What Is a CT Scan?

What to expect when undergoing this test

Computed tomography—sometimes referred to as a CT scan, CAT scan, or computerized axial tomography—involves the use of multiple X-ray images that are translated by a computer and converted to form a three-dimensional image. This allows doctors to look at an organ, injury, or growth from different angles. A CT scan allows for more insightful analyses than other imaging tests without the need for invasive interventions. It is used for a wide variety of reasons, such as detecting tumors, identifying blood clots, assessing a bone fracture, and more.

Over 70 million CT scans are performed in the United States each year, according to research from Columbia University Medical Center in New York.

what to expect during a CT scan
Illustration by Emily Roberts, Verywell

Purpose of Test

A CT scan is a painless, non-invasive procedure that can be used to visualize nearly every part of the body. Since its introduction in 1967, the CT scan has advanced from being a tool for medical diagnosis to one that has applications in disease prevention, screening, and management. It is typically used when an X-ray cannot provide enough detail of an injury or disorder, especially in emergency situations where time is of the essence.

Among the many uses of CT scan:

    Given this, your doctor may order this test for a number of reasons, including the presentation of symptoms related to these or other issues, an event (such as a physical trauma), results from other tests that indicate a need for further evaluation, and/or the need to monitor an already-diagnosed concern.

    Benefits and Limitations

    CT technology offers numerous benefits, particularly in emergency situations where high-contrast images can be produced in literally minutes. The information can tell doctors whether surgery is needed or not.

    Within this context, a CT scan is superior to X-rays, magnetic resonance imagining (MRI), and positron emission tomography (PET). Only an ultrasound can match the CT in terms of speed, but it has limitations in the types of injuries or disorders it can diagnose.

    With that being said, there are situations where a CT may be less effective. For example, an MRI is far better at imaging organs and soft tissues, including joints, ligaments, nerves, and spinal discs. In a non-emergency situation, an MRI may provide more insightful information than a CT scan.

    On the other hand, an MRI costs twice as much and, because it employs strong magnetic waves, may not be appropriate for people with metal implants (including some pacemakers, artificial joints, and cochlear implants).

    By contrast, PET and CT technologies are often combined into a dual-purpose unit, referred to as PET-CT. By providing both anatomic and metabolic information, PET-CT scanners can offer greater insights when diagnosing or staging cancer.

    Risks and Contraindications

    Despite being a valuable tool for diagnosis and screening, a CT scan does carry risks, which relate to increasing your risk of cancer and reacting to contrast agents.

    Cancer Risk

    The main concern that many people have about CT scans is the exposure to "high" levels of radiation and the potential risk of cancer. While it is true that CT scans expose you to anywhere from 100 to 1,000 times more radiation than a conventional X-ray, that doesn’t necessarily translate into a proportional increase in cancer risk.

    According to a report from the National Cancer Research Institute, the risk of cancer from a single CT scan is very small. When compared to the average lifetime risk of cancer among Americans (one in five), the risk from a CT scan is more or less in the range of one in 2,000. The additive impact translates to a lifetime risk of roughly 20.05 percent compared to the general average of 20 percent.

    Children may be at highest risk due to the fact that they have more years to live following the procedure compared to, say, someone in their 50s, 60s, or 70s. However, a 2012 review of studies cast doubt on that and found no clear association between medical radiation and cancer risk in children.

    While this should not suggest there is no risk, when used appropriately, the benefits of a CT scan will almost always outweigh the possible risks. If you've had one or more CT scans in the past, it is important to advise your doctor if a new one is ordered.

    Contrast Agents

    Contrast agents, also known as radiocontrast agents or contrast dyes, are used in CT scans to highlight structures that are difficult to differentiate from their surroundings, such as the brain, spine, liver, or kidneys. Most are iodine-based and are injected intravenously (into a vein) in advance of the scan.

    For certain gastrointestinal investigations, an oral or enema solution may be needed. Barium sulfate and the iodine-based Gastrografin (diatrizoate) are most commonly used.

    Side effects may occur in anywhere from 1 percent to 12 percent of cases, depending on the agent used, according to research published in the International Journal of Angiology. Side effects can range in severity from mild to life-threatening and develop anywhere from one hour to seven days after the dose.

