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The article presents the results of numerical simulation of an experiment on irradiating ex vivo bovine liver sample by the therapeutic array of the MR-HIFU clinical system (Sonalleve V1 3.0T, Profound Medical Corp., Canada). Continuous quasi-linear and pulsed shock-wave exposures with the same time-averaged power are compared. Volumetric thermal lesions were generated by moving the focus of the array in its focal plane along discrete trajectories consisting of two or four concentric circles with a maximum radius of 4 mm. The effect of using the criteria for controlling the thermal dose during treatment and ending the sonication on the shape, volume, and exposure time of generating thermal lesion were analyzed. The acoustic field in tissue was calculated using the Westervelt equation; the temperature field was simulated with the inhomogeneous heat conduction equation; and the lesion boundary was determined according to the thermal dose threshold. In the quasi-linear mode corresponding to the clinical one, thermal diffusion leads to elongation of the lesion by a factor of 2–3 along the beam axis compared to the transverse dimension of the trajectory. The use of pulsed shock-wave exposures with switching off the inner circles of the trajectory as they reach the threshold value of the thermal dose makes it possible to significantly suppress the thermal diffusion effects in the axial direction of the beam and obtain localized thermal lesion of a given shape with a thermal ablation rate comparable to the clinical case. [ABSTRACT FROM AUTHOR]

High intensity focused ultrasound (HIFU) treatments typically involve the ablation of tissue volumes comprising multiple focal sites. One aspect of treatment planning involves the definition of a sequence of ultrasound pulses and corresponding focal sites as the sonication trajectory. Here, numerical simulations of the thermal effects of different trajectories are performed for HIFU exposures delivered to an ex vivo bovine liver sample by a clinical array (Sonalleve V2 3.0T system, Profound Medical Corp., Canada). Simulations consider boiling histotripsy regime with millisecond-long pulses that include shocks. Focusing of the ultrasound beam in tissue was modeled by the Westervelt equation, and the temperature field was modeled by the bioheat equation. To explore different treatment strategies, trajectories were considered with discrete foci located along two or four concentric circles with radii from 2 to 8 mm. Two approaches for traversing these focal sites were compared: In the first approach each discrete focus was sonicated by a sequence of 15 pulses before moving to the next site in the trajectory. In the second approach, each focus was sonicated once before moving to the next site, with sonications over the whole trajectory repeated 15 times. The influence of the trajectory's size and the pulsing strategy on the temperature field was analyzed. It is shown that the structure of the temperature field is more uniform with a longer time interval between repeated irradiation of each focus, and the optimal time interval ranges from three to six pulse repetition periods. [ABSTRACT FROM AUTHOR]