Harnessing cavitation using a tubular transducer geometry for catheter based applications

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The occlusion of large blood vessels is a leading cause of death and disability. These include the acute lodging of (soft) blood clots in coronary, brain or peripheral vessels as well as more complex longer term occlusions (chronic total occlusions) which can have a stiff upstream `caps'. Resolving vascular occlusions to restore blood flow is therefore a high clinical priority, but is in many circumstances poorly addressed by current methods. Standard of care approaches include medical therapy (e.g. intravenously injecting drugs), surgery (e.g. coronary bypass) and catheter procedures (e.g. to retrieve clot or place a stent). `Therapeutic' ultrasound is also under investigation to unblock occlusions. This encompasses a range of methods, many of which involve inducing vibrations (`cavitation') of micron sized circulating bubbles with the ultrasound. We have recently proposed and prototyped a novel design concept for forward directed catheter based ultrasound that involves a tubular transducer (ultrasound source) situated at its tip. This presents a number of unique advantages. Essentially it can be guided over a wire right up to the occlusion, inject the bubbles and drugs through the catheter and transducer hole then expose with ultrasound precisely at the target site. With this new approach and its geometry, there are also many unexplored physical considerations for delivering ultrasound to occlusions. This research program has the objective of investigating basis aspects of cavitation for this situation in order to provide a rational basis for developing exposure protocols. There are two Streams within this program. First, we will examine how bubbles behave within the tube, how the ultrasound can push them out of the tube, and how to `burst' them appropriately to break up the occlusion. These effects will be a function of the spatial pattern of the beam, which is in turn a function of the length of the tube and the frequency of ultrasound that is used. This will be approached with computer simulations, measuring the pressures and using a very high speed microscope to look at the bubble behaviour. This work is relevant to when existing clinically approved bubbles are injected through the catheter tip. In the second Stream, we will investigate how to actually generate bubbles within the tip of the catheter with the ultrasound. This will be done using very small `droplets'- less than a micron in size, as well as a simple solution of lipids which will stabilize any bubbles generated within the tube. The resulting bubbles can then be used for therapy without externally injected bubbles. Together this work will provide the foundation of knowledge that will enable this new approach to be optimized. The end result of the optimized system will be a method to impact the treatment of a significant portion of patients with coronary disease that have chronic total occlusions (20%), as well as other patients that have blood clots in large vessels.