Bias optimization of dual ring CMUT arrays for forward looking IVUS applications

Dual-ring annular CMUT arrays for FL-IVUS which enable independent optimization of transmit (Tx) and receive (Rx) functionalities have been described previously. In this work, we have investigated optimal DC biasing schemes for Tx and Rx elements of identical geometry with separate bias electrodes. Fabricated arrays, 800 μm in diameter, with 32 Rx and 24 Tx elements were designed to operate at 23 MHz and 12 MHz with collapse voltages measured to be at 190 V and 60 V respectively. The samples were coated with 3 μm of Parylene C for electrical isolation and tested in water. Pitch catch experiments with a fixed transmit pulse showed optimal receiver sensitivity close to the measured collapse voltage. The output pressure for different transmitter biasing and pulse combinations were measured using a broadband hydrophone. Experiments using the 12 MHz device and a 5 cycle tone burst showed that without biasing, it was possible to transmit at two times the excitation frequency with harmonic separation of 25 dB. Bipolar and unipolar pulse excitation also showed that the peak pressure can be improved by 14 dB and 5.7 dB with respect to optimal transmission biased close to collapse. Further experimentation with the 23 MHz array showed that it was possible to increase the transmit pressure by 4.2 dB without altering the frequency content though the use of a bipolar pulse and bias greater than the 190 V collapse. These trends are in agreement with a simple SPICE based CMUT model. We have shown that the optimal transmit conditions regarding bias and pulse characteristics do not coincide with the optimal receive bias. Dual ring arrays can take advantage of this optimization to improve the SNR of the system.