Constrained dynamic control of focal trajectory and intensity of phased array ultrasound thermal therapies

Long treatment times and normal tissue damage have been identified as major problems of ultrasound (US) surgery when large tumor volumes are treated, or when a tumor is located in close proximity to critical normal tissues. In this paper, a treatment control system that automatically selects location and intensity of the US focal zone to deliver the prescribed thermal dose to the target in minimum time without violating explicitly-imposed normal tissue safety constraints is developed. The controller is characterized by its time-optimal performance and direct incorporation of normal tissue constraints. To minimize the treatment time, which can be substantial with the explicit safety constraints, the control system applies the maximum US power for as long as possible without violating constraints by automatically changing focal zone locations to continue therapy at the maximum possible power. When there are no untreated locations where the maximum power can be applied without violating safety objectives, the control system selects focal locations and intensities such that the treatment is completed with at least one of the safety constraints active. The developed approach is evaluated in a computer-simulated treatment of a patient-specific three-dimensional breast tumor heated by an US phased array system.