Scan parameter optimization and a temperature controller for scanned focussed ultrasound hyperthermia: A theoretical and experimental study.

Abstract

Maintenance of the treatment 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. Since scanned, focussed ultrasound systems (SFUS) have a great deal of flexibility it is necessary to use such a feedback system in shaping the power field during hyperthermia treatments. For best use of such a system many complicated, interacting decisions must be made to obtain an optimal hyperthermia treatment. This dissertation studies this optimization problem using a simulation program which searches for the optimal scan parameters, and presents a PID plus bang-bang feedback control system which gives a suitable power distribution to meet the above requirements for this hyperthermia system. Several objective functions were studied and compared based on temperature criteria. An extensive objective function study has been done in order to determine; the best formulation for that objection function, the characteristics of that objective function near the optimal operating point, the effects of the scan parameters on that objective function, and the domain of acceptable initial guess points for obtaining a globally optimal solution. A further comprehensive study of the optimal temperature distributions attainable with single and multiple circular scans of a tumor was done. The results show that the optimal scan parameter configuration will allow this SFUS to produce a close to ideal treatment temperature distribution for a wide variety of clinically relevant conditions. To further study the variation of the temporal and spatial blood perfusion, a controller was used to obtain a more suitable power to meet the treatment needs. 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. 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.