The eye is a fascinating structure, with many complex parts that are difficult to examine by conventional means. When ocular disease is present but inaccessible by direct visualization or ultrasound, CT and MR imaging can provide information about structures deep within the eye and orbit, such as the lens, retina, vitreous, and orbital tissues.
Section 1.6 of the Atlas of Small Animal CT and MRI covers inflammatory disorders, trauma, neoplasia, and degenerative disorders of the eye. We hope you enjoy this excerpt and figure from the book!
Introduction
CT is often used to image the structures of the orbit and surrounding skull. It can also be used for imaging the major structures of the globe, including the anterior and vitreous chambers, as well as the lens. MRI is an excellent modality to image the structures of the globe and optic nerve. The cornea, anterior and posterior chambers, ciliary body, lens, vitreous chamber, and retina are visible on standard sequences. The optic nerve can also be evaluated and followed to the optic chiasm, both in transverse images and when dorsal or sagittal plane sequences are oriented obliquely along the long axis of the nerve. The optic nerve is surrounded by cerebrospinal fluid (CSF), which appears hyperintense on T2 and hypointense on T1 images. Fat nullifying sequences, such as STIR, as well as thin collimation can help to suppress the bright fat signal and allow visualization of the CSF and nerve.
Trauma
Trauma to the eye commonly presents as proptosis, with the possibility of additional trauma to the surrounding bones and soft tissues of the orbit. Penetrating trauma to the eye is not commonly evaluated with CT or MRI, but hemorrhage, inflammation, and altered anatomic structures would be expected findings. Foreign bodies and associated inflammation in the sclera can be seen as a mass lesion that deforms the scleral shape with contrast enhancement. Traumatic ruptures of the lens and globe have been reported to be better seen on MR than on CT images.
Inflammatory disorders
Optic neuritis can be seen on MR images as hyperintensity of one or both optic nerves in water-sensitive imaging sequences. The nerve may be hyperintense within the orbit and/or at the level of the optic chiasm. Granulomatous meningoencephalitis has been reported to involve the optic nerves. On MR images, it appears as isointense regions on T1 and T2 weighted images, with intense contrast enhancement. Anterior uveitis is defined as inflammation of the anterior chamber of the eye. On MR images, increased signal intensity can be seen on T1 images, and contrast enhancement may also be observed. Episcleritis involves inflammation of the tissues surrounding the sclera. The periocular tissues are thickened and T1 and T2 hyperintense and are intensely contrast enhancing.
Neoplasia
Melanoma arising from the uvea has been described on MR imaging in dogs. The mass was T1 hyperintense and T2 hypointense with contrast enhancement. T1 hyperintensity is a property of melanin that has been described in other regions of the body. A case of melanoma arising from the choroid and surrounding the optic nerve was T1 and T2 hypointense. 8 On CT images, melanoma may appear hyperattenuatingto the vitreous. Round cell neoplasia such as lymphoma can also primarily involve the eye. Metastatic disease due to other neoplasms can also occur. Neoplasia should be considered in patients with ocular hemorrhage where there is a clear fluid–fluid interface on MR images.
Homework
Check the eyes on some of your head and brain studies, you’ll be surprised at what you can see. And, dicuss advanced imaging with your ophthalmology colleagues next time you have a chance. The Atlas of Small Animal CT and MRI is available through Amazon, and has over 700 figures showing case examples to further your learning. Check it out if this article was helpful!
See you next time,
Allison Zwingenberger
Co-Author, Atlas of Small Animal CT and MRI
P.S.
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