Molecular Contrast Optical Coherence Tomography: A Review¶

Molecular Contrast Optical Coherence Tomography and Its Applications in Medicine

The growing need to understand the molecular mechanisms of diseases has prompted the revolution in molecular imaging techniques along with nanomedicine development. Conventional optical coherence tomography (OCT) is a low-cost in vivo imaging modality that provides unique high spatial and temporal resolution anatomic images but little molecular information. However, given the widespread adoption of OCT in research and clinical practice, its robust molecular imaging extensions are strongly desired to combine with anatomical images. A range of relevant approaches has been reported already. In this article, we review the recent advances of molecular contrast OCT imaging techniques, the corresponding contrast agents, especially the nanoparticle-based ones, and their applications. We also summarize the properties, design criteria, merit, and demerit of those contrast agents. In the end, the prospects and challenges for further research and development in this field are outlined.

Genetically Encodable Contrast Agents for Optical Coherence Tomography

Optical coherence tomography (OCT) has gained wide adoption in biological research and medical imaging due to its exceptional tissue penetration, 3D imaging speed, and rich contrast. However, OCT plays a relatively small role in molecular and cellular imaging due to the lack of suitable biomolecular contrast agents. In particular, while the green fluorescent protein has provided revolutionary capabilities to fluorescence microscopy by connecting it to cellular functions such as gene expression, no equivalent reporter gene is currently available for OCT. Here, we introduce gas vesicles, a class of naturally evolved gas-filled protein nanostructures, as genetically encodable OCT contrast agents. The differential refractive index of their gas compartments relative to surrounding aqueous tissue and their nanoscale motion enables gas vesicles to be detected by static and dynamic OCT. Furthermore, the OCT contrast of gas vesicles can be selectively erased in situ with ultrasound, allowing unambiguous assignment of their location. In addition, gas vesicle clustering modulates their temporal signal, enabling the design of dynamic biosensors. We demonstrate the use of gas vesicles as reporter genes in bacterial colonies and as purified contrast agents in vivo in the mouse retina. Our results expand the utility of OCT to image a wider variety of cellular and molecular processes.

Design and synthesis of lipid-mimetic cationic iridium complexes and their liposomal formulation for in vitro and in vivo application in luminescent bioimaging

Two iridium [Ir(N^C)2(N^N)]+ complexes with the diimine N^N ligand containing a long polymethylene hydrophobic chain were synthesized and characterized by using NMR and ESI mass-spectrometry: N^N - 2-(1-hexadecyl-1H-imidazol-2-yl)pyridine, N^C - methyl-2-phenylquinoline-4-carboxylate (Ir1) and 2-phenylquinoline-4-carboxylic acid (Ir2). These complexes were used to prepare the luminescent PEGylated DPPC liposomes (DPPC/DSPE-PEG2000/Ir-complex = 95/4.5/1 mol%) using a thin film hydration method. The narrowly dispersed liposomes had diameters of about 110 nm. The photophysics of the complexes and labeled liposomes were carefully studied. Ir1 and Ir2 give red emission (λ em = 667 and 605 nm) with a lifetime in the microsecond domain and quantum yields of 4.8% and 10.0% in degassed solution. Incorporation of the complexes into the liposome lipid bilayer results in shielding of the emitters from interaction with molecular oxygen and partial suppression of excited state nonradiative relaxation due to the effect of the relatively rigid bilayer matrix. Delivery of labeled liposomes to the cultured ARPE-19 cells demonstrated the usefulness of Ir1 and Ir2 in cellular imaging. Labeled liposomes were then injected intravitreally into rat eyes and imaged successfully with optical coherence tomography and funduscopy. In conclusion, iridium complexes enabled the successful labeling and imaging of liposomes in cells and animals.

Label-free optical imaging in developmental biology [Invited]

Application of optical imaging in developmental biology marks an exciting frontier in biomedical optics. Optical resolution and imaging depth allow for investigation of growing embryos at subcellular, cellular, and whole organism levels, while the complexity and variety of embryonic processes set multiple challenges stimulating the development of various live dynamic embryonic imaging approaches. Among other optical methods, label-free optical techniques attract an increasing interest as they allow investigation of developmental mechanisms without application of exogenous markers or fluorescent reporters. There has been a boost in development of label-free optical imaging techniques for studying embryonic development in animal models over the last decade, which revealed new information about early development and created new areas for investigation. Here, we review the recent progress in label-free optical embryonic imaging, discuss specific applications, and comment on future developments at the interface of photonics, engineering, and developmental biology.

