MRI Anatomy Flashcards · Neuro

Orbits Anatomy

Learn to identify every labeled structure on a Orbits MRI, plane by plane.

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Orbits anatomy, structure by structure

Orbital MRI is a small-field, high-resolution study, and reading it well starts with knowing the normal anatomy cold. This reference walks through every structure in our orbits flashcard deck across the planes the deck uses (axial and oblique sagittal/axial views angled along the optic nerve), with a plain-language definition, how each structure looks on MRI, and the pathology you will actually run into at the scanner.

Globe and lens

The eyeball itself and its internal contents. Fluid is the orientation cue here: the vitreous is bright on T2 and fills most of the globe.

Globe

The eyeball: a roughly spherical structure made of three layers (sclera, uvea, retina) surrounding the vitreous chamber.

On MRI: A round, well-defined structure at the front of the orbit on axial. The vitreous follows fluid signal (dark on T1, bright on T2), with the thin wall and the lens easily seen against it.

Common pathology: Ocular melanoma, retinoblastoma in children, retinal or choroidal detachment, and globe rupture or phthisis after trauma.

Tip: On axial, compare the two globes for symmetric size and shape; proptosis and microphthalmia are judged by lining them up side to side.

Lens

The transparent biconvex structure behind the pupil that focuses light onto the retina.

On MRI: A small biconvex structure at the front of the globe. It is denser than the surrounding vitreous and reads relatively low signal on T2 against the bright vitreous behind it.

Common pathology: Cataract (usually a clinical diagnosis), lens dislocation after trauma, and a key structure to keep in mind for radiation dose planning.

Vitreous Humor

The clear gel that fills the large chamber of the eye behind the lens and maintains the globe's shape.

On MRI: Fills the bulk of the globe and follows fluid signal: dark on T1, bright on T2. It is the brightest part of the orbit on T2 and a reliable signal reference.

Common pathology: Vitreous hemorrhage (altered signal), endophthalmitis, and asteroid hyalosis; detachments show as bright fluid layering within or behind the vitreous.

Optic pathway

The visual conduction route from the back of the globe to the brain. Oblique planes angled along the nerve show its course best.

Optic Nerve

Cranial nerve II, the white-matter tract that carries visual information from the retina to the optic chiasm.

On MRI: Runs from the back of the globe to the orbital apex, surrounded by bright CSF in its sheath on T2 and outlined by dark fat-suppressed orbital fat. Oblique sagittal best follows its full S-shaped course.

Common pathology: Optic neuritis (T2 bright, enhancing nerve, classic in multiple sclerosis), optic nerve glioma, and meningioma of the nerve sheath.

Tip: Angle the oblique sagittal along the long axis of the nerve so the whole segment from globe to apex sits in one slice; STIR or fat-suppressed T2 makes the nerve and its sheath stand out.

Optic Chiasm

The X-shaped crossing point where fibers from the nasal half of each retina cross to the opposite side.

On MRI: A small midline band of tissue above the pituitary and sella, sitting in the suprasellar cistern. Gray-matter signal, best confirmed on thin coronal or axial images.

Common pathology: Compression by pituitary macroadenoma or craniopharyngioma (causing bitemporal field loss) and chiasmatic glioma.

Optic Tract

The continuation of the visual pathway behind the chiasm, carrying fibers toward the lateral geniculate body.

On MRI: Paired thin bands sweeping back from the chiasm around the midbrain on axial. Gray-matter signal, following the cerebral peduncles.

Common pathology: Compression or infiltration by suprasellar masses and demyelination; lesions cause a contralateral homonymous field defect.

Extraocular muscles

The recti that move the eye. They sit around the optic nerve and are read as a group for symmetry and thickness.

Medial Rectus Muscle

The extraocular muscle on the nasal side of the globe that turns the eye inward (adduction).

On MRI: A soft-tissue band along the medial wall of the orbit on axial, running from the orbital apex to the globe. Intermediate signal, well outlined by orbital fat.

Common pathology: The most commonly enlarged muscle in thyroid eye disease; also involved in orbital cellulitis spreading from the ethmoid sinus.

Tip: On axial, the medial and lateral recti lie in the same slice as the optic nerve, making axial the workhorse plane for comparing these two muscles.

Lateral Rectus Muscle

The extraocular muscle on the temporal side of the globe that turns the eye outward (abduction).

On MRI: A soft-tissue band along the lateral wall of the orbit on axial, mirroring the medial rectus. Intermediate signal against dark fat-suppressed fat.

Common pathology: Thyroid eye disease (though less involved than the medial and inferior recti) and lateral wall fractures or masses.

Superior Rectus Muscle

The extraocular muscle along the roof of the orbit that elevates the eye.

On MRI: A band running across the top of the globe and optic nerve, best seen on coronal or oblique sagittal images above the nerve. Intermediate soft-tissue signal.

Common pathology: Thyroid eye disease and the superior muscle complex involvement in orbital inflammatory disease (orbital pseudotumor).

Inferior Rectus Muscle

The extraocular muscle along the floor of the orbit that depresses the eye.

