Neuroradiology
Core framework
Neuro questions focus on stroke, hemorrhage, tumors, infection, and spine. In acute ischemic stroke, noncontrast CT excludes hemorrhage before thrombolysis; early signs include loss of insular ribbon, obscured lentiform nucleus, and sulcal effacement. ASPECTS scores the MCA territory (10 normal, ≤7 associated with worse outcome). CT angiography identifies large-vessel occlusion for mechanical thrombectomy (roughly up to 24 hours with favorable perfusion imaging in selected patients); CT perfusion assesses core versus penumbra.
Hemorrhage and vascular
Hypertensive hemorrhage favors the basal ganglia, thalamus, pons, and cerebellum; lobar hemorrhage in an elderly patient suggests cerebral amyloid angiopathy. Subarachnoid hemorrhage is detected by noncontrast CT within 6 hours and by lumbar puncture/xanthochromia later. Berry aneurysms arise at branch points of the circle of Willis; the anterior communicating artery is the most common site. Aneurysmal SAH complications: vasospasm (days 3–14) and hydrocephalus.
Tumors and infection
Adult supratentorial: metastasis and glioblastoma (butterfly, necrotic, restricted diffusion in solid portions). Pediatric posterior fossa: medulloblastoma (fills fourth ventricle, restricted diffusion), pilocytic astrocytoma (cyst with mural nodule), ependymoma ("plastic" through foramina). Ring-enhancing lesion with restricted diffusion centrally suggests abscess rather than tumor. Multiple sclerosis: periventricular ovoid lesions perpendicular to ventricles (Dawson's fingers).
Traps and rule of thumb
Traps: calling a hemorrhage an infarct on a noncontrast CT; forgetting that absence of SAH on a CT performed >6 hours after symptom onset has lower sensitivity; mislabelling a thalamic hemorrhage as amyloid angiopathy.
Rule of thumb: restricted diffusion centrally within a ring-enhancing lesion favors abscess; restricted diffusion in the enhancing rim favors tumor.