Theoretical and experimental evaluation of a temperature controller for scanned focused ultrasound hyperthermia

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Abstract
Maintenance of the controlled temperatures at their target levels in the face of disturbances, a uniform temperature distribution within the treatment region, an acceptable temperature rise outside that volume, a fast temperature rise, and stability are desirable characteristics of an optimal hyperthermia treatment control system. This paper presents a proportional-integral-derivative plus bang-bang (power on at either a maximum value or at zero) feedback control system designed to meet the above requirements for a scanned focused ultrasound hyperthermia system. Treatment simulations and analytical results for a first-order approximation of a tumour show that the controller is stable for a wide range of gains and sampling times. It was also found that there is an optimal controller gain which minimizes the peak temperature overshoot and the settling time when a step function input is applied to the system. Both the simulation results and experimental animal results show that the controlled region can be rapidly heated to the target temperature with a small overshoot and maintained at that level in the face of disturbances. The effects of temperature fluctuations due to both the periodic changes caused by the scanning and due to measurement noise can be reduced by the use of an auto regressive moving average approach. In vitro dog kidney model and in vivo dog thigh experiments show that the controller works well in practice, and verify that it can compensate for spatial and temporal blood perfusion variations. As shown in both these experiments and in simulations the controller can be used for controlling a single temperature or multiple temperature points simultaneously, thus allowing relatively uniform temperature fields to be created.