Acousto-optic imaging using focused and plane wave ultrasound pulses

Abstract

Acousto-optic imaging exploits the effects of modulation of light by acoustic waves propagating in an optically scattering medium to determine its optical properties. The imaging resolution is determined by the acoustic pressure field distribution inside the sample. In the ideal case, this distribution should be limited to a relatively small, strictly defined volume within which the light modulation occurs. Practical applications of this technique impose specific requirements on transducers configuration which constitute additional limitations on shaping the acoustic pressure field characteristics. We investigate the possibilities of improving performance of an acousto-optic imaging system utilizing a linear ultrasound array by comparing results obtained using plane wave and focused pulses and different apodization patterns. Acoustic pressure field distributions were determined numerically and with hydrophone measurements. We introduce an ultrasound focus quality coefficient in order to describe and compare the confinement of the expected light modulation volume. Acousto-optic images were obtained experimentally by measuring contrast decrease of the interference patterns of a laser light transmitted through samples and recorded by a camera. The results show that for focused pulses the off-focal pressure field components have significant influence on the determined values, and that this issue can be mitigated by using plane wave imaging.