4D embryonic cardiography using gated optical coherence tomography

Hemodynamics During Development and Postnatal Life

A well-developed heart is essential for embryonic survival. There are constant interactions between cardiac tissue motion and blood flow, which determine the heart shape itself. Hemodynamic forces are a powerful stimulus for cardiac growth and differentiation. Therefore, it is particularly interesting to investigate how the blood flows through the heart and how hemodynamics is linked to a particular species and its development, including human. The appropriate patterns and magnitude of hemodynamic stresses are necessary for the proper formation of cardiac structures, and hemodynamic perturbations have been found to cause malformations via identifiable mechanobiological molecular pathways. There are significant differences in cardiac hemodynamics among vertebrate species, which go hand in hand with the presence of specific anatomical structures. However, strong similarities during development suggest a common pattern for cardiac hemodynamics in human adults. In the human fetal heart, hemodynamic abnormalities during gestation are known to progress to congenital heart malformations by birth. In this chapter, we discuss the current state of the knowledge of the prenatal cardiac hemodynamics, as discovered through small and large animal models, as well as from clinical investigations, with parallels gathered from the poikilotherm vertebrates that emulate some hemodynamically significant human congenital heart diseases.

Experimental assessment of cardiovascular physiology in the chick embryo

High resolution assessment of cardiac functional parameters is crucial in translational animal research. The chick embryo is a historically well-used in vivo model for cardiovascular research due to its many practical advantages, and the conserved form and function of the chick and human cardiogenesis programs. This review aims to provide an overview of several different technical approaches for chick embryo cardiac assessment. Doppler echocardiography, optical coherence tomography, micromagnetic resonance imaging, microparticle image velocimetry, real-time pressure monitoring, and associated issues with the techniques will be discussed. Alongside this discussion, we also highlight recent advances in cardiac function measurements in chick embryos.

Imaging Approaches and the Quantitative Analysis of Heart Development

Heart morphogenesis is a complex and dynamic process that has captivated researchers for almost a century. This process involves three main stages, during which the heart undergoes growth and folding on itself to form its common chambered shape. However, imaging heart development presents significant challenges due to the rapid and dynamic changes in heart morphology. Researchers have used different model organisms and developed various imaging techniques to obtain high-resolution images of heart development. Advanced imaging techniques have allowed the integration of multiscale live imaging approaches with genetic labeling, enabling the quantitative analysis of cardiac morphogenesis. Here, we discuss the various imaging techniques used to obtain high-resolution images of whole-heart development. We also review the mathematical approaches used to quantify cardiac morphogenesis from 3D and 3D+time images and to model its dynamics at the tissue and cellular levels.

OCT based four-dimensional cardiac imaging of a living chick embryo using an impedance signal as a gating for post-acquisition synchronization

Optical coherence tomography (OCT) is a non-invasive imaging modality with high spatial resolution suitable for early embryonic heart imaging. However, the most commonly used OCT systems cannot provide direct 4-D imaging due to acquisition speed limitations. We proposed a retrospective gating 4-D reconstruction method based on spectral domain OCT. A special circuit was designed to measure the impedance change of chick embryos in response to the heart beating. The impedance signal was acquired simultaneously with the OCT B-scan image sequence at several different locations along the heart. The impedance signal was used as a gating for 4-D reconstruction. The reconstruction algorithm includes cardiac period calculation, interpolation from multi-cardiac cycle image sequence into one cardiac cycle, and cardiac phase synchronization among the different locations of the heart. The synchronism of the impedance signal change with the heartbeat was verified. Using the proposed method, we reconstructed the cardiac outflow tract (OFT) of chick embryos at an early stage of development (Hamburger-Hamilton stage 18). We showed that the reconstructed 4-D images correctly captured the dynamics of the OFT wall motion.

Mouse embryo phenotyping with optical coherence tomography

With the explosion of gene editing tools in recent years, there has been a much greater demand for mouse embryo phenotyping, and traditional methods such as histology and histochemistry experienced a methodological renaissance as they became the principal tools for phenotyping. However, it is important to explore alternative phenotyping options to maximize time and resources and implement volumetric structural analysis for enhanced investigation of phenotypes. Cardiovascular phenotyping, in particular, is important to perform in vivo due to the dramatic structural and functional changes that occur in heart development over relatively short periods of time. Optical coherence tomography (OCT) is one of the most exciting advanced imaging techniques emerging within the field of developmental biology, and this review provides a summary of how it is currently being implemented in mouse embryo investigations and phenotyping. This review aims to provide an understanding of the approaches used in optical coherence tomography and how they can be applied in embryology and developmental biology, with the overall aim of bridging the gap between biology and technology.

Dynamic Imaging of Mouse Embryos and Cardiac Development in Static Culture

Dynamic imaging is a powerful approach to assess the function of a developing organ system. The heart is a dynamic organ that undergoes quick morphological and mechanical changes through early embryonic development. Defining the embyonic mouse heart's normal function is important for our own understanding of human heart development and will inform us on treatments and prevention of congenital heart defects (CHD). Traditional methods such as ultrasound or fluorescence-based microscopy are suitable for live dynamic imaging, are excellent to visualize structure and connect gene expression to phenotypes, but can be of low quality in resolving fine features and lack imaging depth and scale to fully appreciate organ morphogenesis. Additionally, previous methods can be limited in accommodating a live imaging apparatus capable of sustaining whole embryo development for extended periods time. Optical coherence tomography (OCT) is unique in this circumstance because acquisition of three-dimensional images without contrast reagents, at single cell resolution make it a suitable modality to visualize fine structures in the developing embryo. OCT setups are highly customizable for live imaging because of the tethered imaging arm, due to its setup as a fiber-based interferometer. OCT allows for 4D (3D + time) functional imaging of living mouse embryos and can provide functional and mechanical information to ascertain how the heart's pump function changes through development. In this chapter, we will focus on how we use OCT to visualize live heart dynamics at different stages of development and provide mechanical information to reveal functional properties of the developing heart.

