KEYWORDS: Red blood cells, Oxygenation, Microscopy, Tissues, Absorption, Third harmonic generation, Sum frequency generation, Spatial resolution, Resonance enhancement, Multiphoton microscopy
We present color third-order sum-frequency generation (color TSFG) microscopy, a multiphoton imaging strategy based on the simultaneous detection of several third-order coherent signals produced by two synchronized femtosecond pulse trains. We demonstrate that it can be used to obtain red blood cell (RBC)-specific label-free contrast in live zebrafish and is a promising tool for probing RBC oxygenation.
Solar ultraviolet longwave UVA1 exposure of human skin has short-term consequences at cellular and molecular level, leading at long-term to photoaging. Following exposure, reactive oxygen species (ROS) are generated, inducing oxidative stress that might impair cellular metabolic activity. However, the dynamic of UVA1 impact on cellular metabolism remains unknown because of lacking adequate live imaging techniques. Here we assess overtime the UVA1- induced metabolic stress response in reconstructed human skin with multicolor two-photon fluorescence lifetime microscopy (FLIM). Simultaneous imaging of the two endogenous biomarkers nicotinamide adenine dinucleotide (NAD(P)H) and flavin adenine dinucleotide (FAD) by wavelength mixing allows quantifying cellular metabolism in function of NAD(P)+/NAD(P)H and FAD/FADH2 redox ratios We measure NAD(P)H and FAD fluorescence lifetime and fraction of bound coenzymes both in keratinocytes in the epidermis basal layer and in fibroblasts in the dermis superficial layer. After UVA1 exposure, we observe an increase of fraction of bound NAD(P)H and decrease of fraction of bound FAD indicating a metabolic switch from glycolysis to OXPHOS or oxidative stress possibly correlated to ROS generation. NAD(P)H and FAD biomarkers have unique temporal dynamics and sensitivities to skin cell types and UVA1 dose. While FAD biomarker is UVA1 dose-dependent in keratinocytes, NAD(P)H biomarker shows earlier time points modulation in fibroblasts, thus reflecting different skin cells sensitivities to oxidative stress. Finally, we show that a sunscreen including a UVA1 filter MCE prevents UVA1 metabolic stress response from occurring.
Many questions in developmental biology and neuroscience require tissue-scale measurements of multiple cell parameters. Recent progress in in-depth and large-scale microscopy approaches are transforming brain imaging, but generally lack efficient color contrast modalities. After introducing the basic concepts of multiphoton microscopy, the talk will discuss ongoing work aiming at augmenting the information content of multiphoton microscopy of tissues.
First, we will present chromatic multiphoton serial (ChroMS) microscopy (1), a recent approach combining trichromatic two-photon (2P) excitation through wavelength mixing and microtome-based serial block-face image acquisition to acquire large-volume images of color-labelled brain tissue. This approach provides micrometric color imaging with constant resolution over the entire imaged volume. We illustrate its potential of ChroMS for several types of measurements, such as color-based morphological, clonal and connectivity analyses. Then, we will discuss three-photon (3P) microscopy, a recent approach providing superior imaging depth than 2P microscopy in highly scattering tissues, but requiring the development of novel infrared MHz OPA sources (2). Finally, we will discuss the combination of multiphoton fluorescence imaging with coherent contrast modalities providing complementary information such as third harmonic generation (THG) (3,4).
References
(1) Abdeladim et al, Nat Comm (2019)
(2) Guesmi et al, Light Sci App (2018)
(3) Morizet et al, Optica (2019)
(4) Morizet et al, Optica (2021)
We propose a framework to quantify photodamage in multiphoton light-sheet microscopy. Using cardiac imaging in live zebrafish embryos, we demonstrate an order of magnitude signal enhancement is safely obtained by adjusting the laser repetition rate.
Light-sheet illumination enables major increase in multiphoton imaging speed for in vivo studies. However, photoperturbation in multiphoton light-sheet microscopy remains poorly investigated. We show here that the heart beat rate of zebrafish embryos is a sensitive probe of linear and nonlinear photoperturbations. By analyzing its behavior with respect to laser power, pulse frequency and wavelength, we derive guidelines to balance signal and photoperturbation. We then demonstrate one order-of-magnitude signal enhancement over previous implementations by optimizing the laser pulse frequency. These results open new opportunities for fast in vivo imaging.
