Minimisation of HIFU pulse heating and interpulse cooling times.

This study presents results from a new optimisation technique that reduces HIFU treatment times by minimising individual heating and interpulse cooling times while adhering to normal tissue constraint limits at each sonication position. The potential clinical usefulness of this technique is demonstrated through its implementation in three dimentsional (3D) simulations of HIFU treatments for a range of tumour geometries, normal tissue constraint values, tissue perfusion levels and focal zone scanning path trajectories, all studied as a function of the applied power magnitude. When compared to typical open loop values the optimised treatment times were lower for all conditions studied, including when treatment-limiting normal tissue thermal build-up was present. While use of this technique guarantees minimum pulse heating and interpulse cooling times for each pulse, the total treatment time gains realised depend on the individual clinical treatment configuration. In combination with a judiciously selected scan path, use of the pulse time optimisation procedure reduced treatment times in a small, superficial tumour by 85%. In addition, in all cases studied the use of an increased applied power always decreased the treatment time, including cases when significant normal tissue thermal build-up was present. Importantly, the power maximisation and pulse time minimisation procedures can be applied independently of the optimisation of the focal zone's scan path, size and shape. Given the basic nature, universal applicability and ready clinical adaptability for use in real time model predictive control, the pulse time minimisation and power maximisation approaches have significant clinical promise for reducing HIFU treatment times.