Embryonic Mouse Cardiodynamic OCT Imaging

The embryonic heart is an active and developing organ. Genetic studies in mouse models have generated great insight into normal heart development and congenital heart defects, and suggest mechanical forces such as heart contraction and blood flow to be implicated in cardiogenesis and disease. To explore this relationship and investigate the interplay between biomechanical forces and cardiac development, live dynamic cardiac imaging is essential. Cardiodynamic imaging with optical coherence tomography (OCT) is proving to be a unique approach to functional analysis of the embryonic mouse heart. Its compatibility with live culture systems, reagent-free contrast, cellular level resolution, and millimeter scale imaging depth make it capable of imaging the heart volumetrically and providing spatially resolved information on heart wall dynamics and blood flow. Here, we review the progress made in mouse embryonic cardiodynamic imaging with OCT, highlighting leaps in technology to overcome limitations in resolution and acquisition speed. We describe state-of-the-art functional OCT methods such as Doppler OCT and OCT angiography for blood flow imaging and quantification in the beating heart. As OCT is a continuously developing technology, we provide insight into the future developments of this area, toward the investigation of normal cardiogenesis and congenital heart defects.

Intravital imaging of host-parasite interactions in organs of the thoracic and abdominopelvic cavities

Infections with protozoan and helminthic parasites affect multiple organs in the mammalian host. Imaging pathogens in their natural environment takes a more holistic view on biomedical aspects of parasitic infections. Here, we focus on selected organs of the thoracic and abdominopelvic cavities most commonly affected by parasites. Parasitic infections of these organs are often associated with severe medical complications or have health implications beyond the infected individual. Intravital imaging has provided a more dynamic picture of the host-parasite interplay and contributed not only to our understanding of the various disease pathologies, but has also provided fundamental insight into the biology of the parasites.

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.

Depth-Resolved Enhanced Spectral-Domain OCT Imaging of Live Mammalian Embryos Using Gold Nanoparticles as Contrast Agent

High-resolution and real-time visualization of the morphological changes during embryonic development are critical for studying congenital anomalies. Optical coherence tomography (OCT) has been used to investigate the process of embryogenesis. However, the structural visibility of the embryo is decreased with the depth due to signal roll-off and high light scattering. To overcome these obstacles, in this study, combined is a spectral-domain OCT (SD-OCT) with gold nanorods (GNRs) for 2D/3D imaging of live mouse embryos. Inductively coupled plasma mass spectrometry is used to confirm that GNRs can be effectively delivered to the embryos during ex vivo culture. OCT signal, image contrast, and penetration depth are all enhanced on the embryos with GNRs. These results show that after GNR treatment, more accurate spatial localization and better contrasting of the borders among organs can be observed on E9.5 and E10.5 mouse embryos. Furthermore, the strong optical absorbance of GNRs results in much clearer 3D images of the embryos, which can be used for calculating the heart areas and volumes of E9.5 and E10.5 embryos. These findings provide a promising strategy for monitoring organ development and detecting congenital structural abnormalities in mice.

Segmentation of Drosophila heart in optical coherence microscopy images using convolutional neural networks

Convolutional neural networks (CNNs) are powerful tools for image segmentation and classification. Here, we use this method to identify and mark the heart region of Drosophila at different developmental stages in the cross-sectional images acquired by a custom optical coherence microscopy (OCM) system. With our well-trained CNN model, the heart regions through multiple heartbeat cycles can be marked with an intersection over union of ~86%. Various morphological and dynamical cardiac parameters can be quantified accurately with automatically segmented heart regions. This study demonstrates an efficient heart segmentation method to analyze OCM images of the beating heart in Drosophila.

Dynamic Imaging of Mouse Embryos and Cardiodynamics in Static Culture

The heart is a dynamic organ that quickly undergoes morphological and mechanical changes through early embryonic development. Characterizing these early moments is important for our understanding of proper embryonic development and the treatment of heart disease. Traditionally, tomographic imaging modalities and fluorescence-based microscopy are excellent approaches to visualize structural features and gene expression patterns, respectively, and connect aberrant gene programs to pathological phenotypes. However, these approaches usually require static samples or fluorescent markers, which can limit how much information we can derive from the dynamic and mechanical changes that regulate heart development. Optical coherence tomography (OCT) is unique in this circumstance because it allows for the acquisition of three-dimensional structural and four-dimensional (3D + time) functional images of living mouse embryos without fixation or contrast reagents. In this chapter, we focus on how OCT can visualize heart morphology at different stages of development and provide cardiodynamic information to reveal mechanical properties of the developing heart.

A method to study the hemodynamics of chicken embryo's aortic arches using optical coherence tomography

Congenital cardiovascular defects are the leading cause of birth defect related death. It has been hypothesized that fluid mechanical forces of embryonic blood flow affect cardiovascular development and play a role in congenital malformations. Studies in small animal embryos can improve our understanding of congenital malformations and can lead to better treatment. We present a feasibility study in which high-resolution optical coherence tomography (OCT) and computational fluid dynamics (CFD) are combined to provide quantitative analysis of the embryonic flow mechanics and the associated anatomy in a small animal model.

Embryonic aortic arch hemodynamics are a functional biomarker for ethanol-induced congenital heart defects [Invited]

The great arteries develop from symmetrical aortic arch arteries which are extensively remodeled. These events are vulnerable to perturbations. Hemodynamic forces have a significant role in this remodeling. In this study, optical coherence tomography (OCT) visualized live avian embryos for staging and measuring pharyngeal arch morphology. Measurements acquired with our orientation-independent, dual-angle Doppler OCT technique revealed that ethanol exposure leads to higher absolute blood flow, shear stress, and retrograde flow. Ethanol-exposed embryos had smaller cardiac neural crest (CNC) derived pharyngeal arch mesenchyme and fewer migrating CNC-derived cells. These differences in forces and CNC cell numbers could explain the abnormal aortic arch remodeling.

