Increasing the HIFU ablation rate through an MRI-guided sonication strategy using shock waves: feasibility in the in vivo porcine liver.

This study investigated whether an MR-guided pulsed HIFU ablation strategy could be implemented under clinical conditions, using a transducer designed for uterine fibroid ablation, to obtain an ablation rate that is sufficiently high for clinical abdominal HIFU therapy in highly perfused organs. A pulsed HIFU ablation strategy, aimed at increasing the energy absorption in the HIFU focal area by local shock wave formation in the non-linear pressure regime, was compared to an energy-equivalent continuous wave sonication strategy in the linear pressure regime. Both ablation strategies were used for transcutaneous sonication of pre-defined treatment cells in the livers of 5 pigs in vivo. Temperature evolution in both the target area as well as the pre-focal muscle layer was monitored simultaneously using MR thermometry. Local energy absorption and thermal dose volumes were shown to be increased using the pulsed ablation strategy, while preserving healthy tissue in the near field of the acoustic beam. Respiratory motion compensation of both acoustic energy delivery and MR thermometry was applied through gating based on MR navigator echoes. Histopathology showed that confluent vacuolated thermal lesions were created when the pulsed ablation strategy was used. Additionally, it was shown that the heat sink effect caused by the presence of larger vessels could be overcome. The pulsed HIFU ablation strategy achieved an ablation rate of approximately 4 ml per hour in the in vivo porcine liver, without causing undesired damage to healthy tissues in the near field.