You will be asked to lie on a narrow table that slides into the center of the CT scanner.

Once you are inside the scanner, the machine's x-ray beam rotates around you. Modern "spiral" scanners can perform the exam without stopping.

A computer creates separate images of the spine area, called slices. These images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of the spine area can be created by adding the slices together.

You must be still during the exam. Movement can cause blurred images. You may be told to hold your breath for short periods of time.

The scan should take only 10 to 15 minutes.

Some exams use a special dye, called contrast that is put into your body before the test starts. Contrast helps certain areas show up better on the x-rays.

Contrast can be given in different ways.

• It may be given through a vein (IV) in your hand or forearm.
• It may be given as an injection into the space around the spinal cord.

If contrast is used, you may also 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 avoid this problem.

Before having the contrast, tell your provider if you take the diabetes medicine metformin (Glucophage). You may need to take extra steps before the test if you take this drug.

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.

You will be asked to remove jewelry and wear a hospital gown during the study as metal can affect the CT images.

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

Contrast given through an IV may cause a slight burning feeling, a metal taste in the mouth, and a warm flushing of the body. These feelings are normal and go away in a few seconds.

CT scans rapidly makes detailed pictures of the lower back. The test may be used to look for:

• Birth defects of the cervical spine in children
• Spine problems, when a spine MRI cannot be used
• Injury to the upper spine
• Bone tumors or cancers
• Fracture of a spine bone
• Disk herniations and compression of the spinal cord
• Healing problems or scar tissue following surgery

This test can also be used during or after an x-ray of the spinal cord and spinal nerve roots (myelography) or an x-ray of the disk (discography).

Results are considered normal if no problems are seen in the lumbar region in the images.

Abnormal results may be due to:

• Degenerative changes due to age
• Birth defects of the spine
• Bone problems
• Fracture of a spine bone
• Lumbar disk herniation
• Lumbar spinal stenosis
• Spondylolisthesis
• Healing or growth of scar tissue after surgery

Risks of CT scans include:

• Being exposed to radiation
• Allergic reaction to contrast dye
• Birth defect if done during pregnancy

CT scans expose you to more radiation than regular x-rays. Having many x-rays or CT scans over time may increase your risk for cancer. However, the risk from any one scan is small. Talk to your provider about this risk and how it weighs against the benefits of the test for your medical problem.

Some people have allergies to contrast dye. Let your provider know if you have ever had an allergic reaction to injected contrast dye.

• 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 or vomiting, sneezing, itching, or hives may occur.
• If you must have this type of contrast, you may get antihistamines (such as Benadryl) or steroids before the test.
• The kidneys help remove iodine out of the body. People with kidney disease or diabetes may need to receive extra fluids after the test to help flush the iodine out of the body.

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

The lumbar CT scan is good for evaluating large herniated disks, but it can miss smaller ones. This test can be combined with a myelogram to get a better image of the nerve roots and pick up smaller injuries.

CAT scan - lumbar spine; Computed axial tomography scan - lumbar spine; Computed tomography scan - lumbar spine; CT - lower back

Grawboski T, Gilbert TM, Larson EP, Cornett CA. Degenerative conditions of the cervical and thoracolumbar spine. In: Miller MD, Thompson SR, eds. DeLee, Drez, & Miller's Orthopaedic Sports Medicine. 5th ed. Philadelphia, PA: Elsevier; 2020:chap 130.

Kapoor G, Toms AP. Current status of imaging of the musculoskeletal system. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology. 7th ed. Philadelphia, PA: Elsevier; 2021:chap 38.

Williams KD. Fractures, dislocations, and fracture-dislocations of the spine. In: Azar FM, Beaty JH, eds. Campbell's Operative Orthopaedics. 14th ed. Philadelphia, PA: Elsevier; 2021:chap 41.

Updated by: C. Benjamin Ma, MD, Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery, San Francisco, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.