A CT scan (also called a CAT scan) takes detailed cross-sectional images of your body using X-rays that rotate around you. The actual scanning takes fewer than five minutes, though your total appointment will likely run about an hour once preparation is factored in. Here’s what happens at each stage.
How the Machine Works
A CT scanner looks like a large donut with a flat bed extending through the center. Inside that donut-shaped structure, called a gantry, an X-ray tube spins in a circle while digital detectors sit directly opposite it. As the tube rotates, it fires narrow beams of X-rays through your body from hundreds of different angles. The detectors on the other side pick up those beams and send the data to a computer.
Each full rotation produces one thin, two-dimensional “slice” of your body. Once a slice is captured, the bed moves forward a small increment and the process repeats. The computer then stacks all those slices together, creating a highly detailed three-dimensional picture that can show bones, organs, blood vessels, and soft tissue in ways a standard X-ray cannot.
Preparing Before Your Appointment
Preparation depends on which part of your body is being scanned. You may be asked to fast for several hours beforehand, especially if contrast material will be used. You’ll also need to remove anything containing metal: jewelry, hair clips, bras with underwire, and clothing with zippers or snaps. Most facilities provide a hospital gown.
If your scan requires contrast injected into a vein, you’ll typically have a blood test beforehand to check how well your kidneys are working. The contrast material is filtered out through your kidneys, and people with reduced kidney function face a higher risk of a temporary kidney complication. Roughly 8% of people with mildly reduced kidney filtration and up to 27% of those with severely reduced function develop this complication after contrast exposure, so the screening step matters.
What Happens During the Scan
You’ll lie on the flat bed, usually on your back. A technologist positions you and may use pillows or straps to help you stay still, since even small movements can blur the images. The technologist then steps into an adjacent room separated by a window and communicates with you through an intercom.
When the scan begins, the bed slides slowly into the gantry opening. You’ll hear a whirring or humming sound as the X-ray tube rotates around you. At certain points, the technologist may ask you to hold your breath for a few seconds so that chest or abdominal movement doesn’t distort the images. The scanning itself is painless, and the gantry never touches you.
Contrast Material: What to Expect
Not every CT scan uses contrast, but many do. Contrast is a substance that makes specific structures show up more clearly on the images by blocking X-rays from passing through. There are three main ways it can be given, depending on which area your doctor needs to see.
- By mouth. You drink a liquid that coats your esophagus, stomach, and intestines. It doesn’t taste great, and you may be asked to drink it up to an hour before the scan so it has time to travel through your digestive tract.
- By IV injection. An iodine-based contrast is injected into a vein in your arm. This is the most common method for imaging blood vessels, the liver, kidneys, and other organs.
- By enema. Contrast is delivered rectally to highlight the colon and rectum. This can cause a feeling of fullness or bloating.
IV contrast produces noticeable sensations. Nearly all patients feel a spreading warmth through the body within seconds of the injection. Many also notice a bitter or metallic taste, and some describe a medicinal or slightly alcoholic flavor. A few people feel like they’ve briefly wet themselves, though they haven’t. These sensations are normal and typically fade within a minute or two.
How Much Radiation Is Involved
CT scans use more radiation than a standard X-ray, but the dose varies widely depending on the type of scan. A chest X-ray delivers about 0.02 millisieverts (mSv). A CT of the head delivers roughly 2 mSv, a CT of the chest about 7 mSv, and a CT of the abdomen around 8 mSv. More specialized scans like a coronary CT angiogram can reach 16 mSv.
To put that in perspective, the FDA estimates that a 10 mSv scan may raise the lifetime chance of fatal cancer from about 400 in 2,000 to approximately 401 in 2,000. The added risk from a single scan is very small compared to the baseline cancer risk everyone already carries, which is why doctors order CT scans when the diagnostic benefit clearly outweighs that incremental risk.
After the Scan
Once the scanning is finished, you can get dressed and go about your day. There’s no recovery period for a standard CT scan. If you received IV contrast, drinking extra water helps your kidneys flush it out. A general guideline is about one cup of water per hour for the next eight hours, though you should follow whatever instructions your facility gives you.
If you fasted beforehand, you can eat normally again right away unless told otherwise.
Getting Your Results
A radiologist reviews the images and writes a report for the doctor who ordered your scan. Turnaround times vary. In emergency settings, preliminary results can be available within a couple of hours. For outpatient scans, you can generally expect to hear from your doctor’s office within a few days. If you haven’t received results within a week, it’s reasonable to call and ask.