Volumetric optical Imaging of viscoelastic properties for Scoliosis and Cancer Observation – VISCO

The current limit on OCT acquisition speed is linked to the need to mechanically scan the sample or certain elements of the imaging system, which limits the use of high speeds.

The approach used in the VISCO project is to use a new OCT configuration that does not require any mechanical scanning: full-field Fourier domain OCT. This configuration is based on hyperspectral detection of the interferometric signal using a camera (in 2D), via a scan of the imaging wavelength so that to measure the interference spectrum for each pixel in the image. Using Fourier transform, it then becomes possible to transform the spectral information into axial information (structures in the depth of the sample).

However, there is no commercial source that allows for fast, high-resolution, high-contrast imaging in full-field Fourier domain OCT.

An initial development of the VISCO project therefore consisted of developing a new imaging source, using a supercontinuum laser source, an acousto-optic tunable filter (AOTF) capable of filtering a given wavelength at over 130 kHz, and a multimode fibre optical system capable of making the source spatially incoherent.

The second main method of the project was the development of a new detection principle for full-field Fourier domain OCT. To achieve imaging at 100 volumes per second, a camera capable of acquiring approximately 20,000 images per second is required. While such ultra-fast cameras do exist, they are about 10 times less sensitive than standard cameras and cannot image the small signals required for OCT. In this project, we have developed a new detection protocol based on the use of synchronous detection cameras, which can demodulate signals up to 250 kHz while maintaining good sensitivity.

We developed a new source module described above and demonstrated its performance. We then used this module, which enables ultra-fast wavelength scanning, to perform OCT imaging in two distinct modes:

-Time-domain OCT, for which rapid wavelength scanning can be used to shape a complex broad spectrum of any arbitrary spectral shape by scanning the camera's exposure time (in 10 ms of exposure time, we can scan up to 3,000 different wavelengths and adjust the relative exposure time of each wavelength to adjust the spectral power density as desired).

We were able to demonstrate 2D plane imaging with different spectra, and were able to demonstrate the advantage of this configuration for spectroscopic contrast imaging, allowing us to add a new imaging contrast.

-OCT in the Fourier domain. We were able to demonstrate 3D volume imaging at a few volumes/second (limited by the camera at this stage) thanks to the fast scanning of the source. Thanks to the source module, we were able to demonstrate a world record for axial resolution in full-field Fourier domain OCT, improving resolution by a factor of 5 compared to the state of the art, enabling the imaging of subcellular structures.

Finally, we were able to demonstrate the use of a new light detection strategy in full-field OCT through the use of a synchronous detection camera and various signal modulation techniques.

By combining these two elements, the aim is to produce 3D volumetric imaging and perform dynamic OCT and 3D elastographic imaging in a matter of seconds, thereby improving imaging speed by a factor of 100 compared to the state of the art.

This will make it possible to measure the viscoelastic properties of samples in parallel with the cellular activity of a tissue, thereby providing a better understanding of the link between cell activity and the mechanical properties of tissues.

The microscope developed will thus enable the correlation between tissue architecture, the physiology of the cells that compose it, and the viscoelasticity of the extracellular matrix and cells, and thus study the relationships and dependencies between these different properties.

One of the key biophysical questions to be studied is how the viscoelastic properties of tissues can be used to transmit biological signals. Two pathological cases, the development of invasive tumours and the development of the body axis in idiopathic scoliosis, will be studied in particular using a zebrafish model. Coming soon

Obtaining a precise quantitative measurement of viscoelastic properties of cells and their microenvironment is of major interest for fundamental biology, medical diagnosis, and physics of wave propagation. Nonetheless, there is no imaging technique available to perform such measurements at the cellular scale. The VISCO project aims to develop 3D label free optical measurement of viscoelastic properties in thick biological tissues at the cellular scale, and combine this measurement with cell physiology. We will build a new optical microscope, based on a high speed volumetric spectral domain full field optical coherence tomography system, to obtain fast 3D displacement maps and reconstruct the local elasticity and viscosity at the cellular scale. We will use this microscope to improve the diagnosis ability of histopathology, including in the case of human tumor breast samples, and evaluate the role of differential muscle elasticity in a zebrafish idiopathic scoliosis model.

More precisely, full field optical coherence tomography relies on 2D recordings of a low coherence interference between the light backscattered by the sample and a reference arm. The low coherence enables to localize the interferences, so that only the signal backscsattered at a given depth can interfere, and be measured. When the spectral dependency of these interference patterns is also measured, the axial information can be recovered by a simple Fourier transform. The VISCO project aims to create a microscope based on such principle, and that will be able to switch between a high resolution configuration and a high speed configuration. Ultimately, acquisitions at 20, 000 volumes/s will be achieved at a resolution of 2x2x4µm3 .

This microscope will achieve high speed and high resolution volumetric optical coherence tomography. The analysis of the signal temporal fluctuations will enable to reveal living cells inside scattering tissue, because the active transport mechanisms inside the cells create fast and organized fluctuations. Such analysis therefore permits to obtain a contrast related with cell metabolism and physiology, which regulate the active transport mechanisms. The VISCO project will demonstrate the first simultaneous 3D dynamic full field optical coherence tomography.

Moreover, acquiring volumetric scattering data enable to measure induced local deformations, giving access to the viscoelastic properties of tissues. The transverse deformations will be measured by 2D image correlations, while nanometric axial deformations will be measurable via the phase differences between successive planes.

Therefore, various mechanical stresses will be applied on the samples, either using large scale shearing, or by generating shear waves. The local mechanical strain tensor will then be measured using the new microscope in order to measure rheological properties of biological tissues at the cellular scale. Thanks to the 3D measurements, the full strain tensor will be accessible, for various applied stresses, enabling to test new models of tissue biomechanics.

The developed microscope will allow correlating the tissue architecture, cell properties, as well as tissue and cell viscoelastic properties, hence to study the relationship between these metrics. One of the essential biophysics questions that will be addressed by the VISCO project is how the viscoelastic properties of tissues can modify, or enable biological signal transduction. Several pathological cases will be studied, including the case of invasive tumor development, and body axis abnormal development, in the case of idiopathic scoliosis.