Orbital Imaging

Introduction

The orbit of the eye comprises the globe with its muscles, blood vessels, lymphatics, cranial nerves 2, 3, 4, 5, and 6, sympathetic and parasympathetic nerves, adipose tissue, connective tissue, and the lacrimal apparatus. The pyramid-shaped cavity has its apex directed posteromedially with its base being the orbital opening. Superiorly is the anterior cranial fossa; medially lie the ethmoid sinuses, sphenoid sinus, and the nasal cavity; inferiorly is the maxillary sinus; and laterally is the temporal fossa. The apex is continuous with the middle cranial fossa. Thus, the orbit is situated in a strategic location and can be involved in many different processes originating from within or from its surrounding tissues. Orbital imaging plays an important role in the diagnosis and management of disorders of the orbit.

This tutorial discusses the various modalities that have been used to diagnose orbital disorders.

Orbital Radiology

Plain radiography has a limited role in the diagnosis of orbital disease. However, it may be of some value in the detection of radio opaque foreign bodies if a computed tomography scanner is not available.

Orbital Venography

Orbital venography was used to detect the presence of an intraorbital mass. Distortion of the orbital vein configuration indicated an abnormality. This modality is no longer used.

Orbital Contrast Studies

In orbital contrast studies, contrast dye is injected into optic nerve sheath to identify optic nerve tumors. These studies are dangerous and, therefore, abandoned as a diagnostic procedure.

Orbital Arteriography

Most orbital lesions do not have a significant blood supply to be revealed by arteriography. However, orbital arteriovenous malformations, dural fistulas, carotid-cavernous fistula, hemangiopericytoma and, sometimes, meningiomas are well demonstrated on angiography.

Orbital Ultrasonography

Ultrasound is inexpensive, easy to use, and readily available in many ophthalmologists’ offices. Primarily used as a screening technique, it is an invaluable component in diagnosis of orbital disease. Ophthalmic ultrasound uses frequencies ranging from 6 MHz to 20 MHz. A lower frequency provides a higher penetration but a lower resolution. Most B-scan units use 10-MHz probes. B-scan allows real time evaluation of the orbital lesions. The orbit is best evaluated with a contact B-mode ultrasound supplemented with an A-scan (standardized echography). Strong echoes are displayed as brighter areas, while weaker echoes are displayed as dimmer areas. The A-mode is a time amplitude display. The vertical amplitude represents the signal’s amplitude. Ultrasonography detects lesions in the anterior one third of the orbit between the globe and the bones and structures immediately behind the globe.

Extraocular muscle size and contour is easily seen by B-scan. Bilateral asymmetric involvement is typically seen in thyroid-associated orbitopathy. Dilation of superior ophthalmic vein is found in high flow carotid cavernous fistulas or dural arteriovenous malformations. A "T sign" is characteristically seen in posterior scleritis, due to the inflammatory scleral thickening with episcleral edema and adjacent optic nerve sheath inflammation. Space occupying lesions such as vascular tumors, solid tumors, cystic lesions and foreign bodies may be detected on ultrasonography.

Use of color duplex ultrasound in the evaluation of temporal arteritis, superior ophthalmic vein occlusion, carotid-cavernous fistula, and ocular ischemic syndrome is being investigated.

Orbital Indications for Ultrasonography

• Orbital mass
• Extraocular muscle evaluation
• Globe evaluation
• Optic nerve drusen
• Orbital inflammation
• Orbital foreign body (wood)
• Cavernous or dural sinus fistulas
• Orbital vascular anomalies

Limitations Ultrasonography is useful for detecting anterior and immediate retro bulbar lesions. It is difficult to assess the apex with ultrasonography. Ultrasonography is a convenient screening tool to be supplemented with more sophisticated imaging techniques.

Computerized Tomography

Computerized tomography (CT) is most likely the most important technologic advance in the evaluation and management of orbital disease. CT images are based on x-ray attenuation coefficient of tissues (Hounsfield scale, –1000 to +1000). It is the modality of choice for bony detail and for detecting calcifications and foreign bodies. Irradiation is a disadvantage. A routine CT examination of the orbit provides 5-mm slices; 3-mm slices are optimal for smaller lesions and 1.5-mm slices are needed when looking for foreign bodies. Direct coronal sections are informative in the assessment of fractures and muscle abnormalities. Contrast material is often necessary to evaluate vascular characteristics, extraorbital extension of an orbital process, and inflammatory lesions, and in differentiation of optic nerve meningioma from gloom. Generally, contrast is unnecessary for evaluating foreign bodies, fractures (trauma), uncomplicated thyroid-associated orbitopathy, dermoid cysts, or bony tumors like osteomas. Axial sections are obtained parallel to the infraorbital metal line. It should include the upper portion of the maxillary sinus and the whole frontal sinus. The coronal sections are roughly perpendicular to the axial section. In some instances they are obtained semicoronal (oblique), especially when patients have many dental fillings.

Orbital Indications for CT Scanning

• Unexplained proptosis
• Unexplained ophthalmoplegia
• Afferent dysfunction believed to be due to an orbital lesion
• Orbital cellulitis
• Orbital mass
• Orbital trauma
• Restrictive dysmotility
• Orbital foreign body

Limitations Radiation exposure is a concern, especially in cases that require repeat scans for monitoring. CT is not the ideal modality for soft tissue evaluation. Direct coronal scans may not be obtainable in acutely injured patients or in patients with cervical spine problems. Apical lesions are not imaged well by CT. Cost of scanning is less of an issue now than in the past.

