CT anatomy of the orbit:

The orbital cavity is a pyramidal-shaped socket of the skull; its walls are formed by the frontal, ethmoid, lacrimal, zygomatic, sphenoid, palatine bones, and maxilla (Fig 1). The orbital cavity is subdivided into four compartments including the eyeglobe, the muscle cone, the intra-, and extraconal spaces. The eye globe is a ball-like structure at the base of the orbital cavity, encircled with the tenon’s capsule with three layers making its wall; the sclera, uvea, and retina. These layers are difficult to distinguish at CT imaging. The lens separates the eyeglobe into anterior and posterior compartments (Fig 2). The muscle cone formed by the extra-ocular muscles separates the intra-conal from the extraconal compartment (Fig 3). The extraconal compartment includes the lacrimal gland (Fig 4), and the orbital fat. The optic nerve with its meningeal and CSF surroundings located within the intraconal space. The orbital cavity is connected to the intracranial cavity through the optic canal, and the superior orbital fissure, while it is connected to the infratemporal fossa through the inferior orbital fissure (Fig 5).

Fig 1: CT anatomy of the orbital walls. Reference: M. Ashour, Cairo/Egypt

Fig 2: CT anatomy of the eyeglobe. Reference: M. Ashour, Cairo/Egypt

Fig 3: CT anatomy of the extra-ocular muscles. Reference: M. Ashour, Cairo/Egypt

Fig 4: CT anatomy of the orbital appendages. Reference: M. Ashour, Cairo/Egypt

Fig 5: CT anatomy of the orbital cavity foramina. Reference: M. Ashour, Cairo/Egypt

Don’t forget the orbit:

Applying a systematic approach and the use of different window settings help to overcome these limitations and to avoid missing orbital findings. For any scan including the orbit in its field, we should use a checklist (Fig 6) for evaluation of the orbit including its different compartments and the periorbital tissues. Changing the display settings for the window level and width is needed to well-assess the fat planes and the orbital walls.

Fig 6: Checklist for interpretation of the orbits in dedicated and non-dedicated...

Review of the orbital pathologies:

• Traumatic pathologies:

• Open Globe Injury: is post-traumatic break down of the scleral or corneal globe outline. On CT images, the eye globe shows deformed contour with reduced volume, a change in anterior chamber depth (Fig 7), intraocular foreign body, and air locules might be seen.

• Intraocular foreign bodies (FBs): CT appearance of the intraorbital FBs depend on their composition. Metallic objects are easily identified at CT (Fig 7), plastic FBs are hyperattenuating but much less than metallic objects, the attenuation of glass is variable.  The attenuation of inorganic FBs as wood varies by time; in the initial phase the wood is hypoattenuating mimicking air over time and is hyperattenuating in the later chronic phase.
  

  Fig 7: Right globe anterior chamber rupture with intraocular hyperdense foreign...

• Orbital Fractures: different fracture types are described in the literature that includes Le Fort II and III fractures, naso-orbital-ethmoid complex fractures, zygomaticomaxillary complex fractures, and orbital blow-out fractures (Fig 8).
  

  Fig 8: Left-sided medial orbital wall blow-out fracture with herniation of the medial...
• Lens dislocation: dislocation may be partial with an abnormal lens orientation. In Complete lens dislocation, the lens usually shows dependent dislocation into the posterior segment while anterior segment dislocation is much less common.

• Traumatic cataract: in post-traumatic cataract, the lens shows diminution in its density compared to the other healthy lens (Fig 9).
  

  Fig 9: Right-sided post-traumatic cataract in a 4-year-old child with acute diminution...
• Retinal Detachment: the retina is firmly attached to the ora Serrata anteriorly and the optic disc posteriorly so retinal detachment usually shows a V-shaped appearance on imaging.
• Choroidal Detachment: usually presents with a biconvex shape sparing the optic disc on imaging (Fig 10).
  

  Fig 10: Right-sided choroidal detachment with a biconvex appearance, seen diverging at...
• Vitreous Hemorrhage: it appears as a hyperdense fluid within the posterior segment (Fig 11).
  

  Fig 11: Left-sided intra-vitreal hyperdense flame-shaped hemorrhage related to the...
• Orbital Compartment Syndrome: is a serious emergency. Its most common cause is post-traumatic hemorrhage. On imaging it appears as severe proptosis, posterior eyeglobe contour tenting, and optic nerve stretching (Fig 12).
  

  Fig 12: Left eyeglobe proptosis with tenting of its posterior contour and stretching of...

