Usefulness of MR Imaging in the Staging of Brain Abscess: Comparison between Experimental Models and Clinical Cases

ABSTRACT

PURPOSE: The purpose of this study is to evaluate the usefulness of MR imaging in the staging of brain abscesses and to determine the correlations between pathologic and MR findings. MATERIALS AND METHODS: Experimental brain abscesses were induced by direct inoculation of 1ml suspension of l06/ml Streptococcus pneumoniae into the brain parenchyma of ten New Zealand white rabbits. The evolution of abscess formation was divided into four stages, based on pathological criteria early cerebritis (days 1 to 5), late cerebritis (days 6 to 10), early capsular (days 11 to 15), and late capsular (day 16 and later). The brain abscess of each animal was examined by MR imaging and light microscopy at 3, 8, 13, and 28 days; T1-weighted, T2-weighted and Gd-enhanced images were obtained. The MR images and pathologic findings of 13 pathologically confirmed clinical cases were compared to MR images of the experimental model. RESULTS: In the experimental model, signal intensity of the abscess content was at all stages hypointense on T1-weighted and hyperintense on T2-weighted images. In all ten cases, Gd-enhanced images showed an ill-defined contrast-enhanced lesion at the early cerebritis stage, and in four of seven cases, irregular ring enhancement at the late cerebritis stage. Pathologic specimens at this latter stage revealed prominent vascular proliferation and infiltration of chronic inflammatory cells. Signal intensity of the abscess wall during the capsular stage showed isointense relative brain parenchyma on T1-weighted images and this was hypointense on T2-weighted images. Gd-enhanced images demonstrated smooth ring enhancement of the abscess wall. At the early capsular stage, pathologic specimens revealed a discrete necrotic center surrounded by infiltration of reticulin and some collagen; at the late capsular stage, these specimens showed marked infiltration of collagen. In clinical cases, the signal intensity of abscess content was at all stages hypointenseon T1-weighted and hyperintense on T2-weighted images. Gd-enhanced images demonstrated ill-defined subtle contrast enhancement at the early cerebritis stage and irregular ring enhancement at the late cerebritis stage. In all cases, signal intensity of the abscess wall during the capsular stage was hypointense on T2-weighted images; at this stage, the abscess wall was showed a pattern of smooth ring enhancement. In clinical cases, hypointensity of the abscess wall, as seen on T2-weighted images, and the enhancement pattern of this wall were identical to these findings in the experimental model. CONCLUSION: In an experimental model, correlation between sequential MR findings can be used to predict the stage of a brain abscess; in clinical cases essential indicators are hypointensity and enhancement pattern of the abscess wall, as seen on T2-weighted images. In cases of brain abscess, MRI is a useful diagnostic modality, and in such cases, also helps determine the most suitable treatment.