A Numerical Acoustic Fluid-structure Model of a Therapeutic Ultrasound Angioplasty Device

Ultrasonic angioplasty involves the use of ultrasonic vibrations delivered to the distal-tip of small diameter wire waveguides and is an emerging technology the may have potential use in the treatment of complicated atherosclerotic plaques during cardiovascular surgery. Complicated plaques, including chronic total occlusions and calcified lesions, seriously reduce success rates during standard intervention involving guidewire access, followed by balloon dilation or stent delivery. The large amplitude (0-150 μm) wire waveguide distal-tip displacements in the low-frequency ultrasonic (18-45 kHz) range have been shown to disrupt plaque material by direct contact ablation and cavitation, acoustic streaming and pressure wave components in adjacent fluid 1. The effects on this surrounding fluid are complex and are related to the distal-tip geometry, frequency of operation, vibration amplitude, as well as the operating environment, including, fluid properties and boundary conditions. While the majority of work to date on ultrasound angioplasty has focused on experimental and clinical studies 2, 3, further understanding of distal-tip effects is necessary. This work describes a numerical fluid-structure model of the wire waveguide distal-tip and is used to predict the pressures developed in the fluid region near the tip wall, the acoustic pressure field and, with the inclusion of appropriate threshold intensity, when cavitation will occur. The model has been validated against experimental acoustic pressure field results reported in the literature. The model can be further used to predict the effects of parameters such as distal-tip geometry, displacement amplitude and frequency of operation and will prove a valuable design aid in the choice of optimum powers to disrupt various biological materials

Fracture and fatigue in High Impact Polystyrene : the influence of secondary phase morphology (Rubber Particle Size and Effective Rubber Phase Volume) on fracture and fatigue behaviour in HIPS

THESIS 7674The subject of the following thesis is the influence of the secondary phase morphology on fracture and fatigue behaviour in High Impact Polystyrene (HIPS). The motivation of the work was the premature failure of parts manufactured from HIPS materials whose micro structures, it was thought, were optimised for toughness and, it was assumed, for fatigue. The objective of the research was to evaluate these assumptions and establish through empirical analysis the relationship between micro-structural morphology, mechanical properties and Fracture and Fatigue behaviour to rationalise the apparent dichotomy that was observed with these HIPS in service applications

An Acoustic Fluid-structure Simulation of a Theraputic Ultrasound Wire Waveguide Apparatus

The use of high-power low-frequency ultrasound transmitted down small diameter wire waveguides is an emerging technology that may have potential in the treatment of complicated atherosclerotic plaques in cardiovascular surgery. This form of energy delivery results in vibrating the distal-tip of the wire waveguide disrupting material by means of direct contact ablation and also cavitation, pressure waves and acoustic streaming in the surrounding fluid. This work describes a numerical acoustic fluid-structure model of the ultrasound wire waveguide and blood surrounding the distal tip. The structural analysis of the model predicts the natural frequencies of the waveguide and shows the extent to which these are affected by the presence of the distal-tip geometry, the surrounding fluid and the length of wire waveguide. These results are validated against experimental results on a 23.5 kHz waveguide apparatus. The acoustic fluid results show the pressure field developed in the surrounding blood and predicts pressure conditions sufficient to cause cavitation in a region close to the distal-tip. These results compare favourably with experimental measurements reported in the literature. The model will prove a valuable design tool in the further development of this potential minimally invasive technology

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, 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

An Acoustic Fluid-structure Simulation of a Therapeutic Ultrasound Wire Waveguide Apparatus

The use of high-power low-frequency ultrasound transmitted down small diameter wire waveguides is an emerging technology that may have potential in the treatment of complicated atherosclerotic plaques in cardiovascular surgery. This form of energy delivery results in vibrating the distal-tip of the wire waveguide disrupting material by means of direct contact ablation and also cavitation, pressure waves and acoustic streaming in the surrounding fluid. This work describes a numerical acoustic fluid-structure model of the ultrasound wire waveguide and blood surrounding the distal tip. The structural analysis of the model predicts the natural frequencies of the waveguide and shows the extent to which these are affected by the presence of the distal-tip geometry, the urrounding fluid and the length of wire waveguide. These results are validated against experimental results on a 23.5 kHz waveguide apparatus. The acoustic fluid results show the pressure field developed in the surrounding blood and predicts pressure conditions sufficient to cause cavitation in a region close to the distal-tip. These results compare favourably with experimental measurements reported in the literature. The model will prove a valuable design tool in the further development of this potential minimally invasive technology

