9B-4 Microbubble Oscillations in Gel Phantom and Ex Vivo Preparation Validate Proposed Mechanisms for Contrast-Based Drug Delivery

The use of ultrasound contrast agents for gene and drug delivery has shown much promise in many recent studies, while simultaneously raising questions regarding the safety of methods used. There is currently no consensus on the optimal safe operating regime, since the mechanisms for contrast- enhanced drug delivery are not well-understood. Here, the mechanisms for increased permeability are investigated by using high-speed microscopy to directly observe microbubbles in a vessel phantom and ex vivo preparation. The vessel phantom was a small block (30 mm times 20 mm times 3 mm) of 0.75% agarose gel with a 200-mum diameter cylindrical channel along the length of the block. For ex vivo work, the preparation used was an excised rat cecum with a cannulated ileocolic vein to allow for microbubble perfusion. Images of microbubble oscillation were acquired using a strobe imaging system with a 30 nsec flash provided by a copper vapor laser. Images of vessel wall disruption and tunnels formed in the wall were acquired with 1, 2.25, and 5 MHz center frequencies using microbubble concentration, duty cycle, and rarefactional pressures that were similar to those used for in vivo drug delivery experiments. Observations from in vitro and ex vivo studies indicate that expansion of microbubbles within small vessels is smaller than in an infinite fluid and that bubbles persist over many transmitted cycles and pulses. In vitro studies over a long pulse train indicate that microbubbles create pores within the tissue phantom and travel long distances along the beam axis. The width and depth of tunnels created by microbubbles at varied frequencies indicate a stronger dependence on insonation frequency than suggested by mechanical index.