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Computed Tomography

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1. What is CT scanning?

Computed tomography (CT) scanning provides images of sections of the body (slices) using special X ray equipment and sophisticated computers.

Radiography
Figure 1: A cross-sectional CT image of the lungs


CT scans of internal organs, bone, soft tissue and blood vessels provide greater clarity and reveal more details than regular X ray exams (like chest X ray or chest radiograph). Radiographs are two dimensional representations of three dimensional objects and CT provides images that show all three dimensions.

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

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2. What are the typical radiation doses associated with CT examinations?

Table 1: Mean effective doses from CT examinations

CT examinations Mean effective dose (mSv) Equivalent number of PA chest radiograph (each 0.02 mSv)
Head [ME] 2 100
Neck [ME] 3 150
Calcium scoring [ME] 3 150
Pulmonary angiography [BR] 5.2 260
Spine [ME] 6 300
Chest [WA] 8 400
Coronary angiography [EI] 8.7 435
Abdomen [WA] 10 500
Pelvis [WA] 10 500
Virtual colonoscopy [ME] 10 500
Chest (pulmonary embolism) [ME] 15 750

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3. Is there a difference in pattern of radiation dose in CT and in conventional X rays (radiographs)?

Figure 2: A conventional X ray table Figure 3: A CT scanner


Yes. With a conventional X ray, the beam enters the body from one plane (front or back or from any side) and the highest dose is at the entrance surface of the body. But in CT, the X ray tube rotates around the patient’s body and thus the radiation is deposited on all entrance points around the body and in addition there is contribution of radiation that penetrates the body from other directions. This results in overall higher radiation absorbed dose to the irradiated parts.

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4. Are CT examinations associated with higher radiation doses?

Yes. All CT examinations deliver higher radiation doses as stated above. The effective dose from CT scans may range from approximately 2 to 20 mSv, which is about the same as a person receives from background radiation in one (2.4 mSv global average) to eight years. Or more simplify, a CT scan with a dose of say 10 mSv is equivalent to 500 chest X rays assuming a dose of 0.02 mSv per chest X ray (0.02 x 500 = 10). Perfusion CT, some CT-guided interventional procedures and some specialized procedures requiring several successive CTs can deliver even higher doses than 20 mSv.

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5. How many CT examinations are unsafe?

Radiography
Figure 4: A doctor performing a CT biopsy


There is no straight answer to this question in terms of specifying a number. At the outset it must be emphasized that there are no prescribed limits on the number of CT examinations that an individual can undergo. As stated earlier, no amount of radiation is considered too much for a patient when the procedure is justified by the doctor and, on the other hand, even a small amount of radiation has the risk of causing cancer. There are well established guidelines and recommendations to help doctors decide on the appropriateness of an examination for a particular disease condition. In essence it amounts to keeping radiation exposure as low as reasonably achievable (the ALARA principle) without compromising on medical benefits.

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6. Do I need yet another CT examination?

You can help avoid unnecessary repeat examinations by keeping the results of your previous examinations (X ray, CT, MRI, ultrasound and other imaging examinations) safely and bringing them to your doctor at the time of consultation. If films are stored in the hospital or kept electronically, it is essential to tell your doctor when the previous examination took place. In the future electronic patient records and smart cards may be helpful for tracking your previous examinations. Not all repeat investigations can be avoided as it may be sometimes appropriate to perform an examination or procedure after treatment for cancer to estimate its effectiveness.

Also a CT examination sometimes involves the injection of an intravenous contrast agent. Imaging can be performed at multiple time points before and/or after the injection of the contrast material. Each image acquisition is referred to as a "series." Although having multiple series can be helpful for some conditions, they are not generally necessary and should only be performed if clinically indicated. You can ask your physician about the risks and benefits of the proposed exam and inquire at the CT facility as to the number of series that will be performed, and if a smaller number of series would be sufficient.

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7. Can I undergo a CT scan while I am pregnant?

CT scan in pregnancy is not forbidden, but if the same information can be obtained from another examination that does not require the use of ionizing radiation, it should be preferred.The main concern is for examinations that require exposure of the fetus (lower abdomen) by the primary beam. For examinations remote from the fetal area, the scattered radiation dose to the fetus will be quite small in a properly conducted examination. The aim is to minimize exposure of the unborn child because it is considered to be more sensitive than adults or older children to potential adverse radiation effects. For many examinations such as CT of the head (including dental CT scans), chest and limbs, where the unborn child is not in the direct X ray beam, the dose to the unborn child can be very low. These examinations can be conducted provided there is medical justification.

If a CT examination requires direct exposure of the pelvic region and the unborn child will be in the direct path of the X ray beam, the examination may still be possible if it is essential to save life or is justified on special consideration of risks and benefits. The doctor might consider delaying the CT examination or using an alternative examination such as ultrasound or MRI.

If CT examination is duly justified, then every effort should be made to optimize so as to perform the examination with minimal radiation dose to the fetus e.g. choosing lower exposure factors and providing shielding where appropriate.

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

Yes. For any examination involving direct exposure of the lower abdomen, pregnancy should be ruled out or pregnancy status verified for justification. As mentioned in question 6 above, the unborn child is more sensitive to radiation than adults or older children.

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

Not really if the examination has been fully justified based on risk and benefit considerations. But it is necessary to establish this by discussion with the doctor why your child needs the CT scan and why it cannot be replaced by another examination like ultrasound or MRI. You might also bring a card for the doctor to write how much dose was given in the CT examination, such as the one that can be downloaded from image gently website. It is important that children get as little radiation exposure as possible because they are more sensitive to radiation than adults and they have a longer life expectancy.

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

No. There is a lack of evidence demonstrating that whole-body CT screening of individuals without symptoms is beneficial. Although there are several ongoing investigational studies of the effectiveness of using CT to screen people, the studies are focused on high-risk groups for specific diseases (e.g. cigarette smokers for lung cancer, cardiac CT for coronary disease and virtual colonography). In such studies only a limited portion of the body is irradiated, not the whole body, and only screening for a specific type of disease is being evaluated, rather than screening for just anything in the body.

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X rays: What patients need to know
 
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