A CT scan (also called a CAT scan) is a medical imaging test that uses X-rays and a computer to create detailed cross-sectional pictures of the inside of your body. The name stands for “computed tomography,” though the older term “computerized axial tomography” is where “CAT scan” comes from. Both names refer to the same test. It’s one of the most common diagnostic tools in medicine, used to examine everything from head injuries to cancer to broken bones.
How a CT Scan Works
A regular X-ray shoots a single beam through your body onto a flat detector, producing a two-dimensional image where structures overlap. A CT scanner works differently. You lie on a table that slides into a large, donut-shaped machine called a gantry. Inside that ring, an X-ray tube rotates around you, firing narrow beams through your body from hundreds of angles.
Each full rotation produces a single “slice,” a cross-sectional image like looking at one thin layer of your body. The machine’s computer stacks these slices together to build a detailed three-dimensional picture. This is why CT scans can reveal things a standard X-ray misses: the precise size and location of a tumor, bleeding inside the brain, or the extent of damage to an internal organ.
What CT Scans Are Used For
CT scans are a workhorse of emergency medicine and cancer care. In the ER, they’re often the first imaging test ordered for head trauma, internal bleeding, and complex fractures because they’re fast and highly detailed. For cancer, CT helps detect tumors, guide biopsies, monitor how well treatment is working, and check for recurrence.
Beyond emergencies and oncology, CT scans are used to evaluate blood vessel problems, diagnose infections, examine the lungs for conditions like pneumonia or blood clots, and assess abnormal brain function in people being evaluated for cognitive decline. They’re also a key tool for planning surgeries, giving surgeons a precise map of the area they’ll be operating on.
What the Experience Is Like
The entire appointment typically takes about an hour, but most of that time is preparation. The actual scan usually takes less than five minutes. You’ll lie still on a motorized table while the machine hums around you. The gantry is open on both ends and much less confining than an MRI machine, so claustrophobia is rarely an issue. A technologist in an adjacent room communicates with you through an intercom and may ask you to hold your breath briefly for certain images.
Some scans require a contrast agent, a special dye that makes certain structures show up more clearly on the images. Contrast can be given in several ways: injected into a vein, swallowed as a drink, or occasionally administered as an enema for lower abdominal imaging. The most common types used in CT are iodine-based compounds and barium sulfate. If you receive IV contrast, you may feel a warm sensation or a metallic taste for a few seconds. Allergic reactions to contrast are possible but uncommon, and severe reactions are rare.
If your scan involves IV contrast, you’ll typically be asked not to eat solid foods for four hours beforehand, though water is usually fine. Abdominal scans sometimes require you to drink water or an oral contrast solution ahead of time so the digestive structures show up clearly. You can continue taking your regular medications, and if you’re diabetic, your doctor’s office will give you specific guidance on managing insulin and food intake during the fasting window.
Radiation Exposure
CT scans do involve more radiation than a standard X-ray, and the dose varies by body region. A head CT delivers roughly 2 millisieverts (mSv), a chest CT about 7 mSv, and an abdominal CT around 8 mSv. For context, the average American receives about 3 mSv per year just from natural background sources like radon and cosmic rays.
For a single scan, the cancer risk from this radiation is extremely small. The concern grows mainly with repeated scans over time, which is why doctors aim to use CT only when the diagnostic benefit clearly outweighs the exposure. Newer scanner technology is helping reduce doses further. The latest generation of machines, called photon-counting CT scanners, use smaller, more efficient detectors that produce sharper images at lower radiation levels across all body parts.
CT Scans During Pregnancy
CT scans aren’t automatically off-limits during pregnancy, but they require careful consideration. For scans outside the abdomen and pelvis (like a head or chest CT), the fetus is exposed only to scattered radiation, which results in a very low dose. A single abdominal or pelvic CT under standard conditions typically delivers about 10 to 25 milligray to the fetus. According to the American College of Radiology, doses under 100 milligray have no identifiable effect on fetal development and do not warrant interrupting a pregnancy. When a CT scan is medically necessary during pregnancy, techniques can be adjusted to minimize the dose, such as reducing the number of image phases through the abdomen.
How CT Compares to MRI
CT and MRI are complementary tools, not interchangeable ones. CT is faster (the scan itself takes about a minute) and excels at imaging bone, detecting bleeding, and providing quick answers in emergencies. MRI takes longer, typically 10 to 30 minutes, but offers superior contrast between different types of soft tissue, making it the better choice for evaluating ligament tears, brain tumors, spinal cord problems, and joint injuries.
CT uses ionizing radiation, while MRI uses strong magnetic fields and radio waves, producing no radiation exposure. That magnetic field, however, means MRI is not safe for people with certain metal implants, pacemakers, or other implanted devices. CT has no such restriction. In practice, your doctor chooses between the two based on what body part needs imaging, how urgently the results are needed, and whether you have any contraindications to either test.