Infections of the Brain and Meninges

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Bacterial Infections

Imaging findings for the various typical bacteria infecting the central nervous system (CNS) are nonspecific and require correlation with laboratory tests to establish a definitive diagnosis. Mycobacterial and spirochetal infections have a clinical presentation and imaging appearance different from those of typical bacteria. Therefore, the following sections first discuss the various stages of pyogenic infections that occur with most bacteria, followed by a description of mycobacterial

Herpes Simplex Encephalopathy

Herpes simplex virus 1 (HSV-1) is the most common worldwide cause of nonepidemic viral encephalitis, which is estimated to represent 10% of all cases of encephalitis infection in the United States.4, 12 The imaging modality of choice is MRI that demonstrates a characteristic pattern of bilateral, but asymmetric, T2/FLAIR hyperintense signal abnormality. This initially affects the hippocampi and mesial temporal lobe structures (Fig. 7) and later progresses to involve the insular cortices and

Fungal Abscess

Fungal infections such as coccidioidomycosis, blastomycosis, and histoplasmosis often occur in specific geographic regions in both immunocompromised and immunocompetent individuals, whereas aspergillosis is predominantly seen in immunocompromised and diabetic patients.2, 24 Parenchymal disease often takes the form of fungal abscess, which is best seen on CT as an irregular, ring-enhancing lesion. On MRI, there often is a T2-hypointense, irregularly enhancing peripheral rim with nonenhancing

Toxoplasmosis

Acquired toxoplasmosis primarily occurs in immunocompromised patients and is the most common opportunistic CNS infection in patients with AIDS.2 Parenchymal lesions are typically multiple, small, and homogenously enhancing lesions at the gray-white junction or in the deep gray nuclei. Larger lesions exhibit ring enhancement, hyperintense signal on T2W images, and absence of restricted diffusion (unlike bacterial abscesses that demonstrate restricted diffusion; Fig. 13). The main differential

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