Feature Of The Week 11/27/11: Langevin Institute and LLTech Investigate In Vivo Cellular Full-Field OCT Imaging with a Rigid Endoscopic Probe
Full-Field OCT (FFOCT) is a particular approach of OCT that directly takes “en face” 2-D images with an isotropic resolution around 1µm . With such a high resolution FFOCT systems can produce images that are similar to that obtained with classical histology procedures and can thus be important tools for pathology . This is why we want to combine the interest of a setup with a needle probe with the performances of FFOCT.
A FFOCT system with probe has to address the problem of keeping the performances of FFOCT in a setup using a miniaturized, medically safe probe. As proposed in  FFOCT systems with a probe rather use two coupled interferometers, a processing interferometer external to the probe and a distal interferometer at the end of the probe in contact with the sample [4,5]. Indeed an endoscopic FFOCT system with only one interferometer would require setting identical probes in both arms of the Linnik interferometer , which would induce very large optical path lengths difficult to balance. On the contrary, in a system with two interferometers the probe is not part of an interferometer arm and is only used to transport an image. It is thus entirely passive and insensitive to its environment. Such a system is to privilege for in situ imaging, where one needs a system able to image outer or inner parts of the body that are difficult to reach.
However to our knowledge no FFOCT setup with probe suitable for in situ and in vivo biomedical endoscopic imaging has already been demonstrated. In our system we decided to use a Xenon arc lamp as a source with very low temporal and spatial coherence. Indeed the temporal incoherence ensures a good axial resolution, whereas the spatial incoherence increases the sensitivity by decreasing cross-talk effects. Moreover we also use a very simple common-path distal interferometer. The advantage compared to scanning system is that it does not require any advanced miniaturized mechanical systems at the tip of the probe, which are likely to increase the diameter as well as the complexity of the probe. Our simple design is well-suited for in situ imaging.
It can be used with different probes without changing the bulk setup. Our first results were obtained with a rigid probe based on a Graded-Refractive-INdex (GRIN) lens assembly with a diameter of 2 mm and a length of 150 mm. The transverse resolution of the system depends on the probe optics, it was measured to be 3.5 µm in air. The axial resolution depends directly on the coherence length of the light source; by measuring the FWHM of the fringe envelope it was found to be 1.8 µm in water. Sensitivity was experimentally evaluated at -80 dB was obtained when averaging 20 images during about 1 second, which should be enough to get signal from biological tissues.
Fixed human breast samples were imaged ex vivo showing strong backscattering connective tissue as well as adipocytes. Preliminary in vivo experiments gave promising results on human skin. Images taken on the lip and forearm of a healthy volunteer reveal large-scale structures of the tissue as well as fine details such as epithelial cells.
The design of our probe, allowing diameters ranging from less than 1 mm to a few mm, makes it suitable for in situ imaging of different accessible areas, such as skin, prostate, bladder, brain. In the mean time this system could also be implemented with a flexible probe based on a fiber bundle. In comparison with the rigid probe the image quality would be degraded due to the pixelation effect of the fibers, but a system with a flexible probe would allow for imaging of areas inaccessible with the rigid probe, for instance in the aero-digestive and gastrointestinal tracts.
For more information see recent Article. Courtesy Anne Latrive from Langevin Institute, ESPCI and LLTech.
1. Dubois and A.C. Boccara, “Full-field optical coherence tomography” in Optical coherence Tomography, E.D. W. Drexler, J. G. Fujimoto (Springer, 2009).
2. M. Jain, N. Shukla, M. Manzoor, S. Nadolny, and S. Mukherjee, “Modified full-field optical coherence tomography: A novel tool for rapid histology of tissues”, Journal of Pathology Informatics 2, (2011).
3. H.D. Ford, R. Beddows, P. Casaubieilh, and R.P. Tatam, “Comparative signal-to-noise analysis of fibre-optic based optical coherence tomography systems” Journal of Modern Optics 52, 1965-1979 (2005).
4. W.-Y. Oh, B.E. Bouma, N. Iftimia, R. Yelin, and G.J. Tearney, “Spectrally-modulated full-field optical coherence microscopy for ultrahigh-resolution endoscopic imaging”, Optics express 14, 8675-8684 (2006).
5. H.D. Ford and R.P. Tatam, “Fibre imaging bundles for full-field optical coherence tomography”, Measurement Science and Technology 18, 2949-2957 (2007).