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.