• Non-traumatic pathologies:

• Inflammatory and Infectious pathologies: Inflammation may be confined to one orbital compartment and may involve more than one of them combined up to panophthalmitis (Fig 13).
  

  Fig 13: Left-sided panopthalmitis involving all left orbital compartments. Reference:...

• Preseptal cellulitis is seen as stranding and swelling of the periorbital tissues anterior to the orbital septum (Fig 14).
  

  Fig 14: Bilateral pre-septal cellulitis in a 4-month-old infant. Reference: M. Ashour,...
• Orbital cellulitis is deep orbital post-septal inflammation. On CT images it is seen as post-septal soft-tissue stranding and edema (Fig 15), it may progress to an abscess.
  

  Fig 15: Non-CE-CT scan, axial section showing left orbital cellulitis and optic...
• Subperiosteal abscess is seen as peripherally enhanced fluid collection along the orbital wall, most commonly secondary to acute ethmoiditis (Fig 16).
  

  Fig 16: Non-CE-CT scan, axial section showing left-sided subperiosteal abscess...
• Endophthalmitis involves an intraocular extension of infection.
• Optic neuritis: it is inflammation of the optic nerve itself due to either infectious or non-infectious causes. On imaging, the optic nerve is enlarged in caliber with post-contrast enhancement in the acute phase (Fig 15), and it may appear atrophic in chronic cases.
• Orbital pseudotumor: it has non-specific imaging features. It can affect the tendons and the muscle bellies of the extra-ocular muscles, and the retro-orbital fat with significant post-contrast enhancement, and it may also appear as an infiltrative lesion.
• Orbital myositis: it appears as thickening and enhancement of one or two extra-ocular muscles and could become atrophic in chronic cases (Fig 17).
  

  Fig 17: Atrophy of the left lateral rectus muscle in a female patient with systemic...

• Endocrine pathologies: Thyroid ophthalmopathy is seen as extra-ocular muscles enlargement with tendinous sparing, and exophthalmos (Fig 18).
  

  Fig 18: Symmetrical enlargement of the extra-ocular muscles bellies with sparing of...

• Neoplastic pathologies:
• Metastases are the commonest intraocular malignancy in adults (Fig 19). Choroidal melanoma is the commonest primary ocular malignancy and it appears as a hyperdense lesion on CT images with post-contrast enhancement.
  

  Fig 19: Bilateral hyperattenuating intra-ocular metastases centered on the retina in a...
• Optic glioma: it appears as a fusiform enlargement of the optic nerve with kinking with variable post-contrast enhancement (Fig 20).
  

  Fig 20: Left fusiform soft tissue thickening of the left optic nerve in a case of optic...
• Orbital meningioma: Meningioma is hyperdense on CT with calcifications, either primary or secondary (Fig 21). Optic nerve meningioma appears as a fusiform enlargement of the optic nerve sheath with 'tram-track' enhancement.
  

  Fig 21: Right sphenoid bone intra-osseous meningioma with extra-osseous soft tissue...
• Orbital lymphoma: usually secondary to systemic non-Hodgkin's lymphoma. Lymphoma is hyperdense on CT with avid post-contrast enhancement (Fig 22).
  

  Fig 22: Non-CE-CT scan, axial section showing bilaterally enlarged and hyperattenuating...

• Vascular pathologies: Vascular lesions are classified into capillary haemangiomas, venous vascular malformations, veno-lymphatic malformations (Fig 23), arterial and arterio-venous lesions.
  

  Fig 23: CE-CT, sagittal reconstruction showing left infra-orbital lobulated enhancing...

Mimics of intra-ocular FBs:

Calcification: commonly occurs in characteristic locations in most cases. Trochlear calcifications have a typical superomedial location within the orbit, and scleral calcifications are seen at the insertion sites of the medial and lateral recti (Fig 24). Optic nerve drusen appear (Fig 25) as punctate calcifications near the optic disc.

Fig 24: Hyperdense scleral calcific foci at the insertion of the medial...

Fig 25: Left optic nerve drusen. Case courtesy of Assoc Prof Frank Gaillard, <a...

Mimics of Open-Globe Injury:

There are congenital and acquired deformities of the globe e.g. coloboma (Fig 26), staphyloma (Fig 27), congenital glaucoma, high myopia, and phthisis bulbi (Fig 28).

Fig 26: Right orbital coloboma presented with a small-sized right eyeglobe with a...

Fig 27: Bilateral increase in the anteroposterior diameter of the eyeglobe in a patient...

Fig 28: Left orbital physis bulbi seen deformed, small-sized with dystrophic...