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CT was discovered 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. A CT scan is a computerised tomography scan. It uses X-rays and a computer to create detailed images of the inside of your body. 

CT scans are also sometimes known as CAT scans, which stands for computerised axial tomography.

During a CT scan, you will usually lie on your back on a flat bed. The CT scanner consists of an X-ray tube that rotates around your body. You will usually be moved continuously through this rotating beam. The rays will be analysed by a detector on the opposite side of your body.

Unlike an MRI scan, where you are placed inside a tunnel, you should not feel claustrophobic.

The images produced by a CT scan are called tomograms and are more detailed than standard X-rays. A CT scan can produce images of structures inside the body including the internal organs, blood vessels, bones and tumours.

The scan is painless and will usually take 10 to 30 minutes, depending on the part of your body being scanned.

Read more about how a CT scan is performed.

When CT scans are used

CT scans can be used to diagnose and monitor a variety of different health conditions including brain tumours, certain bone conditons and injuries to internal organs such as the kidneys, liver or spleen.

They are also often used to look inside the body before another procedure takes place, such as radiotherapy treatment or a biopsy (where a small tissue sample is taken so that it can be examined under a microscope).

Your results

CT scans are usually carried out on an outpatient basis, which means that you will be able to go home on the same day as the procedure.

The results of your scan will not be available immediately. A computer will need to process the information from your scan, which will then be analysed by a radiologist (a specialist in interpreting images of the body).

After analysing the images, the radiologist will write a report and send it to your specialist or GP. This usually takes a few weeks.


CT scans are safe procedures and, in most cases, the benefits of having a scan outweigh any potential risks.

A CT scan does expose you to radiation. The amount is small and will not cause you any harm. However, CT scans are not recommended for pregnant women because there is a small risk that the X-rays that are used could harm the unborn baby.

Children are also more at risk than adults are from developing a build-up of radiation. Therefore, a CT scan will only be recommended if a child has a serious condition that puts them at greater risk.

What is a CT scan?

A CT scan — also called Computerized Tomography or just CT—is an X-ray technique that produces images of your body that visualize internal structures in cross section rather than the overlapping images typically produced by conventional X-ray exams. CT scans use an X-ray unit that rotates around your body and a powerful computer. The result with CT scans is a set of cross-sectional images, like slices, of the inside of your body.

Should I do anything special to prepare for a 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 your clothing and wear a hospital gown. You'll need to remove any metal objects, such as jewelry, that might interfere with image results. Some CT scans require you to drink a contrast liquid before the scan or have contrast injected into a vein in your arm during the scan. A contrast medium blocks X-rays and appears white on images, which can help emphasize blood vessels, bowel or other structures. If your test involves a contrast medium, your doctor may ask you to fast for a few hours before the test.

Can I take my medicine before a CT scan?

Yes, please take medicines before the CT scan, with the exception of diabetic medicines. Consult your physician before the test for instructions.

How long will it take to do a CT scan?

Expect the exam to last no longer than an hour, depending on the preparation needed and whether it includes the use of a contrast medium. The scan itself may take less than a minute on the newest machines. Most scans take just a few minutes to complete.

Will the radiation that I receive from the CT scan hurt me?

CT scans are similar to those of conventional X-rays. During the CT scan, you’re briefly exposed to radiation. But doctors and other scientists believe that CT scans provide enough valuable information to outweigh the associated risks.

What will I experience during and after the procedure?

During the CT scan, you lie on a narrow table that slides through the opening of the gantry. You may lie on your back, side or stomach, depending on the area to be scanned. The table can be raised or lowered. Straps and pillows may help you stay in position. During a CT scan of the head, the table may be fitted with a special cradle that holds your head still. CT scans are painless. If your exam involves use of an intravenous contrast medium, you may feel a brief sensation of heat or experience a metallic taste in your mouth. If you receive the contrast medium through an enema — to help highlight your lower gastrointestinal region — you may feel a sense of fullness or cramping. After the exam you can return to your normal routine. If you were given a contrast medium, your doctor, a nurse or the CT technologist performing the scan may give you special instructions. You may be asked to wait for a short time in the radiology department to ensure that you feel well after the exam. After the scan, you'll likely be told to drink lots of fluids to help your kidneys remove the medium from your body.

Will I have to take a CT contrast or dye, and can I be allergic to it?

It depends on which part of your body is being scanned. Although rare, the contrast medium Involved in a CT scan poses a slight risk of allergic reaction. Most reactions are mild and result in hives or itchiness. For people with asthma who become allergic to the contrast medium, the reaction can be an asthma attack.

In rare instances, an allergic reaction can be serious and potentially life-threatening — including swelling in your throat or other areas of your body. If you experience hives, itchiness or swelling in your throat during or after your CT exam, immediately tell your technologist or doctor.

If you've had a reaction to a contrast medium in the past, and you need a diagnostic test that may require a contrast medium again, talk to your doctor. Be sure to let your doctor know if you have kidney problems, since contrast material that's injected into a vein is removed from your body by your kidneys and could potentially cause further damage to your kidneys.

If you have had a prior reaction to contrast media or have asthma or allergies, there's an increased risk of a reaction to the contrast medium. Diabetes, asthma, heart disease, kidney problems or certain thyroid conditions may increase your risk of a reaction to contrast media.

Will I need someone to drive for me after the CT scan?

No, the CT scan is a safe test that will not affect your ability to drive.

