What Is a CT Scan?

Computed tomography—also called a CT scan, CAT scan, or computerized axial tomography—relies on multiple X-rays to create a 3-D image. It allows healthcare providers to look at organs and helps them to diagnose an injury, such as a fracture, or a condition like cancer.

A CT scan provides more information than some other imaging tests, without the need for invasive techniques. For example, it can be used to complete a biopsy in some cases. It may be done with contrast, an ingestible or injectable type of dye that allows technicians to better visualize a body process.

This article explains how a CT scan works, why you might need one, and how to prepare for it. It also discusses what results mean, and the overall safety of a CT scan for most (but not all) people.

What Can CT Scans Detect?

A CT scan is a painless, non-invasive procedure that can be used to visualize nearly every part of the body. Since the introduction of CT technology in 1967, the imaging procedure first used in diagnosis has advanced with applications in disease prevention, screening, and management.

A CT scan 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 a CT scan are:

• CT scanning of the abdomen may be used to identify masses in the liver, kidney, or pancreas, or to search for causes of bleeding in the urinary tract (hematuria).
• CT scanning of the cardiovascular system can be used to map the flow of blood (CT angiography) and to help to diagnose kidney disorders, aortic aneurysm, atherosclerosis, or pulmonary edema.
• CT scanning of the heart can help diagnose and monitor coronary artery disease (CAD) or aid in valve replacement surgery.
• CT scanning of the head and brain may be used to look for tumors, hemorrhage, bone trauma, blood flow obstruction, and brain calcification (commonly seen in people with Parkinson's disease and dementia).
• CT scanning of the lungs can help detect changes in the lung architecture as a result of fibrosis (scarring), emphysema, tumors, atelectasis (collapsed lung), and pleural effusion.
• CT scanning of the skeletal system can aid in the diagnosis of a spinal cord injury, pathologic fractures, bone tumors, or lesions, and to help evaluate a complex fracture, osteoporosis, or joint damage caused by arthritis.

Given this, your healthcare provider may order this test for a number of reasons, including the need to monitor an already-diagnosed condition.

Benefits and Limitations

A CT scan is superior to X-rays, magnetic resonance imagining (MRI), and positron emission tomography (PET) within the context of emergency care. 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.

There are situations where a CT may be less effective. For example, an MRI is far better at imaging organs and soft tissues and, in a non-emergency situation, may prove more helpful than a CT scan.

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

With CT scans, a main concern 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 the National Cancer Institute (NCI), the risk of cancer from a single CT scan is 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% compared to the general average of 20%.

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, studies have found no clear association between medical radiation and cancer risk in children.

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 agents like Omnipaque (iohexol) are most commonly used.

Contrast agent side effects may occur in anywhere from 1% to 12% of cases, depending on the agent used, and it may be the reason for why some people feel sick after a CT scan.

Side effects can range in severity from mild to life-threatening, and develop anywhere from one hour to seven days after the dose. They 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.

Those with a known allergy to a radiocontrast agent should be pre-treated with antihistamines and steroids before receiving contrast.

Pediatric Considerations

According to the 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 options that do not emit radiation, such as MRI and ultrasound
• 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 healthcare provider and do not hesitate to ask if there are other means to achieve a reliable diagnosis.

CT Scans and Pregnancy

CT scans may pose some risk if you are pregnant. If you are or suspect you may be pregnant, or have been trying to get pregnant, advise your healthcare provider.

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.

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.

If you're breastfeeding, it's important to know that barium is not absorbed into the bloodstream and will not be passed to your baby in breastmilk.

While less than 1% of an iodine-based solution may be transmitted in breastmilk, this amount cannot cause harm to a baby and does not warrant the interruption of breastfeeding.

However, some mothers may prefer a more conservative approach and choose to avoid breastfeeding for 24 to 48 hours following the test. (In such cases, pumping a couple days' supply beforehand can tide you over.)

Contraindications

CT scans are almost always avoided during pregnancy unless the benefits of the test clearly outweigh the potential risks.

Other contraindications may include:

• A known iodine allergy
• Planned radioiodine treatment of thyroid cancer
• Gastrointestinal perforation (for oral or rectal solutions)

From a practical standpoint, obesity may exclude the use of a CT scan given that most machines 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 healthcare provider will provide you with specific instructions based on the aims of the test.

How Long Does a CT Scan Take?

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 should take around three to five minutes to perform.

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 prepared to accommodate for delays when scheduling the test and 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 Do I Need to Do Before a CT Scan?

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 sweatsuit).

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 accessories at home.

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 healthcare provider 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. You may want to bring a sanitary pad to prevent anal leakage after the solution is evacuated from the colon. 

CT Scan Costs

The CT scan cost is between $600 and $1,500 when no contrast is used, but it depends 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 healthcare provider can submit this request on your behalf. 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 healthcare provider 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 in the arm, 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 a contrast solution 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.

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.

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 healthcare provider.

Regardless of the type of CT done, call your healthcare provider if you experience any unusual symptoms, including:

• Fever
• Chills
• Vomiting
• Shortness of breath
• Rapid heartbeat

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.

Interpreting the Results

After a CT scan, you should get results from your healthcare provider 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
• Narrowing (stenosis) of blood vessels
• Air passages
• 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 healthcare provider 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
• Imaging tests
• 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. 

A Word From Verywell

A CT scan is fast and accurate. But if your symptoms continue despite a "normal" result, speak with your healthcare provider about other test options, or ask for a referral to a specialist who may be able to expand the investigation.