Neuroimaging and Diagnostic Investigations - exam-focused guide
Neuroimaging and Diagnostic Investigations tests ordered, interpretation and limitations. The board favours decision rules: which test first, what it rules out, and when it misleads.
Core rules
- Non-contrast CT is first-line in acute trauma, suspected hemorrhage and acute stroke to exclude blood; it is fast but misses early ischemia and small posterior fossa lesions.
- MRI: DWI detects cytotoxic edema within minutes of infarction; FLAIR suppresses CSF and shows edema; GRE/SWI detect hemosiderin and microbleeds; T1 post-contrast shows breakdown of the blood–brain barrier.
- CTA and DSA: CTA screens for aneurysms and large-vessel occlusion; DSA remains the reference standard and allows treatment. Catheter angiography carries stroke, dissection and contrast risks.
- EEG detects seizures and non-convulsive status; it does not exclude structural disease. Evoked potentials (SSEP, BAEP, MEP) assess pathway function and prognosis.
- ICP monitoring is indicated in severe TBI with abnormal CT or selected normal CT with risk factors; waveforms show P1 (arterial), P2 (compliance) and P3 (venous).
Worked example: A 55-year-old with sudden thunderclap headache has a normal CT at 8 hours. CT sensitivity for SAH is very high in the first 6–12 hours but falls later. Lumbar puncture at least 12 hours after onset showing xanthochromia supports SAH; CTA then identifies an aneurysm. If CT is negative and LP is not feasible, CTA may be done, but a negative CTA does not exclude a small aneurysm.
Traps: treating a normal CT as excluding early infarction; ordering contrast CT in renal failure without indication; missing MRI contraindications such as pacemakers; using EEG to rule out a tumour; over-reading ICP waves without clinical correlation.