MRI EVALUATION OF MIDDLE-EAR PATHOLOGIES

Takao Kodama

Department of Radiology, Faculty of Medicine, University of Miyazaki, Miyazaki, Japan

Roles of MRI in the evaluation of middle-ear pathologies

In the imaging of middle-ear pathologies, MRI can play an auxiliary role of CT because of its higher tissue contrast. Some diseases can be diagnosed based on their characteristic signal intensities (e.g., cholesterol granuloma). High spatial resolution is needed for the temporal bone imaging. So, a high magnetic field unit such as 3T and a multi-channel head coil can enhance utility of MRI because of the high signal-to-noise ratio and resultant high spatial resolution. The magnetic susceptibility effect is one of major problems of MRI for the application in the temporal bone, especially with higher magnetic field units. Magnetic susceptibility effects result in artificial signal loss, abnormal signal, distortion of images, and so on. So, pulse sequences should be designed to minimize the susceptibility effects and maximize the spatial resolution.

Essential pulse sequences for the temporal bone

Essential pulse sequences include 2D T1- and T2-weighted imaging (T1WI and T2WI), 3D high-resolution heavily T2WI, and post-contrast T1WI. Using a 3D fast spin echo (FSE) or gradient echo (GRE) acquisition, thin slice T1-weighted images can also be obtained. 3D heavily T2WI and T1WI can be estimated in any directions with multiplanar reconstruction (MPR). Diffusion-weighted imaging (DWI) is also important especially for evaluating cholesteatomas. The following sequences were mainly discussed.

3D high-resolution heavily T2WI

Variable sequences of FSE (SPACE, Cube, etc.) and GRE (true FISP, true SSFP, CISS, FIESTA, etc.) can be applied to obtain high-resolution heavily T2WI. FSE is less sensitive to the magnetic susceptibility.1 Tissue contrast of FSE (SPACE) and true SSFP/CISS may be different.

Post-contrast 3D T1WI

As a high-resolution post-contrast T1WI, 3D GRE sequences have been widely used. GRE, however, is vulnerable to susceptibility effects comparing with SE or FSE sequence. In our experience, the image quality and diagnostic value of 3D SPACE (one of FSE sequence) was superior to 3D FLASH (GRE sequence), especially for lesions of the labyrinth, facial nerve, and tympanic cavity, such as cholesteatoma. This superiority of SPACE was mainly because of its less vulnerability to susceptibility effects (Fig. 1). Signal of vessels is different between FSE and GRE. On the SPACE, most arteries and veins display ‘flow void’ and are associated with faint ghost artifacts along the phase-encoding direction. In contrast, vessels may display hyperintensity and are free from ghost artifacts on the FLASH.

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Fig. 1. A patient with cholesteatoma with labyrinthitis. a: post-contrast 3D SPACE. b: post-contrast 3D FLASH. Abnormal enhancement of the cochlea and vestibule is well visualized on the SPACE images. Signal intensity of artery and veins are different on both images.

DWI

Usefulness of DWI in the evaluation of cholesteatomas and tumorous lesions has been widely recognized. Higher SNR obtained by 3T scanner and a multi-channel coil enables thin-slice and high-resolution DWI. Application of parallel imaging and smaller voxel size can reduce susceptibility effects of EPI. EPI-DWI, however, is not free from susceptibility artifacts especially at 3T. So, non-EPI DWI such as single shot FSE and PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) can be more useful.2

Signal intensities on MRI

Because T1 and T2 of free water is very long, the signal intensity of tissues on MRI largely depend on their water contents. So, hypointensity on T1WI and hyperintensity on T2WI usually represent increased water content of tissues and are non-specific. On the contrary, hyperintensity on T1WI and/or hypointensity on T2WI may reflect some specific pathological conditions.

Hyperintensity on T1WI

Paramagnetic substances: hemoglobin products (methemoglobin), melanin, free radicals, manganese, etc.;

Lipid: lipoma, choristoma such as dermoid, etc.;

High protein content;

Calcification in some conditions (surface effect);

Flow-related enhancement.

