A Study on the Characteristics of Superficial Thermal Ablation Using a Single Transducer Excited by the Superposition of Fundamental and Third-Harmonic Frequencies

To address the issues of complex structure and low ablation efficiency in superficial tissues associated with existing dual-frequency focused ultrasound systems, this paper proposes an enhanced thermal ablation technique based on a single transducer that enables superimposed excitation of the fundamental frequency and its third harmonic. By designing a dual-frequency matching network, a transducer with a center frequency of 4.57 MHz achieves efficient transmission at the third harmonic (13.71 MHz). Under a total acoustic power of 2 W, phantom and ex vivo tissue ablation experiments were conducted by varying the voltage ratio between the fundamental frequency and the third harmonic (from 1:0 to 0:1), and multiphysics simulations were employed to analyze the evolution of the acoustic and thermal fields. The results indicate that as the proportion of the third harmonic increases, the thermal lesion significantly expands, and the boiling-induced cavitation effect—attributable to the high-frequency component—accelerates the temperature rise. At a shallow focal depth of 2 mm, the lesion morphology evolves from an elongated shape to a broad teardrop shape, with an area approximately four times larger than that achieved with the fundamental frequency alone (increasing from 4.41 mm² to 17.44 mm²). At a depth of 4.5 mm, the lesion centroid shifts toward the transducer surface as the harmonic proportion increases (from 7.19 mm to 4.65 mm), exhibiting an axially elongated morphology. This study confirms that introducing third-harmonic energy significantly enhances superficial energy deposition efficiency and enables effective modulation of focal zone morphology, providing a new strategy for the precise treatment of superficial tissue pathologies.