Magnetic Resonance Imaging

In magnetic resonance imaging (MRI), a magnetic field is created and the release of protons (hydrogen in water) by radio frequency signals (pulse frequency) is detected and recorded as a digital image. The resolution of images depends on various parameters.

1. As the amount of mobile protons increases or decreases so does the signal (e.g., bone has little or no signal).
2. T1-weighted (T1W) images provide better spatial resolution, that is, better anatomic detail.
3. T2-weighted (T2W) images show better contrast resolution, that is, better pathologic detail.
4. Short time inversion recovery (STIR) images suppress fat signals and add T1W and T2W information (pathology appears brighter).
5. Fat suppression technique (T1W) is useful in orbital diagnosis.
6. Gadolinium enhancement (T1W). Pituitary gland and venous sinus enhancement is normal.
7. Motion Artifact Suppression Technique (MAST) is helpful for imaging the anterior orbit.
8. Coils (surface and head coils) increase spatial definition and decrease acquisition time.

Typically, 2-mm to 3-mm slices are preferred. Scanning times are 2 to 4 minutes for T1W images and 7 to 10 minutes for T2W images.

Standard orbital MRI protocols should include T1W and T2W axial images, fat suppressed axial T1W images, and contrast enhanced axial and coronal T1W images with fat suppression.

Lymphangiomas and hemangiomas are best evaluated on T2W images.

Indications for MRI

• Optic nerve trauma
• Unusual orbital inflammation
• Posterior or apical orbital tumors
• Afferent dysfunction believed to be due to a posterior orbital lesion
• Optic nerve lesions
• Suspected intracranial extension of an orbital lesion
• Suspected orbital involvement of an intracranial process

Limitations The small gantry poses problems with claustrophobic patients. MRI is contraindicated in people with ferromagnetic implants or foreign bodies. CT provides more accurate bone evaluation. Motion artifact is sometimes a problem. The expense of MRI scanning is also a factor to consider.

Intraconal Lesions

• Cavernous hemangioma
• Optic nerve meningiomas
• Optic nerve glioma
• Neurofibroma/neurilemmoma
• Lymphoma
• Lymphangioma
• Nonspecific orbital inflammatory syndrome
• Varix
• AVM
• Hemangiopericytoma
• Metastasis
• Orbital cellulitis
• Optic neuritis
• Rhabdomyosarcoma

Extraconal Lesions

• Lacrimal gland tumors/lesionsc
• Lymphangioma
• Dermoid and epidermoid cysts
• Lymphoma
• Peripheral nerve tumors
• Rhabdomyosarcoma
• Sarcoidosis
• Enlarged superior ophthalmic vein

Enlarged Extraocular Muscles

• Thyroid associated orbitopathy
• Nonspecific orbital inflammatory syndrome
• Lymphoma
• Hemangiopericytoma
• Arteriovenous malformation
• Metastasis

Sub-periosteal Lesions

• Abscess
• Neoplastic extension from sinuses
• Sinus mucocele
• Spheno-orbital meningioma
• Lymphoma
• Leukemia
• Plasmacytoma
• Osteoma
• Cholesterol granuloma
• Metastasis

Orbital Calcification

• Congenital Fanconi syndrome
• Hyper- or hypoparathyroidism
• Intraocular calcifications

Cataract

Granulomas

Optic nerve drusen

Phthisis bulbi

Retinoblastoma

Tuberous sclerosis

von Hippel-Lindau disease

• Extraocular calcifications

Amyloid

Choristoma

Dermolipoma

Malignant lacrimal gland tumor

Arteriovenous malformation

Varix

Glioma

Cavernous hemangioma

Meningioma

Teratoma

Hemangiopericytoma

Hemorrhage

Bibliography

Kennerdell JS, Cockerham KP. Orbital diagnosis. In: Nesi FA, Lisman RD, Levine MR, eds. Smith’s Ophthalmic Plastic and Reconstructive Surgery. St. Louis, Mo: Mosby Yearbook; 1998:705-712.

Kennerdell JS, Cockerham KP. Ultrasonography of the orbit. In: Nesi FA, Lisman RD, Levine MR, eds. Smith’s Ophthalmic Plastic and Reconstructive Surgery. St. Louis, Mo: Mosby Yearbook; 1998:713-726.

Kennerdell JS, Cockerham KP. Conventional radiology of the orbit. In: Nesi FA, Lisman RD, Levine MR, eds. Smith’s Ophthalmic Plastic and Reconstructive Surgery. St. Louis, Mo: Mosby Yearbook; 1998:727-777.

Kennerdell JS, Cockerham KP. Computed tomography of the orbit. In: Nesi FA, Lisman RD, Levine MR, eds. Smith’s Ophthalmic Plastic and Reconstructive Surgery. St. Louis, Mo: Mosby Yearbook; 1998:778-806

Kennerdell JS, Cockerham KP. Magnetic resonance imaging of the orbit. In: Nesi FA, Lisman RD, Levine MR, eds. Smith’s Ophthalmic Plastic and Reconstructive Surgery. St. Louis, Mo: Mosby Yearbook; 1998:807-827.