Light-sheet fluorescence microscopy is a method of choice for multiscale live imaging. Indeed, its orthogonal geometry results in high acquisition speed, large field-of-view and low photodamage. Its combination with multiphoton excited fluorescence improves its imaging depth in biological tissues. However, it appears femtosecond laser sources commonly used in multiphoton microscopy at an 80 MHz repetition rate may not be optimized to take full advantage of light-sheet illumination during live imaging. Hence, we investigated the nature of induced photodamage in multiphoton light-sheet microscopy and the influence of laser parameters on the signal-to-photodamage ratio. To this end, we used zebrafish embryonic heart beat rate and fluorophore photobleaching as sensitive reporters of photoperturbations. We characterized linear and nonlinear disruptions depending on laser parameters such as laser mean power, pulse frequency or wavelength, and determine their order and relative impact. We found an optimal pulse frequency of ~10 MHz for imaging mCherry labeled beating hearts at 1030 nm excitation wavelength. Thus, we achieved high-speed imaging without inducing additional linear heating or reaching nonlinear photodamage compared to previous implementation. We reach an order-of-magnitude enhancement in two-photon excited fluorescence signal by optimizing the laser pulse frequency while maintaining low both the laser average power and its peak irradiance. It is possible to reach even larger enhancement of 3- photon excited fluorescence using such laser parameters. More generally, using low laser pulse frequency in multiphoton light-sheet microscopy results in a drastic improvement in signal level without compromising live sample, which opens new opportunities for fast in vivo imaging.
Large-scale microscopy approaches are transforming brain imaging, but currently lack efficient multicolor contrast modalities. We address this issue by introducing chromatic multiphoton serial (ChroMS) microscopy, a method combining multicolor multiphoton excitation through wavelength mixing and microtome-assisted serial block-face image acquisition. This approach delivers large-scale micrometric imaging of spectrally distinct fluorescent proteins with constant micrometer-scale resolution and sub-micron channel registration over the entire imaged volume. We achieve multicolor 3D imaging over several cubic millimeters and brain-wide serial 2D multicolor imaging. We illustrate the potential of this method for several novel types of measurements interesting for region-scale or whole brain studies: (i) color-based analysis of astrocyte morphology and spatial interactions in the mouse cerebral cortex, (ii) tracing of densely labeled neurons, and (iii) brain-wide mapping of axonal projections labeled with distinct tracers.
In bone tissue, multiscale interfaces provide the structural basis of essential bone functions and contribute to its macroscopic mechanical properties. The lacuno-canalicular network (LCN) hosting the osteocytes in the bone matrix, in particular, represents a biological signature of the mechanotransduction activity in response to external biomechanical loading. We have demonstrated that label-free third-harmonic generation (THG) microscopy reveals the structure of the LCN in 3D with submicron precision over millimetric fields of view compatible with histology and can be coupled to second-harmonic generation (SHG) signals relating to the collagen organization in the bone matrix. Taking advantage of these label-free imaging methods, we investigate the impact of microgravity on the LCN structure in mice following a 1- month space flight. We show that our current lack of understanding of the extent of the LCN heterogeneity at the organ level hinders the interpretation of such investigations based on a limited number of samples and we discuss the implications for future biomedical studies.
The application range of P-THG microscopy has been so far restricted to studies on molecular order and anisotropy of static specimen removed from their biological environment. Slow polarization commutation limits the investigation of highly dynamic systems because of motion artifacts. Here we have developed a new fast-P-THG microscope enabling efficient in vivo studies in dynamic biological samples. Our P-THG scheme benefits from a built-in EOM that switch polarization states at kilohertz between image lines to provide artefact-free P-THG images with micrometric resolution. Furthermore, we have developed a fast Fourier analysis enabling rapid P-THG processing to quantify lipid order and angular maps. We demonstrated that fast-P-THG is suitable in two major applications. Using first a linear polarization configuration, fast P-THG imaging revealed molecular order changes in MLVs undergoing phase-transition upon heating despite sample distortions. Anisotropy properties of small endogenous microparticles swimming in the otolith cavity embryos were also reported in early zebrafish embryos. A second configuration with linear-circular polarization commutation enabled efficient detection of birefringent media such as anisotropic vesicles in C-elegans gut cells.
