Optimal design of a concave annular high intensity focused ultrasound transducer for medical treatment

In this study, a concave annular array transducer was designed to generate high intensity focused ultrasound for medical therapeutic applications. First, a detailed theoretical equation was derived to determine the sound pressure distribution from the transducer. To facilitate the design, each annulus in the array was simulated by multiple piston sources. Use of the piston sources can generalize the acoustic field analysis of the concave annular transducer and thus it is applicable to any other types of ultrasound transducers. The validity of the equation was verified by comparing the results calculated by the equation with those calculated from finite element analysis. Using this equation, we developed a new design scheme to optimize the structure of the annular transducer, using OptQuest-Nonlinear Programming algorithm, to meet the constraints that the main lobe should exist at a specific region on the normal axis of the transducer and the level of the maximum sidelobe, including the grating lobe, should be lower than the main lobe by −12.0 dB that is a very strict criterion when compared with that in precedent works. For the annular structure, two different schemes were investigated: (1) an equal area channel structure obtained by setting the area of all the channels equal and (2) an equal angle channel structure obtained by setting the angular width of all the channels equal. The optimized structures of both configurations satisfied all the performance constraints for the sidelobe level as well as the geometric and dynamic focusing. The new design method developed in this work can facilitate the analysis and efficient development of HIFU transducers for medical treatment.