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Functional-Morphologic MR Imaging with Ultrasmall Superparamagnetic Particles of Iron Oxide in Acute and Chronic Soft-Tissue Infection: Study in Rats¹

¹ From the Department of Diagnostic Radiology (A.H.K., T.W.) and Institute of Pathology (G.J.), University Hospital Basel, Switzerland; Novartis Pharma, Basel, Switzerland (T.O., P.R.A.); Guerbet, Zurich, Switzerland (J.F.); and the Department of Nuclear Medicine, University Hospital Zurich, Switzerland (G.K.v.S.). Received April 29, 2002; revision requested June 6; revision received July 27; accepted September 23. Supported by Fröhlich Pharma Consulting, Zurich; Novartis-Stiftung, Basel; and Freiwillige Akademische Gesellschaft, Basel. Address correspondence to A.H.K., Brachmattstrasse 6, CH-4144 Arlesheim, Switzerland (e-mail: akaim@uhbs.ch).

PURPOSE: To investigate the use of magnetic resonance (MR) imaging enhanced with ultrasmall superparamagnetic particles of iron oxide (USPIO) to identify acute, early chronic, and late chronic abscess formation in an experimental model of soft-tissue abscess.

MATERIALS AND METHODS: Experimental soft-tissue infection in 15 rats was imaged with an MR imaging unit on days 1 and 2 (acute), days 5 and 6 (early chronic), and days 8 and 9 (late chronic) after inoculation of the infectious agent. All animals were imaged without contrast enhancement and immediately and 24 hours after USPIO administration. MR and histopathologic findings were compared. The changes in relative signal intensity (SI) and in the extent and pattern of contrast enhancement (macrophage distribution) between the animal groups were analyzed. Statistical testing was performed with Kruskal-Wallis analysis of variance and the ² test.

RESULTS: At 24 hours after USPIO administration, the relative SI of the abscess wall and the relative macrophage extent were 0.50 (0.33–0.73) and 1.03 (0.90–1.08), respectively, for acute infection; 0.11 (0.10–0.18) and 0.94 (0.93–1.01) for early chronic infection; and 0.53 (0.44–0.58) and 0.80 (0.77–0.83) for late chronic infection. The changes in enhancement pattern (P < .001), relative SI (P < .001), and relative macrophage extent (P < .05) with time were significant.

CONCLUSION: The macrophage distribution pattern increases the specificity of MR findings in chronic infection and allows differentiation between areas with active inflammation and areas of reparative granulation tissue.

© RSNA, 2003

Index terms: Magnetic resonance (MR), contrast media • Magnetic resonance (MR), functional imaging • Soft tissues, infection • Soft tissues, MR

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