Over the past decade the range of available excitation wavelengths used in nonlinear microscopy has continuously extended within the near infrared window. Nowadays, excitation wavelengths ranging from 750 nm to 1300 nm are routinely used to perform multi-parametric imaging [1,2] and multiple wavelength excitation are used in many techniques, including multicolor 2-photon excited fluorescence imaging, Coherent Anti-stokes Raman Scattering (CARS), Stimulated Raman Scattering (SRS) or sum-frequency generation. While this trend opens new perspectives and applications in the biomedical sciences, it also raises new technical issues. For instance, it calls for new standards for quantifying and comparing the performances of nonlinear microscopes over a broad range of wavelengths. In particular, microscopes equipped with multiple femtosecond sources spanning the entire near-infrared wavelength range are often problematic to characterize with current approaches based on fluorescent probes.
In this study, we present a new and straightforward method to quantify the imaging properties of nonlinear microscopes over a broad range of excitation wavelengths [3]. We show that harmonic generation nanoprobes are a unique tool to map the spatial resolution, field curvature and chromatic aberrations of nonlinear microscopes with a precision below the diffraction limit, across the whole field of view, and with a single calibration sample. We analyze and compare measurements obtained with several microscope objectives designed for multiphoton microscopy over the 850-1100nm wavelength range. Finally, we discuss strategies to minimize the impact of chromatic aberrations during multicolor imaging and we show how our metrology can be used for the post-acquisition correction of chromatic aberrations.
[1] Mahou et al. Nat. Methods 9, 815-18 (2012).
[2] Alexander et al. Curr. Opin. Cell Biol. 25 , 659-71 (2013)
[3] Mahou et al., submitted.
Two-photon laser scanning microscopy has become a standard to map thick and live tissues. However, its application for fast and multicolor imaging remains challenging. To address this issue, we report on the implementation of mixed wavelength excitation in a two-photon light-sheet microscope. We illustrate the potential of the technique by recording sustained multicolor two-photon movies of the beating heart in zebrafish embryos with negligible photobleaching at 28 million pixels/second. In particular, 3D reconstructions of the heart periodic motion are obtained with sufficient spatiotemporal resolution to track the fast movements of individual cells during a cardiac cycle.
Nonlinear optical microscopy is a biocompatible avenue for probing ordered molecular assemblies in biological tissues. As in linear optics, the nonlinear optical response from ordered systems is polarization-sensitive. This dependence can be used to identify and characterize local molecular ordering with micrometer-scale 3D resolution in a nonlinear microscope. In particular, third-harmonic generation (THG) microscopy is a nonlinear optical modality sensitive to the electronic nonlinear susceptibility χ(3) of a material. THG microscopy can be used to map χ(3) spatial variations (i.e. material interfaces), and to probe birefringence. In principle, polarization-resolved THG (P-THG) can therefore be used to probe ordered molecular arrays. However, the orientation, distribution, and nonlinear optical properties of the molecules near the beam focus all affect the detected signal. It is therefore necessary to develop a theoretical method which decouples these effects and permits the extraction of orientational information from P-THG images. In this report, we first present P-THG images of model systems (lipid droplets, multilamellar lipid vesicles) and biological tissues (human skin biopsy) which establish that P-THG is sensitive to lipid ordering and that it is maximized when excitation polarization is parallel to the ordered lipid molecules, giving impetus for the development of a thorough theoretical analysis. We then outline a multiscale model spanning the molecular (nm) and ensemble (μm) scales predicting the PTHG signal, consisting of three main steps: (i) calculation of the molecular electronic hyperpolarizability; (ii) determination of the anisotropic χ(3) for various molecular distribution parameters; and (iii) numerical calculations of the P-THG signal from lipid-water interfaces. This analysis links the measured P-THG response to lipid molecular structure and ordering.