Four-dimensional live imaging of hemodynamics in mammalian embryonic heart with Doppler optical coherence tomography

Hemodynamic analysis of the mouse embryonic heart is essential for understanding the functional aspects of early cardiogenesis and advancing the research in congenital heart defects. However, high-resolution imaging of cardiac hemodynamics in mammalian models remains challenging, primarily due to the dynamic nature and deep location of the embryonic heart. Here we report four-dimensional micro-scale imaging of blood flow in the early mouse embryonic heart, enabling time-resolved measurement and analysis of flow velocity throughout the heart tube. Our method uses Doppler optical coherence tomography in live mouse embryo culture, and employs a post-processing synchronization approach to reconstruct three-dimensional data over time at a 100 Hz volume rate. Experiments were performed on live mouse embryos at embryonic day 9.0. Our results show blood flow dynamics inside the beating heart, with the capability for quantitative flow velocity assessment in the primitive atrium, atrioventricular and bulboventricular regions, and bulbus cordis. Combined cardiodynamic and hemodynamic analysis indicates this functional imaging method can be utilized to further investigate the mechanical relationship between blood flow dynamics and cardiac wall movement, bringing new possibilities to study biomechanics in early mammalian cardiogenesis. Four-dimensional live hemodynamic imaging of the mouse embryonic heart at embryonic day 9.0 using Doppler optical coherence tomography, showing directional blood flows in the sinus venosus, primitive atrium, atrioventricular region and vitelline vein.

Optical Coherence Tomography for Brain Imaging and Developmental Biology

Optical coherence tomography (OCT) is a promising research tool for brain imaging and developmental biology. Serving as a three-dimensional optical biopsy technique, OCT provides volumetric reconstruction of brain tissues and embryonic structures with micrometer resolution and video rate imaging speed. Functional OCT enables label-free monitoring of hemodynamic and metabolic changes in the brain in vitro and in vivo in animal models. Due to its non-invasiveness nature, OCT enables longitudinal imaging of developing specimens in vivo without potential damage from surgical operation, tissue fixation and processing, and staining with exogenous contrast agents. In this paper, various OCT applications in brain imaging and developmental biology are reviewed, with a particular focus on imaging heart development. In addition, we report findings on the effects of a circadian gene (Clock) and high-fat-diet on heart development in Drosophila melanogaster. These findings contribute to our understanding of the fundamental mechanisms connecting circadian genes and obesity to heart development and cardiac diseases.

Applicability, usability, and limitations of murine embryonic imaging with optical coherence tomography and optical projection tomography

We present an analysis of imaging murine embryos at various embryonic developmental stages (embryonic day 9.5, 11.5, and 13.5) by optical coherence tomography (OCT) and optical projection tomography (OPT). We demonstrate that while OCT was capable of rapid high-resolution live 3D imaging, its limited penetration depth prevented visualization of deeper structures, particularly in later stage embryos. In contrast, OPT was able to image the whole embryos, but could not be used in vivo because the embryos must be fixed and cleared. Moreover, the fixation process significantly altered the embryo morphology, which was quantified by the volume of the eye-globes before and after fixation. All of these factors should be weighed when determining which imaging modality one should use to achieve particular goals of a study.

Optical coherence tomography for embryonic imaging: a review

Embryogenesis is a highly complex and dynamic process, and its visualization is crucial for understanding basic physiological processes during development and for identifying and assessing possible defects, malformations, and diseases. While traditional imaging modalities, such as ultrasound biomicroscopy, micro-magnetic resonance imaging, and micro-computed tomography, have long been adapted for embryonic imaging, these techniques generally have limitations in their speed, spatial resolution, and contrast to capture processes such as cardiodynamics during embryogenesis. Optical coherence tomography (OCT) is a noninvasive imaging modality with micrometer-scale spatial resolution and imaging depth up to a few millimeters in tissue. OCT has bridged the gap between ultrahigh resolution imaging techniques with limited imaging depth like confocal microscopy and modalities, such as ultrasound sonography, which have deeper penetration but poorer spatial resolution. Moreover, the noninvasive nature of OCT has enabled live imaging of embryos without any external contrast agents. We review how OCT has been utilized to study developing embryos and also discuss advances in techniques used in conjunction with OCT to understand embryonic development.

Four-Dimensional Imaging of Cardiac Trabeculae Contracting In Vitro Using Gated OCT

Cardiac trabeculae are widely used as experimental muscle preparations for studying heart muscle. However, their geometry (diameter, length, and shape) can vary not only among samples, but also within a sample, leading to inaccuracies in estimating their stress production, volumetric energy output, and/or oxygen consumption. Hence, it is desirable to have a system that can accurately image each trabecula in vitro during an experiment. To this end, we constructed an optical coherence tomography system and implemented a gated imaging procedure to image actively contracting trabeculae and reconstruct their time-varying geometry. By imaging a single cross section while monitoring the developed force, we found that gated stimulation of the muscle was sufficiently repeatable to allow us to reconstruct multiple contractions to form a four-dimensional representation of a single muscle contraction cycle. The complete muscle was imaged at various lengths and the cross-sectional area along the muscle was quantified during the contraction cycle. The variation of cross-sectional area along the length during a contraction tended to increase as the muscle was contracting, and this increase was greater at longer muscle lengths. To our knowledge, this is the first system that is able to measure the geometric change of cardiac trabeculae in vitro during a contraction, allowing cross-sectional stress and other volume-dependent parameters to be estimated with greater accuracy.

In vivo assessment of wall strain in embryonic chick heart by spectral domain optical coherence tomography

The ability to measure in vivo wall strain in embryonic hearts is important for fully understanding the mechanisms of cardiac development. Optical coherence tomography (OCT) is a powerful tool for the three-dimensional imaging of complex myocardial activities in early-stage embryonic hearts with high spatial and temporal resolutions. We describe a method to analyze periodic deformations of myocardial walls and evaluate in vivo myocardial wall strains with a high-speed spectral domain OCT system. We perform four-dimensional scanning on the outflow tract (OFT) of chick embryonic hearts and determine a special cross-section in which the OFT can be approximated as an annulus by analyzing Doppler blood-flow velocities. For each image acquired at the special cross-section, the annular myocardial wall is segmented with a semiautomatic boundary-detection algorithm, and the fluctuation myocardial wall thickness is calculated from the area and mean circumference of the myocardial wall. The experimental results shown with the embryonic chick hearts demonstrate that the proposed method is a useful tool for studying the biomechanical characteristics of embryonic hearts.

