Clinical meaning
Neuroimaging selection in neurological emergencies requires understanding which modality answers the clinical question most effectively and urgently. CT (computed tomography) is the initial imaging modality for acute neurological emergencies because it is fast (seconds), widely available, and highly sensitive for acute hemorrhage (blood appears hyperdense/bright white). CT is FIRST-LINE for: acute stroke (non-contrast CT to rule out hemorrhage before thrombolysis — must be done within minutes), head trauma (identify fractures, epidural/subdural hematomas, contusions, subarachnoid hemorrhage), acute hydrocephalus (ventricular enlargement), and mass effect/midline shift. CT angiography (CTA) rapidly evaluates large vessel occlusion in acute ischemic stroke (guides thrombectomy decisions) and identifies aneurysms in SAH. However, CT has limited sensitivity for early ischemic stroke (<6 hours, ~60% sensitivity), posterior fossa pathology (artifact from surrounding bone), and white matter disease. MRI (magnetic resonance imaging) provides superior soft tissue contrast, higher sensitivity for ischemic stroke (DWI — diffusion weighted imaging detects ischemia within MINUTES of onset, far earlier than CT), posterior fossa lesions, demyelinating disease (MS plaques), brain tumors (detail, edema, enhancement patterns), and spinal cord pathology. MRI contraindications include: ferromagnetic implants (certain pacemakers, cochlear implants, metallic foreign bodies), severe claustrophobia, hemodynamic instability (scan takes 30-60 minutes), and non-MRI-conditional devices. Specific imaging algorithms: thunderclap headache → non-contrast CT (rule out SAH), if negative → LP (xanthochromia); new-onset seizure → contrast-enhanced MRI; suspected MS → brain and spinal cord MRI with gadolinium; acute ischemic stroke → non-contrast CT (exclude hemorrhage) → CTA (identify large vessel occlusion) → MRI-DWI if available.