What Is a CT Scan?

Morsa Images / Getty Images
Morsa Images / Getty Images

Medically reviewed by Anju Goel, MD, MPH

Computed tomography—also known as a CT scan or CAT scan—is a painless medical imaging scan a healthcare provider may use to identify any signs of an injury or illness in your body or to determine if a treatment is working. In most cases, a lab technician will conduct the test while a radiologist (or, a doctor who specializes in medical imaging) will interpret the images from the scan.

The word "tomography" in the CT scan name is from the Greek word "tomos," which means slices or sections. In conjunction, the purpose of a CT scan is to generate sections of images of your organs to help your provider take a closer look at your body and look for concerns or abnormalities.


Healthcare providers can order a CT scan to diagnose a number of conditions. You may be asked to undergo a CT scan if your provider suspects you have symptoms of one of the following conditions:

In addition to diagnosing conditions, a healthcare provider may prescribe a CT scan to assess the effectiveness of certain treatments, such as radiation for a cancerous tumor or the success of a surgical procedure, like an organ transplant.

Types of CT Scans

There are two primary types of CT scans: those that use contrast and those that do not. Contrast is a type of colored dye that your provider can inject into your body or ask you to digest through a liquid or pill that can help the images of the CT scan better visualize your organs.

  • Without contrast: Radiologists can administer some CT scans without using any contrast agents. This may be the case if you have an allergy to the contrast or a history of kidney conditions, which can make the dye harmful for your body to ingest. In some cases, you may not need contrast to diagnose certain conditions, such as a fracture or a broken bone, which can show up in the imaging scan without a contrast dye.

  • With contrast: While X-rays can traditionally produce quality images of bones, the images of soft tissues are more difficult to obtain. For this reason, some healthcare providers order a CT scan with contrast agents. This can help make your organs more visible on a CT scan as well as detect blockages in your blood vessels.

How Does It Work?

A CT scanner uses a specialized device that rotates around your body to use X-rays that generate the images. Each rotation of the CT machine is a "slice" that recreates a portion of the body. This effect allows a CT scanner to recreate a three-dimensional image of your body, which is different from an X-ray—which creates a two-dimensional image instead.

CT scanners can work very rapidly to generate images. Most scans will take 30 minutes or less. For this reason, healthcare providers often prescribe CT scans for children or others who may have a difficult time staying still, such as someone who is severely injured.

Before the Test

In most cases, your healthcare provider will send you to the radiology department in a hospital or clinic to complete the scan. On the day of the test, you may be asked to fill out a consent form to acknowledge the risk and precautions of the CT scan. Be sure to bring your ID and insurance or hospital card to your appointment.

Another note to keep in mind: some CT scanners have weight limitations because they involve using a moving table that must fit into the scanner. If you are concerned about the weight limit, talk to your radiologist about your questions before undergoing the scan. Generally, there is a 450-pound limit on the table.

During the Test

A CT test is a painless and non-invasive (meaning, a lab technician will not be cutting into your skin) scan. While each scan can slightly vary, you can expect the scan to occur in the following steps:

  • Step 1: Get hooked up to an IV if you are going to receive contrast agents.

  • Step 2: Lie down flat on the CT table. If you're uncomfortable, it's OK to ask for a pillow or cushion to place under your head, knees, or back.

  • Step 3: Move into the scanner machine when the lab technician starts the scan as you are lying down on the table.

  • Step 4: Briefly hold your breath and restrict or minimize your movements to ensure the CT scanner obtains the best images possible.

After the Test

Once you complete the test, the lab technician will confirm if the images are of good quality. Because the CT scan sends the images to a computer, the technician can often upload or e-mail the scans to a secure server so your primary healthcare provider has access to them. The radiologist or your provider can later review the images to identify any signs of concern.

When the scan is complete and the technician gives you the all-clear, you can go home. In most cases, people who receive CT scans don't experience any symptoms. However, if you have a fever, nausea, or shortness of breath, it's a good idea to contact your healthcare provider and let them know about your symptoms.

Risks and Precautions

CT scans do use radiation to obtain the images. Exposure to high radiation levels has the potential to cause harmful effects, such as increased risks for cancer. However, a one-time CT scan does not usually generate enough radiation to cause severe adverse effects.

Another risk for CT scan is from the administration of IV contrast agents because some people can have allergic reactions to the dyes. In some cases, contrast has also been known to worsen kidney damage in people who have a history of kidney problems. Make sure to tell your provider and radiologist about your medical history prior to the CT scan, if they haven't already asked.

It's important to note that you can undergo a CT scan if you are pregnant. However, because a CT scan involves a small amount of radiation exposure, a healthcare provider may recommend waiting until you are not in your first trimester or until after you give birth to perform the imaging test if the reason for the scan is not an emergency.

How to Prepare for a CT Scan

You don't usually need to do anything special to prepare for a CT scan. However, you can expect to remove some or all of your clothing, depending on where on your body the CT scan will be taking images. That said, it's a good idea to comfortable clothing that can be easily removed, such as clothes without accessories, zippers, or buttons.

If you are going to be receiving a CT scan with a contrast agent, your provider may recommend to stop eating, drinking, and taking certain medications several hours before your imaging test. However, this varies from person to person and you should ask your provider of any rules you should be following specific to the type of CT scan you're getting.


The timeframe for receiving your results depends upon if the situation is an emergency or not. If you need the results urgently, your provider can look at the scans on the same day of your test. In most cases however, your provider will usually examine your results one to two days after your test and schedule a follow-up appointment via phone or in person to discuss the findings.

While a CT scan can provide helpful information about a number of medical conditions, there are limitations to the scan. For example, a CT scan may not effectively take images of the brain, joints, gallbladder, or pelvic organs.

If your provider lets you know they found something on your CT scan, this can help you reach a diagnosis for a condition or inform them on how to move forward with treatment. If your test results come back inconclusive (meaning, the images didn't lead to a firm conclusion), your provider may recommend additional imaging studies or other diagnostic measures—such as blood or urine tests.

Keep in mind, if your provider doesn't find anything on your CT scan, it doesn't automatically mean you're in the clear. If you're still experiencing symptoms, but your provider is unsure of the underlying cause, they may order additional tests, try certain treatments to help reduce symptoms, or wait until symptoms subside on their own.

A Quick Review

A CT scan is a painless, imaging test that helps your provider take a closer look at your organs and identify signs of concern (e.g., a broken bone, injury, fracture, or tumor). There are two types of CT scans: those that use a contrast dye and those that do not. The type of CT scan you undergo will depend on what your provider is looking for.

If your provider suggests that you may need a CT scan, talk to them about what the procedure of the imaging scan will look like, what you need to do to prepare, and when you can expect results. If you're still experiencing symptoms, but the results of the CT scan don't show anything conclusive, it's important to ask your provider for other testing options.

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