Multilayer transducer for acoustic bladder volume assessment on the basis of nonlinear wave propagation

Catheterization remains the gold standard for bladder volume assessment, but it is invasive, uncomfortable to the patient and introduces the risk of infections and traumas. Acoustic measurement of the bladder volume reduces the need for a urinary catheter. Recently, a new method to non-invasively measure the volume of liquid filled cavities in vivo on the basis of nonlinear wave propagation has been introduced. To implement this method, two different multilayer ultrasound transducers were developed. Both transducers consisted of a first piezo-electric layer of lead zirconate titanate (PZT) to transmit waves at a fundamental frequency (2 MHz) and a second piezo-electric layer (copolymer) to receive a wide range of frequencies including harmonics. To suppress the inherent susceptibility of the film to electromagnetic waves, one of the two transducers, i.e., an "inverted" multilayer transducer, had the copolymer layer located inside the structure. The other multilayer transducer, i.e., a "normal" multilayer transducer, had the copolymer film located on the outside. Both transducers were compared with a commercially available broadband piezo-composite transducer, with respect to their transmit and receive transfer functions, their pulse-echo responses and their electromagnetic susceptibility (EMS) in reception. It was concluded that to measure up to at least the third harmonic frequency component with good sensitivity in combination with high transmit sensitivity at the fundamental frequency, a multilayer structure is preferred. To optimize for the EMS in reception, and hence also the signal-to-noise ratio, an inverted geometry, as proposed in this paper, was proven to be most effective.