Ultrasound medical systems can resolve backscatter signals from individual microbubbles of ultrasound contrast, both in solution and in the targeted immobilized state, implying picogram sensitivity.
Preparation and characterization of targeted microbubbles (ultrasound contrast agents) is described. Specific ligands were attached to the microbubble shell, and ligand-coated microbubbles were selectively attached to various targets, using either an avidin biotin model system or an antigen-antibody system for targeting to live activated endothelial cells. Firm attachment of microbubbles to the target was achieved. Forces necessary to detach microbubbles from the target were estimated to exceed dozens of pN. Microbubbles were bound to the target even in the rapidly moving stream of the aqueous medium. Down to 20 ng of the ultrasound contrast material on the target surface could be detected by the ultrasound imaging with a commercial medical imaging system. At high bubble density on the target surface, strong ultrasound image attenuation was observed.
Higher concentrations of microbubbles, lower ultrasound transmit power settings, and intermittent imaging each can reduce the rate of destruction of microspheres resulting from medical ultrasound insonation.
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