Originally published by Harvard Health.
What is the test?
CT scans are pictures taken by a specialized x-ray machine. The machine circles your body and scans an area from every angle within that circle. The machine measures how much the x-ray beams change as they pass through your body. It then relays that information to a computer, which generates a collection of black-and-white pictures, each showing a slightly different “slice” or cross-section of your internal organs. Because these “slices” are spaced only about a quarter-inch apart, they give a very good representation of your internal organs and other structures. Doctors use CT scans to evaluate all major parts of the body, including the abdomen, back, chest, and head.
A CT scan is an excellent way to view the organs inside your abdomen. It is especially useful for looking at solid organs, such as the liver, pancreas, spleen, kidneys, and adrenal glands. It is also excellent for viewing the large blood vessels that pass through the abdomen (the aorta and vena cava) and for finding lymph nodes in the abdomen. Organs that can change their shape when they are empty or full, such as stomach and intestines, are harder for a CT scan to evaluate well, because it is sometimes difficult for a doctor to tell for sure if they are abnormal. Often the CT can give some information about these organs, though. Abdominal CT scans are often used to look for signs of inflammation or infection inside the abdomen in different organs, to look for cancer, or to look for injury to one or another internal organ.
How do I prepare for the test?
If you will be receiving contrast dye during your study, it is appropriate for you to have a blood test to check your kidney function before the test. (A test that was done during the past six months may be adequate.) People who are taking the diabetes medication metformin (Glucophage) should discontinue this treatment for several days before a CT scan that includes contrast. If your doctor has special concerns about the health of your kidneys, you may be asked to take a protective medicine named n-acetylcysteine (Mucomyst) before the test.
If you are having an abdominal CT scan, you might have to fast 2-4 hours before your test. You also may have to drink a large quantity of oral contrast, a fluid that will show up on the CT scan and help define the lining of some internal organs. Tell your doctor if you’re allergic to x-ray contrast dyes, may be pregnant, or have diabetes and take insulin. Insulin can cause hypoglycemia after missing a meal.
What happens when the test is performed?
The test is done in the radiology department of a hospital or in a diagnostic clinic. You wear a hospital gown and lie on your back on a table that can slide back and forth through the donut-shaped CT machine. If your test requires contrast dye, a technician or other health care professional may insert an IV and inject contrast dye through it. This dye outlines blood vessels and soft tissue to help them show up clearly on the pictures.
The technologist moves the table with a remote control to enable the CT machine to scan your body from all of the desired angles. You will be asked to hold your breath for a few seconds each time a new level is scanned. The technologist usually works the controls from an adjoining room, watching through a window and sometimes speaking to you through a microphone. A CT scan takes about 30–45 minutes. Although it’s not painful, you might find it uncomfortable if you don’t like to lie still for extended periods.
What risks are there from the test?
There are a few risks. The contrast dye used in the test can damage your kidneys, especially if they are already impaired by disease. If kidney damage does occur, this is usually temporary. If you are allergic to the dye used in the procedure, you may get a rash or your blood pressure may drop enough to make you feel faint until you get treatment. As with x-rays, there is a small exposure to radiation. The amount of radiation from a CT scan is greater than that from regular x-rays, but it’s still too small to be likely to cause harm unless you’re pregnant.
Must I do anything special after the test is over?
Your doctor might recommend a follow-up test of your kidney function if you have received contrast and have a history of kidney function problems.
How long is it before the result of the test is known?
The radiologist can probably give you preliminary results within a day. The formal reading of your CT scan might take another day.