Online temperature control of focused ultrasound heating using an adaptive PID feedback loop

Introduction Focused ultrasound induced heating for use in thermal therapy requires reliable temperature control to assure safety and therapeutic accuracy. MR thermometry based on the proton resonance frequency shift (PRFS) [1] in combination with a proportional, integral, derivative (PID) feedback control [2] has been suggested as a precise solution for controlling high intensity focused ultrasound devices in applications such as local drug delivery. Nevertheless, depending on the desired target temperature profile and the measurement noise, overshoots and oscillations can occur if the initially chosen controller gains remain static, which lead to undesired tissue damage. The presented adaptive PID [3] controller continuously adjusts the controller gains as a function of the current temperature error, the measurement noise and an assumed uncertainty in the determination of the absorption coefficient. Its superior performance in comparison to a conventional and an improved PID controller with static controller gains is demonstrated in phantom experiments.