Specific Imaging Findings

Cerebral abscess can occur in any part of the brain, but has a predilection for the supratentorial gray–white matter junction of the frontal and parietal lobes. On CT it typically presents as a subcortical area of prominent hypodense vasogenic edema with mass effect and a central rounded lesion of even lower attenuation, at times with a thin isodense rim, which shows enhancement on post-contrast images. The central portion of an abscess is of low T1 and high T2 signal (slightly brighter than the CSF). The capsule is characteristically T1 hyperintense and T2 hypointense with marked contrast enhancement. The rim enhancement is generally thin and smooth, especially on the outer side. Abscesses tend to expand medially becoming oval in shape, the capsule may accordingly be thinner toward the ventricles and thicker toward the cortex. Diffusion MR imaging is the most accurate imaging technique to differentiate bacterial abscess from other intracranial cystic/necrotic masses. In addition to hyperintensity on FLAIR and T1WI, the abscess center is very bright on DWI and dark on ADC, reflecting reduced diffusivity within the purulent material. Fungal abscesses may be more heterogenous, show higher ADC values, and involve the deep gray matter. MR spectroscopy shows complex spectra within abscesses with multiple abnormal peaks including lactate and lipids, as well as acetate, succinate, and amino acids. Perfusion studies, similar to other inflammatory and infectious processes, typically show decreased to normal cerebral blood volume.