Graham Gavin

High Power, Low Frequency Ultrasound: Meniscal Tissue Interaction and Ablation Characteristics

This study evaluates high power low frequency ultrasound transmitted via a flat vibrating probe tip as an alternative technology for meniscal debridement in the bovine knee. An experimental force controlled testing rig was constructed using a 20 kHz ultrasonic probe suspended vertically from a load cell. Effect of variation in amplitude of distal tip displacement (242-494 μm peak-peak) settings and force (2.5-4.5 N) on tissue removal rate (TRR) and penetration rate (PR) for 52 bovine meniscus samples was analyzed. Temperature elevation in residual meniscus was measured by embedded thermocouples and histologic analysis. As amplitude or force increases, there is a linear increase in TRR (Mean: 0.9 to 11.2 mg/s) and PR (Mean: 0.08 to 0.73 mm/s). Maximum mean temperatures of 84.6°C and 52.3°C were recorded in residual tissue at 2 mm and 4 mm from the ultrasound probe-tissue interface. There is an inverse relationship between both amplitude and force, and temperature elevation, with higher settings resulting in less thermal damage.

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.

Therapeutic ultrasound angioplasty: The risk of arterial perforation. an in vitro study

The use of therapeutic ultrasound delivered via small diameter wire waveguides may represent an emerging minimally invasive approach in the treatment of chronic total occlusions (CTOs), calcified and fibrous plaques. The distal-tip mechanical vibrations (typically 0–210 µm peak-to-peak) have been reported to debulk rigid calcified and fibrous tissues while healthy elastic arterial tissue remains largely unaffected. The risk of arterial (healthy tissue) perforation with energized waveguides is not fully understood. An ultrasonic apparatus capable of delivering a range of wire waveguide distal-tip displacements, up to 80 µm peak-to-peak (p-p), at an operational frequency of 22.5 KHz (+/− 6%) has been developed. For three distal-tip displacement settings (32, 50 and 80 µm p-p) with 1.0 mm diameter waveguides, the force required to perforate healthy porcine aortic tissue was experimentally determined. The results show a distinct two stage perforation, thought to be the result of different mechanical properties of the layers in the arterial wall. The average maximum force (N) required to cause perforation with the 1.0 mm diameter ultrasonic waveguide activated at the three settings was experimentally determined to be 2.7 N (32 µm p-p), 2.6 N (50 µm p-p) and 2 N (80 µm p-p). The force required to cause perforation of the tissue with no ultrasound was found to be approximately 4 N. These results highlight that when ultrasound energy is applied to the waveguide, less force is required to perforate healthy arterial tissue. This reduction in perforation force is more pronounced at higher ultrasonic displacements, similar to those reported in clinical studies for the effective removal of diseased calcified and fibrous plaques.

Soft tissue cutting with ultrasonic mechanical waveguides

The use of ultrasonic vibrations transmitted via small diameter wire waveguides represents a technology that has potential for minimally invasive procedures in surgery. This form of energy delivery results in distal tip mechanical vibrations with amplitudes of vibration of up to 50 μm and at frequencies between 20-50 kHz commonly reported. This energy can then be used by micro-cutting surgical tools and end effectors for a range of applications such as bone cutting, cement removal in joint revision surgery and soft tissue cutting. One particular application which has gained regulatory approval in recent years is in the area of cardiovascular surgery in the removal of calcified atherosclerotic plaques and chronic total occlusions. This paper builds on previous work that was focused on the ultrasonic perforation of soft vascular tissue using ultrasonically activated mechanical waveguides and the applied force required to initiate failure in soft tissue when compared with non-ultrasonic waveguides. An ultras...

Increased Susceptibility of Arterial Tissue to Wire Perforation With the Application of High-Frequency Mechanical Vibrations

High-frequency mechanical vibrations (20-50 kHz), delivered via small diameter flexible wire waveguides represent a minimally invasive technology for the treatment of chronic total occlusions and in other tissue ablation applications. Tissue disruption is reported to be caused by repetitive mechanical contact and cavitation. This work focuses on the effects of vibrating wire waveguides in contact with arterial tissue. An apparatus with clinically relevant parameters was used, characterized as operating at 22.5 kHz and delivering amplitudes of vibration of 17.8-34.3 μm (acoustic intensity, ISATA: 1.03-3.83 W/cm2) via 1.0-mm diameter waveguides. Inertial cavitation (in water at 37 °C) was deter- mined to occur above amplitudes of vibration greater than 31.4 μm (ISATA = 3.21 W/cm2). The energized waveguides were advanced through tissue samples (porcine aorta) and the force profiles were measured for a range of acoustic intensities. The results show that the tissue perforation initiation force, perforation initiation energy, and total energy required to perforate the tissue reduces with increasing acoustic intensity. No significant reduction in perforation force or energy was observed in the inertial cavitation region. Multistage perforation was evident through the force profile and histological examination of the tissue samples post wire waveguide perforation.