    Common side effects include:

    • Flushing
    • Rash
    • Itching
    • Runny nose
    • Coughing
    • Dizziness
    • Abdominal cramps
    • Constipation
    • Nausea
    • Vomiting

    Diabetes, asthma, heart disease, thyroid disorders, and kidney impairment may increase your risk of side effects.

    Life-threatening allergic reactions—known as anaphylaxis—may occur in between 0.01 percent and 0.2 percent of cases. Symptoms include shortness of breath, hives, facial swelling, rapid heart breath, impaired breathing, abdominal cramps, and a feeling of impending doom. If not treated immediately, anaphylaxis can lead to seizures, coma, shock, and even death.

    Pediatric Considerations

    According to the National Cancer Institute (NCI), between 5 million and 9 million CT scans are performed on children in the United States, including infants and toddlers, each year. While the lifetime risk of cancer in children from a single CT scan is low, the NCI recommends that the procedure be adjusted so that the lowest possible radiation dose is delivered to achieve a clear imaging result.

    This would involve:

    • Performing CT scans only when needed
    • Considering other modalities that do not emit radiation, such as ultrasound and magnetic resonance imaging (MRI)
    • Adjusting the radiation level based on the child's size and weight
    • Narrowing the scan to the smallest necessary area
    • Reducing the scan resolution if high-quality images are not absolutely necessary

    If more than one CT scan is recommended, discuss the benefits and risks with your doctor and do not hesitate to ask if there are other means to achieve a reliable diagnosis.

    Pregnancy Considerations

    If you are or suspect you may be pregnant, or have been trying to get pregnant, advise your doctor. Generally speaking, if the abdomen or pelvis are not being scanned, the risk to your fetus is negligible. If the CT scan involves the abdomen or pelvis, the risk to your baby is still considered small, according to guidance from the American College of Obstetricians and Gynecologists (ACOG).

    Similarly, oral and rectal contrast agents are not absorbed in the bloodstream and cannot harm the fetus. While intravenous agents can cross the placenta and enter fetal circulation, animal studies to date have not shown any evidence of harm.

    However small the risk may be, it's always best to speak with your doctor about any procedure performed during pregnancy so that you can make an informed choice.

    With regards to breastfeeding, barium is not absorbed into the bloodstream and will not be passed to your baby in breastmilk. While less than one percent of an iodine-based solution may be transmitted in breastmilk, ACOG has concluded that this amount cannot cause harm to a baby and does warrant the interruption of breastfeeding.

    With that being said, some mothers may prefer a more conservative approach and avoid breastfeeding 24 to 48 hours following the test. (In such case, you can pump a couple days' supply beforehand to tide you over.)

    Contraindications

    CT scans are almost always avoided during pregnancy unless the benefits of the test clearly outweigh the potential risks. The CT scan would also be contraindicated in those with a known allergy to the radiocontrast agent.

    Other contraindications may include:

    From a practical standpoint, obesity may the exclude the use of a CT scan given that the machine can only accommodate weights of less than 425 to 450 pounds and a back-to-belly measurement of fewer than 28 inches. 

    Before the Test

    The preparations for a CT scan can vary based on the type of condition being diagnosed and whether a contrast agent is being used. Your doctor will provide you with specific instructions based on the aims of the test.

    Timing

    From arrival to completion, the appointment should take around one to two hours, depending on the preparations needed. The scan itself without a contrast agent will take around 15 to 30 minutes to perform. Newer machines can run the scan in only a few minutes.

    If a contrast agent is used, it can take anywhere from several minutes to an hour for the solution to fully circulate through the bloodstream or gastrointestinal tract.

    Be sure to accommodate for any delays when scheduling your test and to try to arrive 15 minutes early to sign in.

    Location

    The test is typically conducted in a hospital or an independent radiology facility.

    Newer CT imaging systems are comprised of a large, donut-shaped unit and a motorized scanning table that passes in and out of the scanner. In the center of the tunnel (gantry) are a series of X-ray emitters and detectors. These are far less claustrophobic and loud compared to older systems.

    The radiographer will conduct the CT scan from a radiation-safe control room adjacent to the scanning room.

    What to Wear

    Depending on the part of your body being scanned, some or all of your clothing may need to be removed. Wear comfortable clothing without zippers, buttons, rivets, or snaps (such as a sweat suit).

