Investigation of Confocal Dual-frequency High-intensity Focused Ultrasound with Ultrasound Contrast Agent in Thermal Therapy

Dual-frequency high-intensity-focused-ultrasound (HIFU), due to its lower average acoustic pressure, has better penetration but poorer lesion-producing ability than single-frequency HIFU. However, its ability to produce beating sound wave, of which the frequency is much lower than original ultrasound frequency, may be able to produce a larger thermal lesion when combined with ultrasound contrast agent (UCA) and may be more precisely located at the desired location than those produced by single-frequency HIFU. We studied the hypothesis with poly-acrylamide based egg-white phantom and rat liver in vivo. We used a customized two-element ultrasound transducer to produce single- and dual-frequency ultrasound for experiments. Computer simulation based on Rayleigh integral was used to simulate the acoustic pressure and the transducer was evaluated with hydrophone to check the pressure distribution. Sonovuewith different concentration was applied to phantoms (0%, 0.1%, 0.01%, and 0.001%) and rats (4.5 l/kg, 45 l/kg, and 450 l/kg). The electric power and heating time were 50+50 Watt and 60 sec in phantom experiments and 30+30Watt and 30 sec in rat liver experiments. The experimental results showed that single-frequency HIFU produced larger thermal lesion. Each experiment showed that when UCA applied, the distance of lesion shifted from the focus was shorter in dual-frequency HIFU than in single-frequency HIFU. It may be because the difference acoustic field generated by dual-frequency ultrasound at focal area enhanced the cavitation effect and therefore produced thermal lesion more close to focus.Keywords: beatings, confocal dual-frequency high intensity focused ultrasound, lesion size, lesion shift, ultrasound contrast agent