Biomedical Applications of Acoustoelectric Effect

Abstract

Acousto-electric (AE) effect comes from an interaction between electrical current and acoustic pressure generated when acoustic waves travel through a conducting material. It currently has two main application areas, ultrasound current source density imaging (UCSDI) and AE hydrophone. UCSDI can detect the current direction by modulating the dipole field with ultrasound pulse, and it is now used to form 3D imaging of dipole changing in one period of treatment, such as arrhythmia in the heart and epilepsy in the brain. As ultrasound pulse passes through electrical field, it convolutes or correlates with the inner product of the electric fields formed by the dipole and detector. The polarity of UCSDI is not determined by Doppler effect that exists in pulse echo (PE) signal, but the gradient of lead field potentials created by dipole and recording electrode, making the base-banded AE voltage positive at the anode and negative at cathode. As convolution shifts spectrum lower, the base band frequency for polarity is different from the center frequency of AE signal. The simulation uses the principles of UCSDI, and helps to understand the phenomena in the experiment. 3-D Fast Fourier Transform accelerates the computing velocity to resolve the correlation in the simulation of AE signal. 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. An AE hydrophone requires only a conductive material and can be constructed out of common laboratory supplies to generate images of an ultrasound beam pattern consistent with more expensive hydrophones. Its sensitivity is controlled by the injected bias current, hydrophone shape, thickness and width of sensitivity zone. The design of this device needs to be the tradeoff of these parameters. Simulations were made to optimize the design with experimental validation using specifically fabricated devices composed of a resistive element of indium tin oxide (ITO).