The flow system consisted of a syringe pump (Harvard Apparatus, Holliston, MA) delivering rt-PA (, Activase, Genentech, San Francisco, CA) and/or lipid MBs (, diameter )14 suspended in phosphate buffered saline (PBS) into an open loop polyethylene tubing system containing a cylindrical thrombus formed from whole porcine blood bound to a silk suture10 [Fig. 1(a)]. Two single-element transducers—an unfocused 1-MHz transducer for US delivery, and a focused 2.25-MHz transducer for detection of MB cavitation events (A303S and V306, Olympus NDT, Waltham, MA)—were mounted to a custom-built microscope stage with an acoustic chamber containing degassed water maintained at . The stage was designed for alignment of the two transducers, such that the focal area of the detection transducer overlapped with the center of the US treatment beam with the thrombus positioned at equal distance from the transducers. Target location was confirmed with US pulse/echo technique. MB cavitation was detected using the 2.25-MHz US transducer in passive receiving mode using amplifying and filtering (5073PR, Olympus, Waltham, MA) and a digital oscilloscope (6051A, Lecroy, Chestnut ridge, NY)4,5 [Fig. 1(b)].