Hypointensity on T2WI

Paramagnetic substance: hemoglobin products (deoxyhemoglobin, methemoglobin, hemosiderin), non-hem iron such as ferritin, melanin, free radicals, copper, etc.;

Low proton density: gas, calcification, ossification, tissues with high cellularity and/or high nuclear-cyto-plasmic ratio, fibrous tissue, etc.;

High protein content;

Flow-void.

Fat on MRI

T1 of fatty tissue is short and fat usually displays hyperintensity on T1WI. Fatty bone marrow also displays hyperintensity on T1WI. Bone marrow abnormalities such as osteomyelitis or tumor involvement are more easily diagnosed on MRI compared with CT (Fig. 2).

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Fig. 2. A patient with malignant (necrotizing) otitis media. T1WI shows hypo-intensity of the bone marrow in the left clivus.

Vascular structures on MRI

Signal intensity of vascular structures depends on pulse sequence, slice thickness, location, flow velocity, and so one. High flow vessels usually display signal loss (flow void) especially on T2WI. Intravascular signal may mimic a tumorous lesion especially at jugular bulb (‘pseudolesion’).

Representative MRI findings of middle-ear pathologies

Cholesteatoma

Roles of MRI for evaluating a cholesteatoma include: 1) Precise definition of the borders of lesions and demarcation between a cholesteatoma and other pathologies such as cholesterol granuloma, granulation tissue, scar tissue, inflammatory mucosa, and so on; 2) Depiction of the relationship of the lesion to intracranial structures; 3) evaluating complications such as labyrinthitis, facial-nerve palsy and intracranial complications (meningitis, encephalitis, brain abscess, sinus thrombosis, etc.); and 4) Post-operative follow up.

Signal intensity of a cholesteatoma on T1- and T2WI is non-specific. Although a cholesteatoma usually displays hypointense on T1WI and hyperintense on T2WI, it may display hypointensity on T2WI because of low water content (desiccated material). A cholesteatoma rarely shows hyperintensity on T1WI (white epidermoid), probably because of high protein content or high lipid content (mixed triglycerides and no cholesterol). Intensity on heavily T2WI (MR hydrography) is usually lower and more inhomogeneous compared with standard T2WI.

A cholesteatoma does not typically enhance with gadolinium, except at the margin of the lesion. On the contrary, granulation tissues usually display enhancement. However, it is necessary to obtain delayed contrast-enhanced images with a delay of 30–45 minutes after contrast-material administration.

The usefulness of non-EPI (echo planar imaging) DWI for evaluating a cholesteatoma has been widely recognized.2,3 However, false positive or false negative studies can be seen due to the influence of T2 (T2 shine-through or T2 dark-through). In our experience, apparent diffusion coefficient (ADC) value can differentiate cholesteatomas from other pathological conditions of the middle ear, such as granulation, cholesterol granuloma, inflammatory scar and so on.

In our institute, following imaging protocol has been used for evaluating a cholesteatma; 1) T1WI (SE or 3D FSE); 2) non-EPI DWI; 3) T2WI (FSE); 4) 3D heavily T2WI, 5) 3D T1WI. To obtain delayed post-contrast images, contrast media is injected before T2WI and 3D heavily T2WI (Fig. 3).

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Fig. 3. Protocol for evaluating a cholesteatoma.

Cholesterol granuloma

Cholesterol granulomas arise in aerated spaces such as the mastoid air cells as a result of inadequate ventilation. Hemorrhage in the space without ability to drain leads to inflammatory response. The lesion is usually bright on T1 and T2WI representing blood products especially extracellular methemoglobin.4 A ring of low signal intensity may confirm the presence of hemosiderin-laden macrophages. However, the signal may be variable depending on the blood products (Fig. 4).

