Imaging of the Eye, Orbits, and Visual Tracts

Presented by Dr. Fand Gazer (Gent University Hospital, Belgium)

Contributions by Dr. Stephanie Vomos (St. Lucas Hospital, Gent, Belgium)

Introduction

Indications for Orbital Imaging

• Proptosis: Bulging of the eye
• Visual Disturbances: Important to specify exact nature for best MRI protocol
• Ophthalmoplegia/Diplopia: Suggestive of lesions affecting cranial nerves responsible for eye movement

Orbital MRI Protocols

Proptosis

• Focus on detailed MRI of the orbits rather than brain imaging

Visual Disturbances

• Depends on the exact nature—could require MRI of orbit, pituitary region, or brain

Ophthalmoplegia/Diplopia

• Focus on cranial nerves (III, IV, VI) and brainstem evaluation

Visual System Anatomy

Key Structures

• Eyes: Receive visual information from both visual fields
• Optic Nerve & Chiasm: Information crosses at the chiasm (nasal part crosses, temporal part remains ipsilateral)

Pathways

• From optic chiasm to thalamus (lateral geniculate body), then to occipital cortex via optic radiations

Visual Field Issues

Monocular Blindness

• Total blindness in one eye—MRI of the orbits needed

Bitemporal Hemianopia

• Loss of vision in outer halves of both visual fields; MRI of pituitary needed

Homonymous Hemianopia

• Loss of vision in same side of visual fields of both eyes; MRI of the brain required

Orbital Compartment Anatomy

Components

• Orbital Septum: Fibrous boundary of orbital compartment
• Extraocular Muscles: Found within the conal space
• Fatty Tissue: Contains arterial and neural structures

Compartments

• Intraconal Space: Inside the conal space
• Extraconal Space: Outside the conal space but inside the orbit

Pathologies of the Bony Orbit

Tumors

• Meningioma: Aggressive-looking expansion of the sphenoid wing, causing proptosis
• Bone Hemangioma: Characterized by trabecular thickening, no cortical breakthrough
• Fibrous Dysplasia: Expansile, ground-glass matrix, no cortical breakthrough

Approach to Proptosis

• Assessment using interpupillary line and severity classification (grade 1-3)

Orbital Fractures

Types

• Orbital Floor (Blowout) Fractures: Check for herniation of the inferior rectus muscle
• Zygomaticomaxillary Complex Fractures: Involves lateral orbital wall, may result in cosmetic deformity

Pathologies of the Globe

Anatomy

• Lens: Divider between anterior (aqueous fluid) and posterior segments (vitreous gel)
• Retina: Inner layer, including photoreceptor cells

Common Findings

• Calcified Drusen: Small calcifications near optic disk
• Trabecular Apparatus Calcification: Common in older patients
• Scleral Band & Silicone Injection: Post-surgical findings for retinal detachment

Detachments

• Retinal Detachment: V-shaped, hyperintense on FLAIR images
• Choroid Detachment: Biconvex, not involving optic disc, often post-glaucoma surgery

Tumors

• Uveal Metastasis: Most common intraocular tumor in adults, causes detachments
• Uveal Melanoma: T1 hyperintense if melanocytic
• Retinoblastoma: Calcifications on CT, most common in children

Pathologies of the Conal Space

Key Conditions

• Thyroid Eye Disease: Bilateral symmetrical enlargement of extraocular muscles, can cause apical crowding
• Orbital Pseudotumor: Unilateral, painful, acute, may affect all orbital structures
• Orbital Lymphoma: Often unilateral, hypercellular, shows diffusion restriction

Pathologies of the Intraconal Space

Tumors

• Optic Nerve Sheath Meningioma: Surrounding the optic nerve, causing enhancement, calcifications common
• Optic Nerve Glioma: Mostly in children, diffuse thickening without contrast enhancement

Inflammation

• Optic Neuritis: Common cause of optic neuropathy, associated with demyelinating diseases

Vascular Lesions

• Cavernous Hemangioma: Typically lateral, incidental finding, doesn't cause mass effect
• Venolymphatic Malformation: Combination of venous and lymphatic vessels, often symptomatic
• Orbital Varix: Enlarges with increased venous pressure, may require positional imaging

Tumors

• Orbital Metastasis: Infiltrative, enhancing lesions, often from breast cancer

Pathologies of the Extraconal Space

Infections

• Preseptal Cellulitis: Anterior to orbital septum, treat with oral antibiotics
• Postseptal Cellulitis: Behind orbital septum, necessitates intravenous antibiotics

Tumors

• Dermoid Cyst: Congenital, T1 hyperintense due to fatty content, common in children

Extraorbital Anatomy and Pathology

Optic Pathways

• From Optic Chiasm: Connecting to the occipital cortex; optic radiations via the thalamus
• Pathologies: Tumors, infarctions, and anomalies affecting visual fields

Oculomotor Pathways

• From Brainstem: Through basal cisterns and cavernous sinus to eye muscles
• Pathologies: Tumors, aneurysms, inflammation affecting cranial nerves III, IV, and VI

Pathologies Affecting Optic Pathways

• Cortico-Cavernous Fistula: Leads to pulsatile proptosis, requires angiography for confirmation
• Pituitary Tumors: Compressing the optic chiasm, causing hemianopia
• Occipital Infarctions: Homonymous hemianopia due to PCA infarctions

Clinical Pearl

• Descriptions of visual disturbances help guide appropriate imaging for accurate diagnosis and management.

Conclusion

• MRI protocols and sequences vary based on specific indications and detailed clinical information is critical for effective imaging.
• Thank you to Dr. Stephanie Vomos for contributions and cases shared.