High-resolution, in vivo multimodal photoacoustic microscopy, optical coherence tomography, and fluorescence microscopy imaging of rabbit retinal neovascularization

Photoacoustic microscopy (PAM) is an emerging imaging technology that can non-invasively visualize ocular structures in animal eyes. This report describes an integrated multimodality imaging system that combines PAM, optical coherence tomography (OCT), and fluorescence microscopy (FM) to evaluate angiogenesis in larger animal eyes. High-resolution in vivo imaging was performed in live rabbit eyes with vascular endothelial growth factor (VEGF)-induced retinal neovascularization (RNV). The results demonstrate that our multimodality imaging system can non-invasively visualize RNV in both albino and pigmented rabbits to determine retinal pathology using PAM and OCT and verify the leakage of neovascularization using FM and fluorescein dye. This work presents high-resolution visualization of angiogenesis in rabbits using a multimodality PAM, OCT, and FM system and may represent a major step toward the clinical translation of the technology.

Microvascular contrast enhancement in optical coherence tomography using microbubbles

Gas microbubbles (MBs) are investigated as intravascular optical coherence tomography (OCT) contrast agents. Agar + intralipid scattering tissue phantoms with two embedded microtubes were fabricated to model vascular blood flow. One was filled with human blood, and the other with a mixture of human blood + MB. Swept-source structural and speckle variance (sv) OCT images, as well as speckle decorrelation times, were evaluated under both no-flow and varying flow conditions. Faster decorrelation times and higher structural and svOCT image contrasts were detected in the presence of MB in all experiments. The effects were largest in the svOCT imaging mode, and uniformly diminished with increasing flow velocity. These findings suggest the feasibility of utilizing MB for tissue hemodynamic investigations and for microvasculature contrast enhancement in OCT angiography.

Magnetic and Plasmonic Contrast Agents in Optical Coherence Tomography

Optical coherence tomography (OCT) has gained widespread application for many biomedical applications, yet the traditional array of contrast agents used in incoherent imaging modalities do not provide contrast in OCT. Owing to the high biocompatibility of iron oxides and noble metals, magnetic and plasmonic nanoparticles, respectively, have been developed as OCT contrast agents to enable a range of biological and pre-clinical studies. Here we provide a review of these developments within the past decade, including an overview of the physical contrast mechanisms and classes of OCT system hardware addons needed for magnetic and plasmonic nanoparticle contrast. A comparison of the wide variety of nanoparticle systems is also presented, where the figures of merit depend strongly upon the choice of biological application.

High-resolution in vivo optical imaging of stroke injury and repair

Central nervous system (CNS) function and dysfunction are best understood within a framework of interactions between neuronal, glial and vascular compartments comprising the neurovascular unit (NVU), all of which contribute to stroke-induced CNS injury, plasticity, repair, and recovery. Recent advances in in vivo optical microscopy have enabled us to observe and interrogate cells and their processes with high spatial resolution in real time and in their natural environment deep in the brain tissue. Here, we review some of these state-of-the-art imaging techniques with an emphasis on imaging the interactions among the constituents of the NVU during ischemic injury and repair in small animal models. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

Recent advances in ophthalmic molecular imaging

The aim of molecular imaging techniques is the visualization of molecular processes and functional changes in living animals and human patients before morphological changes occur at the cellular and tissue level. Ophthalmic molecular imaging is still in its infancy and has mainly been used in small animals for pre-clinical research. The goal of most of these pre-clinical studies is their translation into ophthalmic molecular imaging techniques in clinical care. We discuss various molecular imaging techniques and their applications in ophthalmology.

