High resolution spectral-domain optical coherence tomography at 1.3 micron center wavelength using a broadband superluminescent diode light source

International audienceWe present an ultrahigh resolution spectral-domain optical coherence tomography imaging system using a broadband superluminescent diode light source emitting at a center wavelength of 1.3 mm. The light source consists of two spectrally shifted superluminescent diodes that are coupled together into a single mode fiber. The effective emission power spectrum has a full width at half maximum of 200 nm and the source output power is 10 mW. The imaging system has an axial resolution of 3.9 µm in air (3.0 µm in biological tissue), and a lateral resolution of 6.5 µm. The sensitivity and the maximum line rate are 95 dB and 46 kHz, respectively. Images of an infrared viewing card and a cornea from human eye suffering from glaucoma showing Schlemm's canal are presented to illustrate the performance of the system

Chirurgie du segment antérieur de l'oeil et traitement du glaucome par laser femtoseconde et imagerie de tomographie par cohérence optique

Mon projet de thèse est étroitement lié au projet ANR NOUGAT, " Nouvel OUtil pour la chirurgie du Glaucome Assistée par laser femtoseconde et Tomographie par cohérence optique " (ANR-08-TECS-012), coordonné par le groupe " Optique, Photonique, Santé " (OPS) du Laboratoire d'Optique Appliquée (LOA), et dont les partenaires sont : l'hôpital Hôtel-Dieu de Paris, le laboratoire Charles Fabry de l'Institut d'Optique Graduate School, la société Amplitude Systèmes. Le glaucome est une maladie oculaire associée à une pression intraoculaire élevée. Les traitements chirurgicaux classiques visent à créer des canaux filtrants dans la sclère pour baisser la pression, mais les bénéfices ne sont souvent que temporaires. La greffe de cornée est indiquée lors d'opacités du tissu ou pathologies affectant la transparence cornéenne. Elle est largement pratiquée mais certaines limites existent, notamment lorsque la découpe doit se faire en profondeur d'un tissu pathologique. Mes travaux ont d'abord consisté à caractériser les trois principaux éléments constitutifs du segment antérieur de l'oeil : la cornée, la sclère et le cristallin. Suite aux recherches précédentes menées par notre équipe, la diffusion de la cornée est maintenant un phénomène compris et quantifiable. L'étude du tissu scléral a permis de mettre en évidence une fenêtre de relative transparence optique centrée à la longueur d'onde de 1650 nm ; des expériences préliminaires ont été menées sur le cristallin. Grâce à l'optimisation de sources laser existantes au sein de l'équipe, en particulier du générateur paramétrique optique, de nouvelles séries d'incisions ont ensuite été réalisées dans des cornées humaines (obtenues auprès de la Banque Française des Yeux), avec des longueurs d'onde situées entre 1500 et 1800 nm. Les analyses histologiques et par microscopie électronique en transmission et à balayage de ces découpes confirment que l'utilisation d'une longueur d'onde entre 1600 et 1700 nm réduit le phénomène de diffusion dans le tissu par rapport à la longueur d'onde appliquée dans les systèmes commerciaux (1000 nm). Par ailleurs, un dispositif d'imagerie par tomographie par cohérence optique (OCT) a été développé en étroite collaboration avec l'Institut d'Optique Graduate School. Utilisant une longueur d'onde de 1315 nm, ce dispositif a une résolution de l'ordre de 5 m, ce qui est compatible avec notre application ; cependant, il présente encore une rapidité d'acquisition moyenne. Les structures cornéennes ont été imagées avec trois dispositifs OCT disponibles, ce qui a permis de comparer leurs avantages et leurs inconvénients en vue de la combinaison de ce système d'imagerie à la source laser et d'une éventuelle application clinique. Mes travaux de recherche ont finalement permis d'optimiser le développement de systèmes chirurgicaux pour les tissus du segment antérieur de l'oeil et d'approfondir nos connaissances sur l'optique de ces tissus, ce qui vient compléter les études antérieures réalisées au sein de mon équipe de travail et ouvre la voie à des applications futuresMy PhD project is closely related to the ANR project named "NOUGAT" which consists of developing a new tool for glaucoma surgery assisted by femtosecond laser and optical coherence tomography (OCT) [ANR-08-TECS-012]. This project is coordinated by the "Optics -Photonics - Health" group of the Laboratory of Applied Optics (LOA) and our partners are the Hôtel-Dieu hospital, the Charles Fabry Laboratory of the Institut d'Optique Graduate School and the company Amplitude Systèmes. Glaucoma is an ocular disease associated with an increase of the intraocular pressure. Surgical treatments consist in creating filtrating canals in deep sclera in order to lower the pressure; however benefits are often only temporary. Corneal grafting is indicated when reduced corneal transparency (visual acuity) is observed. This is the most common transplant but some limits of the laser procedure exist, especially when the incision has to be made in the depth of a pathological tissue. First, my PhD work consists in the characterization of the three main elements of the ocular anterior segment: the cornea, the sclera and the crystalline lens. The light scattering occurring into the cornea is now well understood and it can be quantified. Studies of the scleral tissue have also shown an optical transparency window around 1650 nm, and some preliminary experiments have been performed on the crystalline lens. Thanks to the optimization of laser sources already developed in the group, especially the optical parametric generator, new series of incisions on human corneas from the French eye bank (Paris), have been performed with wavelengths in the 1500 - 1800 nm spectral range. Analysis by histology and by transmission and scanning electronic microscopy of these new incisions confirm that the use of a longer wavelength in the 1600 - 1700 nm range greatly reduces the light scattering in the tissue compared to the one obtained with commercial systems (at 1000 nm). In addition, a new imaging system based on OCT has been developed in close collaboration with our colleagues from the Institut d'Optique Graduate School. Using a wavelength of 1315 nm, the system has a spatial resolution of 5 m that is compatible with our medical application even if the acquisition rate stills quite low. Corneal structures have been imaged with three different available systems which makes the comparison of the specifications possible. Eventually, my PhD project leads to an optimization of laboratory surgical systems for the anterior segment of the eye and to the consolidation of knowledge on tissue optics, completing anterior studies conducted by my working group and opening new ways for future applicationsPALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Improving contrast and sectioning power in confocal imaging by third harmonic generation in SiOx nanocrystallites

