Multi-wave Imaging : Acousto-optics and Photo-Acoustics

The combination of two types of waves in a same imaging device – one giving the contrast, the other the localization – allows to improve the contrast while maintaining a high spatial resolution. In acousto-optic imaging - or Ultrasound Optical Tomography (UOT) - the tagging of Light by the Ultrasound reveals optical properties of organs in depth. Reciprocally, photo-acoustic method relies on the detection of elastic waves which are generated by the absorption of an optical wave.

Acousto-optic imaging

We have developed this method by digital holography or photosensitive media (adaptive wavefront holography). In the scope of in vivo imaging, a significant contribution has been brought using a photorefractive SPS crystal (Sn2P2S6:Te), which possesses a high gain within the optical diagnosis window (700-1100nm), and a short time response (ms). As an illustration, we have evidenced melanin metastases in a large liver biopsy embedded within a thick scattering (4cm, mu’s=10cm-1), though not detected by ultrasound imaging.

"Multi-modal acousto-optic/ultrasound imaging of ex vivo liver tumors at 790nm using a Sn2P2S6 wavefront adaptative holographic setup, Laudereau J.-B., Benoit a la Guillaume E., Servois V., Mariani P., Grabar A.A., Tanter M, Gennisson J.L., Ramaz F., J. Biophotonics 1–8 (2014)

We have demonstrated the possibility to use a pumped gain medium @1064nm (Nd:YVO4), whose time response of the hologram (50 microseconds) is much shorter than for photorefractive systems. An additional originality is the linear response of phase modulation, due to the small spectral shift (0.1nm) between our probe laser (Nd:YAG) and the center of the absorption line of the crystal. We have thus validated acousto-optic measurements by two and four-wave mixing (phase conjugation), and demonstrated the principle of a pure optical photo-acoustic imaging (by two-wave mixing) in a scattering medium of small thickness.

"Refractive index and gain grating in Nd:YVO4 : application to speckle vibrometry and photoacoustic detection, Baptiste Jayet, Jean-Pierre Huignard, and Francois Ramaz, Opt. Lett. 42, 695-698 (2017)"

In the scope to enhance the Signal-to-Noise ratio (SNR), the axial resolution (linked to the ultrasound excitation), but also the imaging rate, we have considered several excitation schemes for the ultrasound: with digital holography, long sequences (ms) with random phase jumps (0,pi) have brought a significant gain compared to ultrasound bursts excitation.

"Acousto-optical coherence tomography with a digital holographic detection scheme, E. Benoit a la Guillaume, S. Farahi, E. Bossy, M. Gross, and F. Ramaz, 3216 Optics Express 37, 15, (2012)"

Inspired from ultrafast excitation developed by the medical team of the Institute, we have implemented ultrasound plane waves (a patent has been deposited), which allows at equal SNR of focused imaging to improve the acquisition rate by at least a factor of 20.

"Ultrafast acousto-optic imaging with ultrasonic plane waves, J.-B. Laudereau, A. A. Grabar, M. Tanter, J.-L Gennisson, and F. Ramaz, Optics Express 24, 3774, (2016)"