Feasibility of acoustic evaluation of thermal lesions at bone-soft tissue interface of an ex vivo bovine bone exposed to high-intensity focused ultrasound

High-intensity focused ultrasound (HIFU) is a rapidly emerging non-invasive technique to selectively and locally produce thermal ablation of tumors located in various tissues, which could produce thermal lesions in periosteal structures for the thermal treatment of bone metastases that has been approved by FDA. In this work we characterized the surface vibration of an ex vivo bovine bone exposed to HIFU and explored the feasibility of using acoustic parameters to evaluate the thermal lesions at bone-soft tissue interface. The vibration of the bone surface was measured using a laser vibrometer when exposed to HIFU under acoustic pulse excitation and continuous sinusoidal excitation. Thermal lesions were formed at normal and oblique acoustic incidence of HIFU to the surface of the bone. 2-D RF data backscattered from the ablated region were captured by a modified diagnostic ultrasound scanner to construct the ultrasonic B-mode ultrasound images, Nakagami images and differential integrated backscatter (DIBS) images simultaneously, as well as the changes of acoustic apparent integrated backscattering (AIB) and integrated reflection coefficient (IRC) due to the thermal lesions on the bone surface. At oblique incidence of HIFU, the amplitudes of the nano-vibration of the bone surface were higher in the opposite direction. The thermal lesion was 20 mm in width and extended along the front surface of the bone to the right of the HIFU focus. The thermal lesions in the soft tissue were visualized as hyperechoic regions in ultrasonic B-mode images, Nakagami images and DIBS images simultaneously. The mean value of AIB in the region of surface thermal lesions was increased by about 35% compared with the normal level in the control groups and the value of IRC was decreased by about 70% on average. This preliminary study suggested that the presence of bone would directionally change the spatial distribution of surface vibration and thermal effects at bone-soft tissue interface for oblique incidence of HIFU. Ultrasonic parameters, AIB and IRC, may have the potential to evaluate the characteristics of surface thermal injury of bone induced by HIFU ablation.