Implementation of digital synthetic aperture technique for a high frequency annular array transducer

A digital synthetic aperture technique was used to improve the depth of field of a focused, high frequency 5-ring annular array transducer. The transducer was fabricated in-house and has a center frequency of 35 MHz, aperture diameter of 6 mm and focal length of 12 mm. This array was used to scan a wire phantom consisting of seven 25 μm chromium wires. Pulse/echo data were captured for all 25 transmit/receive ring pairs and synthetic aperture methods were applied in the postprocessing stage to the digitized echo data. Time delays for focussing the annuli at different focal depths were computed and added to the received echoes. Simulation wire phantom echo data was also created and analyzed in the same way. Significant improvements in the axial depth of field were observed, with substantial increase in the echo signal amplitude in regions outside the natural focal zone of the transducer.