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Un dispositif de spectroscopie avec resolution du temps de vol des photons en milieu diffus a ete developpe. Celui-ci repose sur l utilisation d un continuum de lumiere blanche genere par focalisation d un laser amplifie (830 nm, 1 kHz, 0.5 W, 170 fs) dans de l eau demineralisee. Afin d optimiser spectralement et en puissance la source blanche sur la fenetre spectrale 600-800 nm, une etude de la mise en forme spatio-temporelle avant autofocalisation de l impulsion laser par le milieu a ete menee. Cette mise en forme est effectuee de maniere spatiale en changeant la focale de la lentille de focalisation et de maniere temporelle en changeant le taux de compression de l impulsion. L etude montre que le cone de lumiere emise possede plus de puissance dans la fenetre spectrale d interet pour des focales longues. Sur la fenetre 600-800 nm, le rendement energetique integre varie de 5%, avec une focale f = 6 cm, a 15%, avec une focale f = 30 cm. La mise en forme temporelle montre des effets similaires avec les memes ordres de grandeur.

International audience; Contrary to the intense debate about brain oxygen dynamics and its uncoupling in mammals, very little is known in birds. In zebra finches, picosecond optical tomography (POT) with a white laser and a streak camera can measure in vivo oxy-hemoglobin (HbO2) and deoxy-hemoglobin (Hb) concentration changes following physiological stimulation (familiar calls and songs). POT demonstrated sufficient sub-micromolar sensitivity to resolve the fast changes in hippocampus and auditory forebrain areas with 250 μm resolution. The time-course is composed of (i) an early 2s-long event with a significant decrease in Hb and HbO2, respectively -0.7 μMoles/L and -0.9 μMoles/L (ii) a subsequent increase in blood oxygen availability with a plateau of HbO2 (+0.3μMoles/L) and (iii) pronounced vasodilatation events immediately following the end of the stimulus. One of the findings of our work is the direct link between the blood oxygen level-dependent (BOLD) signals previously published in birds and our results. Furthermore, the early vasoconstriction event and post-stimulus ringing seem to be more pronounced in birds than in mammals. These results in bird, a tachymetabolic vertebrate with a long lifespan, can potentially yield new insights for example in brain aging.

Contrary to the intense debate about brain oxygen dynamics and its uncoupling in mammals, very little is known in birds. In zebra finches, picosecond optical tomography with a white laser and a streak camera can measure in vivo oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) concentration changes following physiologic stimulation (familiar calls and songs). Picosecond optical tomography showed sufficient submicromolar sensitivity to resolve the fast changes in the hippocampus and auditory forebrain areas with 250 μm resolution. The time course is composed of (1) an early 2-second-long event with a significant decrease in Hb and HbO2 levels of −0.7 and −0.9 μmol/L, respectively, (2) a subsequent increase in blood oxygen availability with a plateau of HbO2 (+ 0.3 μmol/L), and (3) pronounced vasodilatation events immediately after the end of the stimulus. One of the findings of our study is the direct link between blood oxygen level-dependent signals previously published in birds and our results. Furthermore, the early vasoconstriction event and poststimulus ringing seem to be more pronounced in birds than in mammals. These results in birds, tachymetabolic vertebrates with a long lifespan, can potentially yield new insights, e.g., into brain aging.

International audience; By assessing the cerebral blood volume and the hemoglobin oxygen saturation level, near-infrared spectroscopy (NIRS) probes brain oxygenation, which reflects cerebral activity. To develop a noninvasive method monitoring the brain of a songbird, we use an original NIRS device, i.e., a white laser coupled with an ultrafast spectrotemporal detector of optical signals without wavelength scanning. We perform in vivo measurements of the absorption coefficient and the reduced scattering coefficient of the caudal nidopallium area of the head of a songbird (the zebra finch).

A new ultrafast Diffuse Optical Tomography (DOT) has been developed for real time in vivo brain metabolism monitoring in songbird. The technique is based on space resolved time of flight measurements of the photons across the brain tissues. A three dimensional reconstruction of the brain activity is foreseeable by means of a double space and time sampling of the reflectance signal. The setup and the treatment procedure are described in depth and promising preliminary results showing the response of brain tissues to hypercapnia stimulations (increase of CO2) are presented.