Performance Characteristics of a Therapeutic Ultrasound Wire Waveguide

Therapeutic ultrasound angioplasty has been investigated, clinically, by a number of researchers and represents a potentially promising therapy for the treatment of atherosclerotic lesions. To date, there has been no detailed analysis of the effect of mechanical design parameters, such as wire geometry or damping characteristics, on wire waveguide performance. An apparatus capable of delivering therapeutic ultrasound down small diameter nickel–titanium (NiTi) wire waveguides is described. The output peak-to-peak (p–p) displacements at the distal tip of a 1.0mm diameter waveguide were measured experimentally, by means of an optical microscope and image analysis software. The output was measured for a range of waveguide lengths from 118 to 303 mm. Wire waveguide distal tip displacements as high as 98 mm (p–p) at 23.5 kHz were measured. For the range of lengths tested, the experimental measurements show the critical relationship between the length of the waveguide and the output distal tip displacements. A finite element model that can predict the resonant frequencies and distal tip displacements of various wire waveguide geometries and configurations, including the effect of damping, is presented. This numerical model has been validated against the experimental displacement data obtained. This will be a valuable design tool for ensuring the safety and effectiveness of therapeutic ultrasound angioplasty procedures

Development of a Numerical Model to Simulate Pressure Distributions in Ultrasound Angioplasty

Ultrasound Angioplasty has been shown to be effective in the removal and re-canalising of blockages in arteries (Siegel RJ, 1993). The ultrasound is delivered via a wire waveguide to the lesion location. The wire, generally, has a ball-tip at the distal end to increase transmission of the ultrasound to the surrounding fluid by causing the ball-tip to oscillate between 20– 45kHz and with displacements of up to 100mm peak-to-peak (Atar, 1999 and Yock, 1997). Pressure waves, micro streaming, cavitation and direct contact with the oscillating ball-tip affect the blockage. Most work to date has concentrated on a spherical ball-tip geometry at the distal end of the wire waveguide, with the ball-tip diameter between 1-2mm (Steffen, 1994 and Rosenschein, 1996). METHODS To simulate the interaction between the ball-tip and surrounding fluid a Finite Element Acoustic Model using fluid-solid interaction and acoustic elements was developed. In order to validate the FEA model a pulsating sphere was simulated and compared with the analytical solution, given in Equation 1. The radial displacement and frequency were the input loads on the solid ball tip, while outputs included maximum nodal pressures at points in the acoustic field. DISCUSSION The correspondence between the finite element solution and the analytical solution for a pulsating sphere is shown in Figure 1. This is a plot of the maximum pressures at points axially parallel to the tip at a distance of 1mm. This location is similar to that of the arterial wall, although the presence of the wall is ignored here. Areas of cavitation activity may be identified where the maximum pressure amplitude exceeds ambient fluid pressure. This information may aid in the design of the devices. In future work the validated model will be used to simulate an oscillating sphere. This is more representative of actual tip movement during ultrasound angioplast

A Numerical Acoustic Fluid-structure Simulation of Therapeutic Ultrasound Angioplasty

INTRODUCTION Therapeutic ultrasound angioplasty is the delivery of high amplitude ultrasonic displacements to the distal-tip of small diameter wire waveguides with the goal of disrupting atherosclerotic plaques. This is a minimally invasive procedure that may have potential\u27 in the treatment of complicated chronic total occlusions. The disruption of plaque is due to direct contact ablation and also cavitation, pressure waves and acoustic streaming in the fluid surrounding the vibrating waveguide distal-tip [1]. Cavitation appears to play a major role and some authors have suggested that plaque ablation is only evident above the cavitation threshold [2]. Makin and Everbach [3] performed experimental measurements of the acoustic pressures in the field surrounding a vibrating wire waveguide distal-tip (frequency = 22.5 kHz, displacement amplitude = 651lm, tip diameter = 2.46mm). The measured acoustic pressures in the region ahead of the distal-tip are shown in Figure I. No measurements could be made in the region close to the tip but the authors concluded that these pressures would be sufficient to cause the cavitation that was observed, based on the trend in Figure I. This work describes a numerical acoustic fluidstructure model of the wire waveguide and fluid surrounding the distal-tip. The model will predict wire waveguide behaviour to a prescribed ultrasonic input displacement and will predict pressures developed around the distal-tip. METHODS An axisymmetric numerical model of the wire waveguide and fluid surrounding the distal-tip based on the device description by Makin and Everbach [3] was developed in ANSYS©. The model consisted of both structural elements (Plane42) for the waveguide and acoustic elements (Fluid29) for the fluid. A tluidstructure interface was placed at element couplings and infinite acoustic boundary elements (Fluid 129) defined the extremities of the model preventing acoustic reflection RESULTS A comparison of the experimental [3] and the numerical results are shown in Figure l. A close comparison is achieved and, in addition, the numerical model can predict pressures at the fluid-structure interface. With the inclusion of a cavitation threshold (CT), displacements and frequencies required to cause cavitation can be predicted. CONCLUSION The validated model can be used to investigate the effects of changing device parameters such as frequency of operation, displacement amplitudes and geometry, on both waveguide structural response (displacements and stresses) and pressures developed in the surrounding fluid

