Two kinds of iron oxide were studied: USPIO (Meito Sangyo Co, Ltd, Kiyosu, Aichi, Japan) and SPIO (Resovist ® ; I'rom Pharmaceutical Co, Ltd, Shinagawa-ku, Tokyo, Japan). The mean particle diameters and iron concentrations of USPIO and SPIO were 30 nm and 56 mg/mL and 57 nm and 28 mg/mL, respectively. Average diameter of the magnetic core was the same (5 nm) in both particle types. The iron oxide core of both particle types was coated with dextran; their magnetic susceptibility was almost the same (about 0.027 cgs).