ELECTRONICS FOR HIGH IMPEDANCE ULTRASOUND TRANSDUCERS

Whilst linear arrays of PZT composite materials are in common clinical use today, PVdF arrays have had limited commercial success. The prime reason for this lies in PVdF’s lower d33 response and high output impedance. However, PVdF does have advantages in the broadband nature of its response and, potentially its ease of manufacturability into arrays due to its low inter-element cross-talk. In this paper, the design of a 20MHz linear array using PVdF is considered, with respect to optimum elemental size and electronics for array excitation and echo reception. Measurements on transducers of various sizes are made and the system response when connected to real receivers is considered. It is shown that while little can be done to improve the low response, with appropriate system design, problems associated with high output impedance can be circumvented. Introduction In the commercial field, PVdF has been successfully deployed for many hydrophone applications [1]. The use of PVdF in array applications has been more limited and has not reached widespread clinical use [2]. Part of the reason for this is the lower transmit/receive sensitivity relative to PZT based transducers [3], which can compromise the signal to noise of a reconstructed image [4]. However, PVdF does potentially offer a route for the use of straightforward manufacturing techniques to define and interconnect the array elements [5]. Additionally, use of PVdF easily realises transducers of wide operating bandwidth that could potentially yield images with enhanced resolution. Finally, PVdF can be used to couple effectively to human tissue without the need for matching layers, since its acoustic impedance is reasonably close to that of tissue. However, a feature of small PVdF transducers is their high electrical impedance. Connection to a standard 50Ω receiver may not offer the best solution. Impedance matching networks can transform a high impedance to match to a 50Ω receiver, but these can be problematic. With transmission line matching [6] for example, a previously wideband transducer is transformed successfully to 50Ω but with a much poorer impulse response. The clear solution is to employ an amplifier with a high impedance input. Whilst this is effective to a point, diminishing returns on signal strength are obtained when transducer impedance becomes too high [7]. Transducer optimisation When subjected to a pressure wave, the equivalent circuit of a PVdF transducer can be approximated as shown in Figure 1 [8].