Does the patient need a computed tomography (CT) scan of the orbits?

CT Scan of the Orbits: Indications and Recommendations

CT scan of the orbits is indicated when there is clinical suspicion of orbital pathology such as trauma, foreign bodies, infection, or mass lesions, but MRI is generally preferred for soft tissue evaluation and non-traumatic conditions affecting the optic nerve and visual pathway. 1

Indications for Orbital CT Scan

Trauma and Foreign Bodies

• CT is superior to MRI for detecting and localizing metallic foreign bodies with 94.9% sensitivity 2, 3
• Non-contrast thin-section orbital CT with multiplanar reconstructions is the recommended protocol for suspected orbital foreign bodies 2, 4
• CT is the gold standard for evaluating orbital trauma, particularly for identifying fractures and displaced fragments 4, 3
• MRI is contraindicated when metallic foreign bodies are suspected due to potential movement caused by the magnetic field 2, 4

Orbital Infections

• CT orbits with IV contrast is the most useful initial imaging for suspected orbital infections 5
• CT helps differentiate preseptal from postseptal cellulitis and detect orbital abscesses 5, 1
• CT can identify bone erosion and subperiosteal abscess, which may require surgical intervention 1

Mass Lesions and Proptosis

• CT is complementary to MRI in evaluating orbital mass lesions, providing information about adjacent bone involvement 1
• CT is superior for detecting calcifications, which helps differentiate certain tumors like retinoblastoma 5, 6
• CT provides useful information about orbital, muscle, and fat volumes in thyroid eye disease 1

When MRI is Preferred Over CT

Visual Loss and Optic Pathway Disorders

• MRI of the orbits without and with contrast is the preferred modality for evaluating soft-tissue pathology, particularly for optic nerve pathology 1
• For patients with junctional scotoma or bitemporal visual defects, MRI of the brain without and with contrast is recommended 1
• For patients with homonymous hemianopia or quadrantanopia, MRI of the brain without and with contrast is preferred 1

Intraocular and Soft Tissue Lesions

• MRI provides excellent soft-tissue resolution of structures within the orbit, including the globe, muscles, tendons, nerves, and vascular structures 1
• MRI is superior for showing retrolaminar optic nerve infiltrations, orbital invasion, and possible intracranial spread of tumors 5

Clinical Decision Algorithm

1. For trauma or suspected foreign body:

   • Order non-contrast thin-section orbital CT with multiplanar reconstructions 2, 4
   • Add CT head if concurrent head trauma or intracranial abnormality is suspected 2, 4

2. For suspected infection:

   • Order CT orbits with IV contrast 5, 1
   • Add CT head with IV contrast if intracranial complications are suspected 5

3. For visual loss:

   • If monocular and suspecting intraocular mass or optic nerve pathology: MRI orbits without and with contrast 1
   • If bitemporal or homonymous visual field defects: MRI brain without and with contrast 1

4. For proptosis or orbital mass:

   • MRI orbits without and with contrast is optimal for characterization 1
   • CT orbits is complementary for assessing bone involvement 1

Common Pitfalls to Avoid

• Orbital or skull radiographs are insufficient to detect orbital pathology and have been largely replaced by CT 1, 7
• Wooden foreign bodies may appear hypoattenuating on CT and can be mistaken for air 2
• CT head alone without dedicated orbital imaging may miss important orbital pathology 5, 8
• Using MRI as initial imaging when metallic foreign bodies are suspected is dangerous 2, 3