How and when will I get my results?

CT images are stored as electronic data files and usually reviewed on a computer. A radiologist interprets these images and sends a report to your doctor.

What are Contrast Agents?

Contrast agents are used to image tissues and structures that are not normally seen, or not seen very well. Intravenous contrast agents are used to enhance organs and visualize blood vessels. Oral contrast agents are used to visualize the digestive tract.

How do CT scans differ from MRI scans?

CT and MRI images sometimes look very similar, but the equipment used to perform the scans is different. CT uses ionizing radiation just as with a routine X-ray, while MRI uses a magnetic field. Depending on the clinical indications, one may be preferred over the other, or both may be desirable. CT scanners are faster and as a result, claustrophobia and movement are not as problematic as with the MRI scanner.

Who performs the CT scan?

Medical radiation technologists specially trained in the operation of CT scanners perform the procedure.

What will I feel during the scan?

CT scanning causes no pain, just as a routine X-ray is painless. If intravenous contrast is used, you may feel warm and flush and get a metallic taste in your mouth. These sensations normally disappear after a few minutes.

How long will the scan take?

The time required will depend upon the type of scan. If oral contrast is required, about 45 to 60 minutes is needed for the contrast to move through your digestive tract. Actual scan times vary from a few seconds to several minutes. If no oral contrast is required, the examination will take about 15 to 30 minutes, including the time for intravenous preparation and interview. In some cases additional scanning is required as scans are tailored to suit individual diagnostic needs.

Will I need to drink anything?

Most abdominal scans require the patient to drink a barium sulfate oral contrast mixture. This mixture is flavoured and not at all unpleasant. Oral contrast highlights the stomach and upper intestine providing the radiologist with a detailed image for review. If you are scheduled for a CT scan requiring oral contrast, you will be asked to arrive one hour before the scan time.

How long will I have to wait after I arrive?

Every attempt is made to keep procedures and scan appointments on schedule. However there may be fluctuations in appointment times due to emergency patient needs. Trauma patients, and patients whose lives are in jeopardy will take precedence over booked outpatient appointments. We appreciate your understanding and patience when we must attend to these life and death situations.

Can my spouse/friend stay in the room with me?

No. CT scanners use ionizing radiation and only the patient requiring the scan is permitted in the room.

Why does the technologist leave the room?

The technologist must operate the computer system to complete the scanning procedure.

Can I see the images after my scan?

No. In order to stay on schedule, time will not permit a review with the patient. In addition, the technologists are restricted from discussing images with you. While we understand your curiosity and anxiousness, it is in your best interest to discuss the results of your examination with your doctor.

Will I get the results after the scan?

No. In most cases several hundred images are created during the scan, all of which will be reviewed by the radiologist. Previous examinations will also be reviewed and compared if applicable. The radiologist completes an in-depth review of all images and may at times consult with other physicians to provide an accurate report of your examination to your physician. The final report may take several days to complete. Upon completion it will be sent to your referring physician

Should I have a CT scan if I am pregnant?

No. If you are pregnant or suspect that you may be, you should not have a CT scan or any type of X-ray examination. You should inform the technologist if you suspect you may be pregnant. Alternative arrangements may be made to meet your medical needs.

Will I see the X-ray dye in my urine?

No. X-ray dye or intravenous contrast is a colorless compound that is excreted unnoticed in the urine.

Why do some patients need X-ray dye and others not?

Depending upon your condition and the images required to diagnose or rule out pathology, X-ray dye or intravenous contrast may or may not be needed. The radiologist reviews the information sent to us by your physician and decides what contrast is needed to provide the best images.

Are there any instructions I need to follow after the scan?

If no contrast was used, there are no instructions and you may continue with your normal activities. If intravenous contrast or oral contrast is used, you will be instructed to drink water for the rest of the day to help eliminate the contrast.

Will I have to hold my breath?

Depending upon the body part being scanned, you may be required to hold your breath several times during the scan. It is important that you not move during the scan. The technologist will instruct you on breathing prior to the start of the scan.

Can I breastfeed after an injection of intravenous contrast?

You should not breastfeed for 48 hours after an injection of intravenous contrast.

Does the radiation stay in my body?

No. CT uses a thin beam of radiation that is captured by detectors as it exits your body.

If you have a computerised tomography (CT) scan, you will be exposed to radiation in the form of X-rays. The amount of radiation that is used is very small.

Radiation is measured in units called millisieverts (mSv). Different types of CT scan use different amounts of radiation:

  • CT scan of the head: 1.4 mSv
  • CT scan of the chest: 6.6 mSv
  • CT scan of the whole body: 10 mSv

Benefits versus risks

The benefits of having a CT scan to help diagnose a medical condition, or to check the symptoms of an existing condition, will usually outweigh any potential risk. CT scans are quick and accurate and often eliminate the need for invasive surgery.

However, if you do not have any symptoms, the benefits of having a CT scan may not outweigh the risks, particularly if it leads to further unnecessary testing and added anxiety.

Therefore, the benefits and risks should always be weighed up before deciding to have a CT scan. It is recommended that you only have a CT scan following a medical referral.

Pregnant women and children

CT scans are not recommended for pregnant women because there is a small risk that the X-rays may harm the unborn child. Before having a scan, tell your doctor if there's a chance you may be pregnant.

Children are at greater risk from a build-up of radiation than adults and should only have a CT scan if it is justified by a serious condition that puts them at an increased risk.

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