Computerized (Axial) Tomographic Scan (CT or CAT Scan)

The CAT scan, also known as CT (Computed Tomography) imaging, is a safe and non-invasive procedure that uses low radiation X-rays to create a computer-generated, three-dimensional image of the brain. It provides detailed information about the structure of the brain.

The CT scan uses a narrow beam of X-rays, which pass through the head and hit a detector on the opposite side. The beam moves around the head and the detector moves with it. Since different brain tissues have different densities, each will block the X rays from reaching the detector to different degrees. These may reveal abnormalities (blood clots, cysts, tumours, scar tissue, etc.) in the skull or brain which may be related to seizures.

This allows physicians to examine this structure, section by section, as the test is being conducted. The CAT scan helps point to where a person’s seizures originate. The CT scan only shows what the brain looks like, not how it works. The scanning period is brief, safe and painless.

Before a CAT Scan

  • Dress in comfortable, loose-fitting clothing.
  • Remove any metal objects such as jewellery, belts, hairpins, hearing aids, glasses or dentures. These objects may degrade the clarity of the images.
  • If a contrast medium is going to be used, do not eat or drink anything four to eight hours before the test.
    • A contrast medium is a dye that may be given to you orally or injected into your hand or arm. This fluid goes up to the brain and makes the scan easier to read.
    • Some people are allergic to particular contrast mediums. It is important to inform your doctor if you have any allergies, especially to iodine.
      • You should also inform the doctor if you have diabetes, asthma, a heart condition, kidney problems, or thyroid condition. These conditions may indicate a higher risk of iodine reactions.
    • Inform the technician if you feel any itching or shortness of breath after the administration of the contrast medium.
      • After the injection, you may feel flushed, have a metallic taste in your mouth, or feel nauseated.
      • Another mild reaction that can occur following the administration of iodine is itching over various parts of the body. This lasts from several minutes to several hours and is usually treated with medication.
      • More serious but less common reactions may include shortness of breath, swelling of the throat or other parts of the body. These reactions should be treated immediately.
    • A CAT scan still provides valuable information without administering a contrast agent. If you are at risk of reaction to the contrast agent, the physician may decide not to administer it during a CAT scan.

During a CAT Scan

  • After the administration of the contrast agent, you will be asked to lie down on a scanning table.
  • The technician will move to another room and then proceed to move your body into the scanner machine.
  • A series of X-rays will be taken by a camera, which will rotate around your head.
  • It is important that you lie completely still until the entire examination is complete.
    • Any movement will blur the image.
    • You may be asked to hold your breath during the scanning procedure to help to eliminate the blurring of the images caused by the movements of breathing.
  • During the examination, you will hear some noise from the machine and the CT table may move slightly. This will not cause any discomfort.
  • If you have any questions or concerns throughout the examination, you can speak with the technologist via intercom.
  • You cannot have anyone in the room with you during the scanning process. Because the CT scanner uses X-rays, only the person being imaged should be in the CT scanner room during the procedure.
  • A scan can be obtained in about 15 to 20 minutes.

After a CAT Scan

  • You can return to your usual daily activities and resume your regular diet immediately after the scan unless instructed otherwise by the doctor.
  • If you were given an injection and the injection site remains sore, you are encouraged to call your physician.
  • You must have someone drive you home from the test site, as the effects of the sedative may not have worn off yet.
  • A radiologist will interpret the images and send them to your doctor who will review the results with you at a follow-up appointment. Additional tests or a treatment plan will be designed accordingly.

Important Considerations

  • If you have allergies or have experienced discomfort because of any contrast agent previously, inform the technologist and the doctor before the examination.
  • If you are nursing an infant, you should wait 24 hours after receiving the CT contrast injection before breastfeeding again.
  • If you are pregnant, you should not have a CT examination. It is extremely important that you inform your doctor if you are pregnant or think you are pregnant before proceeding with the examination.


  • Radiologists
    • interpret the CT images
    • send a report to the referring physician
    • administer or observe the contrast injection
  • CT technologists
    • operate computerized tomography scanner to produce cross-sectional views of the body section being scanned
    • perform the examination under the supervision of the radiologist
    • may also administer the contrast injection


The CAT Scan is covered by the Ontario Health Insurance Plan (OHIP).

Click here to learn about other types of diagnostic tests.