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To investigate the mechanism of ring enhancement on ferumoxides-enhanced T1-weighted (T1W) gradient echo images in malignant focal hepatic lesions.Eighteen patients with hepatocellular carcinoma (HCC) underwent breath-hold T1-, T2-, and T2*-weighted magnetic resonance (MR) images at 1.5-Tesla after ferumoxides administration. The existence of ring enhancement on T1W, and the maximum size of the area showing decreased phagocytic activity on T2W and T2*W, and that of the area showing ring enhancement on T1W were evaluated. The Kupffer cell (KC) density of HCC itself and peritumoral liver parenchyma was assessed with KC stain sections.Ring enhancement was noted in 13 of 18 HCC (72%). Peritumoral KC density was increased in the ring enhancement (+) group as compared with the ring enhancement (−) group. In the ring enhancement (+) group, the tumor size measured on T2W was smaller than that measured on either T1W or T2*W, suggesting a sustained T1 relaxation effect and a decreased T2* relaxation effect in the peritumoral regions.Ring enhancement on superparamagnetic iron oxide (SPIO)-enhanced T1W may correlate with increased KC density and decreased SPIO clustering in KC in peritumoral regions. J. Magn. Reson. Imaging 2005;21:230–236. © 2005 Wiley-Liss, Inc.