Firing session optimization for dynamic focusing HIFU treatment

A model to predict the size and shape of high intensity focused ultrasound (HIFU)-induced lesions was recently published. This model adapts the classical "bioheat transfer equation" (BHTE) to account for the presence of oscillating microbubbles that naturally form in the tissue during HIFU. Preliminary simulation results have already helped us to better understand how an increase in acoustic power significantly shifts the lesion toward the transducer and may create skin lesions. These simulations are in good agreement with observed clinical results and it can be expected that this approach can be used to improve firing sessions. In the present work, simulations were performed to predict the size, shape and position of lesion induced by an annular array specially designed for transrectal prostate cancer therapy; in-vitro experiments were carried out to test the model. Good agreement was found between the values predicted by the model and the in-vitro results. Then, a target volume was defined and several trials were made to optimize firing parameters (power, shot duration, waiting time between shots, frequency and dynamic focus) to achieve the desired lesion. In a laparotomy procedure, a series of 29 HIFU lesions were induced in 9 pig livers in-vivo. Seven days after treatment, at animal sacrifice, all the experimental lesions were in good agreement with the simulations. The results show that it is possible to predict the shape and the position of HIFU-induced lesions with this model and that it can be used to reduce the number of in-vivo experiments necessary. We believe this model will be useful in setting clinical parameters not only for prostate cancer therapy but also for other HIFU applications.