Computed tomography has transformed diagnosis, but it accounts for a large share of the medical radiation dose received by the population. Clinicians need to understand the numbers well enough to justify each scan and explain it clearly.
Typical effective doses
Effective dose, measured in millisieverts (mSv), allows rough comparison between examinations. Values vary widely with the scanner, protocol and patient size, so treat these as approximate.
Approximate effective doses:
- Chest X-ray – about 0.1 mSv
- Natural background radiation – about 2–3 mSv per year
- CT head – about 2 mSv
- Low-dose CT chest for screening – about 1–1.5 mSv
- CT abdomen and pelvis – about 8–10 mSv
Justification and optimisation
Two principles guide radiation protection. Justification asks whether the scan will change management. Optimisation — often summarised as ALARA, “as low as reasonably achievable” — means using the lowest dose that still answers the clinical question.
Children are different
Children are more sensitive to radiation and have more years of life ahead of them, so pediatric protocols use lower doses and ultrasound or MRI are preferred whenever they can answer the question.
“The risk of a missed diagnosis is almost always greater than the risk of a justified scan.”
Talking to patients
Comparing a scan with months or years of natural background radiation is usually more meaningful to patients than quoting a risk estimate. Be honest that the risk is small but not zero, and explain why the scan matters.
This article is for education only and does not replace clinical judgement or local guidelines.