Direct four-dimensional structural and functional imaging of cardiovascular dynamics in mouse embryos with 1.5 MHz optical coherence tomography

High-resolution three-dimensional (3D) imaging of cardiovascular dynamics in mouse embryos is greatly desired to study mammalian congenital cardiac defects. Here, we demonstrate direct four-dimensional (4D) imaging of the cardiovascular structure and function in live mouse embryos at a ∼43  Hz volume rate using an optical coherence tomography (OCT) system with a ∼1.5  MHz Fourier domain mode-locking swept laser source. Combining ultrafast OCT imaging with live mouse embryo culture protocols, 3D volumes of the embryo are directly and continuously acquired over time for a cardiodynamics analysis without the application of any synchronization algorithms. We present the time-resolved measurements of the heart wall motion based on the 4D structural data, report 4D speckle variance and Doppler imaging of the vascular system, and quantify spatially resolved blood flow velocity over time. These results indicate that the ultra-high-speed 4D imaging approach could be a useful tool for efficient cardiovascular phenotyping of mouse embryos.

4D subject-specific inverse modeling of the chick embryonic heart outflow tract hemodynamics

Blood flow plays a critical role in regulating embryonic cardiac growth and development, with altered flow leading to congenital heart disease. Progress in the field, however, is hindered by a lack of quantification of hemodynamic conditions in the developing heart. In this study, we present a methodology to quantify blood flow dynamics in the embryonic heart using subject-specific computational fluid dynamics (CFD) models. While the methodology is general, we focused on a model of the chick embryonic heart outflow tract (OFT), which distally connects the heart to the arterial system, and is the region of origin of many congenital cardiac defects. Using structural and Doppler velocity data collected from optical coherence tomography, we generated 4D ([Formula: see text]) embryo-specific CFD models of the heart OFT. To replicate the blood flow dynamics over time during the cardiac cycle, we developed an iterative inverse-method optimization algorithm, which determines the CFD model boundary conditions such that differences between computed velocities and measured velocities at one point within the OFT lumen are minimized. Results from our developed CFD model agree with previously measured hemodynamics in the OFT. Further, computed velocities and measured velocities differ by [Formula: see text]15 % at locations that were not used in the optimization, validating the model. The presented methodology can be used in quantifications of embryonic cardiac hemodynamics under normal and altered blood flow conditions, enabling an in-depth quantitative study of how blood flow influences cardiac development.

Imaging of cardiovascular development in mammalian embryos using optical coherence tomography

The cardiovascular system is the first functional organ system to develop within the mammalian embryo. During the early stages of cardiovascular development, the heart and blood vessels undergo rapid growth and remodeling required for embryo viability, proper morphogenesis, and the function of all organ systems. Live imaging of these dynamic events in early mouse embryos is critical to understanding when and how these morphological changes occur during normal development and how mutations and pharmacological agents affect cardiovascular structure and function in vivo. The use of optical coherence tomography (OCT) allows for rapid, three-dimensional structural and functional imaging of mouse embryos at cellular resolution without the aid of contrast agents. In this chapter, we will describe how OCT can be used to assess the morphology of vessels and the heart, dynamic analysis of cardiac function, and hemodynamics within extraembryonic and embryonic blood vessels.

High-frame rate four dimensional optoacoustic tomography enables visualization of cardiovascular dynamics and mouse heart perfusion

Functional imaging of mouse models of cardiac health and disease provides a major contribution to our fundamental understanding of the mammalian heart. However, imaging murine hearts presents significant challenges due to their small size and rapid heart rate. Here we demonstrate the feasibility of high-frame-rate, noninvasive optoacoustic imaging of the murine heart. The temporal resolution of 50 three-dimensional frames per second provides functional information at important phases of the cardiac cycle without the use of gating or other motion-reduction methods. Differentiation of the blood oxygenation state in the heart chambers was enabled by exploiting the wavelength dependence of optoacoustic signals. Real-time volumetric tracking of blood perfusion in the cardiac chambers was also evaluated using indocyanine green. Taken together, the newly-discovered capacities offer a unique tool set for in-vivo structural and functional imaging of the whole heart with high spatio-temporal resolution in all three dimensions.

3D Reconstruction of Chick Embryo Vascular Geometries Using Non-invasive High-Frequency Ultrasound for Computational Fluid Dynamics Studies

Recent animal studies have provided evidence that prenatal blood flow fluid mechanics may play a role in the pathogenesis of congenital cardiovascular malformations. To further these researches, it is important to have an imaging technique for small animal embryos with sufficient resolution to support computational fluid dynamics studies, and that is also non-invasive and non-destructive to allow for subject-specific, longitudinal studies. In the current study, we developed such a technique, based on ultrasound biomicroscopy scans on chick embryos. Our technique included a motion cancelation algorithm to negate embryonic body motion, a temporal averaging algorithm to differentiate blood spaces from tissue spaces, and 3D reconstruction of blood volumes in the embryo. The accuracy of the reconstructed models was validated with direct stereoscopic measurements. A computational fluid dynamics simulation was performed to model fluid flow in the generated construct of a Hamburger-Hamilton (HH) stage 27 embryo. Simulation results showed that there were divergent streamlines and a low shear region at the carotid duct, which may be linked to the carotid duct's eventual regression and disappearance by HH stage 34. We show that our technique has sufficient resolution to produce accurate geometries for computational fluid dynamics simulations to quantify embryonic cardiovascular fluid mechanics.

Algorithms for improved 3-D reconstruction of live mammalian embryo vasculature from optical coherence tomography data

BACKGROUND

Robust reconstructions of the three-dimensional network of blood vessels in developing embryos imaged by optical coherence tomography (OCT) are needed for quantifying the longitudinal development of vascular networks in live mammalian embryos, in support of developmental cardiovascular research. Past computational methods [such as speckle variance (SV)] have demonstrated the feasibility of vascular reconstruction, but multiple challenges remain including: the presence of vessel structures at multiple spatial scales, thin blood vessels with weak flow, and artifacts resulting from bulk tissue motion (BTM).

