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.
Procedure And Posttest Care
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.
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.Enhancement with a contrast medium may provide information about the circulation through abnormal ocular tissues.
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