Orbit CT scan

Definition

A computed tomography (CT) scan of the orbit is an imaging method. It uses x-rays to create detailed pictures of the eye sockets (orbits), eyes and surrounding bones.

Alternative Names

CT scan - orbital; Eye CT scan; Computed tomography scan - orbit

How the Test is Performed

You will be asked to lie on a narrow table that slides into the center of the CT scanner. Only your head is placed inside the CT scanner.

You may be allowed to rest your head on a pillow.

Once you are inside the scanner, the machine's x-ray beam rotates around you but you won't see the x-ray.

A computer creates separate images of the body area, called slices. These images can be stored, viewed on a monitor, or printed on film. The computer can create three-dimensional models of the body area by stacking the slices together.

You must lie still during the exam, because movement causes blurred images. You may be asked to hold your breath for short periods.

The actual scan takes about 30 seconds. The entire process takes about 15 minutes.

How to Prepare for the Test

Before the test:

• You will be asked to remove jewelry and wear a hospital gown during the study.
• If you weigh more than 300 pounds (135 kilograms), find out if the CT machine has a weight limit. Too much weight can cause damage to the scanner's working parts.

Certain exams require a special dye, called contrast, to be delivered into the body before the test starts. Contrast helps certain areas show up better on the x-rays. Contrast can be given through a vein (intravenous- IV) in your hand or forearm.

Before the scan using contrast, it is important to know the following:

• You may be asked not to eat or drink anything for 4 to 6 hours before the test.
• Let your health care provider know if you have ever had a reaction to contrast. You may need to take medicines before the test in order to safely receive this substance.
• Tell your provider if you take the diabetes medicine metformin (Glucophage). You may need to take extra precautions.
• Let your provider know if you have poor kidney function. This is because the contrast can worsen the kidney function.

How the Test will Feel

Some people may have discomfort from lying on the hard table.

Contrast given through an IV may cause a slight burning sensation. You may also have a metallic taste in the mouth and a warm flushing of the body. These sensations are normal and most often go away within a few seconds.

Why the Test is Performed

This test is helpful for diagnosing diseases that affect the following areas around the eyes:

• Blood vessels
• Eye muscles
• Nerves supplying the eyes (optic nerves)
• Sinuses

An orbit CT scan may also be used to detect:

• Abscess (infection) of the eye area
• Broken eye socket bone
• Foreign object in the eye socket

What Abnormal Results Mean

Abnormal results may mean:

• Bleeding
• Broken eye socket bone
• Graves disease
• Infection
• Tumor

Risks

CT scans and other x-rays are strictly monitored and controlled to make sure they use the least amount of radiation. The risk associated with any individual scan is very low. The risk increases as more studies are done.

CT scans are done when the benefits greatly outweigh the risks. For example, it can be more risky not to have the exam, especially if your provider thinks you might have cancer.

The most common type of contrast given into a vein contains iodine.

• If a person with an iodine allergy is given this type of contrast, nausea, sneezing, vomiting, itching, or hives may occur.
• If you have a known allergy to contrast but need it for a successful exam, you may receive antihistamines (such as Benadryl) or steroids before the test.

The kidneys help filter the iodine out of the body. If you have kidney disease or diabetes, you should be closely monitored for kidney problems after contrast is given. If you have diabetes or have kidney disease, talk to your provider before the test to know your risks.

Before receiving the contrast, tell your provider if you take the diabetes medicine metformin (Glucophage) because you may need to take extra precautions. You may need to stop the medicine for 48 hours after the test.

In rare cases, the dye can cause a life-threatening allergic response called anaphylaxis. If you have any trouble breathing during the test, tell the scanner operator right away. Scanners come with an intercom and speakers, so the operator can hear you at all times.

References

Bowling B. Orbit. In: Bowling B, ed. Kanski's Clinical Ophthalmology. 8th ed. Philadelphia, PA: Elsevier; 2016:chap 3.

Chernecky CC, Berger BJ. Cerebral computed tomography-diagnostic. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. Philadelphia, PA: Elsevier Saunders; 2013:310-312.

Guluma K, Lee JE. Ophthalmology. In: Walls RM, Hockberger RS, Gausche-Hill M, eds. Rosen's Emergency Medicine: Concepts and Clinical Practice. 9th ed. Philadelphia, PA: Elsevier; 2018:chap 61.

Poon CS, Abrahams M, Abrahams JJ. Orbit. In: Haaga JR, Boll DT, eds. CT and MRI of the Whole Body. 6th ed. Philadelphia, PA: Elsevier; 2017:chap 20.