Comparative review of interferometric detection of plasmonic nanoparticles

Noble metal nanoparticles exhibit enhanced scattering and absorption at specific wavelengths due to a localized surface plamson resonance. This unique property can be exploited to enable the use of plasmonic nanoparticles as contrast agents in optical imaging. A range of optical techniques have been developed to detect nanoparticles in order to implement imaging schemes. Here we review several different approaches for using optical interferometry to detect the presence and concentration of nanoparticles. The strengths and weaknesses of the various approaches are discussed and quantitative comparisons of the achievable signal to noise ratios are presented. The benefits of each approach are outlined as they relate to specific application goals.

Optical coherence tomography-current technology and applications in clinical and biomedical research

Optical coherence tomography (OCT) is a noninvasive imaging technique that provides real-time two- and three-dimensional images of scattering samples with micrometer resolution. By mapping the local reflectivity, OCT visualizes the morphology of the sample. In addition, functional properties such as birefringence, motion, or the distributions of certain substances can be detected with high spatial resolution. Its main field of application is biomedical imaging and diagnostics. In ophthalmology, OCT is accepted as a clinical standard for diagnosing and monitoring the treatment of a number of retinal diseases, and OCT is becoming an important instrument for clinical cardiology. New applications are emerging in various medical fields, such as early-stage cancer detection, surgical guidance, and the early diagnosis of musculoskeletal diseases. OCT has also proven its value as a tool for developmental biology. The number of companies involved in manufacturing OCT systems has increased substantially during the last few years (especially due to its success in opthalmology), and this technology can be expected to continue to spread into various fields of application.

Microscopic imaging and spectroscopy with scattered light

Optical contrast based on elastic scattering interactions between light and matter can be used to probe cellular structure, cellular dynamics, and image tissue architecture. The quantitative nature and high sensitivity of light scattering signals to subtle alterations in tissue morphology, as well as the ability to visualize unstained tissue in vivo, has recently generated significant interest in optical-scatter-based biosensing and imaging. Here we review the fundamental methodologies used to acquire and interpret optical scatter data. We report on recent findings in this field and present current advances in optical scatter techniques and computational methods. Cellular and tissue data enabled by current advances in optical scatter spectroscopy and imaging stand to impact a variety of biomedical applications including clinical tissue diagnosis, in vivo imaging, drug discovery, and basic cell biology.

Intraoperative evaluation of breast tumor margins with optical coherence tomography

As breast cancer screening rates increase, smaller and more numerous lesions are being identified earlier, leading to more breast-conserving surgical procedures. Achieving a clean surgical margin represents a technical challenge with important clinical implications. Optical coherence tomography (OCT) is introduced as an intraoperative high-resolution imaging technique that assesses surgical breast tumor margins by providing real-time microscopic images up to 2 mm beneath the tissue surface. In a study of 37 patients split between training and study groups, OCT images covering 1 cm(2) regions were acquired from surgical margins of lumpectomy specimens, registered with ink, and correlated with corresponding histologic sections. A 17-patient training set used to establish standard imaging protocols and OCT evaluation criteria showed that areas of higher scattering tissue with a heterogeneous pattern were indicative of tumor cells and tumor tissue in contrast to lower scattering adipocytes found in normal breast tissue. The remaining 20 patients were enrolled into the feasibility study. Of these lumpectomy specimens, 11 were identified with a positive or close surgical margin and 9 were identified with a negative margin under OCT. Based on histologic findings, 9 true positives, 9 true negatives, 2 false positives, and 0 false negatives were found, yielding a sensitivity of 100% and specificity of 82%. These results show the potential of OCT as a real-time method for intraoperative margin assessment in breast-conserving surgeries.

Nonlinear absorption microscopy

For the past two decades, nonlinear microscopy has been developed to overcome the scattering problem in thick tissue imaging. Owing to its increased imaging depth and high spatial resolution, nonlinear microscopy becomes the first choice for imaging living tissues. The use of nonlinear optical effects not only facilitates the signal originating from an extremely small volume defined by light focusing but also provides novel contrast mechanisms with molecular specificity. Nonlinear absorption is a nonlinear optical effect in which the absorption coefficient depends on excitation intensity. As a commonly used spectroscopy tool, nonlinear absorption measurement uncovers many photophysical and photochemical processes correlated with electronic states of molecules. Recently we have been focusing on adapting this spectroscopy method to a microscopy imaging technique. The effort leads to a novel modality in nonlinear microscopy-nonlinear absorption microscopy. This article summarizes the principles and instrumentation of this imaging technique and highlights some of the recent progress in applying it to imaging skin pigmentation and microvasculature under ex vivo or in vivo conditions.

