Axial Anatomy on MRI
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As with CT, the ocular membranes are not able to be
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distinguished among themselves between
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the retina and the choroid.
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However, we do know that the sclera goes over the
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anterior-most portion of the orbit
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and the globe in the cornea.
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Here we see the dark signal intensity lens.
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And laterally, you may see a small dark structure which
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represents the ciliary bodies and the muscles
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that will tense and release the lens.
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Behind the lens is the posterior segment
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and is dominated by the vitreous.
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And we see that both the vitreous humor as well as
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the anterior chamber of the globe have
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signal intensity which simulates CSF
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on this T2-weighted image.
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The muscles on T2-weighted scanning are going to be dark.
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And here we have our lateral rectus muscle
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and our medial rectus muscle.
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Again, lateral rectus innervated by the abducens nerve or
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cranial nerve six and the medial rectus muscle by
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the oculomotor nerve or cranial nerve three.
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Within the muscle cone,
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the dominant structure is the optic nerve.
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In this case, we see both the optic nerve,
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which is cranial nerve two, as well as some of the
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cerebrospinal fluid of the optic nerve sheath.
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Now, the optic nerve sheath cerebrospinal fluid does
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communicate with the intracranial cerebrospinal
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fluid and can lead to infections going both directions.
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In this situation,
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we are also seeing the anterior clinoid process,
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which is a piece of the bone of the lesser
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wing of the sphenoid. And as you can see,
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the optic nerve goes medial to the anterior
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clinoid process in the optic canal.
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Note also the close proximity of the ethmoid sinuses.
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The ethmoid sinuses on T2-weighted scanning
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when they are aerated or dark.
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Here we see a little bit of mucosal thickening,
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which is about normal thickness
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in the ethmoid sinus.
David M Yousem, MD, MBA
Professor of Radiology, Vice Chairman and Associate Dean
Johns Hopkins University
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