On MRI: A band running along the orbital floor beneath the globe and nerve, best seen on coronal or oblique sagittal. Intermediate soft-tissue signal.

Common pathology: One of the first and most commonly enlarged muscles in thyroid eye disease; entrapment in orbital floor (blowout) fractures.

Orbit and sinuses

The fat-filled space around the globe and the air sinuses that border the bony orbit.

Retro Orbital Fat

The orbital fat behind the globe that cushions and surrounds the optic nerve and extraocular muscles.

On MRI: Bright on T1 and T2 when not fat-suppressed, and dark on fat-suppressed sequences. It is the natural contrast medium that outlines the nerve and muscles.

Common pathology: Infiltrated or effaced by orbital cellulitis, pseudotumor, and lymphoma; its loss of normal signal flags an infiltrating process.

Tip: Most dedicated orbit sequences use fat suppression so the bright fat does not mask enhancing or T2-bright disease around the nerve.

Sphenoid Sinus

The deepest paranasal sinus, sitting in the body of the sphenoid bone below the sella turcica.

On MRI: An aerated (dark) midline space behind the posterior ethmoids on axial. Mucosal thickening or fluid stands out as bright T2 signal.

Common pathology: Sinusitis, mucoceles, and a route of spread for infection toward the orbital apex and cavernous sinus.

Maxillary Sinus

The largest paranasal sinus, in the maxilla below the orbital floor.

On MRI: A large aerated dark space below the orbit. Air-fluid levels and mucosal thickening read bright on T2.

Common pathology: Sinusitis and retention cysts; disease or masses here can erode the orbital floor and affect the inferior rectus.

Adjacent structures on orbital axials

The orbital axial slices also cut through the posterior fossa and skull-base vessels; recognize these so you do not mistake them for orbital findings.

Internal Carotid Artery

The paired major artery supplying the anterior circulation of the brain, passing through the skull base near the sella.

On MRI: A round dark flow void on either side of the sella on axial spin-echo images; bright on time-of-flight MR angiography.

Common pathology: Atherosclerosis, dissection, and cavernous or supraclinoid aneurysms that can affect the adjacent optic apparatus.

Basilar Artery

The single midline artery formed by the two vertebral arteries that supplies the brainstem and posterior circulation.

On MRI: A midline dark flow void hugging the front of the pons on axial; bright on MR angiography.

Common pathology: Basilar tip aneurysms, basilar occlusion (a stroke emergency), and dolichoectasia.

Pons

The bulky middle segment of the brainstem that relays signals between the cerebrum and cerebellum.

On MRI: A rounded structure in the center of the posterior fossa on axial, with the dark basilar artery flow void grooving its anterior surface.

Common pathology: Pontine infarcts and hemorrhage, central pontine myelinolysis, and brainstem gliomas.

Cerebellar Vermis

The midline portion of the cerebellum between the two cerebellar hemispheres.

On MRI: A midline folded structure behind the fourth ventricle on axial, showing the finely lined folia of cerebellar cortex.

Common pathology: Medulloblastoma (a midline vermian tumor in children), vermian atrophy, and Dandy-Walker spectrum malformations.

Fourth Ventricle

The diamond-shaped CSF space in the posterior fossa between the brainstem in front and the cerebellum behind.

On MRI: A midline fluid space between the pons and the cerebellar vermis on axial; follows CSF signal (dark on T1, bright on T2).

Common pathology: Obstructed by posterior fossa masses, ependymomas arising within it, and effacement from tonsillar herniation.

Frequently asked questions

What structures are seen on an orbital MRI?

A dedicated orbit MRI shows the globe and its lens and vitreous, the optic pathway (optic nerve, chiasm, and tract), the four recti extraocular muscles, the orbital fat that surrounds them, and the neighboring paranasal sinuses. Because the slices sit at the skull base, orbital axials also catch posterior-fossa structures such as the pons, cerebellar vermis, fourth ventricle, internal carotid artery, and basilar artery. This page labels each one with its MRI appearance.

Which planes are best for studying orbital anatomy?

Axial is the workhorse plane for the globe, the optic nerve, and the medial and lateral recti, since they all lie in the same slice. Oblique sagittal angled along the nerve follows its full course from globe to apex, and coronal is preferred for comparing the four recti and grading muscle thickness in thyroid eye disease.

Why does orbital MRI use fat suppression?

The orbit is packed with fat, which is bright on standard T1 and T2 images and can hide enhancing or T2-bright disease around the optic nerve. Fat-suppressed T2, STIR, and post-contrast fat-suppressed T1 darken the fat so optic neuritis, nerve sheath lesions, and inflammatory change stand out clearly.

How do you tell the lens and vitreous apart on MRI?

The vitreous fills most of the globe and follows fluid signal: dark on T1 and bright on T2, making it the brightest structure in the orbit on T2. The lens is the small biconvex structure at the front of the globe and reads relatively low signal against the bright vitreous behind it.

Do I need an account to use these Orbits MRI flashcards?

No. The interactive flashcards and this full labeled reference are open to use, with no account required to start. Creating an account lets you save your progress across devices and track which packs you have mastered.

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