Enhanced detection of early-stage oral cancer in vivo by optical coherence tomography using multimodal delivery of gold nanoparticles

Contrast in optical coherence tomography (OCT) images can be enhanced by utilizing surface plasmon resonant gold nanoparticles. To improve the poor in vivo transport of gold nanoparticles through biological barriers, an efficient delivery strategy is needed. In this study, the improved penetration and distribution of gold nanoparticles were achieved by microneedle and ultrasound, respectively, and it was demonstrated that this multimodal delivery of antibody-conjugated PEGylated gold nanoparticles enhanced the contrast in in vivo OCT images of oral dysplasia in a hamster model.

Review: Optical Micrometer Resolution Scanning for Non-invasive Grading of Precancer in the Human Uterine Cervix

Management of cervical precancer is archetypal for other cancer prevention programmes but has to consider diagnostic and logistic challenges. Numerous optical tools are emerging for non-destructive near real-time early diagnosis of precancerous lesions of the cervix. Non-destructive, real-time imaging modalities have reached pre-commercial status, but high resolution mapping tools are not yet introduced in clinical settings. The NCBI PubMed web page was searched using the keywords ‘CIN diagnosis’ and the combinations of ‘cervix {confocal, optical coherence tomography, ftir, infrared, Raman, vibrational, spectroscopy}’. Suitable titles were identified and their relevant references followed. Challenges in precancer management are discussed. The following tools capable of non-destructive high resolution mapping in a clinical environment were selected: confocal microscopy, optical coherence tomography, IR spectroscopy, and Raman spectroscopy. Findings on the clinical performance of these techniques are put into context in order to assist the reader in judging the likely performance of these methods as diagnostic tools. Rationale for carrying out research under the prospect of the HPV vaccine is given.

Optical contrast agents and imaging systems for detection and diagnosis of cancer

Molecular imaging has rapidly emerged as a discipline with the potential to impact fundamental biomedical research and clinical practice. Within this field, optical imaging offers several unique capabilities, based on the ability of cells and tissues to effect quantifiable changes in the properties of visible and near-infrared light. Beyond endogenous optical properties, the development of molecularly targeted contrast agents enables disease-specific morphologic and biochemical processes to be labeled with unique optical signatures. Optical imaging systems can then provide real-time visualization of pathophysiology at spatial scales from the subcellular to whole organ levels. In this article, we review fundamental techniques and recent developments in optical molecular imaging, emphasizing laboratory and clinical systems that aim to visualize the microscopic and macroscopic hallmarks of cancer.

Towards automated classification of clinical optical coherence tomography data of dense tissues

The native contrast of optical coherence tomography (OCT) data in dense tissues can pose a challenge for clinical decision making. Automated data evaluation is one way of enhancing the clinical utility of measurements. Methods for extracting information from structural OCT data are appraised here. A-scan analysis allows characterization of layer thickness and scattering parameters, whereas image analysis renders itself to segmentation, texture and speckle analysis. All fully automated approaches combine pre-processing, feature registration, data reduction, and classification. Pre-processing requires de-noising, feature recognition, normalization and refining. In the current literature, image exclusion criteria, initial parameters, or manual input are common requirements. The interest of the presented methods lies in the prospect of objective, quick, and/or post-acquisition processing. There is a potential to improve clinical decision making based on automated processing of OCT data.

Combined Raman spectroscopy and optical coherence tomography device for tissue characterization

We report a dual-modal device capable of sequential acquisition of Raman spectroscopy (RS) and optical coherence tomography (OCT) along a common optical axis. The device enhances application of both RS and OCT by precisely guiding RS acquisition with OCT images while also compensating for the lack of molecular specificity in OCT with the biochemical specificity of RS. We characterize the system performance and demonstrate the capability to identify structurally ambiguous features within an OCT image with RS in a scattering phantom, guide acquisition of RS from a localized malignancy in ex vivo breast tissue, and perform in vivo tissue analysis of a scab.

