Fungal

             Fungal organisms can start as a meningitis or cerebral abscess, or can invade directly from an extracranial compartment. As mentioned above, fungal infections are primarily found in immuno compromised hosts. In immunocompetent patients, fungal abscesses tend to evolve more slowly than bacterial abscesses, but that is not the case in patients with deficient immunity. In general, there are no specific MR imaging features to distinguish the infecting agent.

            Coccidioidomycosis is endemic to the central valley regions of California and desert areas of the southwestern United States. Infection occurs by inhalation of dust from soil usually heavily infected with arthrospores. Primary coccidioidomycosis, a pulmonary infection, is followed by dissemination in only about 0.2% of immunocompetent patients. Central nervous system involvement most often represents a meningitis, but cerebral abscess and granuloma formation can also occur.

            Mucormycosis is seen most often in patients with poorly controlled diabetes. It starts as a necrotizing vasculitis of the nose and sinuses, and spreads by direct invasion of adjacent facial compartments. Extension into the intracranial cavity occurs through the cribriform plate, superior orbital fissure, and basal foramina or indirectly via involvement of vascular structures. Once within the intracranial cavity, it produces a purulent meningitis, cerebral infarction from arterial occlusion, and acute cerebritis due to direct invasion of the olfactory tracts and inferior frontal and temporal lobes. Cranial nerve involvement and cavernous sinus thrombosis are common. The MR findings reflect the observed pathologic changes. Regions of meningeal and cerebral inflammation are hyperintense on T2 and proton density weighted MR images. Infarction and edema account for additional high signal parenchymal abnormalities. Gadolinium-enhanced T1-weighted scans show enhancement of the basal meningeal inflammation, as well as the adjacent cerebral involvement. Coronal scans are especially helpful to display the relationships of the meningeal process to the brain and adjacent extracranial compartments.

            Aspergillosis is an aggressive opportunistic fungal infection. The organism gains entrance with inhalation of infected grains or dusts and results in primarily a pulmonary infection. Pathologic changes include a combination of suppuration and granulomas. Dissemination to the CNS may start as a basal meningitis, but the organism readily invades vascular structures and extends into the brain parenchyma.

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