Biologically degradable superparamagnetic particles for use as nuclear magnetic resonance imaging agents

An improved method for obtaining an in vivo NMR image of an organ or tissue of an animal or human subject, wherein the improvememt comprises administering to such a subject as a contrast agent to enhance such NMR image an effective amount of a dispersoid which comprises coated, biodegradable superparamagnetic metal oxide particles dispersed in a physiologically acceptable carrier, an individual particle (i) comprising a superparamagnetic metal oxide core generally surrounded by a biodegradable polymeric coat, each core comprising one or more biodegradable metal oxide crystals, each crystal about 10 to about 500 angstroms in diameter;

• (ii) having an overll mean diameter, inclusive of the polymeric coat, of about 10 to about 5000 angstroms as measured on a Coulter particle size analyzer;
• an (iii) further characterized as having a retention time in said organ or tissue sufficiently long to permit an image to be obtained and being ultimately biodegraded in said organ or tissue within a period of about 7 days.