Numerical study of the effect of vascular bed on heat transfer during high intensity focused ultrasound (HIFU) ablation of the liver tumor

In this study, the influence of vascular bed comprising terminal arterial branches on heat transfer in a liver tumor exposed to high intensity focused ultrasound (HIFU) is studied numerically. Also, the effect of vascular density on temperature distribution is investigated. A coupled set of acoustics, thermal, and fluid models is used to calculate the temperature distribution in the liver. The numerical model is established based on the Westervelt and bioheat equations along with the Navier-Stokes equations. Moreover, the acoustic streaming effect is included with Newtonian and non-Newtonian flow assumptions. It is found that in a vascular bed comprising terminal arterial branches, the effect of acoustic streaming is negligible because of the small diameter of these vessels, and the non-Newtonian behavior of blood flow reduces the peak streaming velocity. It is also shown that the vascular density (amount of tissue vascular content) has a considerable cooling effect on peak temperature and hence lesion volume in the liver and, by increasing the vascular density, the treatment duration is prolonged. Results show that when the tumor is embedded in the vascular bed, the cancer cells near the vessels walls remain viable. Some approaches are proposed and compared to improve the efficacy of HIFU in a tumor located in the vascular bed. These approaches include increasing the source pressure or transducer gain. It is concluded that for the assumed configuration of the vascular bed, adjusting the transducer gain is preferred to increase the lesion size and to prevent the problems related to skin burns simultaneously.