A computed tomography (CT) scan of the lumbar spine is an imaging method that uses x-rays to create cross-sectional pictures of the lower back (lumbar spine).

You will be asked to lie on a narrow table that slides into the center of the CT scanner. You will lie on your back with your arms up over your head.

Once you are inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam in one continuous motion.)

Small detectors inside the scanner measure the amount of x-rays that make it through the lower back. A computer takes this information and uses it to create several individual images, called slices. These images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of the lumbar spine can be created by stacking the individual slices together.

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

The exam should take 15 - 30 minutes.

Certain exams require a special dye, called contrast, to be delivered into the body before the test starts. Contrast can highlight specific areas inside the body, which creates a clearer image.

Some people have allergies to IV contrast and may need to take medications before their test in order to safely receive this substance.

Contrast can be given in several ways.

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

If contrast is used, you may also be asked not to eat or drink anything for 4-6 hours before the test.

If you weigh more than 300 pounds, have your doctor contact the scanner operator before the exam. CT scanners have a weight limit. Too much weight can cause damage to the scanner's working parts.

Since x-rays have difficulty passing through metal, you will be asked to remove jewelry and wear a hospital gown during the study.

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

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

CT rapidly creates detailed pictures of the lower back. The test may be used to evaluate:

• Birth defects of the spine in children
• The lower spine after severe trauma
• Other spine problems when MRI cannot be used (see: lumbosacral spine MRI)

Lumbar CT scan 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 the lumbar region is normal in appearance.

Abnormal results may be due to:

• Birth defects of the spine
• Bone problems
• Fracture
• Lumbar disk herniation
• Lumbar spinal stenosis
• Spondylolisthesis

CT scans and other x-rays are strictly monitored and controlled to make sure they use the least amount of radiation. CT scans do create low levels of ionizing radiation, which has the potential to cause cancer and other defects. However, the risk associated with any individual scan is small. The risk increases as numerous additional studies are performed.

In some cases, a CT scan may still be done if the benefits greatly outweigh the risks. For example, it can be more risky not to have the exam.

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. Let your doctor know if you have ever had an allergic reaction to injected contrast dye.

If you absolutely must be given such contrast, your doctor may choose to treat you with antihistamines (such as Benadryl) or steroids before the test.

The kidneys help filter the iodine out of the body. Therefore, those with kidney disease or diabetes should receive plenty of fluids after the test, and be closely monitored for kidney problems. If you have diabetes or are on kidney dialysis, talk to your health care provider before the test about your risks.

Before receving the contrast, tell your health care provider if you take the diabetes medication metformin (Glucophage) because you may need to take extra precautions.

Rarely, the dye may cause a life-threatening allergic response called anaphylaxis. If you have any trouble breathing during the test, you should notify the scanner operator immediately. 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.

Curlee PM. Other disorders of the spine. In: Canale ST, Beaty JH. Campbell’s Operative Orthopaedics. 11th ed. Philadelphia, Pa: Mosby Elsevier;2007:chap 21.

Chou R, Qaseem A, Snow V, Casey D, Cross JT Jr., Shekelle P, et al. Diagnosis and treatment of low back pain: a joint clinical practice guideline from the American College of Physicians and the American Pain Society. Ann Intern Med. 2007;147:478-491.