  Thrombotic and chronic occlusions of large blood vessels are a major cause of
mortality and morbidity, and so there is a need for improved treatments in many
clinical circumstances. Endovascular ultrasound approaches have been shown to
hold considerable potential to treat large vessel thrombotic occlusions. Here,
we report the development of a novel forward-looking therapeutic ultrasound
catheter approach. The design concept centers on the use of a radially
polarized hollow cylindrical transducer situated at the distal tip. This
approach enables a compact configuration where the central lumen can
accommodate a guidewire during navigation as well as provide a route to release
cavitation seeds and therapeutic agents adjacent to the site of occlusion.
PZT-5H transducers with outer/inner diameters of 1.35/0.73 mm were evaluated
using simulations and experiments for their capacity to project forward-looking
ultrasound. A length of 2.5 mm operating in the 3rd harmonic of the length mode
resonance (1.85 MHz) was selected. Catheters (1.55 mm outer diameter) were
designed and fabricated with a liner, outer jacket, and braiding, where the
transducers were incorporated with air-backing and a front-face matching layer.
Forward-looking pressures of 2.5 MPa (peak negative) at 0.5 mm were achieved.
Proof of principle vessel phantom experiments were performed demonstrating the
ability to eject microbubbles in proximity to an occlusion and stimulate
inertial cavitation. This approach holds potential for treating thrombotic and
chronic total occlusions.
