Computed tomography of the chest

Computed tomography (CT) of the chest is usually performed after a chest X-ray has identified an abnormality.

CT of the chest is more expensive than chest X-ray and is associated with higher radiation exposure. The effective dose is often around 8 mSv, with variations expected due to technique. A dose of 8 mSv is equivalent to between 80 and 400 chest X-rays or 4 years of natural background radiation. See Radiation exposure in thoracic imaging for more information.

Children are more sensitive to ionising radiation; if a CT scan is considered in a child, consult with a paediatric specialist.

Many requests for CT of the chest, both in hospital and general practice, lack clinical justification and are inappropriately ordered. To avoid extra radiation exposure from an unnecessary repeat scan, seek advice about the appropriate CT scan and its timing in relation to the specialist appointment. The clinical question or indication for ordering the CT scan should be clearly described in the request form.

CT of the chest can be performed as conventional CT chest (with or without contrast), high-resolution CT (HRCT) or CT pulmonary angiography (CTPA). See Summary of types of CT scans of the chest for a summary of these types of CT tests.

Some CT scans of the chest require contrast—consider the possibility of adverse reactions to contrast:

• A history of severe anaphylaxis to iodinated contrast media is an absolute contraindication to iodinated contrast media.
• A history of mild reaction to contrast media (eg pruritis, nausea) occurring immediately at the time of scan increases the risk of reaction to iodinated contrast media; future reactions may be mitigated by premedication with oral prednisolone, with or without antihistamines—consult radiologist for specific advice.
• Food allergy slightly increases the risk of reaction to iodinated contrast media; specific allergy to shellfish does not further increase this risk.
• A history of skin reaction to topical iodine is not associated with an increased risk of reaction to iodinated contrast media.

Historically, there has been concern about the risk of contrast-induced acute kidney injury in patients with impaired kidney function. The risk of contrast-induced acute kidney injury for patients with:

• eGFR greater than 45 ml/min/1.73 m² is likely to be nonexistent
• eGFR 30 to 45 ml/min/1.73 m² is likely to be low or nonexistent—evidence for periprocedural hydration is limited, but it can be considered in patients who are acutely deteriorating
• actively declining kidney function or eGFR less than 30 ml/min/1.73 m² needs to be carefully weighed with the benefit of iodinated contrast media—consider periprocedural kidney protection using intravenous hydration with 0.9% saline.

Discuss any concerns about kidney function with the radiologist, but do not delay emergency investigations (eg CTPA for suspected pulmonary embolism).