A CT scan uses a rotating X-ray machine to take hundreds of images of your body from different angles, then a computer assembles those images into detailed cross-sectional “slices.” The entire scan typically takes about 15 minutes from start to finish, though scans requiring oral contrast can stretch to about 75 minutes. Here’s what happens at each stage.
How the Scanner Actually Works
Inside the large, doughnut-shaped ring (called the gantry), an X-ray tube and a curved row of detectors sit directly across from each other. They rotate together around your body while you lie still on a motorized table that slides through the opening. As the tube spins, it fires a fan-shaped beam of X-rays through you. Different tissues absorb different amounts of radiation: bone blocks most of it, air blocks almost none, and organs fall somewhere in between.
Each time the X-ray beam passes through your body and hits the detectors on the other side, the scanner records one “view.” It collects hundreds of these views from slightly different angles during a single rotation. A computer then uses all of that data to calculate the density of every tiny point within the scanned area and builds a cross-sectional image, like looking at a single slice of bread from a loaf. The table moves forward slightly between rotations so the scanner can capture slice after slice along the length of whatever body part is being examined.
Preparing Before Your Appointment
For a standard CT scan without contrast dye, there’s generally no fasting required. You show up, change into a gown if needed, remove any metal jewelry or accessories, and you’re ready. If your scan does involve contrast, the preparation is a bit more involved. You’ll typically be asked not to eat for two and a half hours beforehand, though clear liquids like water, black coffee, apple juice, or clear broth are usually fine up to two hours before.
You’ll also be asked whether you’ve ever had a reaction to contrast dye or are allergic to iodine. For certain patients, including those over 70, those with diabetes, or those with a history of kidney problems, a blood test checking kidney function is required within 30 days of the scan. This is because the kidneys are responsible for filtering the contrast material out of your body, and the medical team needs to confirm they’re working well enough to handle it safely.
What Happens With Contrast Dye
Not every CT scan uses contrast, but when it does, the goal is to make certain structures (blood vessels, tumors, inflamed tissue) show up more clearly. Contrast can be delivered in a few different ways depending on what part of your body is being scanned.
- By mouth: You drink a liquid that coats and highlights your digestive tract. The taste is mildly unpleasant but tolerable. This is the main reason some CT appointments take over an hour, since you need time to drink the solution and let it move through your system.
- By IV injection: A technologist inserts a small needle into a vein in your arm. When the iodine-based contrast enters your bloodstream, you’ll likely feel a warm, flushed sensation throughout your body and a metallic taste in your mouth. Both fade within a couple of minutes. Some people also feel like they’re urinating, even though they aren’t.
- By enema: Used for imaging the lower digestive tract. You’ll feel abdominal fullness and a strong urge to expel the liquid.
During the Scan Itself
You lie on a flat, narrow table that slides into the center of the gantry. The opening is wide and shallow, more like a large ring than a tunnel, so most people don’t feel enclosed. The technologist positions you carefully, sometimes using straps or pillows to help you stay still, then steps into an adjacent room where they operate the scanner and communicate with you through a speaker.
The table moves slowly through the ring while the X-ray tube spins around you. You’ll hear a whirring or humming sound. At certain points the technologist will ask you to hold your breath for a few seconds, particularly for chest or abdominal scans, because breathing motion can blur the images. These breath holds are rehearsed with you before scanning begins. For chest scans, you might be asked to breathe out gently and then pause, or simply to breathe in very small, quiet breaths. The actual image capture for most body parts takes only 10 to 30 seconds per pass, though you may go through the ring more than once.
There’s no pain involved in the scan itself. You won’t feel the X-rays. The hardest part for most people is simply lying still.
How Long Different Scans Take
A typical CT appointment, from check-in to walking out the door, lasts about 15 minutes. The scanning portion is even shorter. A head CT, for instance, captures all the images it needs in well under a minute of actual scan time. Chest and abdominal scans are similarly fast.
The main exception is when oral contrast is involved. Because you need to drink the contrast solution and wait for it to reach the right part of your digestive system, these appointments can take up to an hour and 15 minutes total. If you’re receiving IV contrast, the injection adds only a few minutes to the process.
Radiation Exposure in Perspective
CT scans do use more radiation than a standard X-ray. A chest X-ray delivers roughly 0.02 millisieverts (mSv), the unit used to measure radiation dose. By comparison, a CT of the head delivers about 2 mSv, a CT of the chest about 7 mSv, and a CT of the abdomen about 8 mSv. Most diagnostic CT scans fall in the 1 to 10 mSv range.
To put that in practical terms, the FDA estimates that a CT scan delivering 10 mSv may increase your lifetime risk of a fatal cancer by roughly 1 in 2,000. The baseline risk of dying from cancer in the U.S. is about 1 in 5, or 400 in 2,000. So a single scan adds a very small increment to a risk that already exists. For children, medical teams follow the principle of using the lowest possible dose by adjusting the scanner’s power settings to the child’s size, limiting the scan to one pass when possible, and scanning only the specific area in question.
What Happens Afterward
Once the scan is done, you can get dressed and leave almost immediately. If you received IV or oral contrast, drinking extra water over the next several hours helps your kidneys flush the dye from your system. For patients with reduced kidney function, a common recommendation is about one cup of water per hour for eight hours after the scan. Most people have no side effects at all, though mild nausea or a brief headache from contrast occasionally occurs.
In rare cases, the IV contrast can leak into the tissue around the injection site instead of entering the vein cleanly. If this happens, you’ll notice swelling or stinging in your arm. Elevating the arm and applying a cold compress typically resolves it.
When You’ll Get Results
The scanner produces images almost instantly, but a radiologist needs to review and interpret them before your results are finalized. For emergency or critical cases, that interpretation usually happens within one to two hours. Routine inpatient scans are typically read within a few hours to 24 hours. If you had an outpatient scan for a non-urgent issue, expect results within 24 to 72 hours, depending on the facility and whether a subspecialist needs to weigh in. Your ordering physician receives the radiologist’s report and then discusses the findings with you, either by phone, through a patient portal message, or at a follow-up visit.