What Is a CAT / CT Scan?

Photo courtesy of Johns Hopkins Medicine Health Library.

By Teresa Rhoades

Procedure overview

Computed tomography (CT or CAT scan) is a noninvasive diagnostic imaging procedure
that — combines a series of Xray views taken from many different angles and computer processing to create crosssectional images of the bones and soft tissues inside your body: muscles, fat, organs, and blood vessels.

The resulting images can be compared to looking down at single slices of bread from a loaf. Your doctor will be able to look at each of these slices individually or perform additional visualization to view your body from different angles. In computed tomography, the X-ray beam moves in a circle around the body. This allows many different views of the same organ or structure. The X-ray information is sent to a computer that interprets the X-ray data and displays it in a two-dimensional (2D) form on a monitor. In some cases, CT images can be combined to create 3D images. CT scan images can provide much more information than do plain X-rays.

CT scans may be done with or without “contrast.” Contrast refers to a substance taken by mouth or injected into an intravenous (IV) line that causes the particular organ or tissue under study to be seen more clearly.


A CT scan has many uses, but is particularly well suited to quickly examine people who may have internal injuries from car accidents or other types of trauma. A CT scan can be used to visualize nearly all parts of the body. One example is a CT scan of the abdomen to assess the abdomen and its organs for tumors and other lesions, injuries, intra-abdominal bleeding, infections, unexplained abdominal pain, obstructions, or other conditions. I had an abdominal CT done.

Risks of the procedure

Radiation exposure: During a CT scan, you’re briefly exposed to much more radiation than you would be during a plain X-ray. This radiation from imaging tests has a very small potential to increase your risk of cancer. Still, CT scans have many benefits that may outweigh potential risks. Doctors use the lowest dose of radiation whenever possible. Talk with your doctor about the benefits and risks of your CT scan. It is a good idea to keep a record of your past history of radiation exposure, such as previous CT scans and other types of X-rays, so that you can inform your doctor.

Harm to unborn babies: Tell your doctor if you’re pregnant. Another type of exam may be recommended, such as ultrasound or magnetic resonance imaging (MRI) to avoid the risk of exposing your fetus to the radiation.

Reactions to contrast material: In certain cases, your doctor may recommend you receive a special dye called a contrast material through a vein in your arm before your CT scan. Although rare, the contrast material can cause medical problems or allergic reactions. Studies show that 85 percent of the population will not experience an adverse reaction from iodinated contrast. Most reactions are mild and result in a rash or itchiness. In rare instances, an allergic reaction can be serious and potentially life-threatening. Tell your doctor if you’ve ever had a reaction to contrast material or kidney problems. Prior to my abdominal CT, my doctor ordered a creatinine serum blood test. The creatinine blood test is used to assess kidney function.

Contrast material

A special dye called a contrast material is needed for some CT scans, to help highlight the areas of your body being examined. The contrast material blocks X-rays and appears white on images, which can help emphasize blood vessels, intestines or other structures. Contrast material can enter your body in a variety of ways:

  •  Injection. Contrast agents can be injected through a vein in your arm to help view your gallbladder, urinary tract, liver, or blood vessels. You may experience a feeling of warmth during the injection or a metallic taste in your mouth.
  • Oral. If your esophagus or stomach is being scanned, you may need to swallow a liquid that contains contrast material. This drink may taste unpleasant.

Before my scans, I was required to drink an Oral Barium Solution. In one instance, the solution was dissolved into ice cold ginger ale. For another instance I was prescribed ReadiCat Smoothie, banana flavor 2% SUS (BEZ0725) 450ML.  If you are ever prescribed ReadiCat Smoothie, I suggest you refrigerate the bottles until the liquid is chilled. This way you can almost believe that you are drinking a regular smoothie. Drinking an unchilled bottle results in a little bit of a ‘chalky’ aftertaste.

Before the procedure

  • If your procedure involves the use of contrast dye, you will be asked to sign a consent form
    that gives permission to do the procedure. Read the form carefully and ask questions if
    something is not clear.
  • Notify the technologist if you have ever had a reaction to contrast dyes or if you are
    allergic to iodine.
  • Generally, there is no fasting requirement prior to a CT scan, unless a contrast dye is to be used. Your doctor will give you special instructions ahead of time if contrast is to be used and if you will need to withhold food and drink.
  • Notify your doctor of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.
  • Notify the technologist if you are pregnant or suspect you may be pregnant.
  • Notify the technologist if you have body piercings on your chest and/or abdomen.

Based on your medical condition, your doctor may request other preparations.


During the CT scan

How you prepare for a CT scan depends on which part of your body is being scanned. You may be asked to remove any clothing, jewelry, or other objects that may interfere with the procedure. If you are to have a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast dye. For oral contrast, you will be given a liquid contrast preparation to swallow.

CT scanners are shaped like a large doughnut standing on its side. You lie on a narrow table that slides into the “doughnut hole,” which is called a gantry. Pillows and straps may be used to prevent movement during the procedure.

A technologist will be nearby, in a separate room where the scanner controls are located. However, you will be in constant sight of the technologist through a window. You will be able to communicate with the technologist via intercom. You will have a call button so that you can let the technologist know if you have any problems during the procedure. The technologist may ask you to hold your breath at certain points to avoid blurring the images. Movement blurs the images and may lead to inaccurate results.

The table will move slowly through the gantry during the CT scan, as the gantry rotates in a circle around you. Each rotation yields several images of thin slices of your body. You may hear buzzing, clicking and whirring noises. The ‘good news’ about the CT scan noises is that they are so much quieter than the noises you hear during an MRI.

The X-rays absorbed by the body’s tissues will be detected by the scanner and transmitted to the computer. The computer will transform the information into an image to be interpreted by the radiologist.


After the CT scan

When the procedure has been completed, you will be removed from the scanner. If an IV line was inserted for contrast administration, the line will be removed.

In some cases, you may be asked to wait for a short time before leaving to ensure that you feel well after the exam. Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation. For example, if you were given a contrast material, you may likely be told to drink lots of fluids to help your kidneys remove the contrast material from your body.

CT images are stored as electronic data files and usually reviewed on a computer screen. A radiologist interprets these images and sends a report to your doctor. After my scan, my doctor contacted my to discuss the results and further treatment needs.

The content provided here is for informational purposes only, and is not designed to diagnose or treat a health problem or disease. Please consult your health care provider with any questions or concerns you may have regarding your condition.

Teresa Rhoades worked at the Central Branch from 2004-2005. During the next two years, she moved out of state and completed a degree in Library & Information Studies. She is currently the Assistant Branch Manager for the East Columbia Branch. She spends much of her spare time being walked by her dog, an extremely energetic German Short-haired pointer.