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, I(SATA): 1.03-3.83 W/cm(2)) via 1.0-mm diameter waveguides. Inertial cavitation (in water at 37 °C) was determined to occur above amplitudes of vibration greater than 31.4 µm (I(SATA) = 3.21 W/cm(2)). 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.

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.

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 microm 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 microm 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 microm 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 microm p-p), 2.6 N (50 microm p-p) and 2 N (80 microm 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.