CT Scans: What They Are and What to Expect

The simple versionExplain▾

Computed tomography — a CT scan — uses special X-ray equipment to create detailed pictures of areas inside the body. Each picture shows the organs, bones, and other tissues in a thin "slice." NCI describes the whole set of pictures like a loaf of sliced bread: you can look at each slice on its own (a 2-D picture) or view the whole loaf together (a 3-D picture).

Modern machines take continuous pictures in a spiral pattern (helical CT), which is faster and produces better 3-D images than older scanners.

What CT is used for in cancerExplain▾

CT has many roles across cancer care. According to NCI, it can be used to:

• screen for cancer
• help diagnose the presence of a tumor
• provide information about the stage of a cancer
• show exactly where to perform a biopsy
• guide certain local treatments, such as ablation or radioactive seed placement
• help plan radiation therapy or surgery
• check whether a cancer is responding to treatment
• detect whether a tumor has come back

What to expectExplain▾

During a CT scan you lie very still on a table that passes slowly through the center of a large donut-shaped machine. You might hear whirring sounds, and at times you may be asked to hold your breath so the images don't blur. Most scanners surround only part of the body, so people are unlikely to feel closed in.

CT is noninvasive and does not cause pain, though lying still can be slightly uncomfortable. It usually takes from a few minutes to about half an hour and is typically done as an outpatient, with no overnight stay.

Contrast material (dye)Explain▾

Sometimes you are given a contrast material, or dye, to highlight specific areas and produce clearer pictures. It may be given by mouth, injected into a vein, or given by enema; iodine and barium are two common dyes.

Serious reactions are very rare. Some people notice mild itching or hives. Tell the technologist right away if you have shortness of breath or swelling, so it can be treated promptly. Very rarely, contrast can cause kidney problems in people with reduced kidney function, which is why a simple blood test may be done first.

Radiation: benefits and risksExplain▾

CT uses X-rays, a form of ionizing radiation, and exposure to ionizing radiation is known to increase cancer risk. The dose from CT is higher than from a plain X-ray, but the added risk from a single scan is still small. For comparison, NCI notes that a typical chest CT (about 6 mSv) is roughly the amount of natural background radiation a person receives in about two years.

The key point from NCI: when a CT scan is used to diagnose cancer or another serious condition in someone with symptoms, the benefits greatly outweigh the potential risks. It's still reasonable to ask your doctor whether a scan is necessary and whether an alternative without radiation would work.

A note about childrenExplain▾

Children are more sensitive to radiation than adults, so doctors are especially careful to use the lowest dose that will do the job. NCI suggests three questions parents can ask: Why is the test needed? Will the results change treatment decisions? Is there an alternative that doesn't involve radiation? When a scan is clinically justified, the benefits outweigh the small long-term risks.