Neuroimaging and Diagnostic Investigations
What the candidate must own
Know the modality decision tree cold. CT is first-line in acute trauma and suspected haemorrhage; CT angiography for aneurysms, vascular malformations, and trauma vascular injury; MRI with diffusion-weighted imaging for acute infarct within minutes; MR angiography and venography for vessel and sinus assessment; and catheter angiography as the reference standard for vascular lesions and for dynamic assessment. Understand the physics that drive the clinical traps: the Hounsfield scale (water 0, acute blood 50-80), why posterior fossa and spinal cord lesions are poorly seen on CT, and why gadolinium enhancement reflects blood-brain barrier disruption rather than tumour grade. Know the sequences: T1 (anatomy, fat bright, CSF dark), T2 (pathology bright, CSF bright), FLAIR (CSF suppressed, perilesional oedema conspicuous), GRE/SWI (haemorrhage and calcification, blooming), DWI/ADC (restricted diffusion in acute infarct, abscess, high-cellularity tumours).
Traps that appear in questions
- A normal CT does not exclude an acute infarct in the first hours - DWI is the correct answer.
- Subarachnoid blood on CT is time-dependent; a negative CT beyond 6 hours with a high clinical suspicion still warrants lumbar puncture or CTA.
- Calcification can mimic haemorrhage on CT; use MRI to distinguish.
- Contrast allergy and renal impairment are the usual distractors for choosing CT over MRI - read the stem for the actual contraindication.
Rule of thumb
Ask three questions in order: Is this an emergency where speed and haemorrhage matter? (CT), Is this soft tissue, posterior fossa, spinal cord, or acute ischaemia? (MRI), Is this a vascular lesion needing dynamic or treatment planning? (catheter angiography). Worked example: a patient with thunderclap headache and a normal CT at 8 hours should proceed to lumbar puncture for xanthochromia or CTA, not be discharged.