Fig. 4.5 Three Tesla (3T) magnetic resonance images demonstrate normal anatomy with (a) 3D FSE, (b) CISS, and (c) true FISP techniques. BA, basilar artery; Co, cochlea; Ve, vestibule; LSC, lateral semicircular canal; CoN, cochlear nerve; VeN, vestibular nerve.

It is imperative to optimize resolution when imaging the small structures of the temporal bone. The field of MRI is constantly changing and several recent hardware and software advances have been particularly useful. Higher-field-strength imaging systems of 3 Teslas (3T) have more recently become available for routine clinical imaging and allow higher spatial resolution than systems operating at a field strength of 1.5 T or lower. Spatial resolution can also be improved with 3D (volume) pulse sequences that provide spatial resolution of less than 1 mm.3 Bilateral surface coils placed over the ears can be used alone or in combination with a head coil to provide more signal and improved resolution when imaging the temporal bone (Fig. 4.6).⁴ Normal MRI anatomy of the temporal bone is illustrated in Fig. 4.7, Fig. 4.8, Fig. 4.9, and Fig. 4.10.

Perilymphatic opacification with gadolinium agents has recently been investigated as a means of visualizing the endolymphatic structures of the inner ear that remain unopacified on 3D FLAIR sequences. This can be achieved by transtympanic injection with unilateral transmission across the round window into the perilymphatic space or by delayed imaging (4 to 8 hours) after intravenous administration to acquire bilateral perilymphatic opacification (Fig. 4.11). This technique has been employed successfully to confirm the diagnosis of inner ear pathologies, such as endolymphatic hydrops in the setting of Meniere’s disease and vestibular migraine.^5,6

Fig. 4.6 Comparison of volume head coil only versus hybrid phased-array combination head and surface coils at 3T. (a) Volume head coil only provides inferior signal-to-noise within the labyrinth compared with (b) combination head and surface coil device.

Fig. 4.7 Three Tesla 3D T2-weighted MRI through the internal auditory canal in the oblique sagittal plane demonstrates the normal nerves. FN, facial nerve; CoN, cochlear nerve; VeN, vestibular nerve.

Fig. 4.8 Three Tesla 3D CISS MRI demonstrates exquisite anatomy of the inner ear and internal auditory canal. M, modiolus; SL, spiral lamina; Ve, vestibule; CoN, cochlear nerve.

Fig. 4.9 Oblique sagittal 3T 3D CISS MRI demonstrates the normal superior semicircular canal (arrow).

Fig. 4.10 Maximum intensity projection (MIP) magnetic resonance image demonstrates the normal inner ear anatomy. B, basal turn cochlea; M, middle turn cochlea; A, apical turn cochlea; V, vestibule; S, semicircular canal.

Fig. 4.11 REAL IR sequence obtained 4 hours after gadolinium demonstrates hypointense signal in the right labyrinth representing opacified perilymph outlining unopacified endolymph in a normal utricle (arrow).

The development of MRA techniques now allows noninvasive imaging of central nervous system (CNS) vascular lesions. Although uncommon, a vascular lesion can present as a vestibular abnormality. Common MRA techniques in use include noncontrast multiple overlapping thin slab acquisition (MOTSA), three-dimensional time-of-flight (3D TOF), and contrast-enhanced MRA (Fig. 4.12). These techniques provide a detailed depiction of the major intracranial arteries, especially near the circle of Willis. Abnormalities identified on MRA are often investigated further with conventional DSA.

Magnetic Resonance Venography

In contrast to MRA, MRV evaluates the intracranial venous structures. MRV techniques in common use include phase-contrast, two-dimensional time-of-flight (2D TOF), and gadolinium bolus imaging. Phase-contrast imaging allows for venous imaging without superimposed arterial structures, but contrast-enhanced MRV provides much higher spatial resolution (Fig. 4.13). The dural venous sinuses, large cortical veins, jugular bulbs, internal jugular veins, and deep cerebral veins are well visualized with MRV. As with MRA, abnormalities on MRV often require definitive evaluation with conventional DSA.

