Introduction
The orbit is a pyramidal cavity in the skull that houses the eyeball (globe), optic nerve, extraocular muscles, vessels, nerves, and supporting connective tissue.
A cross-sectional understanding of orbital anatomy is essential for interpreting CT and MRI in cases of trauma, infection, tumors, vascular lesions, and inflammatory diseases.
The optic nerve is the second cranial nerve, transmitting visual information from the retina to the brain. It is a white matter tract of the CNS, surrounded by meninges and CSF.
Cross-sectional imaging provides:
- Anatomical orientation – precise localization of pathology
- Differentiation of soft tissue, fat, bone, and fluid
- Guidance for surgery and interventions
Applied Anatomy of the Orbit
- Shape: 4-sided pyramid with apex posteriorly and base anteriorly
- Volume: ~30 mL, with the globe occupying about 1/5th of this volume
Bony Boundaries
1. Roof
- Formed by the orbital plate of the frontal bone and the lesser wing of the sphenoid
- Separates orbit from anterior cranial fossa
- Contains fossa for lacrimal gland anterolaterally
2. Floor
- Formed by the maxilla, zygomatic bone, and palatine bone
- Separates orbit from maxillary sinus
3. Medial wall
- Formed by ethmoid (lamina papyracea), lacrimal bone, sphenoid body, and frontal process of maxilla
- Thinnest wall – easily fractured in blow-out injuries
4. Lateral wall
- Formed by zygomatic bone and greater wing of sphenoid
- Thickest wall – provides protection from lateral trauma
Optic Nerve – Segments
The optic nerve is about 50 mm long, divided into 4 segments:
- Intraocular segment (~1 mm)
- Begins at optic disc (1.5 mm diameter)
- Contains unmyelinated retinal ganglion cell axons
- Intraorbital segment (~25–30 mm)
- S-shaped curve allows free eye movement
- Surrounded by retrobulbar fat and extraocular muscles
- Intracanalicular segment (~6–10 mm)
- Passes through optic canal with ophthalmic artery
- Surrounded by bony canal – vulnerable to compressive lesions
- Intracranial segment (~10 mm)
- Extends from optic canal to optic chiasm
Coverings:
- Dura mater (outer)
- Arachnoid mater (middle)
- Pia mater (inner)
- Subarachnoid space contains CSF, continuous with brain
Cross-Sectional Imaging Appearance
CT Features
- Bone: high attenuation
- Retrobulbar fat: low attenuation (dark)
- Optic nerve: isoattenuating linear structure
- Optic nerve sheath complex: seen as a slightly thicker ring if sheath pathology present
- Orbital apex: optic canal + superior orbital fissure well visualized
MRI Features
T1-weighted:
- Fat: high signal (bright)
- Optic nerve: intermediate signal
- Muscles: intermediate to low signal
T2-weighted:
- Fat: intermediate signal
- Optic nerve: intermediate
- CSF in subarachnoid space: bright
- Pathologies (edema, inflammation) often hyperintense
Post-contrast with fat suppression:
- Highlights optic neuritis, tumors, inflammation
- Essential for orbital apex evaluation
Structures Seen in an Axial Section through the Mid-Orbit
- Globe (cornea, lens, vitreous)
- Optic nerve posterior to the globe
- Extraocular muscles:
- Medial rectus (medial side)
- Lateral rectus (lateral side)
- Superior rectus (superior; often seen with levator palpebrae superioris)
- Inferior rectus (inferior)
- Orbital fat around muscles and nerve
- Lacrimal gland in superolateral quadrant
- Orbital apex structures – optic canal, superior orbital fissure
Coronal Cross-Section
- Superior rectus + levator complex seen superiorly
- Inferior rectus below globe
- Medial and lateral recti on either side of optic nerve
- Optic nerve surrounded by intraconal fat within the muscle cone
Common Pathologies on Cross-Sectional Imaging
Optic Nerve Lesions
- Optic neuritis: Nerve thickening, T2 hyperintensity, contrast enhancement
- Optic nerve glioma: Fusiform enlargement, T2 hyperintense
- Optic nerve sheath meningioma: “Tram-track” enhancement on axial images
Extraocular Muscle Pathologies
- Thyroid eye disease: Belly thickening with tendon sparing
- Orbital pseudotumor: Muscle + tendon thickening, painful
Orbital Trauma
- Blow-out fracture: Herniation of orbital contents through floor or medial wall
- Optic canal fracture: Risk of traumatic optic neuropathy
Vascular Lesions
- Carotid-cavernous fistula: Dilated superior ophthalmic vein
- Orbital varix: Enlarges with Valsalva
Radiological Protocol
- CT orbit: Axial + coronal, thin section (1–2 mm), bone and soft-tissue algorithm
- MRI orbit: Axial, coronal, sagittal planes
- T1, T2, STIR/fat suppression
- Post-contrast T1 fat-suppressed sequences
- Optional DWI for optic nerve lesions
Key Points / Summary
- Orbit = pyramidal cavity; optic nerve = CNS white matter tract
- Cross-sectional anatomy essential for interpreting pathology
- CT: excellent for bone, acute trauma
- MRI: superior for soft tissue, optic nerve lesions
- Always use fat suppression in post-contrast orbital MRI
- Knowledge of intraconal vs extraconal compartments is vital for lesion localization
