Bidirectional non-contact ultrasound imaging using MHz-band air-coupled ultrasound transducer for skin assessment: A feasibility study

Non-contact ultrasound imaging provides a valuable alternative for patients in whom direct skin contact is difficult or undesirable, such as those with burns or a high risk of infection. However, clinical adoption has been limited by the lack of a practical air-coupled transducer. In this study, we present a MHz-band air-coupled ultrasound transducer (ACUT) designed specifically for medical use, enabling bidirectional, contact-free imaging of the skin. The device features a compact 7 mm × 7 mm aperture and a 2 MHz center frequency, and incorporates a porous matching layer together with an optimized piezocomposite structure to overcome the severe acoustic impedance mismatch with air. These design choices result in improved transmission efficiency and stable operation at low drive voltages (tens of volts), delivering sufficient acoustic energy for both brightness-mode (B-mode) and acoustic radiation force impulse (ARFI) imaging. To assess performance, tissue-mimicking agar phantoms with different stiffness levels were fabricated, and fully air-coupled B-mode and ARFI imaging were performed. Both reflected intensity and ARFI-induced displacement clearly distinguished stiffness differences. Additionally, temperature measurements during insonification indicated that measurable acoustic energy reached the target surface, consistent with the observed ARFI displacements. Experiments on ex vivo porcine skin with varying degrees of thermal damage further showed that superficial intensity and displacement responses varied consistently with tissue condition. These findings demonstrate that the proposed approach enables simultaneous acquisition of anatomical and biomechanical information from the skin surface without physical contact, offering a promising tool for safe, efficient, and quantitative assessment of skin integrity.