CT Scans: What They Are and What to Expect

Computed tomography, or 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 compares the whole set of pictures to a loaf of sliced bread. You can look at each slice on its own, as a 2-D picture, or view the whole loaf together, as a 3-D picture.

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

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

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. At times you may be asked to hold your breath, so the images do not 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 a little uncomfortable. It usually takes from a few minutes to about half an hour. It is normally done as an outpatient, with no overnight stay.

Sometimes you are given a contrast material, or dye, to highlight specific areas and make the pictures clearer. 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 whose kidney function is already reduced, so a simple blood test may be done first.

CT uses X-rays, which are a form of ionizing radiation. Exposure to ionizing radiation is known to increase cancer risk. The dose from CT is higher than from a plain X-ray. Even so, the added risk from a single scan is still small. For comparison, NCI notes that a typical CT scan of the chest delivers about 6.1 mSv. That is comparable to two years of natural background radiation.

NCI makes the key point plainly. When a CT scan is used to diagnose cancer or another serious condition in someone who has signs or symptoms, the benefits greatly outweigh any potential health risks. It is still reasonable to ask your doctor whether a scan is needed, and whether an alternative without radiation would work.

Children are more sensitive to radiation than adults, so doctors are 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 does not involve radiation? When a scan is clinically justified, the benefits outweigh the small long-term risks.

• Call 911 or go to an emergency department if in the hours after a contrast scan you struggle to breathe, your throat or face begins to swell, or hives spread quickly across your body. NCI describes these reactions as very rare, but shortness of breath and swelling need treating without delay.
• Call your care team the same day if itching or a rash starts once you are home, or you pass much less urine than usual in the days after the dye, since contrast very rarely affects the kidneys.