Abdominal pain is the single most common reason doctors order a CT scan. Among contrast-enhanced scans of the abdomen and pelvis, abdominal pain tops the list of indications, followed by conditions like diverticular disease, liver abnormalities, and blood in the urine. But CT scans serve a wide range of purposes across the body, and the specific reason depends heavily on which body part is being imaged and whether the scan happens in an emergency or a planned outpatient visit.
Roughly 90 million CT scans are performed in the United States each year, making it one of the most frequently used diagnostic tools in medicine. About one in four adults who visit an emergency department will get a CT scan during that visit.
Why Abdominal Pain Leads the List
When you show up to a hospital or clinic with unexplained abdominal pain, a CT scan of your abdomen and pelvis is often the fastest way to figure out what’s going on. The scan can reveal appendicitis, diverticulitis (inflamed pouches in the colon), kidney stones, bowel obstructions, abscesses, and pancreatitis, all in a matter of minutes. It’s also the go-to tool for spotting internal bleeding after a car accident or fall, which is why trauma teams rely on it so heavily.
Beyond acute pain, abdominal and pelvic CT scans are commonly ordered to evaluate cancers of the liver, kidneys, colon, pancreas, bladder, and reproductive organs. Doctors also use them to check whether cancers that started elsewhere, like in the lungs or breast, have spread into the abdomen. Kidney and bladder stones, abdominal aortic aneurysms, and inflammatory bowel diseases like Crohn’s disease and ulcerative colitis round out the frequent reasons for this type of scan.
Head CT Scans and Emergencies
After abdominal imaging, head CT scans are among the most commonly performed. They’re typically the very first test done after a head injury or when someone shows sudden signs of a stroke. Speed matters in both situations: a non-contrast head CT can be completed in seconds, letting doctors quickly identify bleeding, blood clots, skull fractures, or swelling in the brain.
For stroke specifically, CT imaging plays multiple roles. A standard scan shows whether the stroke involves bleeding or a clot. A CT angiogram highlights blood vessels to pinpoint where a clot is lodged. A perfusion scan measures blood flow to determine how much brain tissue can still be saved, which directly guides treatment decisions. Outside of emergencies, head CTs are used to investigate persistent headaches, memory loss, confusion, seizures, and to plan radiation treatment for brain tumors.
That said, not every head CT finds something actionable. For patients with recurring seizures who already have a known seizure disorder, repeat head CTs have a diagnostic yield as low as 2% when the cause is something like medication levels being off or alcohol-related triggers. This is one area where doctors are actively trying to reduce unnecessary scanning.
Chest CT Scans: Lungs and Blood Vessels
The most common reason for a chest CT is an abnormal finding on a prior imaging test, typically a standard chest X-ray that showed something unusual in the lungs. Chest pain and shortness of breath are the next most frequent reasons. Doctors use chest CT to diagnose pneumonia, blood clots in the lungs (pulmonary embolism), interstitial lung disease, and tumors.
Lung cancer screening has become an increasingly important use of chest CT. Low-dose scans are approved for current and former heavy smokers who meet specific age and smoking-history criteria. Because CT can detect very small nodules in lung tissue, it catches lung cancer at earlier, more treatable stages than a standard chest X-ray would. Breast cancer and colon cancer also show up frequently in the data as reasons for chest imaging, since doctors use chest CT to check whether those cancers have spread to the lungs.
Cancer Staging and Monitoring
A large share of CT scans are performed not for initial diagnosis but for tracking cancer over time. In one study of patients with stage II or III colorectal cancer, 67% of all CT scans ordered were for surveillance rather than diagnosis or treatment planning. This pattern holds across many cancer types: once you’ve been diagnosed and treated, your oncologist will schedule regular CT scans at defined intervals to watch for recurrence.
This surveillance imaging adds up. Cancer patients often receive multiple scans per year for several years after treatment, contributing significantly to overall CT volume. Staging scans before treatment are also standard, since knowing whether cancer has spread to lymph nodes or distant organs determines the entire treatment approach.
Who Gets CT Scans Most Often
Older adults account for the majority of CT imaging. At one large academic hospital, patients aged 60 to 90 made up 58% of all CT scans performed in 2024, up from about 50% a decade earlier. More than a quarter of patients in this age group underwent two or more scans per year, and high-frequency use of five or more scans annually was concentrated almost entirely among older adults. The reasons track with the health conditions most common in this group: cancer surveillance, cardiovascular disease, and evaluating fractures or falls.
Younger adults, by contrast, are actually getting fewer repeat scans than they used to. The proportion of younger patients undergoing two or more scans per year dropped from 17.4% to 12.7% over the same ten-year period. When younger adults do get a CT scan, it’s most often in an emergency setting for trauma, appendicitis, or kidney stones.
What the Scan Actually Involves
A CT scan typically takes 10 to 30 minutes from the time you lie down on the table. The scan itself, when the machine is actually capturing images, often lasts under a minute. You’ll lie still on a narrow table that slides through a large, ring-shaped scanner. It’s open on both ends, so it feels less enclosed than an MRI machine.
Some scans require contrast dye, which is either injected into a vein or swallowed as a liquid depending on what part of your body is being imaged. The contrast highlights blood vessels, organs, or areas of inflammation that would otherwise be harder to see. If you receive IV contrast, you might feel a brief warm flush or a metallic taste in your mouth, both of which pass quickly. Abdominal and pelvic scans frequently use contrast, while emergency head CTs for trauma or stroke typically do not, which is part of what makes them so fast.
Results from emergency scans are usually available within an hour or less, since a radiologist reads them in real time. For planned outpatient scans, results typically come back within one to three days.