Worlds of David Darling
Encyclopedia of Science
   
Home > Encyclopedia of Science

chest CT scan





chest CT scan
CT scan of the chest showing both lungs (including structure within the lungs) and the heart lying between them (white area). Image courtesy of Wellcome Photo Library
A chest computed tomography, or CT, scan is a painless, noninvasive test. It creates precise images of the structures in the chest, such as your lungs. "Noninvasive" means that no surgery is done and no instruments are inserted into the body.

A chest CT scan is a type of X-ray. However, a CT scan's pictures show more details than pictures from a standard chest X-ray.

Like other X-ray tests, chest CT scans use a form of energy called ionizing radiation. This energy helps create pictures of the inside of ther chest.


Overview

Chest CT scans are used to:
  • Show the size, shape, and position of the lungs and other structures in the chest.
  • Follow up on abnormalities that are found on standard chest X-rays.
  • Find the cause of lung symptoms, such as shortness of breath or chest pain.
  • See whether there is a lung problem, such as a tumor, excess fluid around the lungs, or a pulmonary embolism (a blood clot in the lungs). The test also is used to check for other conditions, such as tuberculosis, emphysema, and pneumonia.
The chest CT scanning machine takes many pictures, called slices, of the lungs and the inside of the chest. A computer processes these pictures; they can be viewed on a monitor or printed on film. The computer also can stack the pictures to create a very detailed, three-dimensional (3D) model of organs.

Sometimes, a special substance (called contrast dye) is injected into a vein in the arm. This substance highlights areas in the chest, which helps create clearer images.


Outlook

Chest CT scans have few risks. Because the test uses radiation, there may be a slight risk of cancer.

Children are more sensitive to radiation than adults because they are smaller and still growing.

The amount of radiation will vary with the type of CT scan. On average, though, the radiation will not exceed the amount a person is naturally exposed to over 3 years. The benefits of a CT scan should always be weighed against the possible risks.

Rarely, people have allergic reactions to the contrast dye that's sometimes used for chest CT scans. If this happens, medicine is administered to relieve the symptoms.





Types of chest CT scans

A CT scanner is a large machine with a tunnel-like hole in the center. During a chest CT scan, a person lies on a table as it moves small distances at a time through the hole.

An X-ray beam rotates around the body as the person moves through the hole. A computer takes data from the X-rays and creates a series of pictures, called slices, of the inside of the chest.

Different types of chest CT scans have different diagnostic uses.


High-resolution chest CT scan

High-resolution CT (HRCT) scans provide more than one slice in a single rotation of the X-ray tube. Each slice is very thin and provides a lot of details about the organs and other structures in the chest.


Spiral chest CT scan

For this scan, the table moves continuously through the tunnel-like hole as the X-ray tube rotates around the individual. This allows the X-ray beam to follow a spiral path.

The machine's computer can process the many slices into a very detailed, three-dimensional (3D) picture of the lungs and other structures in the chest.


Who needs a chest CT scan?

People who have symptoms of lung problems, such as chest pain or trouble breathing, may need chest CT scans. The scan helps find the cause of the symptoms.

Chest CT scans look for problems such as tumors, excess fluid around the lungs, and pulmonary embolism (a blood clot in the lungs). The scan also checks for other conditions, such as tuberculosis, emphysema, and pneumonia.

A chest CT scan may be ordered if a standard chest X-ray doesn't help diagnose the problem. For example, a chest CT scan may be ordered after a chest X-ray to:
  • See more detailed pictures of the lungs and other structures in the chest
  • Find the exact location of a tumor or other problem
  • Show something that isn't visible on a chest X-ray

What to expect before a chest CT scan


What to wear

Wear loose-fitting, comfortable clothing for the test. In some cases, the CT scan technician (a person specially trained to do CT scans) may ask the subject to wear a hospital gown.

Subjects will be asked to take off any jewelry, eyeglasses, and metal objects that might interfere with the test.

Subjects may be asked to remove hearing aids and dentures as well. The technician should be made aware of any body piercing on the chest.


Pregnancy and other conditions

Tell your doctor whether you're pregnant or may be pregnant. If possible, you should avoid unnecessary radiation exposure during pregnancy. This is because of the concern that radiation may harm the fetus. You and your doctor will decide whether the benefits of a chest CT scan outweigh the possible risks to the fetus, or whether another test might be better. If you do have the chest CT scan, the technician will take extra steps to reduce the fetus' exposure to radiation. You also should tell your doctor whether:
  • You're taking any medicines
  • You have any allergies
  • You've recently been ill
  • You have any medical conditions (for example, heart disease, asthma, diabetes, kidney disease, or thyroid problems)
These factors or conditions may raise your risk for a bad reaction to the test.