Fig. 4.12 (a) Multiple overlapping thin slab acquisition (MOTSA) intracranial MRA clearly depicts the major intracranial arteries on the collapsed whole-head image and (b) an anterior maximum intensity projection image of the posterior fossa. MCA, middle cerebral artery; ACA, anterior cerebral artery; PCA, posterior cerebral artery; VA, vertebral artery; BA, basilar artery; PICA, posterior inferior cerebellar artery; V, vein.

Fig. 4.13 Contrast-enhanced MRV in the lateral projection clearly depicts the venous structures. SSS, superior sagittal sinus; OS, occipital sinus; SS, sigmoid sinus; JV, jugular vein.

Fig. 4.14 Anteroposterior DSA image from a right vertebral artery injection clearly demonstrates an aneurysm at the basilar tip. The arterial lumen is seen with better resolution than with MRA techniques. BT An, basilar tip aneurysm; BA, basilar artery; SCA, superior cerebellar artery.

Digital subtraction angiography is a minimally invasive procedure that directly images the lumen of arteries and veins. Digital subtraction angiography requires the intra-arterial placement of a catheter into the vascular territory of interest and injection of iodinated contrast. Although it is a very safe procedure, it does carry a small risk of bleeding or infection at the vascular access site, as well as a risk of stroke. However, these risks are small and occur in less than 1% of cases. With the advent of modern noninvasive imaging, the initial evaluation and screening of patients with suspected vascular abnormalities is generally performed with MRI or CT. Digital subtraction angiography is now relegated to further defining abnormalities depicted on MRA and MRV studies. Digital subtraction angiography can provide additional information about vascular lesions that cannot be obtained from MRA and MRV. Specifically, the flow dynamics of vascular lesions are best depicted with DSA, which precisely delineates the arterial and venous components of a lesion. Additionally, the spatial resolution of DSA is much higher than MRA and can be important in the evaluation of vascular abnormalities (Fig. 4.14). Furthermore, certain lesions may be treated with endovascular techniques performed at the time of diagnostic cerebral angiography.

Imaging Findings for Various Pathologic Entities in the Vestibular Patient

Dizziness can be the result of a large number of different pathologies. It is important to understand the strengths and weaknesses of the different imaging modalities so the appropriate testing can be pursued based on the differential diagnosis. In general, if the problem is thought to be in the temporal bone (e.g., cholesteatoma), imaging begins with a CT scan, and if it is thought to be an intracranial process (e.g., vestibular schwannoma), MRI is the initial test ordered.

The remainder of this chapter reviews the imaging findings in various pathologic conditions. Dizziness is often not the primary symptom in many of these disorders, but all of the pathologies should be considered when a patient complains of dizziness.

Trauma

Patients with temporal bone trauma are generally imaged with high-resolution CT.⁷ Axial and coronal imaging at 1 mm thickness or less is preferred. As mentioned previously, isotropic imaging with axial and coronal reformats may supplant direct coronal imaging in trauma patients. It may be useful to delay high-resolution CT examinations until there is adequate resorption of any associated hemotympanum that could obscure traumatic ossicular injuries. Magnetic resonance imaging is generally considered only in the face of a normal high-resolution CT in patients with continued clinical symptoms. Entities like labyrinthine hemorrhage, facial nerve injury, and brainstem injury are best seen with MRI.⁸

Fig. 4.15 Axial CT demonstrates a longitudinal fracture (arrows) through the right temporal bone and middle ear cavity.

Temporal bone fractures are usually the result of blunt trauma to the temporoparietal calvaria. Longitudinal fractures extend obliquely through the long axis of the temporal bone and are the most common variety, occurring in 70 to 90% of cases (Fig. 4.15). These fractures often involve the squamous portion of the temporal bone, external auditory canal (EAC), middle ear, petrous apex, and tegmen tympani. Longitudinal fractures tend to extend around the labyrinthine structures due to their dense bony capsule. They can result in disruption of the ossicles and tympanic membrane and often present with a conductive hearing loss. Recent reports indicate that classifying temporal bone fractures as petrous (violation of otic capsule) and nonpetrous (otic capsule intact) may be more useful clinically than the traditional longitudinal, transverse, and mixed descriptions.⁹ Injury of the seventh nerve usually involves the first genu or proximal tympanic segment.¹⁰ Fractures extending through the roof of the tegmen tympani or posterior mastoid air cells can result in a cerebrospinal fluid (CSF) leak or fistula (Fig. 4.16).