Fluorescence interferometry: principles and applications in biology

The use of fluorescence radiation is of fundamental importance for tackling measurement problems in the life sciences, with recent demonstrations of probing biological systems at the nanoscale. Usually, fluorescent light-based tools and techniques use the intensity of light waves, which is easily measured by detectors. However, the phase of a fluorescence wave contains subtle, but no less important, information about the wave; yet, it has been largely unexplored. Here, we introduce the concept of fluorescence interferometry to allow the measurement of phase information of fluorescent light waves. In principle, fluorescence interferometry can be considered a unique form of optical low-coherence interferometry that uses fluorophores as a light source of low temporal coherence. Fluorescence interferometry opens up new avenues for developing new fluorescent light-based imaging, sensing, ranging, and profiling methods that to some extent resemble interferometric techniques based on white light sources. We propose two experimental realizations of fluorescence interferometry that detect the interference pattern cast by the fluorescence fields. This article discusses their measurement capabilities and limitations and compares them with those offered by optical low-coherence interferometric schemes. We also describe applications of fluorescence interferometry to imaging, ranging, and profiling tasks and present experimental evidences of wide-field cross-sectional imaging with high resolution and large range of depth, as well as quantitative profiling with nanometer-level precision. Finally, we point out future research directions in fluorescence interferometry, such as fluorescence tomography of whole organisms and the extension to molecular interferometry by means of quantum dots and bioluminescence.

Harmonically matched grating-based full-field quantitative high-resolution phase microscope for observing dynamics of transparent biological samples

We have developed a full-field high resolution quantitative phase imaging technique for observing dynamics of transparent biological samples. By using a harmonically matched diffraction grating pair (600 and 1200 lines/mm), we were able to obtain non-trivial phase difference (other than 0 degrees or 180 degrees) between the output ports of the gratings. Improving upon our previous design, our current system mitigates astigmatism artifacts and is capable of high resolution imaging. This system also employs an improved phase extraction algorithm. The system has a lateral resolution of 1.6 mum and a phase sensitivity of 62 mrad. We employed the system to acquire high resolution phase images of onion skin cells and a phase movie of amoeba proteus in motion.

Multimodality optical imaging of embryonic heart microstructure

Study of developmental heart defects requires the visualization of the microstructure and function of the embryonic myocardium, ideally with minimal alterations to the specimen. We demonstrate multiple endogenous contrast optical techniques for imaging the Xenopus laevis tadpole heart. Each technique provides distinct and complementary imaging capabilities, including: 1. 3-D coherence microscopy with subcellular (1 to 2 microm) resolution in fixed embryos, 2. real-time reflectance confocal microscopy with large penetration depth in vivo, and 3. ultra-high speed (up to 900 frames per second) that enables real-time 4-D high resolution imaging in vivo. These imaging modalities can provide a comprehensive picture of the morphologic and dynamic phenotype of the embryonic heart. The potential of endogenous-contrast optical microscopy is demonstrated for investigation of the teratogenic effects of ethanol. Microstructural abnormalities associated with high levels of ethanol exposure are observed, including compromised heart looping and loss of ventricular trabecular mass.

Optical coherence tomography: a review of clinical development from bench to bedside

Since its introduction, optical coherence tomography (OCT) technology has advanced from the laboratory bench to the clinic and back again. Arising from the fields of low coherence interferometry and optical time- and frequency-domain reflectometry, OCT was initially demonstrated for retinal imaging and followed a unique path to commercialization for clinical use. Concurrently, significant technological advances were brought about from within the research community, including improved laser sources, beam delivery instruments, and detection schemes. While many of these technologies improved retinal imaging, they also allowed for the application of OCT to many new clinical areas. As a result, OCT has been clinically demonstrated in a diverse set of medical and surgical specialties, including gastroenterology, dermatology, cardiology, and oncology, among others. The lessons learned in the clinic are currently spurring a new set of advances in the laboratory that will again expand the clinical use of OCT by adding molecular sensitivity, improving image quality, and increasing acquisition speeds. This continuous cycle of laboratory development and clinical application has allowed the OCT technology to grow at a rapid rate and represents a unique model for the translation of biomedical optics to the patient bedside. This work presents a brief history of OCT development, reviews current clinical applications, discusses some clinical translation challenges, and reviews laboratory developments poised for future clinical application.

