Dual element transducers for high frequency harmonic imaging

Concentric annular type dual element transducers for second harmonic imaging at 20 MHz / 40 MHz were designed and fabricated to improve spatial resolution with sufficient penetration for ophthalmic imaging applications. The outer ring element was designed to transmit the 20 MHz signal and the inner element was designed to receive the 40 MHz second harmonic signal. Lithium niobate (LiNbO3) was used for piezoelectric material to achieve good impedance matching with its low dielectric constant. Double matching layers and conductive backing were used and optimized by KLM modeling to achieve high sensitivity and bandwidth for harmonic imaging and good time domain characteristics. Prototype transducers were fabricated and evaluated quantitatively. The average measured center frequency for the transmit ring element was 18 MHz with a - 6 dB bandwidth above 60 % and - 28 dB uncompensated insertion loss. The 40 MHz receive element displayed a 35 MHz center frequency, around 60 % bandwidth and - 28 dB uncompensated insertion loss. The lateral beam profile of 20 MHz ring elements at the focus matched the Field II simulated results well. Images of a choroidal nevus of eye were obtained.