it has been discovered microparticles formed from natural or synthetic polymer with thicker walls have significantly enhanced echogenicity as compared with microparticles having thinner walls . the effect of wall thickness has been determined experimentally as well as inserted into a formula for use in predicting the optimum conditions . in the preferred embodiment , the polymers are synthetic biodegradable polymers and the wall thickness is between about 100 and 660 nm , although wall thicknesses from about 20 nm to in excess of 500 nm can be used . the microparticles are manufactured with a diameter suitable for the targeted tissue to be imaged , for example , with a diameter of between 0 . 5 and 8 microns for intravascular administration , and a diameter of between 0 . 5 and 5 mm for oral administration for imaging of the gastrointestinal tract or other lumens . preferred polymers are polyhydroxy acids such as polylactic acid - co - glycolic acid , polylactide or polyglycolide , most preferably conjugated to polyethylene glycol or other materials inhibiting uptake by the reticuloendothelial system . the microspheres may be used in a variety of ultrasound imaging applications including cardiology applications , blood perfusion applications as well as for organ and peripheral vein imaging .