Combination of ultrasound and acousto-optical imaging using a pulsed-ultrasound scanner

Abstract

The acousto-optical sensing (AOS) of a turbid medium is based on the interaction of multiply-scattered coherent laser light with an ultrasonic field. A phase-modulated photon field emanates from the interaction region and carries with it information about the acousto-optical properties of the media. Using a novel technique based on a photorefractive crystal interferometer, it is possible to detect the ultrasound-modulated optical signals generated by short ultrasound pulses. As opposed to continuous-wave (CW) ultrasound, pulsed ultrasound directly provides resolution along the ultrasonic propagation axis. In this work, a commercial ultrasound scanner (Analogic AN2300) was used in pulse mode (5 MHz central frequency) to generate both conventional ultrasound and AO images. Gel-based highly diffusive (μs'=10 cm -1 ) tissue-mimicking phantoms were fabricated, with embedded targets possessing acoustical and/or optical contrast. AO images of 26-mm thick phantoms were generated from optical signals averaged in the time-domain, without further signal processing, and were superimposed on the top of the ultrasound images. Good quality AO images of optical absorbers, intrinsically co-registered with the ultrasound images, were obtained within minutes. The axial resolution of the AO images was given by the spatial length of the ultrasound pulse, typically on the order of one mm in the MHz range. These results show that AO signals can be excited in pulse mode using a commercial scanner, and combined to conventional ultrasound images to provide more information related to the optical properties of the medium.