Computed tomography (CT) - what patients need to know

» What is CT scanning?

Computed tomography (CT) scanning provides images of sections of the body (slices) using special X ray equipment and sophisticated computers. CT scans of internal organs, bone, soft tissue and blood vessels provide greater clarity and reveal more details than X ray or radiograph exams. X ray images are two dimensional representations while CT images show all three dimensions.

Using CT scans, doctors can more easily diagnose problems such as cancers, cardiovascular disease, infectious disease, trauma and musculoskeletal disorders.

» What are the typical radiation doses associated with CT examinations?

CT scans of the head, for example, expose patients to about the equivalence of one year of natural radiation. This is about 100 times the radiation dose from a chest X ray. Other CT examinations involve higher radiation doses ranging up to eight times the dose received from background radiation in one year. 

» Why is the radiation dose in CT higher than in conventional X rays (radiographs)?    

The highest dose is at the body surface where the beam enters the body. In a conventional X ray examination, the beam enters the body from one plane – front, back or any one side.  In CT, the X ray tube rotates around the patient’s body and the beam enters the body at many points, all of which are exposed. This, combined with the radiation that penetrates the body, results in overall higher radiation absorbed dose to the irradiated parts in CT.

» How many CT examinations are unsafe?

There is no straight answer to this question. There are no prescribed limits on the number of CT examinations a patient can undergo. 

No amount of radiation is considered too much for a patient when the procedure is justified by the doctor. This justification takes into account the risk that even a small amount can cause cancer. 

There are well established guidelines and recommendations to help doctors decide on the appropriateness of an examination for a particular disease condition. In general, radiation exposure should be kept as low as reasonably achievable (the ALARA principle) without compromising medical benefits.

» Do I need yet another CT examination?

You can help avoid unnecessary repeat examinations by telling your doctor about previous examinations (X ray, CT, MRI, ultrasound and other imaging examinations) and sharing the results when possible. Repeat investigations are sometimes needed to perform an examination, or after cancer treatment to estimate its effectiveness. A CT examination may sometimes involve the injection of an intravenous contrast agent. Imaging can be performed at multiple time points before and after the injection of the contrast material. Each examination involves sets of images referred to as a "series." Multiple series should be performed only if clinically indicated. A smaller number of series means a smaller radiation dose.

» Can I undergo a CT scan while I am pregnant?

Yes, if medically justified and with certain precautions. 

The aim is to minimize the unborn child’s radiation exposure. An unborn child is considered to be more sensitive than adults or children to potential adverse radiation effects. In many examinations such as CT of the head (including dental CT scans), chest and limbs, the pelvic region is not in the direct beam and the dose to the unborn child can be very low.

Doctors may consider delaying procedures that would put the pelvic region and the unborn child in the direct path of the beam. If the procedure is essential to the mother’s health, the doctors take special actions to keep the dose to the unborn child as low as possible. For example, lower exposure factors can be chosen and pregnant patients can have their pelvic regions shielded during the procedure.

» Is it important to know if I am pregnant for undergoing a CT scan?


For any examination involving direct exposure of the lower abdomen, pregnancy should be ruled out, or the pregnancy status should be verified as part of the justification process. 

» Should I be concerned about radiation if my child has been prescribed a CT?

As part of the justification process, doctors determine whether the benefits of the CT scan outweigh the risks, and they should be able to explain why your child needs a CT scan. Children’s radiation exposure should be as low as possible because they are more sensitive to radiation than adults and they have a longer life ahead of them.

» Should I ask my doctor for a whole-body CT screening?

There is no evidence demonstrating any benefits from whole-body CT screening of individuals. Several investigational studies of the effectiveness of using CT to screen people are focused on high-risk groups for specific diseases. In such studies, only a limited part of the body is irradiated, and the examination screens for a specific type of disease.