Radiation dose is measured in millisieverts (mSv). In Australia we all receive about 1.5–2 mSv a year from natural background radiation. A chest X-ray is only about 0.1 mSv (about 10 days' worth), a hand or foot X-ray is tiny, while a CT of the abdomen and pelvis is around 8 mSv (a couple of years' worth). Ultrasound and MRI use no ionising radiation at all. Knowing these numbers puts any single scan in perspective.
Key takeaways
- Radiation dose is measured in millisieverts (mSv).
- Natural background radiation in Australia is about 1.5–2 mSv per year.
- A chest X-ray is ~0.1 mSv; a CT of the abdomen/pelvis is ~8 mSv.
- Ultrasound and MRI use no ionising radiation.
When people worry about a scan, it’s usually the radiation. The good news is that “radiation exposure” can be measured and compared, which makes it far less mysterious.
Dose is measured in millisieverts
The dose from medical imaging is measured in millisieverts (mSv). The useful thing about this unit is that it lets you compare any scan against the radiation we all receive naturally.[1]
In Australia, natural background radiation — from the ground, the air, space and even our own food — is about 1.5–2 mSv per year.[2] (It’s a little higher in some countries; in the US the average is around 3 mSv, mostly because of more radon gas in homes.)[1] So a helpful way to think about any scan is: how many days, months or years of background radiation is it equivalent to?
How the common scans compare
Typical effective doses for an average-sized adult:[1]
| Scan | Typical dose | Roughly equals |
|---|---|---|
| Hand or foot X-ray | <0.001 mSv | A few hours of background |
| Dental X-ray | ~0.005 mSv | About a day |
| DEXA (bone density) | ~0.001 mSv | A few hours |
| Chest X-ray | ~0.1 mSv | About 10 days |
| Mammogram | ~0.3 mSv | About a month |
| CT head | ~1.6 mSv | About 7 months |
| CT chest | ~6 mSv | About 2 years |
| CT abdomen & pelvis | ~8 mSv | About 2–3 years |
| PET-CT (whole body) | ~14–23 mSv | About 8–9 years |
| Ultrasound / MRI | 0 mSv | No radiation |
These are typical figures and vary with the person, the reason for the scan, and the equipment.[1]
The no-radiation options
It’s worth remembering that ultrasound and MRI use no ionising radiation at all — which is one reason they’re preferred where they can answer the question, especially in children and pregnancy.[3]
Frequently asked questions
How much radiation is in a scan?
It depends on the scan: a chest X-ray is about 0.1 mSv (around 10 days of natural background), while a CT of the abdomen and pelvis is around 8 mSv (a couple of years’). Ultrasound and MRI use none.[1]
What is background radiation?
It’s the natural radiation we’re all exposed to — from the ground, air, space and food — about 1.5–2 mSv a year in Australia. It’s the yardstick used to put scan doses in perspective.[2]
Does the radiation stay in my body?
No. After a plain X-ray or CT there’s no radiation left in you. (Nuclear medicine uses a tracer that fades over hours to a day.)[3]
Sources
- RadiologyInfo.org (RSNA & ACR) — Radiation Dose from X-Ray and CT Exams — www.radiologyinfo.org/en/info/safety-xray
- ARPANSA — Having a scan? A guide for medical imaging (2024) — www.arpansa.gov.au/sites/default/files/documents/2024-05/patienthandout.pdf
- RANZCR / InsideRadiology — Radiation risk of medical imaging — www.insideradiology.com.au/radiation-risk/
