High‐frequency (20‐MHz) high‐intensity focused ultrasound (HIFU) system for dermal intervention: Preclinical evaluation in skin equivalents

Abstract

Background High‐intensity focused ultrasound (HIFU) for non‐invasive treatment of a range of internal pathologies including cancers of major organs and cerebral pathologies is in exponential growth. Systems, however, operate at relatively low frequencies, in the range of 200‐2000 kHz as required for deep axial penetration of the body. HIFU utilizing frequencies in excess of 15 MHz has so far not been explored, but presents an opportunity to extend the HIFU modality to target specific dermal lesions and small animal research. Materials and methods A new 20‐MHz HIFU system (TOOsonix ONE‐R) with narrow focus corresponding to the dermis was studied in acoustic skin equivalents, for example, in a tissue‐mimicking gel and in bovine liver. HIFU lesion geometry, depth, and diameter were determined. The temperature increase in the focal point was measured as a function of acoustic power and the duration of HIFU exposure. Results The system produces highly reproducible ultrasound lesions with predictable and configurable depths of 1‐2 mm, thus corresponding to the depth of the human dermis. The lesion geometry was elongated triangular and sized 0.1‐0.5 mm, convergent to a focal point skin deep. Focal point temperature ranged between 40 and 90°C depending on the chosen setting. Observations were confirmed ex vivo in bovine liver and porcine muscle. Variation of acoustic power and duration of exposure produced linear effects in the range of the settings studied. Thus, effects could be adjusted within the temperature interval and spatial field relevant for clinical therapy and experimental intervention targeting the dermal layer of human skin. Conclusion The tested 20‐MHz HIFU system for dermal applications fulfilled key prerequisite of narrow‐field HIFU dedicated to cutaneous applications regarding reproducibility, geometry, and small size of the applied ultrasound lesions. Controlled adjustment of acoustic lesions within the temperature range 40‐90°C qualifies the system for a range of non‐ablative and ablative applications in dermatological therapy.

High‐frequency (20 MHz) high‐intensity focused ultrasound: New ablative method for color‐independent tattoo removal in 1‐3 sessions. An open‐label exploratory study

“…Preclinical testing indicated a realistic range of operation within a dose range of 0.6-1.5 J/dose in the clinic. 23,26 The method produced the intended thermic insults in the dermis and was confirmed as ablative or non-ablative depending on setting and choice of transducer. In the pig model, scarring was not a problem, albeit histology revealed subclinical fibrous change in cases exposed to high-energy multiple doses positioned in close proximity to each other.…”

“…This requires an operating frequency of at least 15 MHz as confirmed by theoretical considerations and preclinical testing. 22,23 Commercially available HIFU systems for aesthetic wrinkle reduction and body contouring operate at about 3-10 MHz. Such frequencies create focal zones that exceed the total thickness of the skin, and are therefore not feasible for dermatology indications addressing the dermal end epidermal layers only.…”

High‐frequency (20 MHz) high‐intensity focused ultrasound system for dermal intervention: A 12‐week local tolerance study in minipigs

“…It has recently been demonstrated that a new HIFU device operating at 20 MHz shows promise for treating indications requiring much lower focal distances and smaller focal zones. This HIFU modality therefore becomes relevant for the field of dermatology . The principle and relevance of high‐frequency HIFU used in dermatology are illustrated in Figures and .…”

“…In the high‐frequency HIFU introduced in the above mentioned pre‐clinical study, real‐time monitoring is integrated in the system using a digital video camera that operates as a surface microscope or dermascope. Using a pointer in the camera, and observing the skin surface above the lesion, the HIFU shot can be positioned precisely in the target, and a whitening or contraction of the treated point can be observed on the surface after each dose.…”

Skin Tightening

“…This method focuses on applying heat to areas through acoustic energy to areas of the deep dermis, subdermal connective tissue, and fibromuscular layer in targeted microcoagulation zones without effect to the epidermis. 47 The targeted thermal effects and microcoagulation are believed to cause skin tightening through collagen contraction and remodeling. Future studies are needed to determine the overall benefits in skin laxity to achieve approval by the US Food and Drug Administration for use as a treatment option.…”