Emboli detection using a new transducer design.

We have presented, in a previous study, a new approach to detect, characterize and estimate the size of gaseous emboli, based on the nonlinear behavior of gaseous bubbles. In this study, a specific transducer design has been developed to be used for such a purpose. It is composed of two separate transmitting and receiving capabilities. The transmit part, consisting of a lead zirconate-titanate (PZT) material, emits at a frequency of 500 kHz and could generate pressures up to 410 kPa. On the top of the transmit surface, a thin polyvinylidene difluoride (PVDF) layer is glued and used for receiving frequencies from 250 kHz (f0/2) up to 2.5 MHz (5 f0). To evaluate this new design, ultrasonic measurements were carried out with gas bubbles with diameters ranging from 10 microm up to 90 microm and solid particles between 350 microm and 550 microm. The experimental results confirmed our previous findings: gaseous emboli with a diameter close to the resonance size scatter significantly at higher harmonic components (from the second harmonic up to the fifth), and bubbles with a diameter around twice the resonance size produce a subharmonic and/or an ultraharmonic component. Meanwhile, solid particles and other bubble sizes behave only linearly and their scattered spectrum appeared without any harmonics. The study demonstrates the utility of this approach in using a single transducer to detect and characterize selective gaseous emboli from other particles using their nonlinear behavior.