Optical coherence tomography with plasmon resonant nanorods of gold

We explored plasmon resonant nanorods of gold as a contrast agent for optical coherence tomography (OCT). Nanorod suspensions were generated through wet chemical synthesis and characterized with spectrophotometry, transmission electron microscopy, and OCT. Polyacrylamide-based phantoms were generated with appropriate scattering and anisotropy coefficients (30 cm(-1) and 0.89, respectively) to image distribution of the contrast agent in an environment similar to that of tissue. The observed signal was dependent on whether the plasmon resonance peak overlapped the source bandwidth of the OCT, confirming the resonant character of enhancement. Gold nanorods with plasmon resonance wavelengths overlapping the OCT source yielded a signal-to-background ratio of 4.5 dB, relative to the tissue phantom. Strategies for OCT imaging with nanorods are discussed.

Recent developments in optical coherence tomography for imaging the retina

Optical coherence tomography (OCT) was introduced in ophthalmology a decade ago. Within a few years in vivo imaging of the healthy retina and optic nerve head and of retinal diseases was a fact. In particular the ease with which these images can be acquired considerably changed the diagnostic strategy used by ophthalmologists. The OCT technique currently available in clinical practice is referred to as time-domain OCT, because the depth information of the retina is acquired as a sequence of samples, over time. This can be done either in longitudinal cross-sections perpendicular to, or in the coronal plane parallel to the retinal surface. Only recently, major advances have been made as to image resolution with the introduction of ultrahigh resolution OCT and in imaging speed, signal-to-noise ratio and sensitivity with the introduction of spectral-domain OCT. Functional OCT is the next frontier in OCT imaging. For example, polarization-sensitive OCT uses the birefringent characteristics of the retinal nerve fibre layer to better assess its thickness. Blood flow information from retinal vessels as well as the oxygenation state of retinal tissue can be extracted from the OCT signal. Very promising are the developments in contrast-enhanced molecular optical imaging, for example with the use of scattering tuneable nanoparticles targeted at specific tissue or cell structures. This review will provide an overview of these most recent developments in the field of OCT imaging focussing on applications for the retina.

Methods and application areas of endoscopic optical coherence tomography

We review the current state of research in endoscopic optical coherence tomography (OCT). We first survey the range of available endoscopic optical imaging techniques. We then discuss the various OCT-based endoscopic methods that have thus far been developed. We compare the different endoscopic OCT methods in terms of their scan performance. Next, we examine the application range of endoscopic OCT methods. In particular, we look at the reported utility of the methods in digestive, intravascular, respiratory, urinary and reproductive systems. We highlight two additional applications--biopsy procedures and neurosurgery--where sufficiently compact OCT-based endoscopes can have significant clinical impacts.

Molecular contrast optical coherence tomography: A pump-probe scheme using indocyanine green as a contrast agent

The use of indocyanine green (ICG), a U.S. Food and Drug Administration approved dye, in a pump-probe scheme for molecular contrast optical coherence tomography (MCOCT) is proposed and demonstrated for the first time. In the proposed pump-probe scheme, an optical coherence tomography (OCT) scan of the sample containing ICG is first acquired. High fluence illumination (approximately 190 kJ/cm2) is then used to permanently photobleach the ICG molecules--resulting in a permanent alteration of the overall absorption of the ICG. A second OCT scan is next acquired. The difference of the two OCT scans is used to determine the depth resolved distribution of ICG within a sample. To characterize the extent of photobleaching in different ICG solutions, we determine the cumulative probability of photobleaching, phi(B,cum), defined as the ratio of the total photobleached ICG molecules to the total photons absorbed by the ground state molecules. An empirical study of ICG photobleaching dynamics shows that phi(B,cum) decreases with fluence as well as with increasing dye concentration. The quantity phi(B,cum) is useful for estimating the extent of photobleaching in an ICG sample (MCOCT contrast) for a given fluence of the pump illumination. The paper also demonstrates ICG-based MCOCT imaging in tissue phantoms as well as within stage 54 Xenopus laevis.