Modal sensorless adaptive optics relies on the use of an image quality metric to estimate the amplitude of
aberrations, and of a well-suited set of aberration modes to describe the aberration. This set is chosen so that
aberration of one mode does not influence correction in another mode. In this paper, we show how these modes
can be derived experimentally, and investigate the influence of imperfect crosstalk removal on the accuracy of
correction. We show that the resulting error can be mitigated using appropriate algorithms that can incorporate
knowledge of the influence of the modes on the metric and, if available, partial knowledge of the aberrations.
Finally, we derive from these results the minimum time required for correction in various situations.
We investigate the parameters governing the accuracy of correction in modal sensorless adaptive optics for
microscopy. In this paper we focus on the case of two-photon excited fluorescence. Using analytical, numerical
and experimental results, we show that using a suitable number of measurements, accurate correction can
be achieved for up to 2 rad rms initial aberrations even without optimisation of the correction modes. We
demonstrate that this correction can be achieved using low light levels to minimise photobleaching and toxicity,
and we provide examples of such optimised correction.
Accurate control over the phase and amplitude modulation in an adaptive microscope is essential to the quality
of aberration correction that can be achieved. In this paper we present a robust and compact method for
characterising such amplitude and phase modulation in the pupil plane of the focussing objective. This method,
based on phase diversity, permits calibrating the microscope as a whole and thus avoids errors in the alignment
of the wavefront shaping device after calibration and the resulting imprecision in the induced modulation: by
acquiring three 2D images of the point spread function at different distances from the focal plane, we show that
the electric field distribution at the pupil plane can be retrieved using an iterative algorithm. We have applied
this technique to the characterisation of the phase modulation induced by a deformable mirror when conjugated
with the entrance pupil of different objectives, which permits accurate evaluation of the performance of the
mirror for subsequent aberration correction.
Dermal fibroblasts are responsible for the generation of mechanical forces within their surrounding extracellular matrix and can be potentially targeted by anti-aging ingredients. Investigation of the modulation of fibroblast contraction by these ingredients requires the implementation of three-dimensional in situ imaging methodologies. We use multiphoton microscopy to visualize unstained engineered dermal tissue by combining second-harmonic generation that reveals specifically fibrillar collagen and two-photon excited fluorescence from endogenous cellular chromophores. We study the fibroblast-induced reorganization of the collagen matrix and quantitatively evaluate the effect of Y-27632, a RhoA-kinase inhibitor, on dermal substitute contraction. We observe that collagen fibrils rearrange around fibroblasts with increasing density in control samples, whereas collagen fibrils show no remodeling in the samples containing the RhoA-kinase inhibitor. Moreover, we show that the inhibitory effects are reversible. Our study demonstrates the relevance of multiphoton microscopy to visualize three-dimensional remodeling of the extracellular matrix induced by fibroblast contraction or other processes.
We use a vector field model to analyze the third-harmonic generation (THG) emission patterns for isolated objects
illuminated by a Gaussian beam. Simulations and experiments indicate that THG from biological (dielectric) structures is
essentially forward-directed, as opposed to e.g. THG from gold particles. We then address the issue of epidetecting
forward-emitted light backscattered in a turbid medium. We use Monte Carlo simulations and measurements to analyze
the effect of tissue properties (absorption, scattering), and of the geometry of the collecting optics. This analysis provides
guidelines for optimizing epidetection in coherent nonlinear microscopy.
An erratum is attached.
We used intrinsic Second Harmonic Generation (SHG) by fibrillar collagen to visualize the three-dimensional
architecture of collagen fibrosis at the micrometer scale using laser scanning nonlinear microscopy. We showed that
SHG signals are highly specific to fibrillar collagen and provide a sensitive probe of the micrometer-scale structural
organization of collagen in tissues. Moreover, recording simultaneously other nonlinear optical signals in a multimodal
setup, we visualized the tissue morphology using Two-Photon Excited Fluorescence (2PEF) signals from endogenous
chromophores such as NADH or elastin. We then compared different methods to determine accurate indexes of collagen
fibrosis using nonlinear microscopy, given that most collagen fibrils are smaller than the microscope resolution and that
second harmonic generation is a coherent process. In order to define a robust method to process our three-dimensional
images, we either calculated the fraction of the images occupied by a significant SHG signal, or averaged SHG signal
intensities. We showed that these scores provide an estimation of the extension of renal and pulmonary fibrosis in
murine models, and that they clearly sort out the fibrotic mice.
