Design considerations and performance of MEMS acoustoelectric ultrasound detectors

Abstract

Most single-element hydrophones depend on a piezoelectric material that converts pressure changes to electricity. These devices, however, can be expensive, susceptible to damage at high pressure, and/or have limited bandwidth and sensitivity. We have previously described the acoustoelectric (AE) hydrophone as an inexpensive alternative for mapping an ultrasound beam and monitoring acoustic exposure. The device exploits the AE effect, an interaction between electrical current flowing through a material and a propagating pressure wave. Previous designs required imprecise fabrication methods using common laboratory supplies, making it difficult to control basic features such as shape and size. This study describes a different approach based on microelectromechanical systems (MEMS) processing that allows for much finer control of several design features. In an effort to improve the performance of the AE hydrophone, we combine simulations with bench-top testing to evaluate key design features, such as thickness, shape, and conductivity of the active and passive elements. The devices were evaluated in terms of sensitivity, frequency response, and accuracy for reproducing the beam pattern. Our simulations and experimental results both indicated that designs using a combination of indium tin oxide (ITO) for the active element and gold for the passive electrodes (conductivity ratio = ~20) produced the best result for mapping the beam of a 2.25-MHz ultrasound transducer. Also, the AE hydrophone with a rectangular dumbbell configuration achieved a better beam pattern than other shape configurations. Lateral and axial resolutions were consistent with images generated from a commercial capsule hydrophone. Sensitivity of the best-performing device was 1.52 nV/Pa at 500 kPa using a bias voltage of 20 V. We expect a thicker AE hydrophone closer to half the acoustic wavelength to produce even better sensitivity, while maintaining high spectral bandwidth for characterizing medical ultrasound transducers. AE ultrasound detectors may also be useful for monitoring acoustic exposure during therapy or as receivers for photoacoustic imaging.

Simulation-based validation for four- dimensional multi-channel ultrasound current source density imaging

“…In the electric field, because of a distributed current source J I = J I ( x , y , z , t s ), the voltage $V_i^{\mathrm{AE}}$ measured by lead i at coordinates x 0 , y 0 , z 0 , and time t s can be expressed in four dimensions under the assumption of far-field detection of the AE signal. To reduce the noise interference, a band-pass filter h ( t ) with a center frequency similar to the transducer is applied to the AE signal $V_i^{\mathrm{AE}}$ [6], [12]. In the simulations and experiment, the transducer is steered along the x - y plane, and the dipole is near the focus of the transducer; therefore, z 0 is constant and z 0 = − z f (the focal length of the transducer).…”

“…The ultrasound wave field is simulated using Field II software [12], and the pulse can be simplified by $$p(x,y,z,t)=P_{0}b(x,y,z)a(t-z∕c).$$…”

“…UCSDI was used to map cardiac activation waves in the live rabbit heart, with measured conduction velocities estimated from the images to be on the order of 0.12 mm/ms [10], [11]. Its sensitivity is proportional to the amplitude and geometry of the local current density and pressure at the focus [7], [8], [12]. Unipolar ultrasound pulses have also been used in simulation to reconstruct arbitrary current distributions in a conductive volume [13].…”

Comparison of Three Linear Digital Filters Applied to Improve the Quality of the Measured Acoustic Field

“…A band-pass Hamming-window filter at a central frequency of 2.25 MHz and bandwidth of 2.48 MHz (110%) was used to characterize an electroacoustic hydrophone developed in [20]. In [21], a band-pass Hamming window was used to filter the ultrasound signals from an underground mine detector system.…”