Two common scans use no ionising radiation — ultrasound (sound waves) and MRI (magnets and radio waves). Where one of these can answer your doctor's question, it's often preferred, especially for children and in pregnancy. They can't replace X-ray or CT for everything, though — each test has different strengths, and the right choice is the one that answers the question at the lowest risk.
Key takeaways
- Ultrasound and MRI use no ionising radiation.
- They're often first-line, especially for children and in pregnancy.
- They can't replace X-ray or CT for everything — each scan has different strengths.
- Your radiologist helps choose the most appropriate, lowest-risk test.
Not every scan uses radiation. Two of the most common imaging tests use none at all: ultrasound, which uses high-frequency sound waves, and MRI, which uses strong magnets and radio waves.[1] Where one of these can answer your doctor’s question, it’s often the preferred choice.
Ultrasound
Ultrasound is inexpensive, widely available and completely safe, with no known adverse effects.[2] It shows soft tissues in real time, so movement and blood flow can be watched as they happen. Its main limits are that sound can’t pass through bone or gas, and deep structures are harder to see in larger patients.[2]
MRI
MRI produces very detailed images, especially of soft tissues, with no ionising radiation.[3] The trade-offs are different: it takes longer than CT, the strong magnet means metal and implants must be screened carefully, and some people find the enclosed scanner confining (sedation or a wider-bore magnet can help).[3]
When radiation-free tests are preferred
Because children are more sensitive to radiation, ultrasound and MRI are used wherever they can do the job.[1] They’re also the mainstay in pregnancy — ultrasound especially. And for conditions that need repeated monitoring, avoiding repeated radiation is an advantage.
Where they can’t replace X-ray or CT
Radiation-free doesn’t mean “always better.” CT is much faster, which matters in trauma, and is better for acute bleeding, fine bone detail, most fractures and imaging the lungs.[3] Plain X-ray remains the simplest, quickest test for bones and the chest. Often the best approach is to start with the simpler test and only move to more complex imaging if needed.[1]
Your referring doctor and the radiologist choose the most appropriate test for your situation — balancing what needs to be seen against keeping any radiation as low as possible.
Frequently asked questions
Which scans use no radiation?
Ultrasound and MRI. Ultrasound uses sound waves and MRI uses magnets and radio waves — neither uses ionising radiation.[1]
If MRI and ultrasound are radiation-free, why use X-ray or CT at all?
Because each test has different strengths. CT is faster and better for trauma, bleeding, fine bone detail and the lungs; X-ray is the quick, simple choice for bones and the chest. The goal is the right test for the question, with radiation kept as low as reasonably achievable.[3]
Sources
- RadiologyInfo.org (RSNA & ACR) — Radiation Dose from X-Ray and CT Exams — www.radiologyinfo.org/en/info/safety-xray
- RANZCR / InsideRadiology — Ultrasound (Coombs) — www.insideradiology.com.au/ultrasound-hp/
- RANZCR / InsideRadiology — Magnetic resonance imaging (MRI) (McIntyre & Goergen) — www.insideradiology.com.au/mri-hp/
