Improved Signal-to-Noise Ratio in Hybrid 2-D Arrays: Experimental Confirmation

Abstract

2-D array transducers have shown significant promise for medical ultrasound over conventional linear arrays, at the cost of increasing the number of channels, difficulty of fabrication and array element impedance. The increase in element impedance reduces the power coupled to a 2-D array element from a conventional 50 Ω source in transmit mode. If the array is sparse, which is typical of 2-D arrays, then the net power coupled into the front acoustic load is reduced when compared to a fully sampled aperture. Furthermore, the received signal-to-noise ratio (SNR), when measured through a nonideal amplifier, is degraded because the high impedance 2-D array transducer element cannot efficiently drive the coaxial cable. The reduction in transmit sensitivity and received SNR can be circumvented with the application of multilayer piezoelectric elements. The improvement in transmit occurs because the transducer impedance is better matched to the impedance of the source. In receive, multilayer elements allow more of the open circuit received voltage to fall across the input of the high impedance preamplifier. In this case, the same number of layers are used in transmit and receive. Recently, it has been suggested that separate optimization of the transmit channel and receive channel (a hybrid array) would further improve the pulse-echo SNR. In this paper, we fabricated and tested a hybrid array operating at 1 MHz using a multilayer transmit element and single layer receive element. A 7 Ω transmitter and high impedance preamplifier were placed adjacent to the transmit and receive elements within the transducer assembly. The hybrid pulse-echo SNR improved by 26.4 dB over the conventional array. The experimental result showed good agreement with the KLM model. Furthermore, KLM simulations showed that as the operating frequency of the array increases, the overall improvement over the conventional array increases. For example, a 1.5-D array operating at 2 MHz had an improvement of 30 dB whereas a 7.5 MHz 1.5-D array showed an increase of approximately 38 dB. The separate optimization of the transmit and receive channel for 2-D arrays showed even greater improvement than for 1.5-D arrays. For example, a 2 MHz 2-D array had an improvement of over 44 dB.