Computed Tomography

The orbit is particularly well suited to x-ray–based CT imaging because fat provides excellent contrast to the globe, lacrimal gland, optic nerve, and extraocular muscles. Bone and lesions that contain calcium can be easily visualized because of their marked x-ray attenuation. Soft-tissue details can be further enhanced with the injection of iodinated contrast material; this material crosses a disrupted blood–brain barrier and accumulates within a local lesion, revealing inflammatory and neoplastic processes. Iodinated contrast material can be used in patients with a calculated glomerular filtration rate (GFR) greater than 30 mL/min/1.73 m².

Most modern CT scanners image the patient with simultaneous sectioning (ie, spiral or helical) and have multiplanar reconstruction capabilities through the use of a volume-rendering method known as maximum intensity projection (MIP). The advantages of CT include its relatively low cost, rapid image acquisition, wide availability, and excellent spatial resolution. Its speed and ability to accurately identify acute blood or bone abnormalities make CT especially useful when examining trauma patients who may be confused or combative. However, CT scans carry the risk of ionizing radiation. Most national recommendations for dosages are below 150 milliGray (mGy) per scan. The advantages and disadvantages of CT are summarized in Table 2-1.

Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.