High intensity focused ultrasound (HIFU) has shown a great promise in noninvasive cancer therapy.The impact of acoustic cavitation on the lesion formation induced by HIFU is investigated both experimentally and theoretically in transparent protein-containing gel and ex vivo liver tissue samples.A numerical model that accounts for nonlinear acoustic propagation and heat transfer is used to simulate the lesion formation induced by the thermal effect.The results showed that lesions could be induced in the samples exposed to HIFU with various acoustic pressures and pulse lengths.The measured areas of lesions formed in the lateral direction were comparable to the simulated results,while much larger discrepancy was observed between the experimental and simulated data for the areas of longitudinal lesion cross-section.Meanwhile,a series of stripe-wiped-off B-mode pictures were obtained by using a special imaging processing method so that HIFUinduced cavitation bubble activities could be monitored in real-time and quantitatively analyzed as the functions of acoustic pressure and pulse length.The results indicated that,unlike the lateral area of HIFU-induced lesion that was less affected by the cavitation activity,the longitudinal cross-section of HIFU-induced lesion was significantly influenced by the generation of cavitation bubbles through the temperature elevation resulting from HIFU exposures.Therefore,considering the clinical safety in HIFU treatments,more attention should be paid on the lesion formation in the longitudinal direction to avoid uncontrollable variation resulting from HIFU-induced cavitation activity.