METHODS

In order to overcome these challenges, this paper introduces a robust and scalable reconstruction algorithm based on a combination of anomaly detection algorithms and a parametric dictionary based sparse representation of blood vessels from structural OCT data.

RESULTS

Validation results using confocal data as the baseline demonstrate that the proposed method enables the detection of vessel segments that are either partially missed or weakly reconstructed using the SV method. Finally, quantitative measurements of vessel reconstruction quality indicate an overall higher quality of vessel reconstruction with the proposed method.

CONCLUSIONS

Results suggest that sparsity-integrated speckle anomaly detection (SSAD) is potentially a valuable tool for performing accurate quantification of the progression of vascular development in the mammalian embryonic yolk sac as imaged using OCT.

Optically gated beating-heart imaging

The constant motion of the beating heart presents an obstacle to clear optical imaging, especially 3D imaging, in small animals where direct optical imaging would otherwise be possible. Gating techniques exploit the periodic motion of the heart to computationally "freeze" this movement and overcome motion artifacts. Optically gated imaging represents a recent development of this, where image analysis is used to synchronize acquisition with the heartbeat in a completely non-invasive manner. This article will explain the concept of optical gating, discuss a range of different implementation strategies and their strengths and weaknesses. Finally we will illustrate the usefulness of the technique by discussing applications where optical gating has facilitated novel biological findings by allowing 3D in vivo imaging of cardiac myocytes in their natural environment of the beating heart.

Ex vivo 4D visualization of aortic valve dynamics in a murine model with optical coherence tomography

The heart and its mechanical components, especially the heart valves and leaflets, are under enormous strain and undergo fatigue, which impinge upon cardiac output. The knowledge about changes of the dynamic behavior and the possibility of early stage diagnosis could lead to the development of new treatment strategies. Animal models are suited for the development and evaluation of new experimental approaches and therefor innovative imaging techniques are necessary. In this study, we present the time resolved visualization of healthy and calcified aortic valves in an ex vivo artificially stimulated heart model with 4D optical coherence tomography and high-speed video microscopy.

Embryonic heart morphogenesis from confocal microscopy imaging and automatic segmentation

Embryonic heart morphogenesis (EHM) is a complex and dynamic process where the heart transforms from a single tube into a four-chambered pump. This process is of great biological and clinical interest but is still poorly understood for two main reasons. On the one hand, the existing imaging modalities for investigating EHM suffered from either limited penetration depth or limited spatial resolution. On the other hand, current works typically adopted manual segmentation, which was tedious, subjective, and time consuming considering the complexity of developing heart geometry and the large size of images. In this paper, we propose to utilize confocal microscopy imaging with tissue optical immersion clearing technique to image the heart at different stages of development for EHM study. The imaging method is able to produce high spatial resolution images and achieve large penetration depth at the same time. Furthermore, we propose a novel convex active contour model for automatic image segmentation. The model has the ability to deal with intensity fall-off in depth which is characterized by confocal microscopy images. We acquired the images of embryonic quail hearts from day 6 to day 14 of incubation for EHM study. The experimental results were promising and provided us with an insight view of early heart growth pattern and also paved the road for data-driven heart growth modeling.

4D reconstruction of the beating embryonic heart from two orthogonal sets of parallel optical coherence tomography slice-sequences

Current methods to build dynamic optical coherence tomography (OCT) volumes of the beating embryonic heart involve synchronization of 2D+time slice-sequences acquired over separate heartbeats. Temporal registration of these sequences is performed either through gating or postprocessing. While synchronization algorithms that exclusively rely on image- intrinsic signals allow forgoing external gating hardware, they are prone to error accumulation, require operator-supervised correction, or lead to nonisotropic resolution. Here, we propose an image-based, retrospective reconstruction technique that uses two sets of parallel 2D+T slice-sequences, acquired perpendicularly to each other, to yield accurate and automatic reconstructions with isotropic resolution. The method utilizes the similarity of the data at the slice intersections to spatio-temporally register the two sets of slice sequences and fuse them into a high-resolution 4D volume. We characterize our method by using 1) simulated heart phantom datasets and 2) OCT datasets acquired from the beating heart of live cultured E9.5 mouse and E10.5 rat embryos. We demonstrate that while our method requires greater acquisition and reconstruction time compared to methods that use slices from a single direction, it produces more accurate and self-validating reconstructions since each set of reconstructed slices acts as a reference for the slices in the perpendicular set.

Spectral domain optical coherence tomography of multi-MHz A-scan rates at 1310 nm range and real-time 4D-display up to 41 volumes/second

An ultrafast frequency domain optical coherence tomography system was developed at A-scan rates between 2.5 and 10 MHz, a B-scan rate of 4 or 8 kHz, and volume-rates between 12 and 41 volumes/second. In the case of the worst duty ratio of 10%, the averaged A-scan rate was 1 MHz. Two optical demultiplexers at a center wavelength of 1310 nm were used for linear-k spectral dispersion and simultaneous differential signal detection at 320 wavelengths. The depth-range, sensitivity, sensitivity roll-off by 6 dB, and axial resolution were 4 mm, 97 dB, 6 mm, and 23 μm, respectively. Using FPGAs for FFT and a GPU for volume rendering, a real-time 4D display was demonstrated at a rate up to 41 volumes/second for an image size of 256 (axial) × 128 × 128 (lateral) voxels.

In vivo functional imaging of blood flow and wall strain rate in outflow tract of embryonic chick heart using ultrafast spectral domain optical coherence tomography

During cardiac development, the cardiac wall and flowing blood are two important cardiac tissues that constantly interact with each other. This dynamic interaction defines appropriate biomechanical environment to which the embryonic heart is exposed. Quantitative assessment of the dynamic parameters of wall tissues and blood flow is required to further our understanding of cardiac development. We report the use of an ultrafast 1310-nm dual-camera spectral domain optical coherence tomography (SDOCT) system to characterize/image, in parallel, the dynamic radial strain rate of the myocardial wall and the Doppler velocity of the underlying flowing blood within an in vivo beating chick embryo. The OCT system operates at 184-kHz line scan rate, providing the flexibility of imaging the fast blood flow and the slow tissue deformation within one scan. The ability to simultaneously characterize tissue motion and blood flow provides a useful approach to better understand cardiac dynamics during early developmental stages.

