Experimental study on the rim-enhancing lesion of rabbit brain abscess : MR imaging and histopathologic correlation

To evaluate on the basis of histopathologic carrelation the MR findings of mature brain abscess in the rabbit, with particular attention to rim-enhancing lesions. The evolution of abscess formation was obtained by the direct inoculation of Staphylococcus aureus into the gray-white matter junctions of the brains of 16 rabbits. The stages of brain abscesses were divided into four : early cerebritis (days 1 to 5 after inoculation of the organism);late cerebritis (days 6 to 14);early capsular (days 16 to 21);and late capsular (days 22 to 28). The available MR images showed 14 cases at the stage of early cerebritis, seven at the late cerebritis stage, three at the early capsular, and one at the late capsular stage. According to the known pathology of brain abscesses and on the basis of both MR imaging and histopathologic findings, the lesions were grouped according to whether they were found in the central necrotic, border, or peripheral zone. We analyzed the patterns of rim-enhancement (completeness of the rim, thickness, and margin) and the signal intensities of the abscess walls on MR images at each stage. Histopathologic correlation was performed in one case of each stage. We evaluated the presence or absence and degree of infiltration by inflammatory granulation tissue, microhemorrhage, reticulin, collagen, and hemosiderin of the abscess walls. Rim-enhancing lesions were present in three of 14 cases at the late cerebritis stage, in all three cases at the early capsular, in one at the late capsular, but in none at the early cerebritis stage. The enhancing pattern of the late cerebritis stage was irregular-margined incomplete rim-enhancement, with irregular thickness of the abscess walls (3/3). The enhancing pattern of the capsular stages was well-defined, complete rim-enhancement with uniform thickness of the abscess walls (3/4). The signal intensities of the abscess walls at the late cerebritis and early capsular stages were variable. The late capsular stage ws characterized by hypointensity of the abscess wall on both T1- and T2-weighted images. Histopathologically, the capsular stages were distinguished from the late cerebritis stage by the marked infiltration of reticulin and the presence of collagen in the abscess walls. The most conspicuous pathologic finding distinguishing the late from the early capsular stage was abundant infiltration of the abscess wall by collagen and hemosiderin. The enhancing pattern of a brain abscess with mature capsule formation was characterized by a well-defined, complete rim-enhancing abscess wall of uniform thickness. The mature abscess wall was hypointense on both T1- and T2-weighted images, may be explained by marked infiltration by mature collagen and hemosiderin