CT stands for computed tomography, a medical imaging procedure that uses X-rays and computer processing to create detailed cross-sectional pictures of the inside of your body. You may also hear it called a CAT scan (computed axial tomography), which is the same thing. The word “tomography” comes from the Greek for “slice” or “section,” which describes exactly what the technology does: it produces image slices through your body that can be stacked together to build a complete picture.
How a CT Scan Works
During a CT scan, a narrow beam of X-rays rotates quickly around your body while you lie on a table that slides through a doughnut-shaped machine. Detectors on the opposite side of the ring pick up the X-rays after they pass through your tissues, and a computer assembles those signals into cross-sectional images. Different tissues absorb X-rays differently. Bone blocks a lot of the beam and appears white, air lets nearly all of it through and appears black, and everything else falls somewhere in between on a grayscale.
Radiologists quantify this grayscale using Hounsfield units (HU). Water is set at 0 HU, air at negative 1,000, and dense bone around positive 1,000 or higher. Fat registers around negative 60, muscle around 40, and cerebrospinal fluid around 10. These values help distinguish tissues that might look similar on a standard X-ray, which is one reason CT scans reveal far more detail than a plain film.
What CT Scans Are Used For
CT scans are one of the most versatile tools in medicine. They can image bones, muscles, fat, organs, and blood vessels, and they’re especially useful when a standard X-ray or physical exam isn’t conclusive. Common reasons for ordering a CT include evaluating fractures, detecting tumors, diagnosing internal bleeding after trauma, checking for blood clots in the lungs, and assessing infections or organ damage in the abdomen.
A specialized version called low-dose CT (LDCT) is used for lung cancer screening. The U.S. Preventive Services Task Force recommends annual LDCT screening for adults aged 50 to 80 who have a 20 pack-year smoking history and either currently smoke or quit within the past 15 years. A pack-year equals one pack (20 cigarettes) per day for one year, so someone who smoked two packs a day for 10 years would have a 20 pack-year history.
What Happens During the Scan
A typical CT scan takes about one minute, making it one of the fastest imaging options available. You’ll lie still on a motorized table while the machine hums and rotates around you. The table moves through the scanner in small increments so the machine can capture slices at different levels. It’s painless, and the machine is open on both sides, so it’s less confining than an MRI tube.
If your scan requires contrast, you may need to fast for four to six hours beforehand, though fasting policies vary between facilities. Contrast material helps certain structures show up more clearly on the images. Iodine-based contrast is injected into a vein and highlights blood vessels, organs like the liver and kidneys, and tumors. Barium sulfate is swallowed or given as an enema to outline the digestive tract, from the esophagus down to the colon. The intravenous contrast can cause a brief warm flushing sensation and a metallic taste, both of which pass within seconds.
CT vs. MRI
CT and MRI are both cross-sectional imaging tools, but they work differently and excel at different things. CT uses X-rays and is fast, typically finishing in about a minute. MRI uses magnetic fields and radio waves, taking 10 minutes or more even with newer protocols. CT is generally better for bone injuries, bleeding, and lung imaging. MRI provides superior detail for soft tissues like the brain, spinal cord, ligaments, and cartilage.
One practical difference: if you have a pacemaker, certain metal implants, or other implanted devices, you typically can’t have an MRI because of the powerful magnet. CT scans don’t carry that restriction, so they’re often the go-to alternative for patients with metallic hardware.
Radiation Exposure
Unlike MRI or ultrasound, CT does expose you to ionizing radiation. The dose depends on which body part is scanned. A CT of the head delivers roughly 2 millisieverts (mSv), a chest CT about 7 mSv, and an abdominal CT around 8 mSv. For context, the average person absorbs about 3 mSv per year from natural background radiation (cosmic rays, radon in soil, etc.).
Overall, diagnostic CT doses fall in the range of 1 to 10 mSv. The risk from a single scan is very small, but radiation exposure is cumulative over a lifetime, which is why doctors weigh the diagnostic benefit against the dose each time they order one. Low-dose CT protocols, like those used for lung cancer screening, reduce the exposure significantly compared to a standard chest CT.
Contrast and Kidney Safety
Iodine-based contrast is filtered through the kidneys, which raises a concern for people with reduced kidney function. Current evidence shows the risk of contrast-related kidney injury is essentially zero for people whose kidney filtration rate (eGFR) is 45 or above. For those with an eGFR of 30 or higher, contrast-enhanced CT is generally performed without special precautions. Below 30, the risk climbs, with kidney injury occurring in up to 17% of cases in some estimates. If your kidney function is significantly reduced, your medical team will decide whether the diagnostic benefit of contrast outweighs that risk, or whether a non-contrast scan or different imaging method is a better option.