Label-free subcellular 3D live imaging of preimplantation mouse embryos with full-field optical coherence tomography

Early patterning and polarity is of fundamental interest in preimplantation embryonic development. Label-free subcellular 3D live imaging is very helpful to its related studies. We have developed a novel system of full-field optical coherence tomography (FF-OCT) for noninvasive 3D subcellular live imaging of preimplantation mouse embryos with no need of dye labeling. 3D digitized embryos can be obtained by image processing. Label-free 3D live imaging is demonstrated for the mouse embryos at various typical preimplantation stages with a spatial resolution of 0.7 [micro sign]m and imaging rate of 24 fps. Factors that relate to early patterning and polarity, such as pronuclei in zygote, shapes of zona pellucida, location of second polar body, cleavage planes, and the blastocyst axis, can be quantitatively measured. The angle between the two second cleavage planes is accurately measured to be 87 deg. It is shown that FF-OCT provides a potential breakthrough for early patterning, polarity formation, and many other preimplantation-related studies in mammalian developmental biology.

Imaging of oocyte development using ultrahigh-resolution full-field optical coherence tomography

In this paper, a white-light full-field optical coherence tomography is developed to provide three-dimensional imaging of the development of a mouse embryo with ultrahigh-resolution. Spatial resolution of 1.8  μm×1.12  μm (transverse×axial) is achieved owing to the extremely short coherence length of the light source and optimized compensation of dispersion mismatch. A shot-noise limited detection sensitivity of 80 dB is obtained at an acquisition time of 5 seconds per image. To enable in vivo imaging of the mouse embryo development, a homemade incubator is applied to provide appropriate CO2 concentration, temperature, and humidity. An electronic light shutter is used to control the light source in order to avoid unnecessary exposure to the embryo development when the sample is not being scanned. To demonstrate our method, in vivo time series two-dimensional images of the in vitro fertilization process of mouse oocytes at the germinal vesicles stage are presented.

Longitudinal Imaging of Heart Development With Optical Coherence Tomography

Optical coherence tomography (OCT) has great potential for deciphering the role of mechanics in normal and abnormal heart development. OCT images tissue microstructure and blood flow deep into the tissue (1-2mm) at high spatiotemporal resolutions allowing unprecedented images of the developing heart. Here, we review the advancement of OCT technology to image heart development and report some of our recent findings utilizing OCT imaging under environmental control for longitudinal imaging. Precise control of the environment is absolutely required in longitudinal studies that follow the growth of the embryo or studies comparing normal versus perturbed heart development to obtain meaningful in vivo results. These types of studies are essential to tease out the influence of cardiac dynamics on molecular expression and their role in the progression of congenital heart defects.

Sequential Turning Acquisition and Reconstruction (STAR) method for four-dimensional imaging of cyclically moving structures

Optical coherence tomography allows for dynamic, three-dimensional (3D+T) imaging of the heart within animal embryos. However, direct 3D+T imaging frame rates remain insufficient for cardiodynamic analysis. Previously, this limitation has been addressed by reconstructing 3D+T representations of the beating heart based on sets of two-dimensional image sequences (2D+T) acquired sequentially at high frame rate and in fixed (and parallel) planes throughout the heart. These methods either require additional hardware to trigger the acquisition of each 2D+T series to the same phase of the cardiac cycle or accumulate registration errors as the slices are synchronized retrospectively by pairs, without a gating signal. Here, we present a sequential turning acquisition and reconstruction (STAR) method for 3D+T imaging of periodically moving structures, which does not require any additional gating signal and is not prone to registration error accumulation. Similarly to other sequential cardiac imaging methods, multiple fast image series are consecutively acquired for different sections but in between acquisitions, the imaging plane is rotated around the center line instead of shifted along the direction perpendicular to the slices. As the central lines of all image-sequences coincide and represent measurements of the same spatial position, they can be used to accurately synchronize all the slices to a single inherent reference signal. We characterized the accuracy of our method on a simulated dynamic phantom and successfully imaged a beating embryonic rat heart. Potentially, this method can be applied for structural or Doppler imaging approaches with any direct space imaging modality such as computed tomography, ultrasound, or light microscopy.

Optical Coherence Tomography for live imaging of mammalian development

Understanding the nature and mechanism of congenital defects of the different organ systems in humans has heavily relied on the analysis of the corresponding mutant phenotypes in rodent models. Optical Coherence Tomography (OCT) has recently emerged as a powerful tool to study early embryonic development. This non-invasive optical methodology does not require labeling and allows visualization of embryonic tissues with single cell resolution. Here, we will discuss how OCT can be applied for structural imaging of early mouse and rat embryos in static culture, cardiodynamic and blood flow analysis, and in utero embryonic imaging at later stages of gestation, demonstrating how OCT can be used to assess structural and functional birth defects in mammalian models.

Noninvasive imaging of ethanol-induced developmental defects in zebrafish embryos using optical coherence tomography

In this article, we report the use of optical coherence tomography for noninvasive cross-sectional real-time imaging of ethanol-induced developmental defects in zebrafish embryos larvae. For ethanol concentration of over 300 mM, developmental defects of eye (shrinkage and retinal abnormalities), malformation of the notochord and ataxia arising due to the toxic effects of ethanol were observed in OCT images from 3 days post fertilization onwards. The results suggest that OCT could be a valuable tool for noninvasive assessment of birth defects in small animal systems.

Triggered optical coherence tomography for capturing rapid periodic motion

Quantitative cross-sectional imaging of vocal folds during phonation is potentially useful for diagnosis and treatments of laryngeal disorders. Optical coherence tomography (OCT) is a powerful technique, but its relatively low frame rates makes it challenging to visualize rapidly vibrating tissues. Here, we demonstrate a novel method based on triggered laser scanning to capture 4-dimensional (4D) images of samples in motu at audio frequencies over 100 Hz. As proof-of-concept experiments, we applied this technique to imaging the oscillations of biopolymer gels on acoustic vibrators and aerodynamically driven vibrations of the vocal fold in an ex vivo calf larynx model. Our results suggest that triggered 4D OCT may be useful in understanding and assessing the function of vocal folds and developing novel treatments in research and clinical settings.

