IMPROVED SIGNAL-TO-NOISE RATIO IN HYBRID 2-D ARRAYS - EXPERIMENTAL CONFIRMATION

2-D array transducers have shown significant promise for medical ultra sound over conventional linear arrays, at the cost of increasing the n umber of channels, difficulty of fabrication and array element impedan ce. The increase in element impedance reduces the power coupled to a 2 -D array element from a conventional 50 Ohm source in transmit mode. I f the array is sparse, which is typical of 2-D arrays, then the net po wer coupled into the front acoustic load is reduced when compared to a fully sampled aperture. Furthermore, the received signal-to-noise rat io (SNR), when measured through a nonideal amplifier, is degraded beca use the high impedance 2-D array transducer element cannot efficiently drive the coaxial cable. The reduction in transmit sensitivity and re ceived SNR can be circumvented with the application of multilayer piez oelectric elements. The improvement in transmit occurs because the tra nsducer 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. I n this case, the same number of layers are used in transmit and receiv e. Recently, it has been suggested that separate optimization of the t ransmit channel and receive channel (a hybrid array) would further imp rove the pulse-echo SNR. In this paper, we fabricated and tested a hyb rid array operating at 1 MHz using a multilayer transmit element and s ingle layer receive element. A 7 Ohm transmitter and high impedance pr eamplifier were placed adjacent to the transmit and receive elements w ithin the transducer assembly. The hybrid pulse-echo SNR improved by 2 6.4 dB over the conventional array. The experimental result showed goo d agreement with the KLM model. Furthermore, KLM simulations showed th at as the operating frequency of the array increases, the overall impr ovement over the conventional array increases. For example, a 1.5-D ar ray 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 opt imization of the transmit and receive channel for 2-D arrays showed ev en greater improvement than for 1.5-D arrays. For example, a 2 MHz 2-D array had an improvement of over 44 dB. (C) 1997 Dynamedia, Inc.