A Coupled Fluid-Structure Model of a Therapeutic Ultrasound Angioplasty Wire Waveguide

Ultrasonic longitudinal displacements, delivered to the distal tips of small diameter wire waveguides, are known to be capable of disrupting complicated atherosclerotic plaques during vascular interventions. These ultrasonic displacements can disrupt plaques not only by direct contact ablation but also by pressure waves, associated cavitation, and acoustic streaming developed in the surrounding blood and tissue cavities. The pressure waves developed within the arterial lumen appear to play a major role but are complex to predict as they are determined by the distal tip output of the wire waveguide (both displacement and frequency), the geometric features of the waveguide tip, and the effects of biological fluid interactions. This work describes a numerical linear acoustic fluid-structure model of an ultrasonic wire waveguide and the blood surrounding the distal tip. The model predicts a standing wave structure in the wire waveguide, including stresses and displacements, and requires the incorporation of a damping constant. The effects on waveguide response of including an enlarged ball tip at the distal end of the waveguide, designed to enhance cavitation and surface contact area, are investigated, in addition to the effects of the surrounding blood on the resonant response of the waveguide. The model also predicts the pressures developed in the acoustic fluid field surrounding the ultrasonic vibrating waveguide tip and can predict the combinations of displacements, frequencies, and waveguide geometries associated with cavitation, an important event in the disruption of plaque. The model has been validated against experimental displacement measurements with a purpose built 23.5 kHz nickel-titanium wire waveguide apparatus and against experimental pressure measurements from the literature.