Third-harmonic generation (THG) microscopy can provide structural information from unstained biological samples such as developing embryos. However, the contrast mechanisms in THG imaging need to be better characterized in order to develop practical applications. We studied experimentally and theoretically the influence of sample structure and excitation NA (Rayleigh length) on THG signals for various cases (spheres, interfaces). Because the third-harmonic signal critically depends on the spatial distribution of the Gouy shift, the effect of changing the excitation NA depends on the sample geometry within the focal volume. This phenomenon can be used to highlight certain structures within a complex system. Finally, we measured the nonlinear optical properties of several liquids, and we identified lipid bodies as an important source of contrast in biological THG imaging. We show that the technique can be used to characterize lipid accumulation in a variety of cells and tissues.
We recorded multiphoton images of human skin biopsies using endogenous sources of nonlinear optical signals. We detected simultaneously two-photon excited fluorescence (2PEF) from intrinsic fluorophores and second harmonic generation (SHG) from collagen. We observed SHG from fibrillar collagens in the dermis, whereas no SHG was detectable from the non fibrillar type IV collagen in the basal laminae. We compared these distinct behaviours of collagens I and IV in SHG microscopy to polarization-resolved surface SHG experiments on thin films of collagens I and IV molecules. We observed similar signals for both types of molecular films, except for the chiroptical contributions which are present only for collagen I and enhance the signal typically by a factor of 2. We concluded that SHG microscopy is a sensitive probe of the micrometer-scale structural organization of collagen in biological tissues. In order to elucidate the origin of the endogenous fluorescence signals, we recorded 2PEF spectra at various positions in the skin biopsies, and compared these data to in vitro spectroscopic analysis. In particular, we studied the keratin fluorescence and determined its 2PEF action cross section. We observed a good agreement between 2PEF spectra recorded in the keratinized upper layers of the epidermis and in a solution of purified keratin. Finally, to illustrate the capabilities of this technique, we recorded 2PEF/SHG images of skin biopsies obtained from patients of various ages.
The regulation of morphogenetic movements that shape an embryo during its development remains a challenging issue in developmental biology, and may in certain cases involve mechanical sensitivity. Addressing this issue requires novel experimental approaches. We show that the combination of femtosecond laser pulse-induced ablation and multiphoton microcopy can be used to modulate and quantify morphogenetic movements in Drosophila embryos. We characterized the effects of focused nanoJoule pulse trains in developing embryos. We used targeted ablations to locally modify the embryo structural integrity and modulate morphogenesis. Femtosecond-pulse induced ablation was combined with nonlinear microscopy based on two-photon-excited fluorescence (2PEF) and third-harmonic generation (THG).Correlation-based analysis of microscopy data allowed us to track the outcome of ablations and to analyze tissue deformations. These experiments provided insight into the interplay between gene expression and tissue deformations in developing embryos.
Lanthanide-ion doped oxide nanoparticles were functionalized for use as fluorescent biological labels. These nanoparticles are synthesized directly in water which facilitates their functionalization, and are very photostable without emission intermittency. Nanoparticles functionalized with guanidinium groups act as artificial toxins and specifically target sodium channels. They are individually detectable in cardiac myocytes, revealing a heterogeneous distribution of sodium channels. Functionalized oxide nanoparticles appear as a novel tool particularly well adapted to long-term single-molecule tracking.
Animal embryo development exhibits a complex ensemble of cell movements that are tightly regulated by developmental gene expression. It was proposed recently that mechanical factors may also play an important role during development. Investigating these dynamical processes is technically challenging and requires novel in vivo investigation methods. We show that multiphoton microscopy can be used for both perturbing and analyzing morphogenetic movements in vivo. (i) nonlinear microscopy is well adapted for the sustained imaging of early Drosophila embryos despite their highly scattering nature; (ii) femtosecond pulse-induced ablation can be used to process specific tissues in vivo. Combining this approach with multimodal microscopy (two-photon-excited fluorescence (2PEF) and third-harmonic generation (THG)), we report the successful quantitative modulation of morphogenetic movements in vivo. Our data provides insight to the issue of morphogenesis regulation.