Quantifying blood flow and wall shear stresses in the outflow tract of chick embryonic hearts

Wall shear stresses (WSS) exerted by blood flow on cardiac tissues modulate growth and development of the heart. To study the role of hemodynamic conditions on cardiac morphogenesis, here, we present a methodology that combines imaging and finite element modeling to quantify the in vivo blood flow dynamics and WSS in the cardiac outflow tract (OFT) of early chicken embryos (day 3 out of 21-day incubation period). We found a distinct blood flow field and heterogeneous distribution of WSS in the chicken embryonic heart OFT, which have physiological implications for OFT morphogenesis.

Integration of an optical coherence tomography (OCT) system into an examination incubator to facilitate in vivo imaging of cardiovascular development in higher vertebrate embryos under stable physiological conditions

High-resolution in vivo imaging of higher vertebrate embryos over short or long time periods under constant physiological conditions is a technically challenging task for researchers working on cardiovascular development. In chick embryos, for example, various studies have shown that without appropriate maintenance of temperature, as one of the main environmental factors, the embryonic heart rate drops rapidly and often results in an increase in regurgitant flow. Hemodynamic parameters are critical stimuli for cardiovascular development that, for a correct evaluation of their developmental significance, should be documented under physiological conditions. However, previous studies were mostly carried out outside of an incubator or under suboptimal environmental conditions. Here we present, to the best of our knowledge, the first detailed description of an optical coherence tomography (OCT) system integrated into an examination incubator to facilitate real-time in vivo imaging of cardiovascular development under physiological environmental conditions. We demonstrate the suitability of this OCT examination incubator unit for use in cardiovascular development studies by examples of proof of principle experiments. We, furthermore, point out the need for use of examination incubators for physiological OCT examinations by documenting the effects of room climate (22°C) on the performance of the cardiovascular system of chick embryos (HH-stages 16/17). Upon exposure to room climate, chick embryos showed a fast drop in the heart rate and striking changes in the cardiac contraction behaviour and the blood flow through the vitelline circulation. We have documented these changes for the first time by M-mode OCT and Doppler M-mode OCT.

High-speed optical coherence tomography imaging of the beating avian embryonic heart

Congenital heart defects (CHDs) affect thousands of newborns each year in the United States. Recent research using animal model systems indicates that the abnormal function of the early tubular heart precedes structural defects such as septal defects. Optical coherence tomography (OCT) is an imaging modality that can provide high spatial and temporal resolution to study both the structure and the function of the tubular heart. With technical advances in OCT imaging speed, especially with frequency domain OCT and image-based retrospective gating, it is now possible to image a beating avian embryonic heart in three dimensions under physiological conditions and follow morphogenesis over critical periods of developmental time. These technological advances have already revealed novel aspects of heart development. By expanding our understanding of heart development, research using OCT technology combined with other imaging modalities may eventually lead to strategies to predict, treat, and even prevent CHDs.

Arrhythmia caused by a Drosophila tropomyosin mutation is revealed using a novel optical coherence tomography instrument

Background

Dilated cardiomyopathy (DCM) is a severe cardiac condition that causes high mortality. Many genes have been confirmed to be involved in this disease. An ideal system with which to uncover disease mechanisms would be one that can measure the changes in a wide range of cardiac activities associated with mutations in specific, diversely functional cardiac genes. Such a system needs a genetically manipulable model organism that allows in vivo measurement of cardiac phenotypes and a detecting instrument capable of recording multiple phenotype parameters.

Methodology and Principal Findings

With a simple heart, a transparent body surface at larval stages and available genetic tools we chose Drosophila melanogaster as our model organism and developed for it a dual en-face/Doppler optical coherence tomography (OCT) instrument capable of recording multiple aspects of heart activity, including heart contraction cycle dynamics, ostia dynamics, heartbeat rate and rhythm, speed of heart wall movement and light reflectivity of cardiomyocytes in situ. We applied this OCT instrument to a model of Tropomyosin-associated DCM established in adult Drosophila. We show that DCM pre-exists in the larval stage and is accompanied by an arrhythmia previously unidentified in this model. We also detect reduced mobility and light reflectivity of cardiomyocytes in mutants.

Conclusion

These results demonstrate the capability of our OCT instrument to characterize in detail cardiac activity in genetic models for heart disease in Drosophila.

Graphics processing unit accelerated non-uniform fast Fourier transform for ultrahigh-speed, real-time Fourier-domain OCT

We implemented fast Gaussian gridding (FGG)-based non-uniform fast Fourier transform (NUFFT) on the graphics processing unit (GPU) architecture for ultrahigh-speed, real-time Fourier-domain optical coherence tomography (FD-OCT). The Vandermonde matrix-based non-uniform discrete Fourier transform (NUDFT) as well as the linear/cubic interpolation with fast Fourier transform (InFFT) methods are also implemented on GPU to compare their performance in terms of image quality and processing speed. The GPU accelerated InFFT/NUDFT/NUFFT methods are applied to process both the standard half-range FD-OCT and complex full-range FD-OCT (C-FD-OCT). GPU-NUFFT provides an accurate approximation to GPU-NUDFT in terms of image quality, but offers >10 times higher processing speed. Compared with the GPU-InFFT methods, GPU-NUFFT has improved sensitivity roll-off, higher local signal-to-noise ratio and immunity to side-lobe artifacts caused by the interpolation error. Using a high speed CMOS line-scan camera, we demonstrated the real-time processing and display of GPU-NUFFT-based C-FD-OCT at a camera-limited rate of 122 k line/s (1024 pixel/A-scan).

