Computational adaptive optics for broadband optical interferometric tomography of biological tissue.

Aberrations in optical microscopy reduce image resolution and contrast, and can limit imaging depth when focusing into biological samples. Static correction of aberrations may be achieved through appropriate lens design, but this approach does not offer the flexibility of simultaneously correcting aberrations for all imaging depths, nor the adaptability to correct for sample-specific aberrations for high-quality tomographic optical imaging. Incorporation of adaptive optics (AO) methods have demonstrated considerable improvement in optical image contrast and resolution in noninterferometric microscopy techniques, as well as in optical coherence tomography. Here we present a method to correct aberrations in a tomogram rather than the beam of a broadband optical interferometry system. Based on Fourier optics principles, we correct aberrations of a virtual pupil using Zernike polynomials. When used in conjunction with the computed imaging method interferometric synthetic aperture microscopy, this computational AO enables object reconstruction (within the single scattering limit) with ideal focal-plane resolution at all depths. Tomographic reconstructions of tissue phantoms containing subresolution titanium-dioxide particles and of ex vivo rat lung tissue demonstrate aberration correction in datasets acquired with a highly astigmatic illumination beam. These results also demonstrate that imaging with an aberrated astigmatic beam provides the advantage of a more uniform depth-dependent signal compared to imaging with a standard gaussian beam. With further work, computational AO could enable the replacement of complicated and expensive optical hardware components with algorithms implemented on a standard desktop computer, making high-resolution 3D interferometric tomography accessible to a wider group of users and nonspecialists.

Optical parametrically gated microscopy in scattering media.

High-resolution imaging in turbid media has been limited by the intrinsic compromise between the gating efficiency (removal of multiply-scattered light background) and signal strength in the existing optical gating techniques. This leads to shallow depths due to the weak ballistic signal, and/or degraded resolution due to the strong multiply-scattering background--the well-known trade-off between resolution and imaging depth in scattering samples. In this work, we employ a nonlinear optics based optical parametric amplifier (OPA) to address this challenge. We demonstrate that both the imaging depth and the spatial resolution in turbid media can be enhanced simultaneously by the OPA, which provides a high level of signal gain as well as an inherent nonlinear optical gate. This technology shifts the nonlinear interaction to an optical crystal placed in the detection arm (image plane), rather than in the sample, which can be used to exploit the benefits given by the high-order parametric process and the use of an intense laser field. The coherent process makes the OPA potentially useful as a general-purpose optical amplifier applicable to a wide range of optical imaging techniques.

High Resolution Phase-Sensitive Magnetomotive Optical Coherence Microscopy for Tracking Magnetic Microbeads and Cellular Mechanics.

We present a real-time multimodal near-infrared imaging technology that tracks externally induced axial motion of magnetic microbeads in single cells in culture. The integrated multimodal imaging technique consists of phase-sensitive magnetomotive optical coherence microscopy (MM-OCM) and multiphoton microscopy (MPM).MPMis utilized for the visualization of multifunctional fluorescent and magnetic microbeads, while MM-OCM detects, with nanometer-scale sensitivity, periodic displacements of the microbeads induced by the modulation of an external magnetic field. Magnetomotive signals are measured from mouse macrophages, human breast primary ductal carcinoma cells, and human breast epithelial cells in culture, and validated with full-field phase-sensitive microscopy. This methodology demonstrates the capability for imaging controlled cell dynamics and has the potential for measuring cell biomechanical properties, which are important in assessing the health and pathological state of cells.

Multimodal In Vivo Skin Imaging with Integrated Optical Coherence and Multiphoton Microscopy.

In this paper, we demonstrate high-resolution, multimodal in vivo imaging of human skin using optical coherence (OCM) and multiphoton microscopy (MPM). These two modalities are integrated into a single instrument to enable simultaneous acquisition and coregistration. The system design and the OCM image processing architecture enable sufficient performance of both modalities for in vivo imaging of human skin. Examples of multimodal in vivo imaging are presented as well as time lapse imaging of blood flow in single capillary loops. By making use of multiple intrinsic contrast mechanisms this integrated technique improves the ability to noninvasively visualize living tissue. Integrated OCM and MPM has potential applications for in vivo diagnosis of various pathological skin conditions, such as skin cancer, as well as potential pharmaceutical and cosmetic research applications.

Correction of coherence gate curvature in high numerical aperture optical coherence imaging.

We present a method for correcting coherence gate curvature caused by scanning-induced path length variations in spectral-domain high-NA optical coherence imaging systems. These variations cause curvature artifacts in optical coherence tomography and effectively restrict the field of view in optical coherence microscopy (OCM). Here we show that the coherence gate curvature can be measured and corrected by recovering the phase of the analytic signal from a calibration image. This phase information can be used directly to process OCM images allowing the coherence gate curvature, as well as any order of system dispersion, to be corrected in a computationally efficient manner. We also discuss the use of various image quality metrics that can be used to adjust the calibrated phase in order to keep the coherence and confocal gates aligned in tissue.

