A CT scan (sometimes written as “CAT scan” or searched as “ACT 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. Unlike a standard X-ray, which produces a single flat image, a CT scan builds dozens or hundreds of image “slices” that can reveal bones, organs, blood vessels, and soft tissues in far greater detail. The entire scan typically takes about one minute.
How a CT Scan Works
A CT scanner looks like a large donut. You lie on a motorized table that slides slowly through the circular opening, called a gantry. Inside the gantry, an X-ray tube rotates around your body, shooting narrow beams of X-rays from many different angles. Detectors on the opposite side of the ring pick up those beams after they pass through your tissues.
Each full rotation of the X-ray tube produces one two-dimensional image slice. Once that slice is captured and stored, the table moves forward slightly and the process repeats. A computer then assembles all those slices into a detailed picture that a radiologist can examine layer by layer or reconstruct into a three-dimensional view. Dense structures like bone absorb more X-rays and appear white, while air-filled spaces like the lungs appear dark, with organs and soft tissue falling on a spectrum in between.
What CT Scans Are Used For
CT scans are one of the most versatile tools in medicine. They’re commonly ordered to evaluate:
- Trauma and injuries: bone fractures (including subtle ones invisible on a standard X-ray), internal bleeding, and organ damage
- Cancer: detecting tumors, monitoring treatment response, and guiding biopsies
- Blood clots: particularly in the lungs (pulmonary embolism) or brain (stroke)
- Heart disease: checking for coronary artery calcium or structural problems
- Lung and abdominal conditions: infections, kidney stones, appendicitis, and more
Because CT scans are fast and highly detailed, they’re the go-to choice in emergency rooms. A chest or head CT can be completed in roughly a minute, which is critical when doctors need to rule out a stroke or identify internal bleeding after an accident.
What Happens During the Scan
You’ll be asked to remove jewelry and any metal objects that could interfere with the images, and you may change into a hospital gown. A technologist helps you lie down on the scan table, usually on your back with your arms above your head. Pillows or straps may be used to keep the body part being scanned perfectly still, since even small movements can blur the images.
Once positioned, the table glides into the gantry. You’ll hear clicking or whirring sounds as the scanner rotates. The technologist watches from a nearby room through a window and can communicate with you the entire time. The opening of the scanner is much wider and shallower than an MRI machine, so most people don’t feel claustrophobic.
Some scans require contrast dye, a liquid that makes certain structures show up more clearly. It can be given through an IV in your hand or arm, or as a drink if the scan focuses on your digestive tract. Contrast containing iodine can cause a brief warm or flushed feeling, and occasionally a metallic taste in your mouth. Both sensations are normal and fade quickly.
Contrast Dye: What to Know
Most people tolerate contrast dye without any problems. Mild reactions like a brief skin rash, itching, or nausea occur in fewer than 3% of patients. Moderate to severe reactions, such as difficulty breathing or significant swelling, are rare, affecting fewer than 0.04% of people. Delayed skin reactions (redness, rash, mild swelling) can occasionally appear 6 to 12 hours after the injection.
If your scan uses IV contrast, staying well hydrated before and after helps your kidneys clear the dye. Current guidelines from both European and American radiology societies say that routine fasting before a contrast-enhanced CT is not necessary for most patients. If your scan involves imaging the stomach or intestines, though, you may be asked to fast for at least four hours beforehand and drink a large volume of liquid to help fill the digestive tract for better images. Your imaging center will give you specific instructions.
Radiation Exposure
CT scans use more radiation than a standard X-ray, but the doses are still relatively low. A chest X-ray delivers about 0.1 millisieverts (mSv) of radiation. By comparison, a head CT delivers about 2 mSv, a chest CT about 6 mSv, and an abdomen/pelvis CT about 8 mSv. For context, the average person absorbs roughly 3 mSv per year from natural background radiation just by living on Earth.
The risk from a single CT scan is very small, but radiation exposure is cumulative over a lifetime. This is why doctors weigh the diagnostic benefit against the exposure, especially for children or patients who need repeated scans.
CT Scan vs. MRI
CT and MRI are both cross-sectional imaging tools, but they work differently and each has strengths. CT uses X-rays and excels at visualizing bone, detecting bleeding, and producing images quickly. MRI uses powerful magnets and radio waves, producing superior images of soft tissues like the brain, spinal cord, ligaments, and cartilage.
Speed is one of CT’s biggest advantages. A scan that takes one minute on CT might take 30 to 60 minutes in an MRI machine. That speed makes CT the default choice in emergencies. CT is also an option for people who can’t undergo MRI because of metal implants, pacemakers, or other implanted devices that are incompatible with strong magnetic fields.
The Other ACT Test: Activated Clotting Time
If you searched “ACT scan” and none of the above matches what you’re looking for, you may have been asking about the ACT blood test. ACT stands for activated clotting time, a test that measures how many seconds it takes your blood to form a clot. It’s primarily used during procedures like open-heart surgery, cardiac catheterization, or kidney dialysis to monitor the blood-thinning drug heparin.
Without heparin, blood normally clots in 70 to 120 seconds. When heparin is being given, the target range is 180 to 240 seconds. The test helps doctors adjust the dose in real time so blood stays thin enough to prevent dangerous clots but not so thin that uncontrolled bleeding becomes a risk.