Mechanical stress as a regulator of cytoskeletal contractility and nuclear shape in embryonic epithelia

The mechano-sensitive responses of the heart and brain were examined in the chick embryo during Hamburger and Hamilton stages 10-12. During these early stages of development, cells in these structures are organized into epithelia. Isolated hearts and brains were compressed by controlled amounts of surface tension (ST) at the surface of the sample, and microindentation was used to measure tissue stiffness following several hours of culture. The response of both organs was qualitatively similar, as they stiffened under reduced loading. With increased loading, however, the brain softened while heart stiffness was similar to controls. In the brain, changes in nuclear shape and morphology correlated with these responses, as nuclei became more elliptical with decreased loading and rounder with increased loading. Exposure to the myosin inhibitor blebbistatin indicated that these changes in stiffness and nuclear shape are likely caused by altered cytoskeletal contraction. Computational modeling suggests that this behavior tends to return peak tissue stress back toward the levels it has in the intact heart and brain. These results suggest that developing cardiac and neural epithelia respond similarly to changes in applied loads by altering contractility in ways that tend to restore the original mechanical stress state. Hence, this study supports the view that stress-based mechanical feedback plays a role in regulating epithelial development.

Heart wall velocimetry and exogenous contrast-based cardiac flow imaging in Drosophila melanogaster using Doppler optical coherence tomography

Drosophila melanogaster (fruit fly) is a central organism in biology and is becoming increasingly important in the cardiovascular sciences. Prior work in optical imaging of the D. melanogaster heart has focused on static and dynamic structural anatomy. In the study, it is demonstrated that Doppler optical coherence tomography can quantify dynamic heart wall velocity and hemolymph flow in adult D. melanogaster. Since hemolymph is optically transparent, a novel exogenous contrast technique is demonstrated to increase the backscatter-based intracardiac Doppler flow signal. The results presented here open up new possibilities for functional cardiovascular phenotyping of normal and mutant D. melanogaster.

Real-time 4D signal processing and visualization using graphics processing unit on a regular nonlinear-k Fourier-domain OCT system

We realized graphics processing unit (GPU) based real-time 4D (3D+time) signal processing and visualization on a regular Fourier-domain optical coherence tomography (FD-OCT) system with a nonlinear k-space spectrometer. An ultra-high speed linear spline interpolation (LSI) method for lambda-to-k spectral re-sampling is implemented in the GPU architecture, which gives average interpolation speeds of >3,000,000 line/s for 1024-pixel OCT (1024-OCT) and >1,400,000 line/s for 2048-pixel OCT (2048-OCT). The complete FD-OCT signal processing including lambda-to-k spectral re-sampling, fast Fourier transform (FFT) and post-FFT processing have all been implemented on a GPU. The maximum complete A-scan processing speeds are investigated to be 680,000 line/s for 1024-OCT and 320,000 line/s for 2048-OCT, which correspond to 1GByte processing bandwidth. In our experiment, a 2048-pixel CMOS camera running up to 70 kHz is used as an acquisition device. Therefore the actual imaging speed is camera- limited to 128,000 line/s for 1024-OCT or 70,000 line/s for 2048-OCT. 3D Data sets are continuously acquired in real time at 1024-OCT mode, immediately processed and visualized as high as 10 volumes/second (12,500 A-scans/volume) by either en face slice extraction or ray-casting based volume rendering from 3D texture mapped in graphics memory. For standard FD-OCT systems, a GPU is the only additional hardware needed to realize this improvement and no optical modification is needed. This technique is highly cost-effective and can be easily integrated into most ultrahigh speed FD-OCT systems to overcome the 3D data processing and visualization bottlenecks.

Dual optical coherence tomography/fluorescence microscopy for monitoring of Drosophila melanogaster larval heart

This article demonstrates a combined instrument of two imaging modalities to acquire information on cardiac function in larval Drosophila melanogaster: optical coherence tomography (OCT) and laser scanning fluorescence microscopy (LSFM). For this purpose, a dedicated imaging instrument able to sequentially provide cross-sectional OCT and C-scan LSFM images has been developed. With this dual-imaging system, the heart can be easily located and visualized within the specimen and the change of the heart shape in a cardiac cycle can be monitored.

Efficient postacquisition synchronization of 4-D nongated cardiac images obtained from optical coherence tomography: application to 4-D reconstruction of the chick embryonic heart

Four-dimensional (4-D) imaging of the embryonic heart allows study of cardiac morphology and function in vivo during development. However, 4-D imaging of the embryonic heart using current techniques, including optical coherence tomography (OCT), is limited by the rate of image acquisition. Here, we present a nongated 4-D imaging strategy combined with an efficient postacquisition synchronization procedure that circumvents limitations on acquisition rate. The 4-D imaging strategy acquires a time series of images in B mode at several different locations along the heart, rendering out-of-phase image sequences. Then, our synchronization procedure uses similarity of local structures to find the phase shift between neighboring image sequences, employing M-mode images (extracted from the acquired B-mode images) to achieve computational efficiency. Furthermore, our procedure corrects the phase shifts by considering the phase lags introduced by peristaltic-like contractions of the embryonic heart wall. We applied the 4-D imaging strategy and synchronization procedure to reconstruct the cardiac outflow tract (OFT) of a chick embryo, imaged with OCT at early stages of development (Hamburger-Hamilton stage 18). We showed that the proposed synchronization procedure achieves efficiency without sacrificing accuracy and that the reconstructed 4-D images properly captured the dynamics of the OFT wall motion.

High temporal resolution OCT using image-based retrospective gating

High temporal resolution OCT imaging is very advantageous for analyzing cardiac mechanics in the developing embryonic heart of small animals. An image-based retrospective gating technique is presented to increase the effective temporal resolution of an OCT system and to allow visualization of systolic dynamics in 3D. The gating technique employs image similarity measures for rearranging asynchronously acquired input data consisting of a time series of 2D images at each z position along the heart volume, to produce a time sequence of 3D volumes of the beating heart. The study includes a novel robust validation technique, which quantitatively evaluates the accuracy of the gating technique, in addition to visual evaluations by 2D multiplanar reformatting (MPR) and 3D volume rendering. The retrospective gating and validation is demonstrated on a stage 14 embryonic quail heart data set. Using the validation scheme, it is shown that the gating is accurate within a standard deviation of 4.7 ms, which is an order of magnitude shorter than the time interval during which systolic contraction (approximately 50 ms) occurs in the developing embryo. This gating method has allowed, for the first time, clear visualization of systolic dynamics of the looping embryonic heart in 3D.