Page 79 - Atlas of Small Animal CT and MRI

1.5

Orbit

Introduction the canine teeth and results in some obstruction to
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drainage. Similar anatomic changes have been reported
The orbit is bounded by the frontal bones medially, the in brachycephalic dogs. CT is an excellent approach for
zygomatic arch caudolaterally, and the orbital ligament performing dacryorhinocystography to evaluate the
dorsally and contains the globe and associated vascular patency of the lacrimal duct and identify causes of
and glandular structures (Figures 1.5.1, 1.5.2). The obstruction (Figure 1.5.3). Dacryops, developmental
zygomatic arch is curved and regular with a central cysts of the lacrimal system, can obstruct the lacrimal
suture. The orbital ligament spans the frontal process of canal and deform the surrounding bones as they expand
the zygomatic bone and the zygomatic process of the over time and can be imaged with CT or MR. They may
frontal bone and is visible as a hyperattenuating structure contain sedimenting debris, best seen on T2 weighted
on CT images and a hypointense structure on T1 and T2 MR images (Figure 1.5.4). The walls of such cysts exhibit
images on MRI. In cats, the dorsal orbit is mainly bone as mild contrast enhancement.
these processes are close together. Mineralization of the
orbital ligament is common in dogs. Extrinsic ocular Trauma
muscles, the zygomatic salivary gland (dogs), vasculature,
the lacrimal gland and gland of the third eyelid, the globe, Trauma to the skull often affects the orbit, either by
and the optic nerve fill the orbit. The ocular muscles fracturing the bones forming the orbital boundaries or
enhance to a greater degree than surrounding musculature by damaging the soft tissues within. Orbital fractures
on MR images in normal dogs. The nasolacrimal duct can be appreciated best on transverse images, but 3D
1
travels through the lacrimal canal in the lacrimal bone images can also help to describe the displacement of
and maxilla to enter the nasal cavity rostrally and ven­ the bones and alteration to orbital shape (Figure 1.5.5).
trally. The optic nerve passes through the optic canal, Acute trauma results in sharply marginated fracture
2,3
formed by the pterygoid bone, to enter the calvarium. lines that may extend to the nasal cavity and calvarium.
4,5
It is best visualized on MR images in a dorsal oblique Chronic fractures may heal with malunion and deform
plane, parallel to the nerves. Several sequences may be the shape of the orbit by proliferative change or by areas
used for visualization, but 3D T1 weighted images with of disruption of the skull (Figure 1.5.6). The globe may
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a 1–2 mm slice thickness are optimal. be acutely displaced from the orbit (Figure 1.5.7) or
shrunken (phthisis bulbi) as a result of previous trauma.
Developmental disorders
Inflammatory disorders
Head conformation, particularly in brachycephalic
breeds of cats, can alter the path of the normal nasolac­ Inflammation can affect the soft tissues within the orbit,
rimal duct. The dorsal rotation of the facial bones and often manifesting as exophthalmos or periorbital swell­
canine teeth causes the nasolacrimal duct to pass under ing. Penetrating trauma, foreign bodies, and infections

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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