Imaging and identifying early metastases is, to this day, not an easy task: using MRI is expensive and ultrasound is not able to discriminate healthy and diseased tissues. Coupling ultrasound imaging to acousto-optic imaging could be a solution: the additional optical contrast would suppress the indetermination on the origin of the biological tissue.
Acousto-optic imaging is a multi-wave technique which localizes light in highly scattering medium thanks to an acoustic wave: the acousto-optic effect creates frequency-shifted light, carrying local information about the insonified volume. The central challenge of acousto-optic imaging is the detection of the frequency-shifted light, because there are only very few modulated photons and they create a speckle pattern. We choose to explore the detection by spectral filtering using the spectral hole burning phenomenon in a rare earth doped crystal . This filtering technique is intrinsically immune to speckle decorrelation and therefore well adapted to in vivo imaging.
We use a YAG crystal doped with thulium ions under a magnetic field which increases the lifetime of the spectral hole from 10ms to more than a minute. We have undertaken a spectroscopic study to optimize the hole preparation sequence. We will present the first acousto-optic images achieved with a long-lived spectral filter in Tm:YAG, in a scattering medium.
 Li, Y., Zhang, H., Kim, C., Wagner, K. H., Hemmer, P., & Wang, L. V. (2008). Applied Physics Letters, 93(1), 011111.