HIFU treatment time reduction in superficial tumours through focal zone path selection

Purpose: This study evaluates the hypothesis that optimising the path of a high intensity focused ultrasound (HIFU) treatment's N focal zone heating pulses can significantly reduce treatment time, and identifies the underlying bio-thermal principles. Materials and methods: Thirty-one scanning paths were investigated using 3D simulations, with a minimum thermal dose delivered to every tumour position. Treatment time was calculated as the sum of the N, individually optimised heating and cooling periods. Tumours were superficial (skin to tumour distance ranging from 1.3 to 2.5 cm), but always deep enough so that the pre-tumour normal tissue was routinely heated to its constraint temperature (range: 42-45 degrees C). Properties were uniform and constant, and a range of blood perfusion and phased array powers were studied. Results: The best paths significantly reduced treatment times, with the largest gains occurring when (1) temperature superposition inside the tumour was maximised by successively heating the focal zone positions located in a 'stack' along the transducer's axis, and (2) the focal zone was moved laterally to an optimised location and another stack was applied. Stacking takes advantage of the focal zone's elongated shape, which produces axial temperature superposition within the tumour. Reduced tumour heating times also reduced energy deposition in the normal tissues, thus reducing or eliminating the need for inter-pulse cooling. Conclusions: HIFU treatment times can be significantly reduced by taking advantage of axial temperature superposition in tumours. Further reductions are obtained by correct choice of the transverse scan path.