Intense Focused Ultrasound: Evaluation of a New Treatment Modality for Precise Microcoagulation within the Skin

Abstract

Focused ultrasound can produce thermal and/or mechanical effects deep within tissue. We investigated the capability of intense focused ultrasound to induce precise and predictable subepidermal thermal damage in human skin. Postmortem human skin samples were exposed to a range of focused ultrasound pulses, using a prototype device (Ulthera Inc.) emitting up to 45 W at 7.5 MHz with a nominal focal distance of 4.2 mm from the transducer membrane. Exposure pulse duration ranged from 50 to 200 ms. Thermal damage was confirmed by light microscopy using a nitroblue tetrazolium chloride assay, as well as by loss of collagen birefringence in frozen sections. Results were compared with a computational model of intense ultrasound propagation and heating in tissue. Depth and extent of thermal damage were determined by treatment exposure parameters (source power, exposure time, and focal depth). It was possible to create individual and highly confined lesions or thermal damage up to a depth of 4 mm within the dermis. Thermal lesions typically had an inverted cone shape. A precise pattern of individual lesions was achieved in the deep dermis by applying the probe sequentially at different exposure locations. Intense focused ultrasound can be used as a noninvasive method for spatially confined heating and coagulation within the skin or its underlying structures. These findings have a significant potential for the development of novel, noninvasive treatment devices in dermatology.