Orbital Computed Tomography

Orbital computed tomography (CT) allows visualization of abnonnalities not readily seen on standard radiographs, delineating their size, position, and relationship to adjoining structures. A series of tomograms reconstructed by a computer and displayed as anatomic slices on a monitor, the orbital CT scan identifies space-occupying lesions earlier and more accurately than other radiographic techniques and provides three-dimensional images of orbital structures, especially the ocular muscles and the optic nerve.

Purpose

• To evaluate pathologies of the orbit and eye-especially expanding lesions and bone destruction
• To evaluate nactures of the orbit and adjoining structures
• To detennine the cause of unilateral exophthalmos
• To aid diagnosis of intracranial lesions that affect vision
• To evaluate a patient with conditions such as suspected circulatory disorders, hemangioma, or subdural hematoma (with contrast enhancement)

Patient Preparation

• Describe the procedure to the patient, and explain that this test visualizes the anatomy of the eye and its surrounding structures.
• If contrast enhancement isn't scheduled, inform him that he needn't restrict food or fluids. If contrast enhancement is scheduled, withhold food and fluids from the patient for 4 hours before the test.
• Tell the patient that a series of X-ray films will be taken of his eye and who will perform the test and where.
• Reassure him that the test will cause him no discomfort and will take 15 to 30 minutes to perform.
• Tell the patient that he'll be positioned on an X-ray table and that the head of the table will be moved into the scanner, which will rotate around his head and make loud clacking sounds.
• If a contrast medium will be used for the procedure, tell the patient that he may feel flushed and warm and experience a transient headache, a salty or metallic taste, and nausea or vomiting after the contrast medium is injected. Reassure him that these reactions to contrast media are typical.
• Check his history for hypersensitivity reactions to iodine, shellfish, or contrast media.
• Instruct the patient to remove jewelry, hairpins, or other metal objects in the X-ray field to allow for precise imaging of the orbital structures.

Procedure And Posttest Care

• The patient is placed in a supine position on the X-ray table, with his head immobilized by straps if required. Ask him to lie still.
• The head of the table is moved into the scanner, which rotates around the patient's head taking radiographs.
• Information obtained is stored on magnetic tapes, and the images are displayed on a monitor. Photographs may be made if a permanent record is desired.
• When this series of radiographs has been taken, contrast enhancement is performed. The contrast medium is injected and a second series of scans is recorded.
• If a contrast medium was used, watch for its residual adverse effects, including headache, nausea, or vomiting. Following the procedure, advise the patient that he may resume his usual diet.

Precautions

• Use of contrast enhancement is contraindicated in those patients with known hypersensitivity reactions to iodine, shellfish, or contrast media used in other tests.

Normal Findings

Orbital structures are evaluated for size, shape, and position. Dense orbital bone provides a marked contrast to less dense periocular fat. The optic nerve and the medial and lateral rectus muscles are clearly defined. The rectus muscles appear as thin dense bands on each side, behind the eye. The optic canals should be equal in sizes.

Abnormal Findings

Orbital CT scans can identify intraorbital and extraorbital space-occupying lesions that obscure the normal structures or cause orbital enlargement, indentation of the orbital walls, or bone destruction. This test can also help determine the type of lesion. For example, infiltrative lesions, such as lymphomas and metastatic carcinomas, appear as irregular areas of density. However, encapsulated tumors, such as benign hemangiomas and meningiomas, appear as clearly defined masses of consistent density. CT scans can also visualize intracranial tumors that invade the orbit, thickening ofthe optic nerve that may occur with gliomas, meningiomas, and secondary tumors that may cause enlargement of the optic canal.

In evaluating fractures, CT scans allow a complete three-dimensional view of the affected structures. In determining the cause of unilateral exophthalmos, CT scans can show early erosion or expansion of the medial orbital wall that may arise from lesions in the ethmoidal cells. It can also detect space­occupying lesions in the orbit or paranasal sinuses that cause exophthalmos. CT scans can also show thickening of the medial and lateral rectus muscles in proptosis resulting from Graves' disease.

Interfering Factors

• Head movement
• Failure to remove metallic objects from examination field (possible poor imaging)