Radiation, availability, contrast risk and what the question is
Expansion
First line by clinical question
| Question | First test |
|---|---|
| Gallstones, biliary dilatation | Ultrasound |
| Renal tract obstruction | Ultrasound, then CT KUB for stones |
| Suspected renal colic | Non-contrast CT KUB |
| Acute abdomen | CT with contrast |
| Suspected pulmonary embolism | CT pulmonary angiogram |
| Acute stroke | Non-contrast CT head, then angiography |
| Cord compression, cauda equina | MRI whole spine, urgently |
| Soft tissue, marrow, brain detail | MRI |
| Bone cortex, fracture | Radiograph, then CT |
| Deep vein thrombosis | Doppler ultrasound |
| Thyroid or breast lump | Ultrasound |
| Occult infection or malignancy staging | CT, then PET-CT |
Radiation dose, in rough equivalents of chest radiographs: a chest radiograph is 1, an abdominal radiograph about 35, a CT head about 100, and a CT abdomen and pelvis about 500, which is roughly three years of background radiation. Ultrasound and MRI use none, which is why they are preferred in children, in pregnancy and for repeated imaging.
Contrast cautions
- Iodinated CT contrast: renal impairment, previous reaction, and metformin should be reviewed. Hydration is the main protective measure
- Gadolinium: avoided in severe renal impairment because of nephrogenic systemic fibrosis
- MRI safety: pacemakers and implanted devices, metallic foreign bodies especially intraocular, cochlear implants, and claustrophobia
Pregnancy: ultrasound and MRI first. Where CT is genuinely necessary, particularly for suspected pulmonary embolism or major trauma, it should not be withheld, since the maternal risk of a missed diagnosis exceeds the fetal risk.
Always give the radiologist the clinical question, not just the anatomy; the request determines the protocol and the report.