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Ultrasonic longitudinal displacements, delivered to the distal tips of small diameter wire waveguides, have been shown to be capable of disrupting complicated atherosclerotic plaques during vascular interventions. These ultrasonic displacements can disrupt plaques 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 the stresses and the displacements, with the inclusion of a validated damping constant. The effects 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 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 required to cause 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.
Gavin, G., Dolan, F., Hashmi, M., McGuinness G.: A Coupled Fluid-Structure Model of a Therapeutic Ultrasound Angioplasty Wire Waveguide. Journal of Medical Devices, Vol. 1, , pp 254-263. December, 2007.