Acute and chronic otitis media

MRI may be useful to estimate following complications.5

Coalescent otomastoiditis: The pneumatic cells coalesce into larger cavities filled with purulent exudates and granulations, resulting in empyema;

Subperiosteal, bezold, or perisinus abscess;

Gradenigo’s syndrome;

Labyrinthitis;

Intracranial complication: epidural abscess, meningitis, encephalitis, brain abscess, dural sinus thrombophlebitis, etc.

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Fig. 4. Cholesterol granuloma. The lesion displays hyperintensity on T1WI. T2WI shows hyper- and hypo-intensity with fluid-fluid levels.

Labyrinthitis

Labyrinthitis can complicate a cholesteatoma or otitis media and may be associated with labyrinthine fistula. MRI plays an important role. Abnormal enhancement can be seen in the acute or subacute phase (Fig. 1). Fibrosis of the labyrinth can be detected with high-resolution heavily T2WI. In the chronic stage of the laby-rinthitis, calcification or ossification occurs (labyrinthitis ossificans). CT is better for detecting the condition.

Tumorous lesions of the middle ear

Facial-nerve schwannoma (Fig. 5)

Although the lesion can be seen anywhere along the course of the facial nerve, it is most commonly originated from the geniculate ganglion.6 Facial-nerve palsy is associated in less than 50% of the cases. Morphology depends on its location. Lesions in the internal auditory canal mimicked acoustic tumors. Lesion in the tympanic or mastoid segment displays ‘sausage-like’ configuration.

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Fig. 5. Facial-nerve schwannoma. Post-contrast T1WI displays enlarged labyrinth segment of the left facial nerve with abnormal enhancement.

Glomus tympanicum tumor, paraganglioma

The lesion usually located at the cochlear promontory and is usually small (millimeters to two cm). Characteristics of a glomus tumor is its hypervascular nature.7

Retrotympanic ‘vascular’ lesions

Different diagnoses include the following pathologies;

Congenital lesions: aberrant internal carotid artery, persistent stapedial artery, dehiscent jugular bulb;

Inflammatory lesions: cholesterol granuloma;

Benign tumors: Glomus tympanicum and jugulare.

Facial-nerve palsy (FNP)

Classic Bell palsy requires no imaging in initial stages. If decompressive surgery is anticipated, however, MR imaging is warranted for excluding lesions causing FNP. Atypical FNP requires search for underlying lesion.

Conclusion

MRI can play an auxiliary role of CT because of its higher tissue contrast and some diseases can be diagnosed based on their characteristic signal intensities.

Pulse sequences should be designed to minimize the susceptibility effects and maximize the spatial resolution.

References

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2.Foer BD, Vercruysse JP, Spaepen M, Somers T, Pouillon M, Offeciers E, Casselman JW. Diffusion-weighted magnetic resonance imaging of the temporal bone. Neuroradiology 52:785–807, 2010

3.Foer BD, et al. Middle ear cholesteatoma: non-echo-planar diffusion-weighted MR Imaging versus delayed gadolinium-enhanced T1-weighted MR imaging – value in detection. Radiology 255(3):866–872, 2010

4.Martin N, Sterkers O, Mompoint D, Julien N, Nahum H. Cholesterol granuloma of the middle ear cavities: MR imaging. Radiology 172(2):521–525, 1989

5.Vazquez E, et al. Imaging of complications of acute mastoiditis in children. RadioGraphics 23(2):359–372, 2003

6.Wiggins RH 3rd, Harnsberger HR, Salzman KL, Shelton C, Kertesz TR, Glastonbury CM. The many faces of facial nerve schwannoma. AJNR 27(3):694–699, 2006

7.Noujaim SE, Pattekar MA, Cacciarelli A, Sanders WP, Wang AM. Paraganglioma of the temporal bone: role of magnetic resonance imaging versus computed tomography. Top Magn Reson Imaging 11(2):108–122, 2000


Address for correspondence: Takao Kodama, tkodama@med.miyazaki-u.ac.jp

Cholesteatoma and Ear Surgery – An Update, pp. 195–200

Edited by Haruo Takahashi

2013 © Kugler Publications, Amsterdam, The Netherlands