Improving ultrasound transducer performance using FEA-assisted designs and digitized waveform compensation techniques

Diagnostic ultrasound imaging relies on a serial signal processing chain. Frequently, the transducer is the weak link in this chain, and improvements in transducer performance directly map to improved spatial and contrast image qualities.; Capacitive Micromachined Ultrasound Transducers (CMUTs) are finding growing applications in medical ultrasound imaging because of their broad bandwidth and the resultant better image resolution compared with the conventional PZT transducers. However, CMUTs have a relatively low electromechanical coupling coefficient. Two improved structure designs were created using Finite Element Analysis (FEA). In the first design, trenches are formed at the edge region on the membrane. The maximum membrane displacement on axis was increased by 528% and the on-axis acoustic pressure output was improved by 122% when using 4 trenches. In the second design, the shape of the support structure is modified by using a more compliant 'dogleg' shape. The maximum membrane displacement in the CMUT element was increased by 112%, and the acoustic pressure output was improved by 23%. FEA results demonstrated that both methods achieved larger 'high-yield' membrane areas, higher Figure of Merit values, more uniform membrane deformation, and most importantly, higher effective electromechanical coupling coefficients (k). A collapsed mode of operation was also investigated, and resulted in improved coupling coefficient by approximately 16% when the DC bias was 90% of collapsing voltage.; Due to their non-linear operating mechanism, CMUTs inherently produce harmonics. This characteristic makes them unsuitable for tissue and contrast harmonic imaging. Therefore, an 'iterative cancellation' approach was developed to reduce this inherent harmonic generation. Both FEA simulations and experimental results are presented. The experiment was accomplished on a Sensant single element CMUT transducer. This iterative cancellation approach achieved 18.6dB 2nd harmonic reduction in FEA simulations and 20.7dB 2nd harmonic reduction in the experiment.; Undesired acoustic crosstalk presents in all transducer arrays, and is particularly troublesome in CMUT arrays. A new crosstalk reduction method in the transmit mode was investigated. A matrix was created to describe the transfer function mapping input on a single element to outputs from that element and its neighbors. The required excitation and cancellation waveforms to minimize the crosstalk were calculated by solving this matrix equation by imposing idealized outputs. FEA simulation on PZT arrays produced a 9dB crosstalk reduction, and a transmitting experiment also indicated that the sidelobes caused by the crosstalk in the single element angular response was reduced by 6dB. This transfer function matrix method was also tested on CMUTs in FEA. It provided an 11dB crosstalk reduction when the AC excitation is much smaller than the DC bias. At high AC excitation amplitude levels, it achieved a similar crosstalk reduction result (11dB) when the "iterative cancellation" approach was applied together with this crosstalk reduction approach to also minimize the transmitted harmonics.