Operational verification of a 40-MHz annular array transducer.

An experimental system to take advantage of the imaging capabilities of a 5-ring polyvinylidene fluoride (PVDF)-based annular array is presented. The array has a 6-mm total aperture and a 12-mm geometric focus. The experimental system is designed to pulse a single element of the array and then digitize the received data of all array channels simultaneously. All transmit/receive pairs are digitized and then the data are post-processed with a synthetic-focusing technique to achieve an enhanced depth of field (DOF). The performance of the array is experimentally tested with a wire phantom consisting of 25-microm diameter wires diagonally spaced at 1-mm by 1-mm intervals. The phantom permitted the efficacy of the synthetic-focusing algorithm to be tested and was also used for two-way beam characterization. Experimental results are compared to a spatial impulse response method beam simulation. After synthetic focusing, the two-way echo amplitude was enhanced over the range of 8 to 19 mm and the 6-dB DOF spanned from 9 to 15 mm. For a wire at a fixed axial depth, the relative time delays between transmit/receive ring pairs agreed with theoretical predictions to within +/- 2 ns. To further test the system, B-mode images of an excised bovine eye were rendered.