Miniature polymer transducers for high frequency medical imaging

High frequency (30-100 MHz) polymer transducers have been used in a variety of medical imaging applications(1,2) since they were first introduced by Sherar and Foster(3) in the late 1980s. The transducers are intrinsically broadband and the flexibility of the polymer material makes fabrication relatively easy. Unfortunately, piezoelectric polymer materials have a low dielectric constant. Unless a large aperture is used, the electrical impedance of the transducer will be high, and the receiver sensitivity will be poor. This problem can be avoided by placing a high impedance pre-amplifier inside the transducer housing. Placing the preamplifier close to the transducer is important to avoid standing waves between the high output impedance of the transducer and the high input impedance of the pre-amplifier. We have recently developed a process for fabricating high frequency spherically shaped polymer transducers in which an integrated circuit die is mounted just beneath the surface of the transducer. In this paper we describe a theoretical and experimental analysis of the noise performance of these devices. The signal-to-noise ratio at the output of the pre-amplifier is estimated by combining a simple noise model for the amplifier with a KLM model of the transducer. This analysis provides a useful way of evaluating different transducer/pre-amplifier combinations. Excellent agreement between the model predictions and experimental results proves the validity of this approach.