Thermal dose control of ultrasound therapies using MR thermometry images:: An in-vitro phantom study

An MR-based thermal treatment controller that delivers a specified thermal dose to a selected target has been developed and evaluated in-vitro using a magnetic resonance (MR) compatible focused ultrasound heating system. The thermal treatment control system has a cascade structure with the main nonlinear dose controller continuously generating the reference temperature trajectory for the secondary constrained, model predictive temperature controller. The control system allows the physician to impose constraints on the maximum allowable temperature elevation at the selected normal tissue location. To reflect hardware imitations and to prevent tissue cavitation, constraint on the maximum transducer power can also be imposed. The combination of a surface coil designed for in-vitro experiments, a gradient echo sequence with k-space reduction and a 3T scanner allowed for fast (1.45s) and low noise (+/- 0.5 degrees C) MR temperature acquisition. The noninvasive thermal images are used during the pre-treatment heating session to characterize the spatial distribution of applied power and effective perfusion, and for the online feedback control of target thermal dose. During the in-vitro phantom experiments the prescribed thermal dose was delivered to the target while restricting the temperature elevation at a selected normal tissue location to below a specified limit. The results demonstrate the robustness of the developed MR-based thermal dose controller in terms of target dose delivery and normal tissue safety.