Performances of high numerical aperture water and oil immersion objective in deep-tissue, multi-photon microscopic imaging of excised human skin

Deep-skin multiphoton microscopy in vivo excited at 1600 nm: A comparative investigation with silicone oil and deuterium dioxide immersion

Multiphoton microscopy (MPM) excited at the 1700-nm window has enabled deep-tissue penetration in biological tissue, especially brain. MPM of skin may also benefit from this deep-penetration capability. Skin is a layered structure with varying refractive index (from 1.34 to 1.5). Consequently, proper immersion medium should be selected when imaging with high numerical aperture objective lens. To provide guidelines for immersion medium selection for skin MPM, here we demonstrate comparative experimental investigation of deep-skin MPM excited at 1600 nm in vivo, using both silicone oil and deuterium dioxide (D₂ O) immersion. We specifically characterize imaging depths, signal levels and spatial resolution. Our results show that both immersion media give similar performance in imaging depth and spatial resolution, while signal levels are slightly better with silicone oil immersion. We also demonstrate that local injection of fluorescent beads into the skin is a viable technique for spatial resolution characterization in vivo.

Comparison of Transparency and Shrinkage During Clearing of Insect Brains Using Media With Tunable Refractive Index

Improvement of imaging quality has the potential to visualize previously unseen building blocks of the brain and is therefore one of the great challenges in neuroscience. Rapid development of new tissue clearing techniques in recent years have attempted to solve imaging compromises in thick brain samples, particularly for high resolution optical microscopy, where the clearing medium needs to match the high refractive index of the objective immersion medium. These problems are exacerbated in insect tissue, where numerous (initially air-filled) tracheal tubes branching throughout the brain increase the scattering of light. To date, surprisingly few studies have systematically quantified the benefits of such clearing methods using objective transparency and tissue shrinkage measurements. In this study we compare a traditional and widely used insect clearing medium, methyl salicylate combined with permanent mounting in Permount (“MS/P”) with several more recently applied clearing media that offer tunable refractive index (n): 2,2′-thiodiethanol (TDE), “SeeDB2” (in variants SeeDB2S and SeeDB2G matched to oil and glycerol immersion, n = 1.52 and 1.47, respectively) and Rapiclear (also with n = 1.52 and 1.47). We measured transparency and tissue shrinkage by comparing freshly dissected brains with cleared brains from dipteran flies, with or without addition of vacuum or ethanol pre-treatments (dehydration and rehydration) to evacuate air from the tracheal system. The results show that ethanol pre-treatment is very effective for improving transparency, regardless of the subsequent clearing medium, while vacuum treatment offers little measurable benefit. Ethanol pre-treated SeeDB2G and Rapiclear brains show much less shrinkage than using the traditional MS/P method. Furthermore, at lower refractive index, closer to that of glycerol immersion, these recently developed media offer outstanding transparency compared to TDE and MS/P. Rapiclear protocols were less laborious compared to SeeDB2, but both offer sufficient transparency and refractive index tunability to permit super-resolution imaging of local volumes in whole mount brains from large insects, and even light-sheet microscopy. Although long-term permanency of Rapiclear stored samples remains to be established, our samples still showed good preservation of fluorescence after storage for more than a year at room temperature.

Deep-brain three-photon microscopy excited at 1600 nm with silicone oil immersion

Three-photon microscopy excited at the 1700-nm window (roughly covering 1600-1840 nm) is especially suitable for deep-brain imaging in living animals. To match the brain refractive index, D₂ O has been exclusively used as the immersion medium. However, the hygroscopic property of D₂ O leads to a decrease of transmittance of the excitation light and as a result a decrease in three-photon signals over time. Solutions such as replacing D₂ O from time to time, wrapping both the objective lens and the immersion D₂ O, and sealing D₂ O with paraffin liquid have all been demonstrated, which add to the system complexity. Based on our recent characterization of immersion oils, we propose using silicone oil as a potential alternative to D₂ O for deep-brain imaging. Excited at 1600 nm, our comparative deep-brain imaging using both D₂ O and silicone oil immersion show that silicone oil immersion yields 17% higher three-photon signal in third-harmonic generation imaging within the white matter. Besides, silicone oil immersion also enables three-photon fluorescence imaging of vasculature up to 1460 μm (mechanical depth) into the mouse brain in vivo acquired at 2 seconds/frame. Together with the nonhygroscopic physical property, silicone oil is promising for long-span three-photon brain imaging excited at the 1700-nm window.

