Multi-layer composite transducer arrays for improved signal-to-noise ratio and bandwidth in medical ultrasound

Increasing transducer bandwidth and signal-to-noise ratio (SNR) is fundamental to improving the quality of medical ultrasound images. In this dissertation, I describe a range of array transducers using new materials to improve both parameters. These new materials are stacked multi-layer composites of piezoelectric ceramics and polymer epoxies. The first transducer consists of 2 layers of posts (piezoelectric ceramic, PZT-5H) surrounded by soft epoxy. Experimentally, this 2 layer 1–3 composite transducer, yielded increased pulse-echo SNR by 5.2 dB and increased −6 dB bandwidth by a factor of 1.3, compared to the PZT-5H control. However, this structure required precision alignment of the posts greater than 90% of the post pitch (0.125mm) and a thin film bond line between the layers.; Thus, I developed a new multi-layer structure that will not require post alignment and would ideally be fabricated using thick film technology capable of volume production. Starting from a PZT-5H multi-layer transducer, cuts were made through the top layer and back-filled with epoxy, forming a PZT/epoxy composite layer on top of PZT layers, referred to as a multi-layer composite hybrid transducer. Finite element simulations (FEM) showed that for a 2 MHz phased array element with a single acoustic matching layer, the 3 layer hybrid structure increases the pulse-echo SNR by 11 dB compared to a single layer PZT-5H control element and increases −6 dB pulse-echo fractional bandwidth from 46% to 65%, a factor of 1.4, for the hybrid element. I fabricated a hybrid transducer array and obtained improvement in SNR by 11 dB over a PZT-5H control and increased −6 dB bandwidth from 54% to 59%, a factor of 1.1. However, the material properties of currently available thick film multi-layer transducers limit the performance of these hybrid arrays and need further refinement before simulated results can be matched experimentally.; Additional FEM simulations were performed to further improve the transducer array designs. These simulations showed that for a 2 MHz phased array element with a single matching layer, the improved 3 layer hybrid structure increased the pulse-echo SNR by 16 dB and −6 dB pulse-echo fractional bandwidth from 58% to 75%, a factor of 1.3, for the hybrid element versus the PZT-5H control. Analogous FEM simulations of single crystal material (PZN-8%PT), showed increased pulse-echo SNR by only 3.1 dB versus the PZT-5H control and a −6 dB bandwidth of 108%.