Experimental abscess in the thigh of rabbit : magnetic resonance imaging and pathologic correlation

To understand MR imaging characteristics, of abscesses by correlation with pathologic findings, with emphasis on the hypointense rim of the abscess wall on T2-weighted images. We experimentally induced twenty abscesses on both thighs of ten New Zealand white rabbits by innoculation of E. coli. Spin-echo axial T1-weighted images(T1WI), proton density weighted images(PDWI), T2-weighted images(T2WI) and gadolinium enhanced T1WI of two rabbits were each obtained at 1 and 3 days, and at 1, 2 and 4 weeks following innoculation of pathogens. Rabbits were sacrificed after MR imaging, and freezing, sectioning along MR imaging planes and histopathologic examination were subsequently carried out. MR-pathologic correlation was performed, with emphasis on the MR signal characteristics of the abscess wall. In 19 abscesses, necrotic portions except gas showed hypointensity or hyperintensity on T1WI and hyperintensity on T2WI. Peripheral inflammatory reaction showed hypointensity on T1WI, hyperintensity on PDWI and T2WI, and contrast enhancement. Abscess wall showed slightly increased signal intensity on T1WI. A hypointense rim on PDWI and T2WI appeared from 3 days after pathogen innoculation. The rim was thickest at 1 week, and showed multilayers at 2 weeks and double layers at 4 weeks after pathogen innoculation. Cellular and necrotic debris was accumulated at the inner portion of the abscess wall. Inflammatory cells were mainly polymorphonuclear leukocytes and lymphocytes. Macrophages, which appeared at 3 days, suggested active phagocytosis at 1 week after innoculation. Thereafter, inflammatory reactions decreased and fibrosis progressed. The hypointense rim of the abscess wall on T2WI reflects the accumulation of cellular debris, the paramagnetic effect of free radicals due to active phagocytosing macrophages during the abscess forming stage, and fibrosis during the maturation stage