Noise and element impedance in ultrasonic arrays with small elements

Transducer arrays with elements that are small compared to the acoustic wavelength in two dimensions are of interest to make 2D ultrasonic arrays. Such arrays will increase the flexibility of ultrasonic imaging, but also pose new problems to the array designer. We have here investigated a noise path from the element amplifier to the focused signal, which to our knowledge is not investigated in detail before. It involves noise signals that in reception is transmitted backwards from the element receiver amplifier, and reflected partly at the element tissue interface, and partly absorbed in neighbour elements. Both effects are enhanced in elements that are small compared to the acoustic wavelength. This will increase the coupling between neighbour elements, a process which also changes the impedance the elements see from the medium when excited alone, compared to when they are excited in phase. The noise through the two additional signal paths is found to cancel each other for transducer elements ideally matched to plane waves. The suppression is however to some degree spoiled when the elements are not acoustically matched, and in sparse arrays, where some of the neighbour element positions are not used.