    While a locked storage space may be provided, leave any valuables at home. Since you will need to remove anything made of metal from the scanning site (including eyeglasses, jewelry, and piercings), it is best to leave any non-essential accessory at home.

    Food and Drink

    Food and drink restrictions may be needed for some CT procedures, especially those involving contrast agents. In such cases, you may be asked to stop eating or drinking six to eight hours beforehand.

    Certain medications may also need to be temporarily stopped. Advise your doctor about any and all drugs you may be taking, whether they are prescription, over-the-counter, or recreational.

    If a rectal contrast agent has been ordered, you will need to undergo bowel preparation a day before the procedure, which involves food restrictions and laxatives to ensure the bowel is entirely clear of fecal matter.

    What to Bring

    Be sure to bring your ID and health insurance card with you when signing in at the lab. If your child is undergoing the scan, you may want to bring a soft toy along if he or she is especially anxious.

    If a rectal contrast agent is ordered, you may want to bring a sanitary pad to prevent anal leakage after the solution is evacuated from the colon. 

    Cost and Health Insurance

    The cost of a conventional CT scan without a contrast agent is between $600 and $1,500 depending on the state you live in and the facility you choose. A more extensive evaluation with a contrast agent may be as high as $5,000.

    As a rule, CT scans will require some form of insurance pre-authorization. Your doctor can submit this request on your behalf. If the scan is denied, ask for the reason in writing. You can then take the letter to your state consumer protection office for assistance in submitting an appeal. Your doctor should also provide additional motivation as to why the test is crucial.

    If approved, be sure to find out what your out-of-pocket expenses will be. If you are uninsured or underinsured, shop around for the best prices. By and large, hospital radiology units are more expensive than independent ones.

    You can also ask if the lab offers flexible payment options. If you are uninsured, ask if they have a patient assistance program with a tiered price structure.

    During the Test

    The test will be performed by a specially trained radiographer in a scanning room. A nurse may also be present.

    Pre-Test

    On the day of the test, after signing in and confirming your insurance information, you may be asked to sign a consent form stating that you understand the purpose and risks of the procedure. You will then be led to a changing room to change out of your clothes. If you are having a conventional CT, you are now ready to get into position in the scanning room. But if your doctor has ordered that your test be done with a contrast agent, you will need to undergo some additional preparations:

    • If an IV contrast agent is ordered, you will be positioned on the table in the scanning room and an IV line will be inserted into a vein, usually in the arm or groin, after which a contrast agent is injected. In some cases, the agent may be injected directly into the joint (arthrogram) or lower spine (myelogram). You may experience brief flushing or a metallic taste in your mouth. Depending on the part of the body being scanned, you may need to wait in a reclined or prone position for several minutes or more. The IV line is kept in place until the end of the scan.
    • If an oral contrast agent is ordered, you will be asked to drink either a chalky (barium) or watery (Gastrografin) substance before entering the scanning room. Depending on the part of the body being assessed, you may need to wait 30 to 60 minutes before the scan can be performed. Let the nurse or radiographer know if you experience nausea or distress of any sort.
    • If a rectal contrast agent is ordered, you will be positioned on the table in the scanning room and your rectum will be lubricated. An enema tube will be inserted to gradually fill your colon with the contrast agent (and sometimes air). To ease muscle spasms, you may be given a shot of Buscopan (butylscopolamine). A balloon at the tip of the tube is then inflated to prevent leakage and kept there until the scan is complete.

      Depending on the investigation, you may be asked to lie on your back, side, or stomach. The table can be raised or lowered, and straps and pillows may be used to keep you in position and help you stay still during the test. While not moving as the scan is being performed is imperative, newer multi-detector CT systems are fast and easy, reducing the amount of time you need to hold your position.

      If you are accompanying your child, you will need to wear a protective apron to minimize radiation exposure. During the actual scan, you will stay in the control room with the technologist but will be able to communicate with your child through the two-way speaker.

      Throughout the Test

      When it is time to begin, the technician will let you know by communicating with you through the speaker. At first, the motorized table will move in and out of the scanner quickly. This is to ensure that the table is in the right starting position and that the scan will cover the entire body part being investigated. You will also see special light lines projected onto your body to ensure that you are in the correct position.

