Thermal efficiency in sonotherapy array design

The use of ultrasound for therapeutic applications involving innovative drug delivery methodologies is a promising area of research to enhance the effectiveness of drug treatment to treat a wide range of diseases, including cancer, peripheral vascular disease, and stroke. Rather than use destructive heating, we have designed and built a family of multi-functional arrays which can ultrasonically identify a target tissue and produce mild heating within this specific tissue site for activated delivery of drug-encapsulated vehicles. These triple array probes are truly multifunctional, operating on a standard commercial imaging system. As sonotherapeutic devices, however, the internal heating and heat dissipation pathways are a concern for design optimization. We have compared three different triple-array probe designs for thermal efficiency and heat transfer characteristics. Each of these multifunctional arrays are comprised of a single center array row of 128 elements operating at 5.3 MHz for imaging, and two 1.5 MHz, 64 element outer arrays operated in parallel for sonotherapy. The laboratory verified KLM model for the low frequency array pair in each probe design has been used to derive estimates of array transmission power and efficiency. Using the key parameters of each design, we can determine the array heat source function and apply analytical expressions to explain the heat dissipation pathways in the probe itself. Our analytical techniques use simplifying assumptions and include the use of both equivalent lumped element thermal circuits for the transducer front port path, and a solution of the PDE heat equation for transient conduction with constant surface heat flux boundary conditions at the transducer backing. The metalized traces of the flex circuits act as the third heat escape pathway. The multifunctional array probes can be used to deliver up to 5 Watts of total acoustic power in mild hyperthermia experiments with small animals. Our analytical expressions have been verified with temperature measurements made in the array backing material and on the array surface. Our early design, the G3, has a triple layer array stack and flex circuit interface connections to support these layers; a later design, the G4, has only a single layer array piezoceramic but has an optimized front port for heat dissipation. The G5 has a further optimized front port and changes in array interconnects. These three probe designs have produced very different thermal dissipation power path distributions; the front, back and flex trace heat path distributions for the three are a) 16%, 2%, 82%, b) 42%, 7%, 51%, and c) 86%, 8%, 6% for the G3, G4, and G5 designs respectively. These results can be directly related to optimized design features which improve the heat routing in sonotherapy array devices.