To obtain D-USPIO, 84 g of carboxydextran (molecular weight: 2500, approximately) was dissolved in 150 mL of water. To this was added an aqueous solution obtained by dissolving 17 g of ferrous chloride tetrahydrate in 130 mL of a 1 M aqueous ferric chloride solution under a nitrogen gas stream. Then, 220 mL of a 3 N aqueous solution of sodium hydroxide was added, with heating and stirring. The mixture was adjusted to pH 7.0 by adding 6 N hydrochloric acid, and then refluxed for 1.5 hours. The reaction mixture was subjected to centrifugation at 2500 × g for 1 hour, and the supernatant liquid then subjected to ultrafiltration (Centramate™; Pall Corporation, Long Island, NY) (molecular weight cut-off [MWCO]: 100,000) to obtain 1 L of D-USPIO.