We report two novel applications of multiphoton microscopy for pharmacological studies of unstained cardiovascular tissue. First, we show that second harmonic generation (SHG) microscopy of unstained cardiac myocytes can be used to determine the sarcomere length with sub-resolution accuracy, owing to the remarkable contrast of the SHG signal originating from myosin filaments. A measurement precision of 20 nm is achieved, taking the sample variability into account. We used this technique to measure sarcomere contracture in the presence of saxitoxin, and results were in agreement with mechanical measurements of atrial tissue contracture. Second, we characterized multiphoton microscopy of intact unlabeled arteries. We performed simultaneous detection of two-photon-excited fluorescence (2PEF) from elastin laminae and SHG from collagen fibers upon 860 nm excitation. Combined 2PEF/SHG images provide a highly specific, micron scale description of the architecture of these two major components of the vessel wall. We used this methodology to study the effects of lindane (a pesticide) on the artery wall structure and evidenced structural alteration of the vessel morphology.
Animal embryo development exhibits a complex choreography of cell movements highly regulated both in time and space. This sequence of morphogenetic movements is initiated at gastrulation and is tightly controlled by a cascade of developmental gene expression. We have recently reported that developmental gene expression can in turn be mechanically regulated by morphogenetic movements during Drosophila melanogaster early development. In order to study this phenomenon of mechanically induced gene expression, it is necessary to develop new techniques of in vivo investigation. We show that the combination of femtosecond pulse intratissue surgery and two-photon-excitation fluorescence (2PEF) microscopy is a powerful tool for (i) disrupting natural morphogenetic movements and (ii) imaging native and disrupted morphogenetic movements during Drosophila development. (i) First, non-linear-absorption-mediated photo-disruption makes it possible to perform controlled intra-vital micro-dissections resulting in the modulation of morphogenetic movements and subsequent mechano-sensitive gene expression. (ii) Second, in vivo 2PEF microscopy of transgenic GFP systems appears to be an excellent technique for long-term in vivo imaging of the complex morphogenetic movements involved in normal or perturbed Drosophila gastrulation. Together, these two techniques provide a powerful novel approach to study embryo development.
The use of chiral harmonophores in second harmonic generation (SHG) microscopy of lipid bilayers should enable one to obtain a signal even when the distribution of the chromophores is centrosymmetric. In order to determine optimal chiral molecules, we performed polarization-resolved second harmonic reflection experiments. We found that chirality must arise from an excitonic coupling rather than from an asymmetric center. We selectively labeled giant unilamellar lipid vesicles and cell membranes with such a molecule, namely an acridine substituted Troger's base, as demonstrated by two-photon-excited fluorescence microscopy. We performed preliminary SHG microscopy experiments, but the poor efficiency of the current form of our molecule does not allow us to demonstrate chirality effects.
We investigate tissue and instrument parameters affecting the penetration depth in two-photon microscopy. We show that the temporal redistribution of the same average power into fewer pulses of higher peak energy by means of a regenerative amplifier results in an increase in excitation depth by approximately 2-3 scattering mean free paths. We then measure the excitation scattering mean free path in vitro, using rat brain slices, as a function of the excitation wavelength and tissue age. We find that young-animal tissue (< P18) is two-fold less scattering than adult tissue (P90). We quantify the fall-off of the collected fraction of generated fluorescence in a backward detection geometry, in vivo. At large depths, we observe that the collected fraction scales as the angular acceptance squared (related to the effective field-of-view) of the detection optics. Matching the angular acceptance of the detection optics to that of the objective (63X NA-0.90) results in a factor 3-4 of the collected fluorescence. The collection efficiency can be further increased (10X) by using an objective with large field-of-view and high numerical aperture (20X NA-0.95). These gains translate into approximately 120 micrometers additional depth penetration when working in the rat brain in vivo with a standard Ti:sapphire source.
We present an interference microscope for non-invasive 3D imaging of biological tissues with a resolution better than 2 micrometer in the 3 dimensions. Our microscope is based on a Linnik interferometer associated with a parallel lock-in detection on a CCD camera and produces en face tomographic images with full-field illumination. We describe the performances of our microscope and we present images obtained from various biological tissues.