Detecting intrinsic scattering changes correlated to neuron action potentials using optical coherence imaging.

We demonstrate how optical coherence imaging techniques can detect intrinsic scattering changes that occur during action potentials in single neurons. Using optical coherence tomography (OCT), an increase in scattering intensity from neurons in the abdominal ganglion of Aplysia californica is observed following electrical stimulation of the connective nerve. In addition, optical coherence microscopy (OCM), with its superior transverse spatial resolution, is used to demonstrate a direct correlation between scattering intensity changes and membrane voltage in single cultured Aplysia bag cell neurons during evoked action potentials. While intrinsic scattering changes are small, OCT and OCM have potential use as tools in neuroscience research for non-invasive and non-contact measurement of neural activity without electrodes or fluorescent dyes. These techniques have many attractive features such as high sensitivity and deep imaging penetration depth, as well as high temporal and spatial resolution. This study demonstrates the first use of OCT and OCM to detect functionally-correlated optical scattering changes in single neurons.

Dual-spectrum laser source based on fiber continuum generation for integrated optical coherence and multiphoton microscopy.

A single-laser dual-spectrum source designed for integrated optical coherence and multiphoton microscopy is demonstrated. The source implements the laser characteristics needed to optimally perform both modalities while extending the spectral range for this imaging technique. It consists of a widely tunable, mode-locked, Ti-sapphire laser with a portion of its output spectrally broadened via continuum generation in a photonic crystal fiber. The continuum-broadened beam allows for enhanced optical sectioning with optical coherence microscopy, while the unbroadened beam from the ultrashort-pulse Ti-sapphire laser optimally excites fluorescent markers. The noise power of the continuum-broadened beam is less than 1.1 dBmHz higher than the Ti-sapphire laser in the range from 1 Hz to 25 MHz, and the fiber shows no sign of damage after approximately 100 h of use. We demonstrate the use of this source across a wide spectral range by imaging green fluorescent protein-transfected mouse fibroblast cells costained with fluorescent dyes that are maximally excited at various wavelengths. Images of unstained in vivo human skin are also presented. This source extends the feasibility of this integrated imaging modality and will facilitate new investigations in in vivo microscopy, tissue engineering, and cell biology.

In Vivo Assessment of Engineered Skin Cell Delivery with Multimodal Optical Microscopy.

The healing process is often significantly impaired under conditions of chronic or large area wounds, which are often treated clinically using autologous split-thickness skin grafts. However, in many cases, harvesting of donor tissue presents a serious problem such as in the case of very large area burns. In response to this, engineered biomaterials have emerged that attempt to mimic the natural skin environment or deliver a suitable therapy to assist in the healing process. In this study, a custom-built multimodal optical microscope capable of noninvasive structural and functional imaging is used to investigate both the engineered tissue microenvironment and the in vivo wound healing process. Investigation of various engineered scaffolds show the strong relationship among the microenvironment of the scaffold, the organization of the cells within the scaffold, and the delivery pattern of these cells onto the healing wound. Through noninvasive tracking of these processes and parameters, multimodal optical microscopy provides an important tool in the assessment of engineered scaffolds both in vitro and in vivo.

Longitudinal label-free tracking of cell death dynamics in living engineered human skin tissue with a multimodal microscope.

We demonstrate real-time, longitudinal, label-free tracking of apoptotic and necrotic cells in living tissue using a multimodal microscope. The integrated imaging platform combines multi-photon microscopy (MPM, based on two-photon excitation fluorescence), optical coherence microscopy (OCM), and fluorescence lifetime imaging microscopy (FLIM). Three-dimensional (3-D) co-registered images are captured that carry comprehensive information of the sample, including structural, molecular, and metabolic properties, based on light scattering, autofluorescence intensity, and autofluorescence lifetime, respectively. Different cell death processes, namely, apoptosis and necrosis, of keratinocytes from different epidermal layers are longitudinally monitored and investigated. Differentiation of the two cell death processes in a complex living tissue environment is enabled by quantitative image analysis and high-confidence classification processing based on the multidimensional, cross-validating imaging data. These results suggest that despite the limitations of each individual label-free modality, this multimodal imaging approach holds the promise for studies of different cell death processes in living tissue and in vivo organs.

In vivo multimodal microscopy for detecting bone-marrow-derived cell contribution to skin regeneration.

