What Is a CT Scan?
What Is CT Scan?
CT, or CAT scans, are special X-ray tests that produce cross-sectional images of the body using X-rays and a computer. CT scans are also referred to as computerized axial tomography. CT was developed independently by a British engineer named Sir Godfrey Hounsfield and Dr. Alan Cormack. It has become a mainstay for diagnosing medical diseases. For their work, Hounsfield and Cormack were jointly awarded the Nobel Prize in 1979.
CT scanners first began to be installed in 1974. CT scanners have vastly improved patient comfort because a scan can be done quickly. Improvements have led to higher-resolution images, which assist the doctor in making a diagnosis. For example, the CT scan can help doctors to visualize small nodules or tumors, which they cannot see with a plain film X-ray.
Picture of CT scan machine.
CT Scan Facts
- CT scan images allow the doctor to look at the inside of the body just as one would look at the inside of a loaf of bread by slicing it. This type of special X-ray, in a sense, takes "pictures" of slices of the body so doctors can look right at the area of interest. CT scans are frequently used to evaluate the brain, neck, spine, chest, abdomen, pelvis, and sinuses.
- CT is a commonly performed procedure. Scanners are found not only in hospital X-ray departments, but also in outpatient offices.
- CT has revolutionized medicine because it allows doctors to see diseases that, in the past, could often only be found at surgery or at autopsy. CT is noninvasive, safe, and well-tolerated. It provides a highly detailed look at many different parts of the body.
- If one looks at a standard X-ray image or radiograph (such as a chest X-ray), it appears as if they are looking through the body. CT and MRI are similar to each other, but provide a much different view of the body than an X-ray does. CT and MRI produce cross-sectional images that appear to open the body up, allowing the doctor to look at it from the inside. MRI uses a magnetic field and radio waves to produce images, while CT uses X-rays to produce images. Plain X-rays are an inexpensive, quick test and are accurate at diagnosing things such as pneumonia, arthritis, and fractures. CT and MRI better to evaluate soft tissues such as the brain, liver, and abdominal organs, as well as to visualize subtle abnormalities that may not be apparent on regular X-ray tests.
- People often have CT scans to further evaluate an abnormality seen on another test such as an X-ray or an ultrasound. They may also have a CT to check for specific symptoms such as pain or dizziness. People with cancer may have a CT to evaluate the spread of disease.
- A head or brain CT is used to evaluate the various structures of the brain to look for a mass, stroke, area of bleeding, or blood vessel abnormality. It is also sometimes used to look at the skull.
- A neck CT checks the soft tissues of the neck and is frequently used to study a lump or mass in the neck or to look for enlarged lymph nodes or glands.
- CT of the chest is frequently used to further study an abnormality on a plain chest X-ray. It is also often used to look for enlarged lymph nodes.
- Abdominal and pelvic CT looks at the abdominal and pelvic organs (such as the liver, spleen, kidneys, pancreas, and adrenal glands) and the gastrointestinal tract. These studies are often ordered to check for a cause of pain and sometimes to follow up on an abnormality seen on another test such as an ultrasound.
- A sinus CT exam is used to both diagnose sinus disease and to detect a narrowing or obstruction in the sinus drainage pathway.
- A spine CT test is most commonly used to detect a herniated disc or narrowing of the spinal canal (spinal stenosis) in people with neck, arm, back, and/or leg pain. It is also used to detect a fracture or break in the spine.
CT Scan Risks
CT scan is a very low-risk procedure.
- The patient will be exposed to radiation when undergoing a CT scan. However, it is a safe level.
- The biggest potential risk is with a contrast (also called dye) injection that is sometimes used in CT scanning. This contrast can help distinguish normal tissues from abnormal tissues. It also helps to help distinguish blood vessels from other structures such as lymph nodes. Like any medication, some people can have a serious allergic reaction to the contrast. The chance of a fatal reaction to the contrast is about 1 in 100,000. Those at increased risk may require special pretreatment and should have the test in a hospital setting. Anyone who has had a prior contrast reaction or severe allergic reaction to other medications, has asthma or emphysema, or has severe heart disease is at increased risk for a contrast reaction and is referred to a hospital X-ray department for the exam. Besides an allergic reaction, the intravenous dye can damage the kidneys, particularly if an individual already has marginal kidney disease. Usually, the patient is advised to drink plenty of fluids to help flush the dye out of their system.
