A CT scan takes about an hour from check-in to walking out, but the actual scanning part usually lasts fewer than five minutes. During that time, you lie on a motorized table that slides through a large, doughnut-shaped machine while an X-ray tube spins around you, capturing hundreds of images from different angles. A computer then assembles those images into detailed cross-sectional “slices” of whatever body part your doctor needs to see. Here’s what happens at each stage.
How the Machine Works
The CT scanner looks like a large ring (called a gantry) with a hole in the center big enough for a person to pass through. Inside that ring, an X-ray tube and a set of detectors sit on opposite sides. In modern scanners, both the tube and detectors rotate together around you in a fan-beam pattern, firing X-rays through your body from every angle as you pass through. The detectors measure how much of each X-ray beam makes it through your tissues: bone absorbs a lot, air absorbs almost none, and organs fall somewhere in between.
A computer takes all of that absorption data and runs it through a mathematical process called filtered backprojection, which reconstructs it into thin image slices. The radiologist can adjust how thick or thin those slices are, which affects how much detail the images capture. Thinner slices show finer detail but produce more data and slightly more radiation. These slices can also be stacked to create 3D views of bones, blood vessels, or organs.
Preparing Before Your Scan
When you arrive, you’ll be asked to remove anything metal that could interfere with the images. This includes jewelry, watches, glasses, hairpins, hearing aids, underwire bras, dentures, and medication patches like nicotine or pain patches. Even clothing labeled as antimicrobial or using “silver technology” contains enough metal to cause problems. You’ll typically change into a hospital gown.
If your scan requires contrast dye (more on that below), you’ll need to stop eating solid foods four hours beforehand. Water is fine, and some facilities also allow juice or black decaffeinated coffee or tea during that fasting window. The fasting helps prevent nausea from the contrast and keeps your stomach clear for abdominal imaging.
Your care team will also check your kidney function before giving IV contrast. The kidneys are responsible for filtering the contrast dye out of your blood, so they need to be working well enough to handle the load. Patients with a kidney filtration rate below 30 generally cannot receive contrast safely, while those between 30 and 45 require a radiologist’s review. Above 45 is considered safe.
What Happens if You Need Contrast
Not every CT scan requires contrast, but many do. Contrast is an iodine-based liquid that makes blood vessels, organs, and abnormal tissues stand out more clearly on the images. It can be given in three ways depending on what part of your body is being scanned: through an IV in your arm (most common), swallowed as a drink, or in rare cases administered rectally for lower bowel imaging.
For IV contrast, a technologist or nurse places a small catheter, usually in the inside of your elbow or forearm. Before injecting contrast, they flush the line with saline to make sure it’s properly positioned and flowing without resistance. The contrast is then delivered through a power injector at a controlled rate. You may feel a warm, flushing sensation spreading through your body, and some people notice a metallic taste in their mouth or a brief feeling like they’ve wet themselves. These sensations are normal and pass within a minute or two.
If you’re drinking oral contrast, you’ll typically receive it 30 to 60 minutes before the scan to give it time to coat your digestive tract.
During the Scan
You’ll lie on a flat, padded table, usually on your back. The technologist positions you based on what’s being scanned. For a head CT, you’ll be centered at chin level with your head kept as straight as possible. For a spine scan, the centering point depends on which section: the neck, mid-back, or lower back each have a different landmark. Abdominal scans are typically done while lying face-up with arms raised above your head to keep them out of the image field.
Once positioned, the table slides slowly through the gantry. The technologist operates the scanner from an adjacent room with a window, communicating through a speaker. For chest or abdominal scans, you’ll hear instructions to take a deep breath and hold it for several seconds. This keeps your organs still and prevents blurring. Head, neck, and spine scans typically don’t require breath-holding because those structures don’t move with breathing.
The machine makes whirring or clicking sounds as the X-ray tube rotates, but it’s much quieter than an MRI. You won’t feel anything from the X-rays themselves. The key thing you need to do is stay completely still. Even small movements can blur the images and require repeating part of the scan.
How Much Radiation Is Involved
CT scans use more radiation than a standard X-ray but far less than levels that would cause immediate harm. The dose varies significantly depending on the body part. A brain CT delivers about 1.6 millisieverts (mSv), roughly equivalent to six months of natural background radiation from the environment. A head and neck scan is slightly lower at 1.2 mSv. Chest CT delivers about 6.1 mSv, and an abdomen and pelvis scan is the highest common exam at 7.7 mSv.
These are averages for a typical adult. Larger patients may receive slightly higher doses because more radiation is needed to penetrate thicker tissue. Pediatric scans use lower settings. For context, the average American receives about 3 mSv per year from natural background sources like radon and cosmic rays, so even the highest-dose CT scans represent a modest additional exposure relative to what you’re already getting.
After the Scan
If you didn’t receive contrast, there’s nothing to recover from. You can eat, drink, drive, and go about your day immediately. The entire experience, from the scanning portion itself, likely lasted under five minutes.
If you received IV contrast, the technologist flushes the catheter with saline after the injection, inspects the IV site for any swelling, and then removes it. Your kidneys will filter the contrast dye out of your bloodstream over the next several hours. Drinking extra water helps speed that process along. Most people feel completely normal, though mild nausea or a slight headache occasionally occurs and resolves on its own.
Your images are reviewed by a radiologist, who writes a report and sends it to the doctor who ordered the scan. Turnaround time depends on the facility and urgency. Emergency scans are often read within minutes, while routine outpatient scans may take a day or two to be formally reported.