The right scan depends on what's being investigated, your age, and whether you're pregnant — and your doctor makes the call. As a rough guide: kidney stones → low-dose CT or ultrasound; gallstones → ultrasound; a lung clot → CTPA or VQ scan; appendicitis → CT (or ultrasound/MRI); bones → X-ray; soft tissue → MRI; pregnancy → ultrasound. Radiation-free tests (ultrasound, MRI) are preferred where they can answer the question.
Key takeaways
- The right scan depends on the question, your age and pregnancy status.
- Radiation-free tests (ultrasound, MRI) are preferred where they'll work.
- Some conditions have a clear first-line scan; others need a combination.
- Your doctor and the radiologist choose — this guide explains the usual logic.
“Which scan do I need?” is one of the most common questions — and the honest answer is that your doctor decides, based on your symptoms, age and situation. But it helps to understand the usual logic. This guide maps common problems to the scan typically used, with links to fuller explanations.
Imaging by symptom or condition
| If it’s about… | Scan usually used | Why |
|---|---|---|
| Kidney stones | Low-dose CT (CT KUB) or ultrasound | CT is most accurate; ultrasound avoids radiation. More → |
| Gallstones / gallbladder pain | Ultrasound (± MRCP or HIDA) | Radiation-free and accurate for stones. More → |
| A blood clot on the lung | CTPA, or a VQ scan | CTPA is fast; VQ avoids contrast. CTPA → · VQ → |
| Appendicitis | CT (adults); ultrasound/MRI (children, pregnancy) | Balances accuracy and radiation. More → |
| A broken bone | X-ray | Quick and excellent for bone. More → |
| A knee, shoulder or spine problem | MRI | Best for soft tissue, no radiation. More → |
| Pregnancy / the baby | Ultrasound | No radiation, the mainstay of pregnancy. More → |
| Breast screening | Mammogram | Finds cancer early, before symptoms. More → |
| Bone density (osteoporosis) | DXA scan | Low-dose, measures bone strength. More → |
| Cancer staging | PET-CT | Shows activity and spread. More → |
The logic behind the choices
A few principles run through all of it:[2]
- Match the scan to the tissue — bone → X-ray/CT; soft tissue → MRI/ultrasound; function → nuclear medicine.
- Prefer radiation-free where it works — ultrasound and MRI first, especially for children and in pregnancy.
- Speed matters in emergencies — CT is fast, which is why it’s common in trauma and acute illness.
- Start simple — often a plain X-ray or ultrasound first, moving to CT or MRI only if needed.
Important
This guide explains the usual approach — it isn’t medical advice, and it can’t replace your doctor’s judgement for your specific situation.[1] If you have symptoms, see your doctor, who will choose the right test (and provide the referral you’ll need for a Medicare rebate). For a broader overview, see which scan for what and our introduction to radiology.
Frequently asked questions
Can I choose which scan I have?
Not directly — your doctor selects the most appropriate scan and provides a referral. You can always ask why a particular scan was chosen and whether a radiation-free option would work.[2]
Why might I need more than one scan?
Scans are often complementary — one may show that a closer or different look is needed. Combining them (for example ultrasound then MRI) frequently gives the clearest answer.[1]
Sources
- RadiologyInfo.org (RSNA & ACR) — Tests & Treatments — www.radiologyinfo.org/en/test-treatment
- RANZCR / InsideRadiology — Radiation risk of medical imaging — www.insideradiology.com.au/radiation-risk/