Development and Performance Characteristics of an Ultrasound Angioplasty Device

INTRODUCTION The effect of atherosclerosis is well documented and many procedures such as balloon angioplasty and stentimplantation have been developed to reopen occluded arteries. However, there are considerable differences in the material properties of various atherosclerotic lesions as they develop. Many authors have suggested that rigid calcified plaques may require specific procedures that target this rigid material through de-bulking or complete removal (Salunke et al, 1997). Ultrasound angioplasty is the delivery of high power low frequency ultrasound via a wire waveguide to the lesion location. This results in distal tip wire displacements of up to 100um peak-to-peak (p-p) at frequencies of between 20-45 kHz (Atar, 1999 and Yock, 1997). Ultrasound angioplasty was shown to be effective in the ablation of fibrous and calcified blockages in arteries (Siegel, 1993). At the displacements and frequencies mentioned the pressure field developed can produce disruptive cavitations around the distal tip (Gavin et al, 2004). The principal objective of this study was to develop an ultrasound angioplasty device and investigate its performance characteristics both experimentally and numerically. MATERIALS AND METHODS Ultrasound was generated by a piezoelectric transducer driven by an ultrasonic generator. The ultrasonic generator drives the transducer at its resonant frequency, in the case of the present device, 22.5 kHz with output displacements of between 3-18 um (p-p). This output was passed into an acoustic horn with a wire waveguide attached to the distal end. These amplify the displacement characteristics and the waveguide has a working flexibility similar to present balloon catheters and a working length of approximately 500mm. The wire waveguide was ensheathed in a catheter with the distal end of the wire protruding at the tip. The output (p-p) displacements at the distal end of the wire waveguide were measured using an optical microscope with video acquisition and measurement. The characteristics of the system along the length of the wire waveguide have also been numerically simulated using FEA RESULTS The output displacement characteristics of the wire waveguide showed achievable peak-to-peak output displacements of 15- 90 um (p-p) at 22.5 kHz. Fig. 1 shows an image obtained by the optical measurement system of the distal tip of a 1mm wire waveguide subjected to ultrasonic energy. DISCUSSION This initial testing of tip displacements has proved promising and the comparison of the FEA and experimental results has shown good agreement. Future studies involve identifying cavitations and testing is to be carried out on various materials that simulate plaque with a focus on rigid calcified lesions

Pressure Distribution around Spherical Distal Ball-tip in Ultrasound Angioplasty

INTRODUCTION Ultrasound Angioplasty has been shown to be effective in the removal and re-canalising of blockages in arteries (Siegel RJ, 1993). By delivering therapeutic ultrasound to the blockage, via a wire waveguide to a ball-tip, the lesion or thrombus is affected by pressure waves, micro streaming, cavitation and direct contact with the oscillating ball-tip. Most work to date has concentrated on a spherical ball-tip geometry at the distal end of the wire waveguide (Steffen, 1994 and Rosenschein, 1996). Tip displacements usually lie between 10 - 100m (peak-to peak) and ball tip diameters between 1 - 2mm (Atar, 1999 and Yock, 1997). The analytical solution of an oscillating sphere is given in Equation 1 and has previously been used to describe pressures in ultrasound angioplasty (Siegel, 1996). METHODS To simulate the interaction between the ball-tip and surrounding fluid a Finite Element Acoustic Model using fluid-solid interaction and acoustic elements was developed. The displacement and frequency were the input loads on the solid ball tip, while outputs included maximum nodal pressures at points in the acoustic field. From this numerical solution a comparison was performed with the analytical solution to validate the model. DISCUSSION The correspondence between the finite element solution and the analytical solution for an oscillating sphere is shown in Figure 1. This is a plot of the maximum pressures at points axially parallel to the tip at a distance of 1mm. This location is similar to that of the arterial wall, although the presence of the wall is ignored here. Areas of cavitation activity may be identified where the maximum pressure amplitude exceeds ambient fluid pressure. This information may aid in the design of the devices such as desirable ball-tip size, geometry and exposure time. In future work the validated model will be used to solve the pressure distribution around more complex geometries to determine possible advantages in the use of non-spherical ball-tips