RecruitingDevice studyUltrasound Cavitation Therapy for CLIIn this study, we will explore how ultrasound exposure of ultrasound contrast agents, which produces beneficial shear-mediated bioeffects, can be used to treat patients with severe non-healing ulcers secondary to peripheral arterial disease (PAD). The primary outcome measure is whether ultrasound exposure to microbubble contrast agents in the inflow artery and at the wound site can accelerate wound healing. A secondary outcome measure is whether cavitation-related changes occur in tissue perfusion in the treated limb and wound measured by ultrasound perfusion imaging and skin flow.