High-intensity micro-focused ultrasound (HIFU)
High-intensity micro-focused ultrasound (HIFU) acoustic energy, known to propagate much deeper through tissue than laser or RF energy, has been previously investigated for use in bulk heating for the treatments of solid organ tumors8-10 and recently adapted for the treatment of subcutaneous lipoly-sis. The ultrasound waves penetrate into tissue, leading to vibration in molecules at the site of beam focus. The friction between tissue molecules produces heat and thermal injury at the focal site of the beam. Penetration depth is determined by frequency in which higher frequency waves produce a shallow focal injury zone and lower frequency waves have a greater depth of penetration to produce focal thermal injury zones (TIZs) at deeper layers. HIFU allows for more precise energy delivery as a result of advances within the system to better address the needs of skin laxity.
The HIFU device is able to penetrate deeper into tissue than its nonsurgical predecessors in an effort to affect superior tissue tightening and longevity of results by selectively targeting the superﬁcial musculoaponeurotic system (SMAS). The SMAS lies deep to the subcutaneous fat, envel-ops the muscles of facial expression, and extends superﬁcially to connect with the dermis.15 The SMAS layer is composed of collagen and elastic ﬁbers similar to the dermal layer of the skin; however, it has more durable holding property and less delayed relaxation after lifting procedures than skin alone.15 Thus, the SMAS is a desirable target for noninvasive skin tightening procedures.