Acousto-Optical Coherence Tomography (AOCT)

The combination of light and ultrasound to measure local optical properties through thick and highly scattering media is a tantalizing approach for breast cancer detection. Light is highly scattered within biological tissues, while the ultrasounds are ballistic. Thus, thanks to the acousto-optic effect, we can get the optical contrast information given by light and get the spatial localization from the ultrasound longitudinal waves. However, resolution can remain poor along the ultrasounds axis.

To get a millimetric axial resolution, the most common configuration is to apply a microsecond pulse regime to the ultrasounds.
We introduce here a new technique called Acousto-Optical Coherence Tomography (AOCT) that enables to get this millimetric axial resolution with continuous US and light beams by applying random phase jumps on both of them. The set up we use is based on the photorefractive effect to perform a self-adaptive wavefront holography with a GaAs bulk crystal and a single large area photodetector.

This technique is performed by application of a stochastic phase modulation on light and ultrasounds. By this way, we get a short coherence length ultrasound source that explores the sample. A time delay between the ultrasound and light modulation enables to select the active zone along the ultrasound column, where the acousto optic interferometric signal remains coherent in time.

It is a low coherence imaging technique like OCT (Optical Coherence Tomography) that involves “low coherence acousto optic sources”, i.e an optical source and an ultrasound source, which are both incoherent in time, but fully correlated together.

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