International audienceWe present a new optical microscope in which the light transmitted by a sample-scanned transmission confocal microscope is frequency-tripled by SiOx nanocrystallites in lieu of being transmitted by a confocal pinhole. This imaging technique offers an increased contrast and a high scattered light rejection. It is demonstrated that the contrast close to the Sparrow resolution limit is enhanced and the sectioning power are increased with respect to the linear confocal detection mode. An experimental implementation is presented and compared with the conventional linear confocal mode

Mécanismes d'intéraction du laser femtoseconde avec le tissu

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Multiphoton microscopy at an excitation wavelength of 1.26 µm

International audienceWe present a multiphoton microscope using a Cr4 + :forsterite femtosecond laser with an emission wavelength of 1260 nm for the excitation of the multiphoton processes. This wavelength is well adapted to the "optical window" in biological tissues and permits to reach higher imaging depths than systems using more conventional titanium:sapphire laser sources. The paper describes the experimental set-up and reports on first results on human cornea and skin samples

Towards New Indices of Arterial Stiffness Using Systolic Pulse Contour Analysis: A Theoretical Point of View

International audienceTotal arterial stiffness plays a contributory role throughout aging and in numerous cardiovascular diseases, including hypertension. Aortic stiffening is responsible for an increased characteristic impedance (ie, the impedance to the left ventricular pulsatile flow), thus increasing the forward pressure-wave amplitude that contributes to pulse pressure elevation. Aortic stiffening also increases pulse wave velocity, and this results in anticipated and enhanced wave reflections, further augmenting central pulse pressure. Unfortunately, there is no simple time-domain estimate of characteristic impedance. Furthermore, recent guidelines have reviewed the limitations of diastolic pulse contour analysis to estimate arterial stiffness in the time domain. The present theoretical article proposes that systolic pulse contour analysis may provide new, simple time-domain indices quantifying pulsatile load in resting humans. Our proposal was mainly based on 2 simple, validated assumptions: (1) a linear aortic pressure-flow relationship in early systole and (2) a triangular aortic flow wave during systole. This allowed us to describe new time-domain estimates of characteristic impedance, pulsatile load (waveguide ratio), total arterial compliance, and total arterial stiffness. It is demonstrated that total arterial stiffness may be estimated by the following formula: [(Pi - DAP) × ST] / (SV × Δt), where Pi is the aortic pressure at the inflection point (peak forward pressure wave), DAP is diastolic aortic pressure, ST is systolic ejection time, SV is stroke volume, and Δt is the time-to-Pi. A mathematical relationship among time intervals and indices of pulsatile load is demonstrated, and the clinical implications are discussed in terms of cardiovascular risk and stroke volume prediction

Fourier-domain OCT at 1300 nm for glaucoma surgery monitoring

International audienc

Self-focusing and spherical aberrations in corneal tissue during photodisruption by femtosecond laser

International audienceThe use of ultrashort pulse lasers is current in refractive surgery and has recently been extended to corneal grafting (keratoplasty). When performing keratoplasty, however, permanent degradation of the optical properties of the patient's cornea compromises the penetration depth of the laser and the quality of the incisions, therefore causing unwanted secondary effects. Additionally, corneal grafting needs considerably higher penetration depths than refractive surgery. Little data are available about the interaction processes of the femtosecond pulses in the volume of pathological corneas—i.e., in the presence of spherical aberrations and optical scattering. We investigate the influence of the focusing numerical aperture on the laser–tissue interaction. We point out that at low numerical apertures (NAs), tissue damage is produced below and above the focal region. We attribute this phenomenon to nonlinear self-focusing effects. On the other hand, at high NAs, spherical aberrations become significant when focusing at high depths for posterior surgeries, which also limit the cutting efficiency. As high NAs are advisable for reducing unwanted nonlinear effects and ensure accurate cutting, particular attention should be paid to aberration management when developing clinical femtosecond lasers

Quantitative measures of corneal transparency, derived from objective analysis of depth-resolved corneal images, demonstrated with full-field optical coherence tomographic microscopy

International audienceLoss of corneal transparency, as occurs with various pathologies, infections, immune reactions, trauma, aging, and surgery, is a major cause of visual handicap worldwide. However, current means to assess corneal transparency are extremely limited and clinical and eye-bank practice usually involve a subjective and qualitative observation of opacities, sometimes with comparison against an arbitrary grading scale, by means of slit-lamp biomicroscopy. Here, we describe a novel objective optical data analysis-based method that enables quantifiable and standardized characterization of corneal transparency from depth-resolved corneal images, addressing the demand for such a means in both the laboratory and clinical ophthalmology setting. Our approach is based on a mathematical analysis of the acquired optical data with respect to the light attenuation from scattering processes in the corneal stroma. Applicable to any depth-resolved corneal imaging modality, it has been validated by means of full-field optical coherence tomographic microscopy (FF-OCT or FF-OCM). Specifically, our results on ex-vivo corneal specimens illustrate that 1) in homogeneous tissues, characterized by an exponential light attenuation with stromal depth (z), the computation of the scattering mean-free path (ls) from the rate of exponential decay allows quantification of the degree of transparency; 2) in heterogeneous tissues, identified by significant deviations from the normal exponential z -profile, a measure of exponential-decay model inadequacy (e.g., by computation of the Birge ratio) allows the estimation of severity of stromal heterogeneity, and the associated depth-dependent variations around the average ls enables precise localization of the pathology