Introduction

Central nervous system (CNS) infections account for 1% of primary hospital admissions and 2% of nosocomial infections and when encountered, require prompt diagnosis and initiation of specific treatment. The brain has some unique peculiarities like absence of lymphatics, lack of capillaries in the subarachnoid space, and presence of cerebrospinal fluid (CSF), which is an excellent culture medium for dissemination of infectious processes, in the subarachnoid space and into the ventricular system. The normal brain responds to these insults in a limited and stereotypical fashion, and in most cases there is concomitant abnormality of the blood-brain barrier, with associated enhancement. Therefore, the imaging findings are mostly nonspecific with respect to the causative pathogen. However, imaging techniques are sensitive for detecting an abnormality, localizing it, and in many cases categorizing the lesion into infectious or inflammatory disease versus neoplastic. Contrast enhancement is generally useful, and in the light of clinical history and examination findings the radiologist can provide a probable differential diagnosis. Whereas analysis of CSF remains the gold standard to identify the infectious agent, neuroimaging plays a pivotal role not only in diagnosis but also in monitoring therapeutic response. This article begins by briefly describing the anatomy of cranial meninges and extra-axial spaces of the brain. Characteristic findings and recent advances in neuroimaging of meningitis and its complications and ventriculitis are then summarized, and certain noninfectious causes of meningitis and meningitis mimics are described.