New contrast agents

PCT No. PCT/GB93/00470 Sec. 371 Date Jan. 17, 1995 Sec. 102(e) Date Jan. 17, 1995 PCT Filed Mar. 5, 1993 PCT Pub. No. WO93/17718 PCT Pub. Date Sep. 16, 1993The present invention relates to contrast agents comprising gas-containing or gas-generating polymer microparticles and/or microballoons, in which the polymer is a biodegradable polymer containing units of formula -[-(O)m-CO-O-C(R1R2)-O-CO-(O)n-]- . R1 and R2 each represent hydrogen or a carbon-attached monovalent organic group, or together form a carbon-attached divalent organic group, and m and n are each independently zero or one. The contrast agents may be used in diagnostic applications such as ultrasound and MR imaging.