Neuroimaging and Neuroradiology
What you must own
Know the physics and clinical use of each modality. CT: fast, excellent for acute haemorrhage and bone; radiation; hyperdense acute blood, hypodense ischaemia after hours. MRI sequences: T1 (anatomy, fat bright, CSF dark), T2 (fluid bright), FLAIR (CSF suppressed, oedema bright), DWI/ADC (restricted diffusion in acute infarct, abscess, high-grade tumour), GRE/SWI (haemorrhage, calcification), T1 post-gadolinium (blood-brain barrier breakdown).
Vascular imaging: CT angiography, MR angiography, digital subtraction angiography (gold standard), and CT perfusion for penumbra. Understand the concept of the ischaemic penumbra (mismatch between core and hypoperfused tissue).
Spine: know the difference between disc herniation, spondylosis, and epidural abscess on MRI; recognise cord signal change as a marker of myelopathy. Recognise hydrocephalus (enlarged ventricles with transependymal oedema) versus atrophy (ex vacuo).
Traps
Questions ask whether a lesion restricts diffusion (DWI bright, ADC dark) versus T2 shine-through (both bright). Another trap: distinguishing acute blood (hyperdense on CT, signal varies by age on MRI) from calcification. Beware questions that imply gadolinium is safe in renal failure - it carries nephrogenic systemic fibrosis risk.
Rule of thumb
For acute stroke: CT excludes haemorrhage; DWI confirms infarct; perfusion mismatch identifies salvageable penumbra.
Worked example
A patient presents 3 hours after onset with a normal non-contrast CT and a DWI-positive lesion with a large perfusion mismatch. The CT is normal because early ischaemia is not visible; the mismatch suggests a salvageable penumbra and supports reperfusion therapy.