Page 37 - Atlas of Small Animal CT and MRI

1.2

Ear

Normal ear images. Exudates are typically hyperintense on MR T2
images and of variable intensity on T1 images depending
The ear is divided into external, middle, and internal on the cellular and macromolecular content of the exu­
components. The external ear includes the pinna, the dative fluid. Canal lining hyperplasia is strongly contrast
external ear canal, and external acoustic meatus. The enhancing on CT and MR images because of the high
middle ear includes the tympanic membrane, the vascular density of the inflamed canal wall (Figure 1.2.3).
osseous bulla, and the auditory ossicles. The inner ear,
located within the temporal bone, includes the semicir­ Inflammatory polyps
cular canals, vestibule, and cochlea. Together these
structures define the labyrinth. CT and MR appearance Inflammatory polyps may arise from the external ear
of the normal ear have been previously reported canal epithelium in association with otitis externa.
(Figures 1.2.1, 1.2.2). 1,2 Polyps are typically vascular, resulting in moderate
Although the normal tympanic membrane can be enhancement and increased conspicuity on contrast‐
seen on CT and MR images, it is usually obscured by the enhanced CT and MR images (Figure 1.2.4).
presence of adjacent horizontal canal or bulla effusion in Polyps may also arise from the epithelial lining of the
patients with external and/or middle ear disease. The tympanic membrane or within the auditory canal
auditory ossicles and labyrinth are likewise visible on extending into the nasopharynx and most commonly
normal CT and MR images but are typically more occur in the cat (Figure 1.2.5). Polyps may not be readily
3,4
difficult to accurately identify in the presence of middle distinguished from surrounding fluid on unenhanced
and inner ear disease. Thinly collimated CT images CT and MR images but are easily detected on contrast‐
(1 mm or less) and 3D sequences to produce thin MR enhanced images. Neoplastic masses may also occasion­
images are recommended to fully evaluate the labyrinth ally arise within or adjacent to the tympanic bulla and
if subtle disease is suspected. should be distinguished from inflammatory polyps.

Inflammatory disorders Cellulitis, abscesses and fistulae
Pericanalicular cellulitis, abscesses, and fistulae may
Otitis externa occur secondary to otitis externa when the canal wall is
Uncomplicated otitis externa is characterized by breached. Abscesses have a typical cavitary appearance
inflammation of the external ear canal. Hyperplastic with a fluid‐filled center that appears hypoattenuating
thickening of the canal lining occurs as a response to on CT, hyperintense on MR T2 images, and of variable
chronic inflammation, causing ceruminous and aqueous intensity on T1 images, depending on the cellular and
exudates to fill the canal lumen. Exudates are generally macromolecular content of the exudative fluid. Abscess
hypoattenuating to adjacent canal epithelium on CT walls and surrounding cellulitic tissue are highly contrast

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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