The CT machine

The CT scanner is a large machine with a tunnel-like hole in the center. You lie on a table that goes through the hole.

Tell your doctor if you're afraid of tight or closed spaces. He or she may give you medicine to help you relax. This medicine may make you sleepy, so you'll need to arrange for a ride home after the test.


Contrast dye

Your doctor may give you a special substance (called contrast dye). This substance highlights areas of your chest and helps create clearer images.

The contrast dye will be injected into a vein in your arm. You may feel some discomfort when the needle is inserted. As the substance is injected, you also may feel warm and have a metallic taste in your mouth. These feelings last only a few minutes.

Your doctor may ask you not to eat or drink for a few hours before the test, especially if contrast dye is part of the test.

Some people are allergic to the contrast dye. If you have allergic symptoms, such as itching or hives, tell the technician or doctor right away. He or she can give you medicine to relieve the symptoms.

The most common type of contrast dye used in lung CT scans contains iodine. Let your doctor know if you're allergic to iodine.

If you're breast-feeding, ask your doctor how long you should wait after the test before you breast-feed. The contrast dye can be passed to your baby through your breast milk.

You may want to prepare for the test by pumping and saving milk for 24 to 48 hours in advance. You can bottle-feed your baby in the hours after the CT scan.


What to expect during a chest CT scan

A chest CT scan takes about 30 minutes, which includes preparation time. The actual scanning time is much shorter, only a few minutes or less.

The CT scanner is a large machine that has a tunnel-like hole in the middle. You will lie on a narrow table that moves through the hole.

While you're inside the scanner, an x-ray tube moves around your body. You will hear soft buzzing, clicking, or whirring noises as the scanner takes pictures.

The CT scan technician who controls the machine will be in the next room. He or she can see you through a glass window and talk to you through a speaker.

Moving your body can cause the pictures to blur. The technician will ask you to lie still and hold your breath for short periods. These measures help make the pictures as clear as possible.

The scan itself doesn't hurt, but you may feel anxious if you get nervous in tight or closed spaces. Your doctor may give you medicine to help you relax.


What to expect after a chest CT scan

You usually can return to your normal routine right after a chest CT scan.

If you got medicine to help you relax during the CT scan, your doctor will tell you when you can return to your normal routine. The medicine may make you sleepy, so you'll need someone to drive you home.

If contrast dye was used during the test, you may have a bruise where the needle was inserted. Your doctor may give you special instructions, such as drinking plenty of liquids to flush out the contrast dye.

If you're breast-feeding, ask your doctor how long you should wait after the test before you breast-feed. The contrast dye can be passed to your baby through your breast milk.

You may want to prepare for the test by pumping and saving milk for 24 to 48 hours in advance. You can bottle-feed your baby in the hours after the CT scan.


What does cardiac CT show?

A chest CT scan provides detailed pictures of the size, shape, and position of your lungs and other structures in your chest. Doctors use this test to:
  • Follow up on abnormalities that are found on standard chest X-rays.
  • Find the cause of lung symptoms, such as shortness of breath or chest pain.
  • See whether you have a lung problem, such as a tumor, excess fluid around the lungs, or a pulmonary embolism (a blood clot in the lungs). The test also is used to check for other conditions, such as tuberculosis, emphysema, and pneumonia.

What are the risks of cardiac CT?


Radiation

You will receive radiation during a chest CT scan. The amount will vary with the type of CT scan. On average, though, the amount of radiation will not exceed the amount you're naturally exposed to over 3 years. The radiation from the test is gone from the body within a few days.

Children are more sensitive to radiation because they're smaller than adults and still growing.

Exposure to radiation is associated with a risk of cancer. However, it's not known whether the amount of radiation from a chest CT scan puts you at higher risk than the amount of radiation you're naturally exposed to.

You and your doctor will decide whether the benefits of the CT scan outweigh any possible risks. Your doctor also will try to avoid ordering repeated CT scans over a short period.


Allergic reaction

The contrast dye used in some chest CT scans can cause an allergic reaction, such as hives or trouble breathing. The risk of this happening is slight. If you do have an allergic reaction, your doctor can give you medicine to relieve it.

The most common contrast dye used in CT scans contains iodine. Tell your doctor if you're allergic to iodine.


Related categories

   • HEALTH AND DISEASE
   • MEDICAL TESTS

Source: U.S. National Heart, Lung and Blood Institute