Fast accurate optical measurement of medical ultrasonic field in combination with numerical simulation of nonlinear propagation

Abstract

Fast and accurate measurement of ultrasonic field is necessary to ensure the safety and efficacy of therapeutic as well as diagnostic applications of medical ultrasound. The most common method for the purpose is hydrophone scanning. However, it requires a long scanning time and potentially disturbs the field, which is limiting the efficiency of developing such applications. This study proposes an optical phase contrast method in combination of a CT algorithm. Ultrasonic pressure field modulates the phase of the light passing through the field. A phase plate was employed to shift the phase of the non-diffracted component of the light typically by 90 degrees. The phase modulation of the diffracted component was converted to amplitude modulation through interference with the phase-shifted non-diffracted component and then measured by camera. From the measured projected 2D data, the 3D pressure field was reconstructed by a CT algorithm. An upstream field, in which the optical phase does not wrap and the effect of nonlinear propagation can be ignored, was thereby quantified. Nonlinear ultrasonic propagation was simulated based on a pseudo spectral method using the upstream pressure field as the input. Both pressure waveform and absolute pressure from the proposed method agreed well with those directly from hydrophone scanning.