A CT scan uses a rotating X-ray source to take hundreds of images of your body from different angles, then a computer assembles those images into detailed cross-sectional “slices” of your organs and tissues. The actual scanning takes less than five minutes, though your total time in the imaging suite is typically about an hour once preparation is factored in.
How the Machine Works
The CT scanner looks like a large doughnut standing on its side, with a motorized table that slides you through the opening. Inside that ring, an X-ray source rotates around you, completing a full circle in about one second. As it spins, it sends out a narrow, fan-shaped beam of X-rays that passes through one thin section of your body at a time.
On the opposite side of the ring, a curved row of detectors picks up the X-rays that have passed through you. Because different tissues (bone, muscle, fluid, air) absorb X-rays at different rates, each detector reading captures a unique “snapshot” of that slice from one angle. Over the course of a single rotation, the machine collects snapshots from hundreds of angles. A computer then stitches all of those snapshots together into one highly detailed cross-sectional image. As the table moves you forward continuously, the scanner repeats this process for each new slice, building a complete picture of the area being examined.
Preparing for Your Scan
Preparation depends on which part of your body is being scanned and whether contrast dye is needed. For scans without contrast, you may not need to do anything special beforehand. If contrast is involved, some facilities ask you to fast for four hours or more before the appointment, though recent research suggests that skipping food may not reduce nausea or vomiting rates compared to eating normally. Your imaging center will give you specific instructions.
You’ll be asked to remove jewelry, eyeglasses, belts, and any metal objects that could interfere with the images. Most facilities provide a gown, though some allow you to keep your own clothes on if they’re free of metal zippers or snaps. If you have any metal implants, let the technologist know, as they won’t prevent the scan but may need to be accounted for.
What Happens During the Scan
A technologist positions you on the table, usually lying on your back. For a chest CT, for example, you’d lie with your arms raised above your head so they don’t block the view. The technologist adjusts the table height so you’re centered in the scanner opening and uses laser alignment lights to mark the correct starting position.
First, the machine takes one or two quick “scout” images, similar to a standard X-ray, so the technologist can verify the area to be scanned. They’ll then set the exact range, such as from the top of the lungs down through the upper abdomen. Once everything is set, the table glides smoothly through the ring while the scanner does its work. For many scans, you’ll hear a brief instruction to hold your breath for a few seconds. This prevents motion blur, especially in the chest and abdomen. The scanning portion itself is typically done in under five minutes.
You won’t feel the X-rays. The machine hums and may make a whirring sound as the X-ray source rotates, but it’s not as loud as an MRI. You can communicate with the technologist through an intercom the entire time.
Contrast Dye: What It Does and How It Feels
Not every CT scan requires contrast, but when it does, the dye helps certain structures stand out more clearly. The most common method is an IV injection, usually through a small catheter placed in a vein near your inner elbow or forearm. A power injector pushes the contrast through at a controlled rate to ensure it reaches the target area at the right moment.
For abdominal scans, you may also be asked to drink a flavored liquid contrast an hour or two before the scan to help outline the stomach and intestines. In some cases, contrast is given rectally to highlight the lower bowel.
When IV contrast enters your bloodstream, you’ll likely feel a sudden warm, flushing sensation that spreads through your body. Many people also notice a metallic or salty taste in their mouth. Some feel a brief wave of nausea or a mild headache. These sensations are normal and typically pass within seconds. A small number of people experience an allergic-type reaction to contrast, so you’ll be asked in advance about any history of allergies or previous reactions.
Radiation Exposure
CT scans use significantly more radiation than a standard X-ray. A single chest CT delivers a dose of roughly 8 millisieverts, equivalent to about 400 chest X-rays. Abdominal and pelvic scans can range from 5 to 30 millisieverts depending on the protocol. For context, the average person absorbs about 3 millisieverts per year from natural background sources like soil, air, and cosmic rays.
Modern scanners use dose-reduction techniques, such as adjusting the beam intensity based on your body size and limiting exposure to the smallest area necessary. The radiation from a single scan is not considered dangerous, but repeated scans over time add up, which is why doctors weigh the diagnostic benefit against cumulative exposure before ordering one.
After the Scan
If you didn’t receive contrast, you can get up and resume your normal activities immediately. If contrast was used, you’ll typically be encouraged to drink extra water over the next several hours to help your kidneys filter the dye out of your system. The IV catheter is removed right after the scan, and any mild warmth or metallic taste fades quickly on its own.
The images are sent to a radiologist, who reviews the cross-sectional slices and creates a written report. Turnaround time varies: emergency scans are often read within minutes, while routine outpatient scans may take one to three business days. Your referring doctor receives the report and discusses the findings with you. In many health systems, you can also view results through an online patient portal once they’re finalized.