An annular CMUT array beamforming system for high-frequency side looking IVUS imaging

A CMUT annular array system for Side-Looking Intravascular Ultrasound (SL-IVUS) with fixed transmit and dynamic receive focusing capabilities has been developed. The system was experimentally characterized and validated through analytical models that simulate the beamformed transducer behavior. An 840 μm diameter, 35MHz array was fabricated, characterized, and used in experiments. The array consists of curved 18μm by 60μm CMUT membranes that form 8 ring transducer elements with approximately equal areas. The beamforming system uses an IC chip consisting of 8 transimpedance amplifiers and delay elements for receive beamforming with adjustable delays between 2ns and 4ns, that are constant up to 50 MHz with close to unity gain. Transmit focusing is implemented with an FPGA controlled, high voltage pulser board that can generate adjustable electrical pulses with delays as small as 2ns. The system is characterized by measuring the radiation patterns of individual CMUT annular array elements as well as the unfocused and fixed transmit focused arrays. The results show predicted behavior including acoustic crosstalk effects at certain frequencies. For transmit-receive beamforming characterization, a 25μm gold wire was imaged using 4 beamformed transmit elements and 4 beamformed receive elements with different delay values. The results show improved lateral resolution and lower side lobes with proper beamforming.