Refractive index and pulse broadening characterization using oil immersion and its influence on three-photon microscopy excited at the 1700-nm window

Three-photon microscopy excited at the 1700-nm window enables deep-tissue penetration. However, the refractive indices of commonly used immersion oils, and the resultant pulse broadening are not known, preventing imaging optimization. Here, we demonstrate detailed characterization of the refractive index, pulse broadening and distortion for excitation pulses at this window for commonly used immersion oils. On the physical side, we uncover that absorption, rather than material dispersion, is the main cause of pulse broadening and distortion. On the application side, comparative three-photon imaging results indicate that 1600-nm excitation yields 5 times higher three-photon signal than 1690-nm excitation.

In vivo nonlinear optical imaging to monitor early microscopic changes in a murine cutaneous squamous cell carcinoma model

Early detection of cutaneous squamous cell carcinoma (cSCC) can enable timely therapeutic and preventive interventions for patients. In this study, in vivo nonlinear optical imaging (NLOI) based on two-photon excitation fluorescence (TPEF) and second harmonic generation (SHG), was used to non-invasively detect microscopic changes occurring in murine skin treated topically with 7,12-dimethylbenz(a)anthracene (DMBA). The optical microscopic findings and the measured TPEF-SHG index show that NLOI was able to clearly detect early cytostructural changes in DMBA treated skin that appeared clinically normal. This suggests that in vivo NLOI could be a non-invasive tool to monitor early signs of cSCC. In vivo axial NLOI scans of normal murine skin (upper left), murine skin with preclinical hyperplasia (upper right), early clinical murine skin lesion (lower left) and late or advanced murine skin lesion (lower right).

A pragmatic guide to multiphoton microscope design

Multiphoton microscopy has emerged as a ubiquitous tool for studying microscopic structure and function across a broad range of disciplines. As such, the intent of this paper is to present a comprehensive resource for the construction and performance evaluation of a multiphoton microscope that will be understandable to the broad range of scientific fields that presently exploit, or wish to begin exploiting, this powerful technology. With this in mind, we have developed a guide to aid in the design of a multiphoton microscope. We discuss source selection, optical management of dispersion, image-relay systems with scan optics, objective-lens selection, single-element light-collection theory, photon-counting detection, image rendering, and finally, an illustrated guide for building an example microscope.

Full-depth epidermis tomography using a Mirau-based full-field optical coherence tomography

With a Gaussian-like broadband light source from high brightness Ce(3+):YAG single-clad crystal fiber, a full-field optical coherence tomography using a home-designed Mirau objective realized high quality images of in vivo and excised skin tissues. With a 40 × silicone-oil-immersion Mirau objective, the achieved spatial resolutions in axial and lateral directions were 0.9 and 0.51 μm, respectively. Such a high spatial resolution enables the separation of lamellar structure of the full epidermis in both the cross-sectional and en face planes. The number of layers of stratum corneum and its thickness were quantitatively measured. This label free and non-invasive optical probe could be useful for evaluating the water barrier of skin tissue in clinics. As a preliminary in vivo experiment, the blood vessel in dermis was also observed, and the flowing of the red blood cells and location of the melanocyte were traced.

High-throughput nonlinear optical microscopy

High-resolution microscopy methods based on different nonlinear optical (NLO) contrast mechanisms are finding numerous applications in biology and medicine. While the basic implementations of these microscopy methods are relatively mature, an important direction of continuing technological innovation lies in improving the throughput of these systems. Throughput improvement is expected to be important for studying fast kinetic processes, for enabling clinical diagnosis and treatment, and for extending the field of image informatics. This review will provide an overview of the fundamental limitations on NLO microscopy throughput. We will further cover several important classes of high-throughput NLO microscope designs with discussions on their strengths and weaknesses and their key biomedical applications. Finally, this review will close with a perspective of potential future technological improvements in this field.

