P2D-5 Time-Reversal of Photo-Acoustic Waves Generated by Optical Contrasts in an Optically Diffusive Tissue Phantom

Time-reversal of ultrasonic waves allows focusing ultrasound through complex media, such as highly aberrating or highly diffusive media. Time-reversal is based on the detection and re-emission of waves generated by ultrasound sources located within the investigated medium. Classically, these sources consist of high acoustic contrasts echoing ultrasonic waves generated by an incident ultrasonic field, or directly by point-like transducers inserted in the medium. In this work, we use contrast of optical nature as sources of photo-acoustic waves to perform time-reversal experiments. Briefly, photo-acoustic waves are ultrasonic waves generated by the thermoelastic expansion following the absorption of a light pulse. A tissue phantom with optical contrast was fabricated by embedding an optically absorbing gel sphere (with a diameter of approximately 1 mm and an optical absorption coefficient on the order of 0.5 mm^-1) in an optically diffusive intralipid solution (reduced scattering coefficient on the order of 10 cm^-1). A Q-switched pulsed Nd:YAG laser was used to illuminate the tissue phantom with 80 mJ nanosecond laser pulses. A 1.5 MHz ultrasound array connected to a 64-channel time-reversal electronics was used to detect, record and time-reverse the photo-acoustic signals back towards the absorbing gel sphere. The quality of the focusing was assessed in the presence of a strong acoustically aberrating medium, and was found to be identical to that obtained without aberrator. As an example of application, B-mode images of several nylon wires were built in the presence of the aberrator, based on the time-reversed and steered wavefront generated by an absorbing gel sphere hung to one of the wire