Dual modality imaging of a novel rat model of ovarian carcinogenesis

Ovarian cancer is the fifth leading cause of cancer death in women, in part because of the limited knowledge about early stage disease. We develop a novel rat model of ovarian cancer and perform a pilot study to examine the harvested ovaries with complementary optical imaging modalities. Rats are exposed to repeated daily dosing (20 days) with 4-vinylcyclohexene diepoxide (VCD) to cause early ovarian failure (model for postmenopause), and ovaries are directly exposed to 7,12-dimethylbenz(a)anthracene (DMBA) to cause abnormal ovarian proliferation and neoplasia. Harvested ovaries are examined with optical coherence tomography (OCT) and light-induced fluorescence (LIF) at one, three, and five months post-DMBA treatment. VCD causes complete ovarian follicle depletion within 8 months after onset of dosing. DMBA induces abnormal size, cysts, and neoplastic changes. OCT successfully visualizes normal and abnormal structures (e.g., cysts, bursa, follicular remnant degeneration) and the LIF spectra show statistically significant changes in the ratio of average emission intensity at 390:450 nm between VCD-treated ovaries and both normal cycling and neoplastic DMBA-treated ovaries. Overall, this pilot study demonstrates the feasibility of both the novel animal model for ovarian cancer and the ability of optical imaging techniques to visualize ovarian function and health.

Quantitative evaluation of optical coherence tomography signal enhancement with gold nanoshells

Nanoshell-enhanced optical coherence tomography (OCT) is a novel technique with the potential for molecular imaging and improved disease detection. However, optimization of this approach will require a quantitative understanding of the influence of nanoshell parameters on detected OCT signals. In this study, OCT was performed at 1310 nm in water and turbid tissue-simulating phantoms to which nanoshells were added. The effect of nanoshell concentration, core diameter, and shell thickness on signal enhancement was characterized. Experimental results indicated trends that were consistent with predicted optical properties-a monotonic increase in signal intensity and attenuation with increasing shell and core size. Threshold concentrations for a 2-dB OCT signal intensity gain were determined for several nanoshell geometries. For the most highly backscattering nanoshells tested-291-nm core diameter, 25-nm shell thickness-a concentration of 10(9) nanoshells/mL was needed to produce this signal increase. Based on these results, we discuss various practical considerations for optimizing nanoshell-enhanced OCT. Quantitative experimental data presented here will facilitate optimization of OCT-based diagnostics and may also be relevant to other reflectance-based approaches as well.

Three-dimensional optical coherence tomography of the embryonic murine cardiovascular system

Optical coherence tomography (OCT) is an emerging high-resolution real-time biomedical imaging technology that has potential as a novel investigational tool in developmental biology and functional genomics. In this study, murine embryos and embryonic hearts are visualized with an OCT system capable of 2-microm axial and 15-microm lateral resolution and with real-time acquisition rates. We present, to our knowledge, the first sets of high-resolution 2- and 3-D OCT images that reveal the internal structures of the mammalian (murine) embryo (E10.5) and embryonic (E14.5 and E17.5) cardiovascular system. Strong correlations are observed between OCT images and corresponding hematoxylin- and eosin-stained histological sections. Real-time in vivo embryonic (E10.5) heart activity is captured by spectral-domain optical coherence tomography, processed, and displayed at a continuous rate of five frames per second. With the ability to obtain not only high-resolution anatomical data but also functional information during cardiovascular development, the OCT technology has the potential to visualize and quantify changes in murine development and in congenital and induced heart disease, as well as enable a wide range of basic in vitro and in vivo research studies in functional genomics.

Multimodal imaging of mouse development: Tools for the postgenomic era

With the sequence of the mouse genome known, it is now possible to create or identify mutations in every gene to determine the molecules necessary for normal development. Consequently, there is a growing need for advanced phenotyping tools to best understand defects produced by altering gene function. Perhaps nothing is more satisfying than to directly observe a process in action; to disturb it and see for ourselves how the process changes before our very eyes. No doubt, this desire is what drove the invention of the very first microscopes and continues to this day to fuel progress in the field of biological imaging. Because mouse embryos are small and develop embedded within many tissue layers within the nurturing environment of the mother, directly observing the dynamic, micro- and nanoscopic events of early mammalian development has proven to be one of the greater challenges for imaging scientists. Here, I will review some of the imaging methods being used to study mouse development, highlighting the results obtained from imaging.