      From there on, the table will move slowly through the scanner. The gantry will spin around you as the X-ray emitters produce an array of beams. The beams will pass through your body and be received by corresponding detectors.

      During each scan, remember to remain still. In some cases, you may be asked to hold your breath. Your position may also be altered to obtain different views. Unlike CT scanners of old, newer units only emit slight buzzing, whirring, or clicking sounds. You will feel no pain from the scan itself.

      If you need to sneeze or itch, or you are cramping up, let the technician know. There is no problem with stopping the test momentarily. In some cases, the technician may be able to make you more comfortable without obstructing the imaging.

      A computer will then translate the signals into a series of cross-sectional (tomographic) images called slices. Using geometric digital processing, the two-dimensional slices can be converted into the final, 3D image.

      Post-Test

      Once the scans are completed, the radiographer will double-check to ensure the images are clearly visualized.

      • If an IV contrast agent was used, the IV line will be removed and the puncture wound bandaged.
      • If an oral contrast agent was used, you will be given a glass of water and encouraged to drink plenty of fluids.
      • If a rectal contrast agent was used, the solution will be extracted from the colon through the enema tube. Once the tube is removed, you will be lead to the restroom to expel the rest into the toilet. A sanitary pad may be provided to protect your clothing from leakage. A laxative may also be offered to help clear the bowel and prevent constipation.

      In most cases, you can change back into your clothes and drive yourself home or to work.

      After the Test

      Most iodine-based contrast agents have a half-life of between two to four hours, meaning that they are fully cleared from your body in a day or two. Much of the solution will be excreted in urine, so drink plenty of fluids.

      If you were given a barium solution, you may experience short-term constipation, and your stools may be chalky for a day or two. If you don’t have a bowel movement after two days, call your doctor. A barium enema can sometimes cause impaction and lead to bowel obstruction. A special enema may be needed to clear the impaction.

      The radiation from the CT scan will not remain in the body, and you will be of no harm to anyone you touch, kiss, or stand close to.

      Regardless of the type of CT done, call your doctor if you experience any unusual symptoms, including fever, chills, vomiting, shortness of breath, or rapid heartbeat.

      Interpreting the Results

      Your doctor should receive the CT scan results within a day or two. In addition to the images, the radiologist will prepare a detailed report outlining the normal and abnormal findings.

      A CT scan can sometimes provide definitive evidence of a disorder, particularly fractures; kidney stones; clots; or the narrowing (stenosis) of blood vessels, air passages, or the intestines.

      At other times, a scan can only suggest what is happening. This is especially true with abnormal growths, lesions, and tumors. Further investigation is often needed to determine whether the growth is benign or malignant and what types of cells may be involved.

      In some cases, no problem may be found. This doesn't mean that you are necessarily in the clear. It simply indicates that nothing was detected based on the limitations of the CT technology.

      Based on the results, your doctor may either offer a treatment plan or recommend further testing.

      Follow-Up

      If further investigation is needed, the diagnosis may involve blood tests, urine tests, cultures, tissue biopsies, other imaging tests, or even exploratory surgery.

      If cancer is suspected, a combination PET-CT scan may provide more definitive evidence of a malignancy alongside a biopsy of the growth itself. 

      Disorders of the brain often benefit from a functional MRI (fMRI), which measures changes in blood flow, or a single-photon emission computerized tomography (SPECT) scan, a specialized form of PET that can pinpoint where seizures are triggered.

      Cardiac disorders may require a Holter monitor, a portable electrocardiogram (ECG) unit that you wear for 24 to 72 hours. A cardiac stress test may be used to see how your heart responds to physical activity. Cardiac catheterization, which involves the insertion of a narrow tube into a vein or artery, may be used to measure blood flow or check for abnormalities in the heart valves. 

      A Word From Verywell

      As accurate and fast as a CT scan can be, the results are sometimes open to interpretation. If your symptoms continue despite a "normal" result, speak with your doctor about other test options, or ask for a referral to a specialist who may be able to expand the investigation. Don't ever be afraid to seek a second opinion or to ask that your files be forwarded to another doctor.

      CT images today are stored as electronic data files and can be delivered via email or other means when needed.

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