Matrices of detectors carrying up to a few millions of pixels have changed in many aspects the science of imaging from astronomy to popular photography as well as from X rays to infrared. Most of the time they have been used for signal acquisition but rarely as an active part of the signal processing. A few years ago we have proposed ' touse such sensors, which convert radiations into charges and store them before reading, in order to achieve parallel lock-in or heterodyne detection at frequencies much higher than the speed of image acquisition. In the usual single channel lock-in detection scheme the (amplified) periodical noisy signal as multiplied by a noiseless synchronous reference signal and then filtered by a low-pass filter. In our approche 2 roughly speaking, the low-pass filter is replaced by the charge integration during one image acquisition whereas the multiplication is replaced by a synchronous excitation of the source (typically each half or quarter of period). Let us point out that the gain in term of signal-to-noise ratio is about the same in the two approaches (as long as we have to face a white noise such as the shot noise) but in the last one acquisition time can be i04 to iO times faster. In the two examples that we will describes below we will take advantage of the new promising field of biomedical imaging to demonstrate how helpful is our approach for imaging inside biological tissues which are strong light scatterers.
KEYWORDS: Modulation, Speckle, Signal detection, Ultrasonics, CCD cameras, Tissues, Transducers, CCD image sensors, Ultrasonography, Signal to noise ratio
We describe here the detailed signal processing technique that we have introduced to detect ultrasonic speckle modulation using a CCD array. We show that this new approach leads to a better signal to noise ratio than the single detector one not only because all the speckle grains signals are used but also because averaging over a few 104 grains smoothes this random signal. Finally a few examples of signal generated by locally absorbing volume immersed in real biological tissues are given.
We have built an interference microscope that produces in real-time images of cross-section slices located at adjustable depths inside 3-D objects. The microscope is based on a Michelson-type polarization interferometer. A light emitting diode (LED), used as an optical source at (lambda) equals 840 nm with short coherence length, provides optical sectioning ability with better than 10 micrometer resolution in the depth dimension. By using high numerical aperture objective lenses (NA equals 0.95), the depth resolution can be improved to better than 1 micrometer, in good agreement with theory. Images can be produced at the rate of 50 per second using a multiplexed lock-in detection and MMX assembler-optimized calculation routines. Cross-section images inside an onion and at different depths in a multilayer silicon integrated circuit are presented.
We present a new microscopy system based on the OCT principle, that uses a multiplexed lock-in detection scheme to generate a 2D head-on image in parallel without lateral scanning. Our 'full-field optical coherence microscopy' comprises a Michelson interferometer built with a polarizing beam splitter, and uses a photoelastic birefringence modulator to modulate the optical path difference between the two orthogonal polarizations. A novel signal processing method is used to achieve a demodulation in parallel on every pixel of a 256 X 256 CCD camera. A 840 nm electroluminescent diode with 20 micrometers coherence length is used to illuminate the field of view through the microscope objective lens. In-depth exploration of the sample is realized by changing the plane of focus. The lateral resolution of the images is limited by the camera pixel size and is 2 micrometers . The axial sectioning ability is approximately 8 micrometers . Having validated our setup on model samples, we now evaluate its performance on biological structures. As an example, images of onion cells from 50-400 micrometers below the surface are obtained in 1 s with 100 dB sensitivity.
KEYWORDS: Control systems design, Imaging systems, Microscopy, Tissues, Signal processing, CCD cameras, Computing systems, Multiplexing, Cameras, Signal detection
We describe the design of a versatile electronic system performing a lock-in detection in parallel on every pixel of a 2D CCD camera. The system is based on a multiplexed lock- in detection method that requires accurate synchronization of the camera, the excitation signal and the processing computer. This device has been incorporated in an imaging setup based on the optical coherence tomography principle, enabling to acquire a full 2D head-on image without scanning. The imaging experiment is implemented on a modified commercial microscope. Lateral resolution is on the order of 2 micrometers , and the coherence length of the light source defines an axial resolution of approximately 8 micrometers . Images of onion cells a few hundred microns deep into the sample are obtained with 100 dB sensitivity.
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