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Nowadays imaging the early liver metastases has to be improved in order to have an easier setup than MRI or to be more discriminant than ultrasound between healthy and diseased tissues. Acousto-optic imaging could solve these issues by coupling itself with ultrasound modality: the additional optical contrast would suppress the indetermination on the health of the biological tissue.
Acousto-optic imaging is a multi-wave technique which localizes light in very scattering media 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 process in rare earth doped crystal [1].
Spectral hole burning consists in creating a sub-MHz-wide transparency window in the wide absorption spectrum of a rare earth doped crystal: the crystal becomes transparent at the wavelength of the spectral hole and thus can filter the modulated light. 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 longer than a minute. We have undertaken a spectroscopic study to optimize the hole preparation sequence. The long lifetime simplifies the optimization of fast imaging sequences, making real-time acousto-optic imaging reachable. We will present the first acousto-optic images achieved with a long-lived spectral filter in Tm:YAG, in a scattering medium.
[1] Li, Y., Zhang, H., Kim, C., Wagner, K. H., Hemmer, P., & Wang, L. V. (2008). Pulsed ultrasound-modulated optical tomography using spectral-hole burning as a narrowband spectral filter. Applied physics letters, 93(1), 011111.
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