Advances in contrast enhancement for optical coherence tomography

Contrast in optical coherence tomography (OCT) images is often limited, particularly when pathological tissue is morphologically or optically similar to normal tissue. In recent years, there has been increasing interest in developing methods for enhancing OCT contrast. In general, contrast can be enhanced by the administration of passive or targeted exogenous contrast agents, or by exploiting linear and nonlinear techniques for sampling the endogenous molecular composition of tissue. Many exogenous agents, in addition to being targeted to specific cells and tissues, can also serve as multifunctional agents, delivering or facilitating therapy as well as providing enhanced contrast for imaging and localization. This paper and presentation will discuss novel OCT contrast enhancing methods designed to selectively identify tissues of interest.

Spectral domain optical coherence tomography: a better OCT imaging strategy

This paper reviews the current state of research in spectral domain optical coherence tomography (SDOCT). SDOCT is an interferometric technique that provides depth-resolved tissue structure information encoded in the magnitude and delay of the back-scattered light by spectral analysis of the interference fringe pattern. There are two approaches to SDOCT—one that uses a broadband source and a spectrometer to measure the interference pattern as a function of wavelength and the other that utilizes a narrowband tunable laser that is swept linearly in k ∼ 1/λ space during spectral fringe data acquisition. Unlike time domain (TD) OCT, the reference arm is stationary in both SDOCT methods, which allows for ultra high-speed OCT imaging. Owing to its high speed and superior sensitivity, SDOCT has become indispensable in biomedical imaging applications. After a brief introduction and a discussion on sensitivity advantage, methods of implementation of the two SDOCT schemes will be presented. The two peer approaches are compared in speed, scan depth range, complexity, spectral regions of operation, and methods of detection. The review also discusses OCT enhancements and functional methods based on SDOCT format and concludes with possible directions that this research may take in the near future.

Correction of artefacts in optical projection tomography

A new imaging technique called optical projection tomography (OPT), essentially an optical version of x-ray computed tomography (CT), provides molecular specificity, cellular resolution and larger specimen coverage ( approximately 1 cubic centimetre) than was previously possible with other imaging techniques. It is ideally suited to gene expression studies in small animals. Reconstructed OPT images demonstrate several artefacts which reduce the overall image quality. In this paper, we describe methods to prevent smear artefacts due to illumination intensity fluctuation, ring artefacts due to CCD pixel sensitivity variation and a new 'detector edge' artefact caused by non-zero background signal. We also present an automated method to align the position of the rotational axis during image reconstruction. Finally, we propose a method to eliminate bowl artefacts due to projection truncation using a lower resolution OPT scan of the same specimen. This solution also provides OPT with the ability to obtain a high-resolution reconstruction from a region of interest of a specimen that is larger than the field of view. Implementation of these corrections and modifications increases the accuracy of the OPT imaging technique and extends its capabilities to obtain higher resolution data from within a whole specimen.

Optical probes and techniques for molecular contrast enhancement in coherence imaging

Optics has played a key role in the rapidly developing field of molecular imaging. The spectroscopic nature and high-resolution imaging capabilities of light provide a means for probing biological morphology and function at the cellular and molecular levels. While the use of bioluminescent and fluorescent probes has become a mainstay in optical molecular imaging, a large number of other optical imaging modalities exist that can be included in this emerging field. In vivo imaging technologies such as optical coherence tomography and reflectance confocal microscopy have had limited use of molecular probes. In the last few years, novel nonfluorescent and nonbioluminescent molecular imaging probes have been developed that will initiate new directions in coherent optical molecular imaging. Classes of probes reviewed in this work include those that alter the local optical scattering or absorption properties of the tissue, those that modulate these local optical properties in a predictable manner, and those that are detected utilizing spectroscopic optical coherence tomography (OCT) principles. In addition to spectroscopic OCT, novel nonlinear interferometric imaging techniques have recently been developed to detect endogenous molecules. Probes and techniques designed for coherent molecular imaging are likely to improve the detection and diagnostic capabilities of OCT.