Bone-marrow (BM)-derived cells have been shown to be capable of aiding skin regeneration in vivo by differentiating into keratinocytes. However, the conditions under which this occurs are not fully understood. Characterizing innate mechanisms of skin regeneration by stem cells in vivo is important for the area of stem cell biology. In this study, we investigate the use of novel in vivo imaging technology for characterizing the contribution of BM-derived cells to regeneration of the epidermis in mouse skin in vivo. In vivo imaging provides the ability to non-invasively observe the spatial positions and morphology of the BM-derived cells. Using a GFP BM-transplanted mouse model and in vivo multimodal microscopy, BM-derived cells can be observed in the skin. Our in vivo imaging method was used to search for the presence and identify the 3D spatial distribution of BM-derived cells in the epidermis of the skin under normal conditions, following wound healing, and after syngeneic skin grafting. We did not observe any evidence of BM-derived keratinocytes under these conditions, but we did observe BM-derived dendritic cells in the skin grafts. In vivo multimodal imaging has great potential for characterizing the conditions under which BM-derived cells contribute to skin regeneration.

Imaging and tracking of bone marrow-derived immune and stem cells.

Bone marrow (BM)-derived stem and immune cells play critical roles in maintaining the health, regeneration, and repair of many tissues. Given their important functions in tissue regeneration and therapy, tracking the dynamic behaviors of BM-derived cells has been a long-standing research goal of both biologists and engineers. Because of the complex cellular-level processes involved, real-time imaging technologies that have sufficient spatial and temporal resolution to visualize them are needed. In addition, in order to track cellular dynamics, special attention is needed to account for changes in the microenvironment where the cells reside, for example, tissue contraction, stretching, development, etc. In this chapter, we introduce methods for real-time imaging and longitudinal tracking of BM-derived immune and stem cells in in vivo three-dimensional (3-D) tissue environments with an integrated optical microscope. The integrated microscope combines multiple imaging functions derived from optical coherence tomography (OCT) and multiphoton microscopy (MPM), including optical coherence microscopy (OCM), microvasculature imaging, two-photon excited fluorescence (TPEF), and second harmonic generation (SHG) microscopy. Short- and long-term tracking of the dynamic behavior of BM-derived cells involved in cutaneous wound healing and skin grafting in green fluorescent protein (GFP) BM-transplanted mice is demonstrated. Methods and algorithms for nonrigid registration of time-lapse images are introduced, which allows for long-term tracking of cell dynamics over several months.

Long-term time-lapse multimodal intravital imaging of regeneration and bone-marrow-derived cell dynamics in skin.

A major challenge for translating cell-based therapies is understanding the dynamics of cells and cell populations in complex in vivo environments. Intravital microscopy has shown great promise for directly visualizing cell behavior in vivo. However, current methods are limited to relatively short imaging times (hours), by ways to track cell and cell population dynamics over extended time-lapse periods (days to weeks to months), and by relatively few imaging contrast mechanisms that persist over extended investigations. We present technology to visualize and quantify complex, multifaceted dynamic changes in natural deformable skin over long time periods using novel multimodal imaging and a non-rigid image registration method. These are demonstrated in green fluorescent protein (GFP) bone marrow (BM) transplanted mice to study dynamic skin regeneration. This technology provides a novel perspective for studying dynamic biological processes and will enable future studies of stem, immune, and tumor cell biology in vivo.

In vivo imaging of immune cell dynamics in skin in response to zinc-oxide nanoparticle exposure.

Zinc oxide (ZnO) nanoparticles (NPs) are widely used in cosmetic and sunscreen products which are applied topically to the skin. Despite their widespread use, the safety and biological response of these particles remains an active area of investigation. In this paper we present methods based on in vivo multiphoton microscopy (MPM) in skin to address relevant questions about the potential toxicity and immunological response of ZnO NPs. Registration of time-lapse volumetric MPM images allows the same skin site to be tracked across multiple days for visualizing and quantifying cellular and structural changes in response to NP exposure. Making use of the unique optical properties of ZnO enables high contrast detection of the NPs in the presence of strong autofluorescence and second harmonic generation (SHG) background from the skin. A green fluorescent protein (GFP) bone marrow (BM) transplanted mouse model is used to visualize and assess the dynamic response of BM-derived immune cells. These cells are visualized to assess the potential for ZnO NPs to interact with immune cells and elicit an immune reaction in skin. We investigate both topical and dermal exposure of the ZnO NPs. The methods and findings presented in this paper demonstrate a novel approach for tracking ZnO NPs in vivo and for visualizing the cellular response of the exposed tissue to assess the immunological response and potential toxicity of these particles.

Integrated multimodal optical microscopy for structural and functional imaging of engineered and natural skin.

