5C-2 20-MHz Annular Arrays for Ophthalmic Imaging

High-frequency annular arrays allow for an improved depth of field (DOF) versus single-element transducers. A P(VDF-TrFE) based, 20-MHz annular array was designed with five, equal-area annuli, a geometric focus of 31 mm, and a total aperture of 10 mm. Digitized data were acquired for all 25 transmit/receive ring pairs and the data were then processed with a synthetic-focus algorithm to create a composite image consisting of a series of focal zones. After acoustical and electrical characterization, the array performance was validated with a wire phantom. With impedance matching, the center frequency of the array elements was ap 17 MHz and the -6-dB bandwidths were ap30%. Synthetic focusing was found to improve the DOF from a span of 4 mm to nearly 18 mm. -6-dB lateral beamwidths ranged from 0.16 to 0.27 mm. Ex vivo scans of a bovine and human cadaver eye demonstrated the improved imaging potential of 20-MHz annular arrays versus fixed-focus transducers