Fig. 4.16 High-resolution CT coronal reformat showing a defect in the tegmen tympani (arrow) with fluid in the mastoids and middle ear.

High-resolution CT imaging after the instillation of intrathecal contrast can be used to detect such an injury. Transverse fractures of the temporal bone are less common but more severe than longitudinal fractures (Fig. 4.17). These fractures occur along the short axis of the temporal bone, extending through the foramen magnum, occipital bone, jugular fossa, petrous bone, and body of the sphenoid bone. Transverse fractures often spare the middle ear but disrupt the vestibule, vestibular nerve, cochlear nerve, and facial nerve. This can cause severe impairment, including complete loss of cochlear and vestibular function. Air within the inner ear can result from fractures through the mastoid air cells or middle ear. Extension into the carotid canal is not uncommon and can result in injury to the internal carotid artery. Facial nerve injury occurs in 50% of transverse fractures and generally involves the labyrinthine segment proximal to the geniculate ganglion.¹¹ A perilymphatic fistula can result from disruption of the ossicles with stapes mobilization or fractures extending into the footplate of the stapes or through the round or oval windows. Explosive injuries or barotrauma may result in oval or round window membrane rupture.

Fig. 4.17 Axial CT demonstrates a transverse fracture (arrow) through the left temporal bone and vestibule.

Magnetic resonance imaging should be used to evaluate traumatic CNS lesions, such as brainstem contusion or temporal lobe encephalocele (Fig. 4.18).¹² T2-weighted images of the posterior fossa reveal contusions as areas of abnormal increased signal, with hemorrhagic components demonstrating areas of increased T1 and heterogeneous T2 signal.

Inflammatory Disorders

Inflammatory disorders causing dizziness include a diverse range of etiologies with varied imaging findings. Inflammatory disorders with imaging abnormalities include otitis media, mastoiditis, cholesteatoma, cholesterol granuloma, sarcoidosis, Wegener granulomatosis, pachymeningitis, neuronitis, and labyrinthitis.

Acute mastoiditis and its complications are well visualized with CT or MRI.^13,14 Typically, there is fluid within the mastoid air cells and middle ear cavity. In severe cases, destruction of the bony mastoid septations can occur (Fig. 4.19). The diagnosis of mastoiditis is often made clinically, with imaging reserved for complications, such as venous sinus thrombosis, petrous apicitis, and epidural or brain abscess. Computed tomography is generally better suited for observing air–fluid levels as well as bony destruction. Magnetic resonance imaging should be used to evaluate brain and epidural abscesses (Fig. 4.20).

Fig. 4.18 High-resolution coronal T2-weighted image shows a traumatic defect of the right tegmen tympani (arrows) with CSF in the mastoid air cells. A small temporal lobe cephalocele was found at surgery.

Fig. 4.19 Axial high-resolution CT shows fluid in the middle ear cavity (upper arrow) in this patient with acute mastoiditis and facial nerve palsy. Fluid in the mastoid air cells with destruction of the bony septa is noted (lower arrow).

Fig. 4.20 Axial T2-weighted MRI shows fluid in the right mastoids (upper arrow) and an associated right temporal lobe abscess (lower arrow) in this patient with mastoiditis.

Fig. 4.21 (a) A source image and (b) collapsed MIP image from 2D TOF MRV in a patient with mastoiditis shows absence of flow in the thrombosed right transverse sinus (white arrows). Normal flow-related signal is seen in the superior sagittal and left transverse sinuses (black arrows).

