Patients often arrive asking for a specific scan. Usually the more useful conversation is about the symptom, because the right modality follows from the question being asked.
Ultrasound — fast, safe, operator-dependent
Best for: pregnancy, gallbladder, kidneys, thyroid, testes, superficial lumps, blood flow in vessels, and any question in a child where radiation should be avoided. Real-time, radiation-free and inexpensive. Its limitation is that sound does not travel through bone or gas, so the brain in adults, the lungs and gas-filled bowel are essentially invisible.
CT — fast, geometric, excellent for bone and bleeding
Best for: trauma, acute stroke, suspected bleeding, lung parenchyma, kidney stones, bowel obstruction and vascular studies. A whole-body CT takes seconds, which is why it is the default in an emergency where the patient cannot lie still. Its limitation is soft-tissue contrast — subtle differences between similar soft tissues can be hard to resolve.
MRI — unmatched soft-tissue detail, slower
Best for: brain and spinal cord, joints, ligaments and cartilage, nerves, bone marrow, liver lesion characterisation, prostate and pelvic organs. No ionising radiation. Its limitations are time, cost, sensitivity to patient movement, and contraindication in certain implants.
Worked examples
- Knee injury with locking and swelling: MRI, to see meniscus and cruciate ligaments.
- Sudden severe headache: CT first, to exclude bleeding within minutes.
- Right upper abdominal pain after fatty food: ultrasound, for gallstones.
- Persistent back pain with leg numbness: MRI, for disc and nerve root.
- Fall with suspected wrist fracture: X-ray first, CT only if it remains unclear.
The practical advice
Bring the referral and describe the symptom rather than requesting a modality. If a different study would answer your question better or more safely, our radiologists will tell you before anything is booked.