■ Fast in depth Ultrasound Optical Tomography (UOT) of biological tissues with structured ultrasonic excitation

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Description of the PhD project

Imaging in depth biological tissues with Light for medical diagnosis in a major issue, e.g. early tumors detection, since acoustical and optical contrasts deliver complementary information.
Due to multiple scattering processes, a conventional optical imaging is limited beyond the mm range, corresponding to a transport mean free path (l*). The coupling between Light and Ultrasound (US) opens access to a local optical information (absorption, scattering) localized by the ultrasound, ballistic for frequencies beneath 20MHz. Typically, we perform images with 500 microns-resolution or less through scattering objects of several cm.
We have developed at Institut Langevin a coherent technique - Ultrasound Optical Tomography (UOT) - based on the acousto-optic effect: light crossing the US is partly shifted from the US frequency (so called the tagged-photons). UOT consists in counting the tagged-photons – a few in thick biological tissues: if the US locates in an optical absorber, their amount is reduced (light is absorbed), thus a scan of the US over the volume images the OPTICAL properties of the sample, LOCALIZED by the US. Detection of the tagged-photons is quite complex, since one has to compose with a speckle character at the output of the sample. We have developed adaptive holography techniques in real time (two-wave mixing) using non-linear materials (e.g. photorefractive crystals) sensitive within the diagnosis optical window (650-1100nm). The technique uses a US imaging device, meaning that we are able to recover an ultrasound and an optical image (bi-modal technique). Until recently, the US system performed a focused excitation, not optimized in terms of acquisition speed, and sensitivity. This is partly solved in exciting the US under plane waves with multi-angle incidences (international patent deposited), improving the acquisition speed by a factor of 30 [1], though with a loss of transverse resolution. We plan to recover this resolution still with a US structured plane waves excitation (2 patents have been recently deposited).
The development of those techniques will be an important point during the PhD (software, experimental development), in particular in designing custom UOT-probes (combination of US probes and optical input/output fibers), in the aim to push the method in-vivo on small animals with the help of biologist colleagues (collaborations are under progress). The concept will be used with an adaptive holographic [2] setup, but also with a more recent version that directly selects the tagged-photons using an ultra-narrow filter (10^-6nm) created within a rare-earth doped inorganic crystal at Helium temperature (spectral holeburning mechanism).

[1] Ultrafast acousto-optic imaging with ultrasonic plane waves,
Laudereau et al., Vol. 24, No. 4 | OPT. EXP. 2016
[2] Two-color interpolation of the absorption response for quantitative acousto-optic imaging,
M. Bocoum, Opt. Lett. 43, 399-402, 2018


UOT, Two-wave mixing, Phase-conjugation, Ultrasound Imaging, Photorefractive effect, Spectral Holeburning

Research unit

Langevin Institute Waves and Images – CNRS UMR 7587 – 1, rue Jussieu 75005 Paris

Description of the research Unit/subunit

The Institut Langevin is affiliated to ESPCI-ParisTech, CNRS UMR 7587. The Institut Langevin is a world leading team in the fields of ultrasound and optics. His first director, Mathias Fink, member of the French Academy of Science, has created an exemplary laboratory for innovation at the crossroad of different field of applications as medical imaging and therapy, non-destructive testing, audible acoustic, underwater acoustic or telecommunications. The Institut Langevin is the inventor of the concept of ultrasound ultrafast imaging, and is amongst pioneers in UOT techniques.

Name of the supervisor
François RAMAZ - Langevin Institute (francois.ramaz (arobase) espci.fr)

Name of the co-supervisor
Maïmouna BOCOUM - Langevin Institute (maimouna.bocoum (arobase) espci.fr)

3i Aspects of the proposal

The photonics project we plan to develop is potentially applicable in biotechnology, but some developments need to be performed further in laboratory in order to be mature. Once demonstrated in vivo (under progress), the different techniques we develop can have an important impact in biomedical imaging. In particular, a multi-wavelength UOT measurement can lead to important information on intrinsic markers as haemoglobin/oxy-haemoglobin (oxygen saturation ratio measurement), encountered in many pathologies. Our team has regular contacts with Thalès Research and Technology (TRT), where close problematics occur in ultrafast Radio-Frequency detection with an optical carrier, and LIDAR technologies.
This experimental project is at the crossroad of many fields, since it involves the monitoring of specific acoustic waves, optical methods of non-conventional wavefront adaptive holographic techniques using non-linear photosensitive crystals. From a technological point, the integration of optics and acoustics is an important aspect in the aim to realize a compact UOT probe, e.g. using optical fibers. At term, the optical detection could be implemented within a commercial ultrasound imaging device, in order to obtain a bi-modal imaging system. In a first step the project will be tested on calibrated scattering phantoms; at present, we are launching a process in order to be able to perform measurements on small animals and biopsies with biologist colleagues we are collaborating with.
The project is part of a national fundings (Inserm – Plan cancer) we have obtained last year, with four laboratories from the Paris Area, with whom we are in close contact for many years. Though without official fundings yet, our team is in close contact with European labs working on the subject (University College London - UK, Nottingham University – UK, DTU Fotonik Roskilde- Denmark, University of Lund – Sweden, Tyndall University Cork - Ireland). François Ramaz has spent two weeks at the University of Lund in november 2017 as an invited professor, in the group of Dr Stefan Kröll.

Expected Profile of the candidate

The project is mainly experimental. The candidate must be a physicist, he will have to manipulate light and ultrasound waves, in a lesser extent biological samples. A knowledge on physical optics is necessary for this project, since unconventional optical detections, photosensitive materials, are involved. The US sequences applied to the ultrasound imaging device are coded in a Matlab environment, as well as the optical detection via a fast acquisition board. The candidate needs to be familiar with such interfaces.

Important dates

Call for applications : from July 16th to September 17th 2018
Eligibility check results : Late September
3i Committee evaluation results : Late October
Interviews from the shortlisted candidates with the Selection Committee : Mid-December (week of December 10th)
Final results : Late December

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