Development of a Water bag System for Focal Depth Control of Single Transducer based Focused Ultrasound

Herein, the development of an applicator for coupling between the ultrasound transducer and human skin in a focused transcranial ultrasound (FUS) system is proposed. Generally, applicators that use a single element have the problem of a fixed-depth stimulating point because the focal point of ultrasound is around the transducer radius of curvature. This study proposes a method to increase the degree of freedom of selecting a stimulus depth, including in the coupling method, using a single-element transducer. This is achieved by developing a pump system, which connects to the water bag-based applicator to actively change the amount of water in the water bag and, thus, the position of the focal point. Herein, the water-bag-based applicator was developed and it was verified that there was no ultrasonic loss through sound field experiments. Additionally, a control system was developed to actively change the amount of water in the water bag when the desired stimulation depth is determined by deriving a relationship between the depth of the focal point of the transducer according to the volume change of the water bag. An average error of 0.26 mm was obtained from experiments. Lastly, a cooling system was developed to cool the water bag, and it was observed that water at 70 °C could be cooled to 40.3° C in 10 min.