CT scan (computed tomography)

What is it?

According to NIBIB: computed tomography is an X-ray imaging procedure in which "a narrow beam of X-rays is directed at the patient and rotates quickly around the body", producing cross-sectional images (slices) that can be stacked to form a three-dimensional image of the bones, organs and tissues.

According to the NHS it is a test that takes detailed pictures of the inside of the body, and is usually performed in hospital by a specialist called a radiographer. MedlinePlus describes it as an imaging method that uses X-rays to produce pictures of cross-sections of the body.

The FDA states that CT gives "more clarity and detail than conventional X-ray images".

Why is it done?

According to NIBIB it is used to detect disease or injury in several areas: the head (injuries, tumours, clots, bleeding and stroke), the lungs (tumours, clots, fluid, emphysema and pneumonia), the abdomen (tumours and lesions), the heart (when heart disease is suspected), and the bones (complex fractures, bone tumours and joint erosion in better detail than conventional X-rays).

According to the NHS it helps examine bones and organs after injury, diagnose conditions such as cancer, monitor the effectiveness of treatment, and guide procedures such as taking a biopsy or draining an abscess. The FDA adds that it may sometimes replace exploratory surgery.

Preparation

According to the NHS you may be asked to follow dietary restrictions, take certain medicines, fast for several hours, and avoid metal objects. Tell the centre if you are or might be pregnant, or if you have kidney or thyroid problems or diabetes, or have previously had an allergy to a contrast agent.

MedlinePlus states that if contrast is used you may be asked not to eat or drink for 4 to 6 hours, to tell the team if you take the diabetes medicine metformin, or have kidney problems or an allergy to iodine, and to remove jewellery and wear a gown.

The exact instructions differ according to the organ imaged and the centre, so follow what your centre sends you. The NHS says the centre should provide you with instructions before the appointment.

How is it done?

According to MedlinePlus you lie on a narrow table that slides into the machine, usually on your back with your arms raised, the X-ray beam rotates around you and the computer forms separate images called "slices". You must stay still because movement blurs the image. It states that modern machines complete the scan in under 30 seconds (the scan time itself).

According to the NHS the appointment usually takes 10 to 20 minutes. According to the NCI the duration ranges from a few minutes to half an hour, and most scans are done as outpatients without an overnight stay.

The contrast agent (dye) may be given by mouth, by injection or by enema depending on the area imaged. The NHS and MedlinePlus state that it may cause a temporary sensation of warmth and flushing or a metallic taste, and may cause a mild burning sensation.

Understanding the result

According to the NHS the result of the scan usually takes one to two weeks to arrive and may take up to 4 weeks; this describes the hospital system in the UK, timing differs at your centre, and emergencies are read faster.

MedlinePlus says that the result is considered normal if the organs and structures examined look normal. The meaning of any finding in the report is interpreted by the doctor who requested the test according to your condition.

Risks and side effects

Radiation: according to NIBIB and the FDA, CT uses X-rays, and all X-rays produce ionising radiation that may slightly increase the lifetime risk of cancer, and this risk is "generally small" and accumulates with repeated tests. The NCI says that the increase in risk from a single scan is still small, and the FDA says: "when used appropriately, the benefits of the test far outweigh its risks". This does not mean avoiding a medically requested test, but discussing the benefit and risks with the doctor.

Children: NIBIB and the FDA state that children are more sensitive to radiation and have a longer life expectancy, so scan settings are adjusted to suit them instead of adult settings.

Contrast agent: according to NIBIB it may cause an allergy in some patients, and in rare cases temporary kidney failure. MedlinePlus lists the risks of an allergic reaction and an effect on kidney function, and in very rare cases a severe allergic reaction called anaphylaxis. The NHS describes allergy as rare and it may cause weakness, sweating and difficulty breathing, so you may be asked to stay for 30 minutes for observation.

Pregnancy: according to NIBIB there are no known risks to the fetus if the area imaged is not the abdomen or pelvis, and for abdominal or pelvic imaging alternatives without radiation such as MRI or ultrasound are preferred, and CT becomes acceptable if the alternatives do not give the necessary information or in an emergency. The NCI says the radiation level is thought to be too low to harm the fetus. The NHS warns to tell the hospital if pregnancy is possible. The decision is for the medical team.

Limits of the test

According to NIBIB soft tissues appear less clearly than dense tissues such as bone unless a contrast agent is used.

The NCI states that whole-body CT as a general screening test has limited effectiveness, as most abnormal findings in it do not indicate a serious health problem, and may require follow-up with other tests.

Common questions