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A method for time-optimal, direct control of thermal dose in thermal therapies is developed and experimentally validated using a focused ultrasound transducer and a phantom patient. State constraint on the maximum allowable temperature in a selected spatial location is imposed to prevent damage to critical normal tissues. A saturation constraint on the ultrasound power is imposed to reflect hardware limitations. It is shown that to achieve the minimum time treatment it is necessary to control the therapy with either saturated ultrasound power or active normal-tissue temperature constraints. The special cases for which the necessary condition are also sufficient for time optimality are also established. The model-based treatment control system is then designed that ensures that the necessary condition for time optimal treatment is satisfied throughout the treatment. During validation experiments, the ultrasound specific absorption rate and thermal response models of the phantom, needed for the operation of the designed treatment control system, were identified using temperature measurements. The performance of the treatment control system during the experiments demonstrates that the proposed approach is effective at delivering the desired thermal dose in a near-minimum time without violating safety constraints imposed in healthy tissues.
Date of Publication: Nov. 2007