Meningitis
- Diagnosis = CSF analysis (LP). Imaging before LP only with red flags: GCS ≤ 12 or falling, focal deficit, new seizures, papilloedema, immunocompromise, signs of raised ICP.
- MRI > CT: sulcal FLAIR hyperintensity, leptomeningeal enhancement (post‑contrast FLAIR most sensitive), complications: hydrocephalus, subdural effusion/empyema (crescentic, rim‑enhancing, restricts), ventriculitis (ependymal enhancement, layering debris), cerebritis/abscess, venous sinus thrombosis, vasculitic infarcts, labyrinthitis.
- Pachymeningitis (dura) vs leptomeningitis (pia‑arachnoid; commoner). Hypertrophic pachymeningitis (IgG4, TB, granulomatosis, idiopathic): thick enhancing dura, "Eiffel tower by night" sign along the falx and tentorium on coronal MRI.