Although dural sinus thrombosis secondary to acute mastoiditis can be suggested on CT, this entity is best demonstrated with MRI (Fig. 4.21). Direct extension of infection to the sigmoid sinus can result in thrombophlebitis. Aseptic propagation of venous thrombosis can involve the sigmoid, transverse, and petrosal sinuses, as well as the internal jugular vein. On contrast-enhanced CT, dural sinus thrombosis is manifested as incomplete or absent enhancement, and noncontrast CT demonstrates increased attenuation in the involved dural sinus (Fig. 4.22). T2-weighted MRI shows lack of a normal flow void in the dural sinus, and T1-weighted MRI reveals isointense or hyperintense thrombus within the sinus. However, MRV provides the best evaluation of the dural venous sinuses. Contrast-enhanced MRV demonstrates lack of flow in the occluded dural sinus, confirming the diagnosis.

Fig. 4.22 (a) Noncontrast axial CT reveals increased density in the right transverse sinus (arrows) from right dural sinus thrombosis. (b) Contrast-enhanced head CT in another patient shows thrombosis of the left sigmoid sinus with lack of sinus enhancement (black arrow). Normal enhancement of the right sigmoid sinus is noted (white arrow).

Chronic mastoiditis produces varying amounts of fluid in the middle ear cavity and mastoid air cells. Ossicular erosion of the incus and stapes can be seen with high-resolution CT, as well as sclerosis of the mastoids. Fibrous tissue may be noted in the middle ear but is difficult to differentiate from fluid in the setting of complete opacification.

Cholesteatomas can be evaluated with CT or MRI, although CT is the preferred modality. Cholesteatomas can arise from both the pars tensa and the pars flaccida. Cholesteatomas are generally associated with a retracted and thickened tympanic membrane. The acquired cholesteatoma originates from the pars flaccida in the Prussak space and classically produces erosion of the scutum. As it enlarges, the soft tissue extends superiorly into the epitympanum or attic and displaces the malleus and incus medially. These findings are especially well depicted on high-resolution coronal CT images.15 Cholesteatomas of the pars tensa tend to involve the sinus tympani and facial nerve recess. These lesions are best visualized on axial CT images and often displace the ossicles laterally. Both types of cholesteatoma can produce erosion of the ossicles. With MRI, cholesteatomas have nonspecific signal intensities with decreased T1 and increased T2 signal.³ Cholesteatomas generally do not enhance after intravenous (IV) contrast administration, which can help differentiate them from other lesions, such as tumors or granulation tissue.

Recently, diffusion-weighted imaging has been suggested as a possible method to differentiate between recurrent or residual cholesteatoma and granulation tissue in the postoperative patient.¹⁶ This technique can be useful as an alternative to “second look” operations to exclude recurrent disease after canal wall up procedures in which the postoperative clinical exam is limited^17,18 Cholesteatomas will demonstrate bright signal on diffusion-weighted images, whereas granulation tissue does not (Fig. 4.23). However, the best indicator that a lesion in the middle ear is a cholesteatoma is the presence of secondary findings, such as erosions of the ossicles and scutum. Labyrinthine fistulae can result from cholesteatomas eroding through the lateral semicircular canal or, very rarely, the cochlear promontory. High-resolution CT clearly depicts the bony defect, and this is an important finding to be aware of at the time of surgery (Fig. 4.23).¹⁹

Cholesterol granuloma is a lesion particularly well suited to evaluation by MRI, although CT can provide important information (Fig. 4.24).²⁰ Cholesterol granuloma results from an inflammatory response involving the deposition of cholesterol crystals in the middle ear, mastoid, or aerated petrous apex. As a part of this inflammatory response, hemorrhagic products including methemoglobin are produced. These products appear as increased signal intensity on T1- and T2-weighted sequences. Occasionally, the appearance can be confounded by the presence of hemosiderin, which produces heterogeneity and regions of decreased T1 and T2 signal. Cholesterol granulomata tend to be smoothly marginated, with little associated mass effect. Computed tomography shows a well-circumscribed, expansile mass with bony remodeling in the petrous apex or middle ear.

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