Extra- and intracellular accumulation of ultrasmall superparamagnetic iron oxides (USPIO) in experimentally induced abscesses of the peripheral soft tissues and their effects on magnetic resonance imaging

The effects of ultrasmall superparamagnetic iron oxide (USPIO) particles on magnetic resonance imaging (MRI) were studied in an animal abscess model and the findings compared with microscopic sections of the abscesses. Staphylogenic abscesses of the right hind leg were induced in six Sprague–Dawley rats. The USPIO particles consisted of polyethylene-glycol-coated Fe 3O 4 with a mean size of 26 nm and were injected intravenously (i.v.), with three animals receiving a dose of 50 μmol/kg and three animals a dose of 150 μmol/kg. Before and immediately after i.v. administration of the particles, MR data were acquired with fast gradient-echo technique FLASH sequences applied over a period of 60 min. The Fe 3O 4-induced signal changes were registered in regions of interest (ROIs) placed over the margin and center of the abscess, over the perifocal granulation tissue and over corresponding sites of the contralateral healthy muscle. Microscopic sections were prepared using the conventional paraffin technique and, in part, a kryohistologic method before staining of the specimen with hematoxylin and Berlin-blue reaction. In addition to the mostly perfusion dependent loss of signal intensity within 8 s after injection, a signal reduction, which could be rather pronounced, was observed in the abscess margin and perifocal granulation tissue, with the underlying mechanism mainly attributed to extravasation and predominantly extracellular deposition of Fe 3O 4. Conclusion: The USPIO particles used in this study lead to a prolonged demarcation of abscesses in the peripheral soft tissues due to particle extravasation and accumulation in the periphery of the abscesses. Besides the known accumulation of already characterized USPIO in the RES of liver, spleen and lymph nodes the study could demonstrate ultrasmall iron oxide deposition (BY 818) in abscesses of the peripheral soft tissue.