Acousto-optic imaging techniques in scattering media

Acousto-optic imaging is a hybrid method that uses light to visualize objects embedded in thick diffusing media (> cm), with applications such as medical imaging for tumor detection.

Optical information is relevant because it complements data obtained by other medical imaging techniques by providing additional contrast to aid diagnosis, such as metabolism or species identification.

One parameter, considered important in many cases, is the oxygen saturation level in the blood, which can be obtained by measuring at several wavelengths, given that hemoglobin (Hb) and oxygenated hemoglobin (HbO) have different absorption spectra in the near infrared. However, the phenomenon of multiple scattering in these media prevents well-resolved direct imaging (i.e. sub-millimeter) when centimeter scanning is desired. We can, however, obtain local information by combining light and ultrasound, and take advantage of the ballistic nature of the latter to guide the optical measurement inside the medium by scanning their position, as a standard ultrasound scanner does.

However, this method, known as "acousto-optic imaging" or UOT for "Ultrasound Optical Tomography", is tricky to implement, as the optical signals collected are weak. What's more, the light emitted by this type of medium has a random spatial wavefront (known as "speckle"), which requires special processing.

We will begin by presenting the background to this type of imaging, with a brief review of existing medical imaging techniques. In order to provide as broad an overview as possible, we will outline the techniques of diffuse optical tomography and photoacoustic imaging, which are competing methods with acousto-optic imaging.

We will then give the theoretical foundations of the acousto-optic effect, in order to understand how we can access local optical information in the medium, by selecting "tagged" photons using ultrasound. These notions will enable us to understand the experimental configurations currently under development, namely adaptive interferometry with photorefractive crystals, digital holography, or spectral filtering of ultrasound-tagged photons based on the phenomenon of spectral holeburning.

The final chapter looks at the various forms of ultrasonic excitation applied to the medium, which can significantly improve the signal-to-noise ratio of the measurement.