- Any time an injection is done into a vein, there is a risk of the contrast leaking outside of the vein under the skin. If a large amount of contrast leaks under the skin, in rare cases, this can cause the skin to break down.
CT Scan Preparation
If a patient is going to have a contrast injection, he or she should not have anything to eat or drink for a few hours before the CT scan because the injection may cause stomach upset. To receive the contrast injection, an IV is inserted into the arm just prior to the scan. The contrast then enters the body through the IV.
Prior to most CT scans of the abdomen and pelvis, it is important to drink an oral contrast agent that contains dilute barium. This contrast agent helps the radiologist identify the gastrointestinal tract (stomach, small and large bowel), detect abnormalities of these organs, and to separate these structures from other structures within the abdomen. The patient will be asked to drink slightly less than a quart spread out over 1.5 to 2 hours.
What Is an MRI?
An MRI is similar to a CT scan (computed tomography) in that it produces cross-sectional images of the body. MRI uses a strong magnetic field and radio waves to produce very clear, detailed computerized images of the inside of the body, while a CT scan uses X-rays to produce the images.
During the Procedure
Most CT scans are conducted as an outpatient procedure. Since they do not require hospitalization, the patient has the test and then goes home.
- The CT scanner looks like a large donut with a narrow table in the middle. Unlike MRI, in which the patient would be placed inside the tunnel of the scanner, when undergoing a CT scan, the patient rarely experiences claustrophobia because of the openness of the doughnut shape of the scanner. Typically the patient lies on their back on the table, which moves through the center of the machine. The patient moves through the scanner either head first or feet first, depending on the part of the body being scanned. For certain scans such as sinuses and middle ear, the patient would lie on their stomach and go through head first.
- The patient must remain motionless for the length of the study, which is typically just a few minutes. The entire procedure, which includes set-up, the scan itself, checking the pictures, and removing the IV if needed, takes 15 to 45 minutes depending on what part of the body is being scanned.
- For some studies, the patient will be asked to hold their breath for up to 20 seconds.
- No metal may be worn.
- What clothing the patient wears depends on the nature of the study. For a CT scan of the chest, abdomen, or pelvis, for example, usually the patient will change into a hospital gown. For a head CT scan, the patient can wear normal street clothes.
- Sedation is rarely necessary. The machine is quiet, so the patient hears during the test is a quiet whirr.
- The technologist is in the next room and can observe the patient through a large window.
After the Procedure
If the patient received a contrast injection, the IV is removed from the arm before going home. There should be no ill effects from the scan or the contrast injection. In the rare circumstance that the patient received sedation, they will be sent home once they are awake and alert. However, someone will have to drive the patient home.
The CT scan is interpreted by a radiologist, a medical doctor trained to interpret various X-ray studies. The results are forwarded to the doctor. How soon the doctor receives the report depends on the imaging center where the study is performed.
When to Seek Medical Care
The reaction to the contrast is almost always immediate, so it is very rare to have a reaction after the patient leaves the facility. However, if a patient thinks they are having a delayed reaction to the contrast, they should call the facility where they had the exam.
Symptoms include itching and difficulty breathing or swallowing. If contrast leaked under the skin, the patient should look for increased redness, swelling, or pain. Patients will often be asked to come back the next day so their skin can be checked. There are no side effects of the exam itself, but patients who have multiple CT scans should discuss the radiation exposure with their physician.
Reviewed on 11/20/2017
Stark, Paul MD. "Principles of computed tomography of the chest." UptoDate. Updated Jun 2, 2015.
"CT Scan - CAT Scan Imaging - What is a CT Scan." Imaginis.com. Accessed Sept 19, 2016.
Levy, Jason MD. "CT Scan." Medline. Updated Oct 22, 2014.