A new multifrequency transducer for microemboli detection and classification

Introduction: The classification of circulating microemboli as gaseous or participate matter is essential to establish the relevance of the detected embolic signals. The TransCranial Doppler (TCD) technology has not fully succeeded yet to characterize unambiguously the composition of microemboli. Recently, the authors have proposed a new approach to detect, characterize and size gaseous emboli. The method is based on the nonlinear properties of gaseous bubbles. It has been established that for specific ultrasonic exposure conditions, a gaseous embolus vibrates nonlinearly, leading to the generation of harmonic (2f0, 3f0,⋯) and/or subharmonic (f0/2) components in the frequency spectrum of the backscattered embolic signal. The application of this approach requires the use of a dedicated transducer with the ability of transmitting the adequate frequencies and receiving simultaneously the high frequency scattered nonlinear components. Method: This study presents a multifrequency emboli transducer composed of two independent transmitting elements and a separate receiving part. The transmitting elements, made of piezocomposite, consist of a central piston element with a 20 mm diameter operating at a center frequency of 360 kHz and an outer ring element, of 3.7 mm width, transmitting at 130 kHz. The transducer has a total aperture of 30 mm. The receiving element consists of a PVDF layer 110 mm thick. The maximal acoustic pressure generated at the regions of interest ranged from 145 kPa up to 370 kPa, sufficient to induce nonlinear behavior of gaseous emboli. The capabilities of this new transducer to detect and classify emboli have been tested in vitro with gaseous emboli with a diameter between 10 μm up to 120 μm. Results: The transmitting part can cover a frequency band between 100 kHz and 600 kHz. The reception of the signal is performed by a 110 μm PVDF layer sensitive over a frequency band ranging from 50 kHz up to 2 MHz. The experimental results showed that a specific range of gaseous embolus size was detected by each transmitting element. Using the 130 kHz outer element in transmission, microemboli between 35 μm and 105 μm, could be discriminated through their second harmonic or subharmonic emissions while gaseous microemboli between 10 μm and 40 μm were accurately classified using the 360 kHz inner element. The in vitro results demonstrate that nonlinear properties of microemboli combined to a new transducer generation offer a real opportunity to characterize and size microemboli.

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