Pulsed high-intensity focused ultrasound exposure decreases shear wave speed of rabbit´s Achilles tendons

Tightening and contracture of dense fibrous tissues including tendons and ligaments are debilitating conditions frequently encountered in clinical practice. Conservative management for loosening of the tightened tissues usually has limited effectiveness and invasive interventions are inevitable in most protracted conditions. Pulsed high-intensity focused ultrasound (HIFU) holds the promise of being an alternative nonsurgical option for releasing of the tightened tissues. In this work, we evaluated the feasibility of combining pulsed-HIFU and supersonic shear imaging (SSI) as a novel platform for loosening of tightened tissues in a controllable manner. An ex vivo model using fresh porcine tendons was first employed to determine the acoustic parameters set for the delivered HIFU pulses that are optimized for decrease of shear wave speed (SWS) in the tendons measured by SSI. Pulsed-HIFU insonation was then applied to the Achilles tendons of New Zealand white rabbits using the acoustic setting optimized in the ex vivo model. We found that pulsed-HIFU exposure with a pulse repetition frequency of 500 Hz, a pulse cycle of 50, and an exposure duration of 1 min significantly reduced SWS of the Achilles tendons in vivo without rising of the tissue temperature. By contrast, application of conventional continuous-wave HIFU in the ex vivo samples markedly increased tissue SWS and temperature. These findings suggest that pulsed-HIFU may loosen the treated tissue via non-thermal mechanisms. Our results indicate that pulsed-HIFU combing SSI as a promising therapeutic alternative for contracture of dense fibrous tissues.