Unequal heating duration to reduce the treatment time in high-intensity focused ultrasound therapy

Cooling periods are usually intervened between two consecutive sonications to avoid near-field heating during high-intensity focused ultrasound (HIFU) therapy, which leads to a long treatment time. A possible strategy to shorten the treatment time is to eliminate the cooling intervals. An unequal heating duration method without the cooling time is proposed to reduce the treatment time during HIFU. The starting focal point in the target region is set with the longest duration. Two unequal periods with a long-short pattern are prepared for two sequential focal points begun with that starting point; with the two points, the previous one is heated for a long duration while the latter is heated for a short duration. The durations for the rest focal points are set by analogy. If the distance between a point and its previous heated point is more than double focal spacings, it is set as a new starting point with a long heating period. Computer simulations and ex vivo experiments with the focus scanned by a 90-element spherical phased array had been carried out. An H-shaped lesion consisting of 17 single lesions about 2 mm in diameter was generated with a 2-mm focal spacing using the proposed method, and average time per point was reduced by 65% and 76% respectively in simulation and experiment. Moreover, in simulation 2 separate prefocal lesions (< 3 mm in diameter) were generated 2 mm proximal of the focal plane while no lesion was generated 5 mm proximal of the focal plane. The results demonstrated the proposed method was able to effectively shorten the treatment time by setting unequal heating durations for consecutive focal points and sparing the cooling interval through sonications. The results also indicated that it was promising to alleviate nontarget surrounding tissue damage in HIFU treatments by optimizing heating duration distributions of focal points.