Computed tomography (CT) and magnetic resonance imaging (MRI) are the primary studies for evaluation of orbital disorders. Ultrasonography (echography) may also be helpful for some disorders.
Computed tomography is essential in the management of orbital disorders. Tissues in a tomographic plane are assigned a density value proportional to their coefficient of absorption of x-rays. Either 2- or 3-dimensional images are digitally constructed from these density measurements.
CT is the most valuable technique for delineating the shape, location, extent, and character of lesions in the orbit (Fig 2-4). CT helps refine the differential diagnosis; and when orbitotomy is indicated, CT helps guide the selection of the surgical approach by showing the relationship of the lesion to the surgical space or spaces of the orbit. The resolution and soft-tissue contrast of CT are adequate for visualizing nearly all pathologic processes in the orbit, and the bony resolution is superior to that provided by any other modality, making CT the imaging technique of choice for orbital trauma and bony tumors. Orbital CT scans are usually obtained in 3-mm sections (as opposed to the thicker 5-mm sections typically utilized in head CT scans). For greater detail, fine cuts at 1.0-mm intervals may be requested.
The visualization of tumors that are highly vascularized (eg, meningioma) or that have altered vascular permeability is improved by the use of intravenous contrastenhancing agents. Contrast is also helpful to identify an orbital abscess. If contrast is desired, it must be specifically ordered as part of the study. The resolution and tissuecontrast capabilities of CT allow imaging not only of bone but also of soft tissue and foreign bodies.
Figure 2-4 Orbital bones, internal views. A, Lateral wall. B, Medial wall.
(Courtesy of Bobby S. Korn, MD, PhD.)
Orbital images can be obtained in the axial plane, parallel to the course of the optic nerve; in the coronal plane, showing the eye, optic nerve, and extraocular muscles in cross section; or in the sagittal plane, parallel to the nasal septum. CT scanners use software to reconstruct (reformat) any section in any direction (axial, coronal, or sagittal). Modern spiral (helical) CT scanners have multiple detector ports, and the scanner and the collecting tube move in a spiral fashion around the patient, generating a continuous data set. This results in rapid acquisition of a larger volume of data that allows highly detailed reconstructions in all imaging planes. Direct coronal scans are ideal for evaluation of the optic nerve and extraocular muscles as well as the bony roof and floor of the orbit. Three-dimensional CT allows reformatting of CT information into 3-dimensional projections of the bony orbital walls (Fig 2-5). Because this type of imaging requires thin sections and additional computer time, it is typically reserved for use in preparation for craniofacial surgery or repairs of complex orbital fractures.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.