Measurement and correction of in vivo sample aberrations employing a nonlinear guide-star in two-photon excited fluorescence microscopy

We demonstrate that sample induced aberrations can be measured in a nonlinear microscope. This uses the fact that two-photon excited fluorescence naturally produces a localized point source inside the sample: the nonlinear guide-star (NL-GS). The wavefront emitted from the NL-GS can then be recorded using a Shack-Hartmann sensor. Compensation of the recorded sample aberrations is performed by the deformable mirror in a single-step. This technique is applied to fixed and in vivo biological samples, showing, in some cases, more than one order of magnitude improvement in the total collected signal intensity.

Experimental and molecular dynamics investigation into the amphiphilic nature of sulforhodamine B

Sulforhodamine B (SRB), a common fluorescent dye, is often considered to be a purely hydrophilic molecule, having no impact on bulk or interfacial properties of aqueous solutions. This assumption is due to the high water solubility of SRB relative to most fluorescent probes. However, in the present study, we demonstrate that SRB is in fact an amphiphile, with the ability to adsorb at an air/water interface and to incorporate into sodium dodecyl sulfate (SDS) micelles. In fact, SRB reduces the surface tension of water by up to 23 mN/m, and the addition of SRB to an aqueous SDS solution induces a significant decrease in the cmc of SDS. Molecular dynamics simulations were conducted to gain a deeper understanding of these findings. The simulations revealed that SRB has defined polar "head" and nonpolar "tail" regions when adsorbed at the air/water interface as a monomer. In contrast, when incorporated into SDS micelles, only the sulfonate groups were found to be highly hydrated, suggesting that the majority of the SRB molecule penetrates into the micelle. To illustrate the implications of the amphiphilic nature of SRB, an interesting case study involving the effect of SRB on ultrasound-mediated transdermal drug delivery is presented.

Analysis of optical properties of the mouse cranium--implications for in vivo multi photon laser scanning microscopy

Trans-cranial imaging is the least invasive method for optical in vivo studies of structures in the mouse brain and has found wide application over the last few years. An important issue is how and to what extent the cranium and the tissue between the cranium and the focal point detract from the quality of the recorded images. Here we address this issue by recording transmission images in wild type mice at five wavelengths in the visible and near-infrared spectrum. The recorded laser scanning microscopic images were analyzed pixel by pixel in order to quantify the light attenuation and shading as function of the location of the focal point relative to the cranium. Additional images demonstrate the effects of the mouse crania on the images of fluorescent microspheres in the low micrometer range. The results of this study demonstrate that light attenuation by the cranium, though with typical losses of less than 20% of the incident light, induces shading effects during the imaging process. Geometrical shapes and sizes in the images of the recorded objects may differ substantially depending on whether they have been recorded trans-cranially or not. This is true even for comparatively large structures such as cell somata. Our results call for a more realistic appraisal of the potential of the trans-cranial imaging approach, particularly when it comes to absolute measurements of sizes and shapes of small objects. As trans-cranial imaging has found wide use in contemporary research it is important that the results be interpreted with due caution.

Application of nonlinear optical microscopy for imaging skin

Recent advances in the use of nonlinear optical microscopy (NLOM) in skin microscopy are presented. Nonresonant spectroscopies including second harmonic generation, coherent anti-Stokes Raman and two-photon absorption are described and applications to problems in skin biology are detailed. These nonlinear techniques have several advantages over traditional microscopy methods that rely on one-photon excitation: intrinsic 3D imaging with <1 microm spatial resolution, decreased photodamage to tissue samples and penetration depths up to 1,000 microm with the use of near-infrared lasers. Thanks to these advantages, nonlinear optical spectroscopy has become a powerful tool to study the physical and biochemical properties of the skin. Structural information can be obtained using the response of endogenous chemical species in the skin, such as collagen or lipids, indicating that optical biopsy may replace current invasive, time-consuming traditional histology methods. Insertion of specific probe molecules into the skin provides the opportunity to monitor specific biochemical processes such as skin transport, molecular penetration, barrier homeostasis and ultraviolet radiation-induced reactive oxygen species generation. While the field is quite new, it seems likely that the use of NLOM to probe structure and biochemistry of live skin samples will only continue to grow.

