A CT scan is one of the quickest imaging tests you can have. The actual scan takes fewer than five minutes, though you should expect to spend about an hour at the facility once you factor in check-in, preparation, and any contrast dye that needs to be administered. Here’s what the process looks like from start to finish.
Whether You Need to Fast
Fasting requirements depend entirely on which body part is being scanned and whether you’ll receive contrast dye. Scans that use contrast generally require fasting beforehand, while scans without contrast typically don’t.
For abdominal and pelvic soft tissue scans (looking at organs like the liver, kidneys, pancreas, or bowels), you’ll usually need to stop eating six hours before the exam. For chest scans checking for blood clots, neck soft tissue scans, kidney and bladder scans, and carotid artery imaging, the fasting window is four hours.
Many common scans require no fasting at all. These include scans of the spine (cervical, thoracic, or lumbar), lungs, sinuses, skull, facial bones, and any extremity like an arm, leg, foot, or shoulder. If your scan is for flank pain or a bone fracture, you can eat and drink normally beforehand. Your scheduling team will tell you which category your scan falls into when you book the appointment.
How Contrast Dye Works
Contrast dye makes certain tissues, blood vessels, or organs show up more clearly on the images. Not every CT scan requires it, but when it’s used, it can be given in a few different ways: swallowed as a drink, injected into a vein through an IV, administered as an enema, or injected into a specific body cavity.
Oral contrast is the most common type you’ll notice. It’s a barium-sulfate liquid you drink before the scan, and you may be asked to arrive early so the liquid has time to move through your digestive tract. If contrast is given rectally, your care team may ask you to follow a special diet or use a bowel prep beforehand. IV contrast is injected right before or during the scan and can cause a brief warm, flushed feeling or a metallic taste in your mouth. Both sensations pass quickly.
For people with kidney disease, diabetes, or dehydration, IV contrast carries a small risk of acute kidney injury. Your care team may check your kidney function with a blood test before the scan and provide extra IV fluids before and after the procedure to reduce that risk.
What Happens During the Scan
You’ll be asked to change into a hospital gown and remove any metal objects like jewelry, glasses, belts, or hairpins, since metal can interfere with the images. A technologist will then position you on a narrow, padded table, usually lying on your back. Depending on what’s being scanned, you might have a pillow under your knees or a strap across your body to help you stay still.
The table slides into a large, ring-shaped machine called a gantry. Unlike an MRI, the CT scanner is open on both ends and relatively shallow, so most people don’t feel enclosed. The machine rotates around you while taking X-ray images from many angles, and a computer assembles those images into detailed cross-sectional slices of your body.
During the scan, you’ll hear a soft humming or whirring sound. The technologist watches from an adjacent room through a window and communicates with you through a speaker. You may be asked to hold your breath for 10 to 20 seconds at a time so that breathing movement doesn’t blur the images. The actual image acquisition is fast, often finishing in under five minutes.
Radiation Exposure
CT scans use X-rays, so they do involve radiation. The dose varies depending on which part of your body is scanned. A head CT delivers roughly 2 millisieverts (mSv), a chest CT about 7 mSv, and an abdominal CT about 8 mSv. For context, the typical range for diagnostic CT procedures falls between 1 and 10 mSv, and the dose can vary by a factor of ten depending on patient size and the specific machine settings used.
These doses are well below the levels associated with measurable harm from a single scan. The concern with radiation is cumulative exposure over a lifetime, which is why doctors weigh the diagnostic benefit against the radiation each time a CT is ordered.
CT Scans During Pregnancy
The radiation from a CT scan is much lower than the dose associated with fetal harm. The American College of Obstetricians and Gynecologists states that if a CT scan is the most appropriate or most readily available diagnostic tool, it should not be withheld from a pregnant patient. That said, doctors will typically consider ultrasound or MRI first since neither uses ionizing radiation. When a CT is necessary, the benefit of getting an accurate diagnosis outweighs the very small theoretical risk.
After the Scan
Once the images are captured, you can get dressed and leave. There’s no recovery period for the scan itself. If you received IV contrast, you’ll be encouraged to drink extra water for the rest of the day to help flush it from your system. Some people experience mild nausea or a headache after contrast, but this usually resolves within a few hours.
A radiologist reviews the images and sends a report to the doctor who ordered the scan. Results are often available within 24 to 48 hours, though urgent scans in an emergency setting can be read much faster. Your doctor will then go over the findings with you and discuss any next steps.