HIFU treatment time reduction through heating approach optimisation

Purpose: This study evaluated the HIFU treatment time reductions attainable for several scan paths when optimising the heating approach used (single, discrete pulses versus volumetric scanning) and the paths' focal zone heating locations'; number (N-FZL), spacings, sequencing order, number of heating cycles (N-CYCLES), and heating times. Also evaluated were the effects of focal zone size, increased tissue absorptivity due to heating, and optimisation technique. Materials and methods: Treatments of homogeneous constant property tumours were simulated for several simple generic tumour shapes and sizes. The concentrated heating approach (which delivered the desired thermal dose to each location in one discrete heating pulse (N-CYCLES = 1)) was compared to the fractionated heating approach (which dosed the tumour using multiple, shorter pulses repeatedly scanned around the heating path (i.e. 'volumetric scanning' with N-CYCLES > 1)). Treatment times were minimised using both simultaneous, collective pulse optimisation (which used full a priori knowledge of the interacting effects of all pulses) and sequential, single pulse optimisation (which used only the information from previous pulses and cooling of the current pulse). Results: Optimised concentrated heating always had shorter treatment times than optimised fractionated heating, and concentrated heating resulted in less normal tissue heating. When large, rapid tissue absorptivity changes were present (doubled or quadrupled immediately after heating) the optimal ordering of the scan path's sequence of focal zone locations changed. Conclusions: Concentrated heating yields significant treatment time reductions and less normal tissue heating when compared to all fractionated scanning approaches, e.g. volumetric scanning.