Design and implementation of a sensitive high-resolution nonlinear spectral imaging microscope

Live tissue nonlinear microscopy based on multiphoton autofluorescence and second harmonic emission originating from endogenous fluorophores and noncentrosymmetric-structured proteins is rapidly gaining interest in biomedical applications. The advantage of this technique includes high imaging penetration depth and minimal phototoxic effects on tissues. Because fluorescent dyes are not used, discrimination between different components within the tissue is challenging. We have developed a nonlinear spectral imaging microscope based on a home-built multiphoton microscope, a prism spectrograph, and a high-sensitivity CCD camera for detection. The sensitivity of the microscope was optimized for autofluorescence and second harmonic imaging over a broad wavelength range. Importantly, the spectrograph lacks an entrance aperture; this improves the detection efficiency at deeper lying layers in the specimen. Application to the imaging of ex vivo and in vivo mouse skin tissues showed clear differences in spectral emission between skin tissue layers as well as biochemically different tissue components. Acceptable spectral images could be recorded up to an imaging depth of approximately 100 microm.

Characterization of optical-aberration-induced lateral and axial image inhomogeneity in multiphoton microscopy

The effects of off-axis optical aberration in multiphoton microscopy and the resulting lateral and axial image inhomogeneity are investigated. The lateral inhomogeneity of the scanning field is demonstrated by second harmonic generation (SHG) imaging of fasciae and two-photon fluorescence (TPF) microscopy of thin fluorescent samples. Furthermore, refractive index mismatch-caused intensity attenuation of the TPF signal at central and peripheral regions of the scanning frame is measured using homogeneous 10-microM sulforhodamine B samples with refractive indexes of 1.33 and around 1.465. In addition to characterizing image field convexity, we also found that image resolution degrades away from the optical axis. These effects need to be accounted for in both qualitative and quantitative multiphoton imaging applications.

Improving Signal Levels in Intravital Multiphoton Microscopy using an Objective Correction Collar

Multiphoton microscopy has enabled biologists to collect high-resolution images hundreds of microns into biological tissues, including tissues of living animals. While the depth of imaging exceeds that possible from any other form of light microscopy, multiphoton microscopy is nonetheless generally limited to depths of less than a millimeter. Many of the advantages of multiphoton microscopy for deep tissue imaging accrue from the unique nature of multiphoton fluorescence excitation. However, the quadratic relationship between illumination level and fluorescence excitation makes multiphoton microscopy especially susceptible to factors that degrade the illumination focus. Here we examine the effect of spherical aberration on multiphoton microscopy in fixed kidney tissues and in the kidneys of living animals. We find that spherical aberration, as evaluated from axial asymmetry in the point spread function, can be corrected by adjustment of the correction collar of a water immersion objective lens. Introducing a compensatory positive spherical aberration into the imaging system decreased the depth-dependence of signal levels in images collected from living animals, increasing signal by up to 50%.

Deep Tissue Microscopic Imaging of the Kidney with a Gradient-Index Lens System

Intravital microscopy using two-photon excitation is proven to be a valuable tool for studying the kidney and associated disease processes. However, routine performance of intravital kidney imaging is limited by the fact that fluorescence signal is attenuated by the tissue and at certain tissue depth lost its strength completely. For most of the animal tissues, this finite imaging depth is limited to a few hundred microns. Currently it is not possible to non-invasively image the kidney beyond the superficial tissue layers of the cortex. This has imposed significant limitations on the animal models one can use for imaging since structure such the glomerulus is typically located below the superficial layer of the cortex that can not be imaged using a conventional fluorescence microscope. Here we report the use of a needle-like lens system based on gradient-index (GRIN) microlenses capable of transferring high quality fluorescence images of the tissue through a regular microscope objective for deep tissue imaging of the kidney. By combining this GRIN lens system with a Zeiss LSM 510 NLO microscope, we are able to extend the imaging depth for kidney tissues far beyond the few hundred microns limit. This GRIN lens imaging system provides an alternative microendoscopic imaging tool that will enhance current intravital kidney imaging techniques for studying structural and functional properties of local tissues at locations below the superficial layers of the kidney.