Songbirds' vocal communication behaviour has been a favourite research domain for years. In order to study brain activation in response to acoustic stimuli, two different in-vivo methods that are able to assess neuronal activity based on detection of haemodynamic changes were compared within identical experimental settings. Optical imaging (non-invasive in-vivo broadband time-resolved spectroscopy) and functional magnetic resonance imaging (Blood Oxygenation Level Dependent MRI) were both performed in zebra finch brain upon a hypercapnic challenge known to induce a standard haemodynamic response. The use of these two methods offers the opportunity to compare their sensitivity and to correlate local variations in CBV and haemoglobin saturation level (obtained from optical data) with local BOLD signal variations (providing overall information on CBF, CBV and haemoglobin oxygenation). Both for optical imaging and fMRI, four adult female zebra finches (Taeniopygia guttata) were anaesthetized with 2% isoflurane while spontaneously breathing, and immobilized in a stereotactical device equipped with essential monitoring devices. Each bird underwent three runs of 5 minute hypercapnia (600 ml/min 7% CO2, 21% O2, 72% N2), interleaved by 20 minutes normocapnia (600 ml/min 21% O2, 79% N2). Optical imaging was performed with an ultrafast white laser and time-resolved spectrometer. With the 95% PL (i.e. mean path length of the light detected by the camera at 95% of the measured temporal profile) at 2 spectral windows centered at 748 nm and 793 nm, we quantify respectively the haemoglobin saturation level and CBV. MR imaging was performed at 300 MHz on a 7 Tesla in-vivo NMR microscope with horizontal bore and actively shielded gradient-insert (210 mT/m). The optical imaging data demonstrated that hypercapnia induced a significant increase of the hemoglobin saturation level in run1 (p=.01) and run2 (p=.01). No significant CBV changes were observed during run1 (p=.1), run2 (p=.75) and run3 (p=.5). The fMRI data demonstrated the existence of significant BOLD signal changes for run1 (p

In the field of biophotonics the main goals are the control and processing of in vivo biological tissues and the monitoring of biomolecule dynamics. Two particular “pitfalls” are present: the dynamic multiscale organization and the photostress of the medium. Until now the state of the art of the pico-femtosecond systems designed to these applications shows that the changing laser technology has been only used as an add-on. Our approach is based on a bottom-up procedure and on the medium-centered knowledge. The range of neurobiological applications of ultrafast photonics extends from TRP (time-resolved propagation) to linear and non-linear TRE (time-resolved emission). The device combines a one kilohertz chirp pulse amplification laser system and a single shot streak camera. For discrete wavelength applications (TRE), the set-up is a SHG/OPG/OPA 3 /SHG design. In the case of TRP, the beam is focused into pure water to generate a white light continuum. After propagation through tissue, a single-shot streak camera with single photo-electron counting capability performs the picosecond time-resolved spectroscopy of the collected photons. Depending on the acceptable level of photostress, the integration time can extend from 33ms up to several minutes with a real-time control of the jitter and time drifts. The meaning of the TRE spectro-temporal image is particularly detailed in the 450-480nm excitation window in regards to the contributions of mitochondrial flavoproteins. This optical system fulfills the reliability and the sensitivity, conditions required for measuring opto-electronic quantities from freely moving animal at low irradiation.

International audience; La spectroscopie résolue en temps utilisant un laser blanc femtoseconde est appliquée à la mesure in vivo des principaux absorbeurs du cerveau. Après génération adéquate du continuum de lumière blanche femtoseconde (50mW/[580-756nm] à 1 Hz), cette source se propage dans la calvaria, les méninges et le cortex chez le rat anesthésié. La transmission est étudiée sur 7mm de distance entre l impact laser et la fibre optique de collection. Le signal transmis est analysé dans la fenêtre 580-760nm, par un spectromètre couplé à une caméra à balayage de fente permettant la décorrélation de l absorption et de la diffusion.

Our purpose is to spectrally probe the main brain absorbers. The determination of their spatial distribution remains a challenge. According to anatomical data, the proposed 3D model of the rat pial-cortical vascular networks is divided into three parts: (1) the pial vessels could be approximated by a dense layer of around 250 micrometers depth; (2) the penetrating vessels repartition is described as periodic hexagonal prisms with three modules; (3) the capillary network is modelized using a periodic tiling of polyhedron with a density of 817mm.mm-3 and a branching pattern of 10000mm-3. With anaesthetized rats under stereotaxic conditions, in vivo time-resolved brain spectroscopy experiments are presented. The setup is designed to allow broadband time-resolved spectroscopy using a streak camera. A femtosecond white light continuum is produced by focusing 800nm pulses (0.5mJ, 1kHz, 150fs) in an adapted third order non linear medium. In the case of water, the spectrum expands over 380-780nm with an efficiency of 20 percent. Mathematical homogenization techniques could be applied to the radiative transfer equation with this geometrical vascular architecture and might be useful to analyze in depth time-resolved spectroscopy of such complex media.