An integrated multimodal optical microscope is demonstrated for high-resolution, structural and functional imaging of engineered and natural skin. This microscope incorporates multiple imaging modalities including optical coherence (OCM), multi-photon (MPM), and fluorescence lifetime imaging microscopy (FLIM), enabling simultaneous visualization of multiple contrast sources and mechanisms from cells and tissues. Spatially co-registered OCM/MPM/FLIM images of multi-layered skin tissues are obtained, which are formed based on complementary information provided by different modalities, i.e., scattering information from OCM, molecular information from MPM, and functional cellular metabolism states from FLIM. Cellular structures in both the dermis and epidermis, especially different morphological and physiological states of keratinocytes from different epidermal layers, are revealed by mutually-validating images. In vivo imaging of human skin is also investigated, which demonstrates the potential of multimodal microscopy for in vivo investigation during engineered skin engraftment. This integrated imaging technique and microscope show the potential for investigating cellular dynamics in developing engineered skin and following in vivo grafting, which will help refine the control and culturing conditions necessary to obtain more robust and physiologically-relevant engineered skin substitutes.

Guide-star-based computational adaptive optics for broadband interferometric tomography.

We present a method for the numerical correction of optical aberrations based on indirect sensing of the scattered wavefront from point-like scatterers ("guide stars") within a three-dimensional broadband interferometric tomogram. This method enables the correction of high-order monochromatic and chromatic aberrations utilizing guide stars that are revealed after numerical compensation of defocus and low-order aberrations of the optical system. Guide-star-based aberration correction in a silicone phantom with sparse sub-resolution-sized scatterers demonstrates improvement of resolution and signal-to-noise ratio over a large isotome. Results in highly scattering muscle tissue showed improved resolution of fine structure over an extended volume. Guide-star-based computational adaptive optics expands upon the use of image metrics for numerically optimizing the aberration correction in broadband interferometric tomography, and is analogous to phase-conjugation and time-reversal methods for focusing in turbid media.

In Vivo Multiphoton Microscopy for Investigating Biomechanical Properties of Human Skin.

The biomechanical properties of living cells depend on their molecular building blocks, and are important for maintaining structure and function in cells, the extracellular matrix, and tissues. These biomechanical properties and forces also shape and modify the cellular and extracellular structures under stress. While many studies have investigated the biomechanics of single cells or small populations of cells in culture, or the properties of organs and tissues, few studies have investigated the biomechanics of complex cell populations in vivo. With the use of advanced multiphoton microscopy to visualize in vivo cell populations in human skin, the biomechanical properties are investigated in a depth-dependent manner in the stratum corneum and epidermis using quasi-static mechanical deformations. A 2D elastic registration algorithm was used to analyze the images before and after deformation to determine displacements in different skin layers. In this feasibility study, the images and results from one human subject demonstrate the potential of the technique for revealing differences in elastic properties between the stratum corneum and the rest of the epidermis. This interrogational imaging methodology has the potential to enable a wide range of investigations for understanding how the biomechanical properties of in vivo cell populations influence function in health and disease.

Imaging and analysis of three-dimensional cell culture models.

Three-dimensional (3D) cell cultures are important tools in cell biology research and tissue engineering because they more closely resemble the architectural microenvironment of natural tissue, compared to standard two-dimensional cultures. Microscopy techniques that function well for thin, optically transparent cultures, however, are poorly suited for imaging 3D cell cultures. Three-dimensional cultures may be thick and highly scattering, preventing light from penetrating without significant distortion. Techniques that can image thicker biological specimens at high resolution include confocal microscopy, multiphoton microscopy, and optical coherence tomography. In this chapter, these three imaging modalities are described and demonstrated in the assessment of functional and structural features of 3D chitosin scaffolds, 3D micro-topographic substrates from poly-dimethyl siloxane molds, and 3D Matrigel cultures. Using these techniques, dynamic changes to cells in 3D microenvironments can be non-destructively assessed repeatedly over time.

Imaging engineered tissues using structural and functional optical coherence tomography.

As the field of tissue engineering evolves, there will be an increasingly important need to visualize and track the complex dynamic changes that occur within three-dimensional constructs. Optical coherence tomography (OCT), as an emerging imaging technology applied to biological materials, offers a number of significant advantages to visualize these changes. Structural OCT has been used to investigate the longitudinal development of engineered tissues and cell dynamics such as migration, proliferation, detachment, and cell-material interactions. Optical techniques that image functional parameters or integrate multiple imaging modalities to provide complementary contrast mechanisms have been developed, such as the integration of optical coherence microscopy with multiphoton microscopy to image structural and functional information from cells in engineered tissue, optical coherence elastography to generate images or maps of strain to reflect the spatially-dependent biomechanical properties, and spectroscopic OCT to differentiate different cell types. From these results, OCT demonstrates great promise for imaging and visualizing engineered tissues, and the complex cellular dynamics that directly affect their practical and clinical use.