Role of three-dimensional bleach distribution in confocal and two-photon fluorescence recovery after photobleaching experiments

The quantitative analysis of fluorescence perturbation experiments such as fluorescence recovery after photobleaching (FRAP) requires suitable analytical models to be developed. When diffusion in 3D environments is considered, the description of the initial condition produced by the perturbation (i.e., the photobleaching of a selected region in FRAP) represents a crucial aspect. Though it is widely known that bleaching profiles approximations can lead to errors in quantitative FRAP measurements, a detailed analysis of the sources and the effects of these approximations has never been conducted until now. In this study, we measured the experimental 3D bleaching distributions obtained in conventional and two-photon excitation schemes and analyzed the deviations from the ideal cases usually adopted in FRAP experiments. In addition, we considered the non-first-order effects generated by the high energy pulses usually delivered in FRAP experiments. These data have been used for finite-element simulations mimicking FRAP experiments on free diffusing molecules and compared with FRAP model curves based on the ideal bleach distributions. The results show that two-photon excitation more closely fits ideal bleaching patterns even in the event of fluorescence saturation, achieving estimations of diffusion coefficients within 20% accuracy of the correct value.

Mid infrared microspectroscopic mapping and imaging: a bio-analytical tool for spatially and chemically resolved tissue characterization and evaluation of drug permeation within tissues

The combination of the two classical biophysical methods, microscopy and infrared spectroscopy, has led to the development of a potent analytical technology termed infrared microspectroscopy. It combines high lateral resolution as obtained by microscopy and the chemical identification of the sample components by infrared spectroscopy. The two approaches mainly utilized in microspectroscopy are the mapping and the imaging techniques, which are introduced and presented. Especially, since the development of so called focal plane array detectors, which are implemented in the imaging methods (microspectroscopic imaging) has become a promising bio-analytical tool for ultrastructural medical diagnostics, due to the fact that the time required for analyzing a sample has been reduced dramatically and the lateral resolution improved to approximately 4 microm. Mid infrared microscopy allows a direct access to spatially resolved molecular and structural information of the analyzed area. The image contrast is generated on the basis of the tissue's intrinsic biochemical composition. The current investigation shows how mid infrared microspectroscopic mapping and imaging is used for the bio-analytical characterization and identification of specific molecular components of a tissue sample at high lateral resolution of a few microns (approaching the mid infrared diffraction limit). Furthermore, the potential of these methods for monitoring the penetration and distribution of drugs within biological tissues are presented. Due to the fact, that mid infrared microspectroscopy is a noninvasive, nondestructive technique for the analyzed sample, requiring no complicated and time consuming staining procedures, it is a convenient method for histological and pathological investigations, allowing the generation of a huge amount of biochemical information not yet available with other nonvibrational techniques. The strength of the presented microscopic technique is the fact that the infrared images are directly comparable to outcomes of classical histological staining procedures and can be interpreted by nonspectroscopists.

Effects of objective numerical apertures on achievable imaging depths in multiphoton microscopy

Multiphoton microscopy is a powerful technique for achieving three-dimensional submicron imaging in biological specimens. However, specimen optical parameters such as refractive indices and scattering coefficients can result in the loss of image resolution and decreased signal in depth. These factors are coupled to the focusing objective's numerical aperture (NA) in limiting the achievable imaging depths. In this work, we performed multiphoton imaging on aqueous fluorescent solution, human skin, and rat tail tendon to show that, under the same immersion condition, lower NA objectives can examine more deeply into biological specimens and should be used when optimal imaging depths is desired.

Non-invasive glucose measurement technologies: an update from 1999 to the dawn of the new millennium

There are three main issues in non-invasive (NI) glucose measurements: namely, specificity, compartmentalization of glucose values, and calibration. There has been progress in the use of near-infrared and mid-infrared spectroscopy. Recently new glucose measurement methods have been developed, exploiting the effect of glucose on erythrocyte scattering, new photoacoustic phenomenon, optical coherence tomography, thermo-optical studies on human skin, Raman spectroscopy studies, fluorescence measurements, and use of photonic crystals. In addition to optical methods, in vivo electrical impedance results have been reported. Some of these methods measure intrinsic properties of glucose; others deal with its effect on tissue or blood properties. Recent studies on skin from individuals with diabetes and its response to stimuli, skin thermo-optical response, peripheral blood flow, and red blood cell rheology in diabetes shed new light on physical and physiological changes resulting from the disease that can affect NI glucose measurements. There have been advances in understanding compartmentalization of glucose values by targeting certain regions of human tissue. Calibration of NI measurements and devices is still an open question. More studies are needed to understand the specific glucose signals and signals that are due to the effect of glucose on blood and tissue properties. These studies should be performed under normal physiological conditions and in the presence of other co-morbidities.