Page 80 - Atlas of Small Animal CT and MRI

70 Atlas of Small Animal CT and MRI

of the eye may be initiating causes. Cellulitis or myositis tumors include carcinomas (adenocarcinoma, squa­
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results in increased soft‐tissue attenuating material or mous cell carcinoma), sarcoma (fibrosarcoma, liposar­
T2 hyperintensity of the orbital tissues. The volume coma, rhabdomyosarcoma, osteosarcoma), round‐cell
of the tissues appears greater, and there is loss of defini­ neoplasia (lymphoma, mast cell tumor) and meningi­
tion of the normal fat and extraocular muscles oma. 11–13 These tumors may primarily involve the tissues
(Figures 1.5.8, 1.5.9). The eyelids and surrounding of the orbit, extend from the nasal cavity and maxilla
tissues may also be affected. On contrast‐enhanced (see Chapters 1.1, 1.4), or represent metastasis. 3,4,13
images, there is diffuse enhancement of the tissues Imaging features include local bone destruction, irregu­
surrounding the globe. Abscesses may also form in lar bone production, and increased soft‐tissue mass
the tissues of the orbit, resulting in fluid‐attenuating, within the orbit. Tumors are heterogeneously to
or T2 hyperintense and T1 hypointense, collections. intensely contrast enhancing (Figures 1.5.13, 1.5.14,
These lesions are peripherally contrast enhancing 1.5.15, 1.5.16, 1.5.17, 1.5.18, 1.5.19). The surrounding
(Figure 1.5.10). In addition to the extraocular muscles structures should be evaluated to determine the involve­
within the orbit, the pterygoid muscle is located medial ment of bone, nasal cavity, optic nerve, and cranium.
to the zygomatic salivary gland and may also be affec­ Myxosarcoma has a predilection to the orbit in dogs,
ted by inflammatory disease (Figure 1.5.11). Rarely, the with CT and MR imaging characteristics of extensive,
inflammation within the orbit may extend intracranially. fluid‐filled cavities within the orbit and surrounding
MR imaging features of intracranial extension include fascial planes. They can extend to the temporomandibu­
T2, STIR, and FLAIR hyperintensity of the tissues in the lar joints and mimic a salivary mucocele. 14
skull foramina and orbital fissure, which do not extend Osteoma and multilobular tumor of bone also occur
into the brain or meninges. 10 in this location. Osteoma has a characteristic smooth,
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Zygomatic sialadenitis, often with sialocele formation, uniform attenuation on CT images and may enlarge to
can be a cause of inflammation and exophthalmos in the affect adjacent structures by mass effect.
orbit (see Chapter 1.7) (Figure 1.5.12). Restrictive orbital myofibroblastic sarcoma of cats,
which was previously named idiopathic sclerosing
orbital pseudotumor, is an invasive, low­grade neo­
Neoplasia
plasm affecting the orbital tissues. CT and MR images
Neoplastic disease may arise from the soft tissues show diffuse thickening of the orbital tissues, sclera,
or osseous structures surrounding the orbit. On CT and eyelids with intense contrast enhancement. 16,17 The
images, neoplasia usually has more clearly defined disease often affects both eyes and/or the oral cavity
margins compared to inflammatory disease. Common (Figure 1.5.20).

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