Real-time optical gating for three-dimensional beating heart imaging

PAAQ: Paired Alternating AcQuisitions for virtual high frame rate multichannel cardiac fluorescence microscopy

In vivo fluorescence microscopy is a powerful tool to image the beating heart in its early development stages. A high acquisition frame rate is necessary to study its fast contractions, but the limited fluorescence intensity requires sensitive cameras that are often too slow. Moreover, the problem is even more complex when imaging distinct tissues in the same sample using different fluorophores. We present Paired Alternating AcQuisitions, a method to image cyclic processes in multiple channels, which requires only a single (possibly slow) camera. We generate variable temporal illumination patterns in each frame, alternating between channel-specific illuminations (fluorescence) in odd frames and a motion-encoding brightfield pattern as a common reference in even frames. Starting from the image pairs, we find the position of each reference frame in the cardiac cycle through a combination of image-based sorting and regularized curve fitting. Thanks to these estimated reference positions, we assemble multichannel videos whose frame rate is virtually increased. We characterize our method on synthetic and experimental images collected in zebrafish embryos, showing quantitative and visual improvements in the reconstructed videos over existing nongated sorting-based alternatives. Using a 15 Hz camera, we showcase a reconstructed video containing two fluorescence channels at 100 fps.

Live Imaging of Heart Injury in Larval Zebrafish Reveals a Multi-Stage Model of Neutrophil and Macrophage Migration

Neutrophils and macrophages are crucial effectors and modulators of repair and regeneration following myocardial infarction, but they cannot be easily observed in vivo in mammalian models. Hence many studies have utilized larval zebrafish injury models to examine neutrophils and macrophages in their tissue of interest. However, to date the migratory patterns and ontogeny of these recruited cells is unknown. In this study, we address this need by comparing our larval zebrafish model of cardiac injury to the archetypal tail fin injury model. Our in vivo imaging allowed comprehensive mapping of neutrophil and macrophage migration from primary hematopoietic sites, to the wound. Early following injury there is an acute phase of neutrophil recruitment that is followed by sustained macrophage recruitment. Both cell types are initially recruited locally and subsequently from distal sites, primarily the caudal hematopoietic tissue (CHT). Once liberated from the CHT, some neutrophils and macrophages enter circulation, but most use abluminal vascular endothelium to crawl through the larva. In both injury models the innate immune response resolves by reverse migration, with very little apoptosis or efferocytosis of neutrophils. Furthermore, our in vivo imaging led to the finding of a novel wound responsive mpeg1+ neutrophil subset, highlighting previously unrecognized heterogeneity in neutrophils. Our study provides a detailed analysis of the modes of immune cell migration in larval zebrafish, paving the way for future studies examining tissue injury and inflammation.

Adaptive prospective optical gating enables day-long 3D time-lapse imaging of the beating embryonic zebrafish heart

Three-dimensional fluorescence time-lapse imaging of the beating heart is extremely challenging, due to the heart's constant motion and a need to avoid pharmacological or phototoxic damage. Although real-time triggered imaging can computationally "freeze" the heart for 3D imaging, no previous algorithm has been able to maintain phase-lock across developmental timescales. We report a new algorithm capable of maintaining day-long phase-lock, permitting routine acquisition of synchronised 3D + time video time-lapse datasets of the beating zebrafish heart. This approach has enabled us for the first time to directly observe detailed developmental and cellular processes in the beating heart, revealing the dynamics of the immune response to injury and witnessing intriguing proliferative events that challenge the established literature on cardiac trabeculation. Our approach opens up exciting new opportunities for direct time-lapse imaging studies over a 24-hour time course, to understand the cellular mechanisms underlying cardiac development, repair and regeneration.

Non-invasive in vivo quantification of the developing optical properties and graded index of the embryonic eye lens using SPIM

Graded refractive index lenses are inherent to advanced visual systems in animals. By understanding their formation and local optical properties, significant potential for improved ocular healthcare may be realized. We report a novel technique measuring the developing optical power of the eye lens, in a living animal, by exploiting the orthogonal imaging modality of a selective plane illumination microscope (SPIM). We have quantified the maturation of the lenticular refractive index at three different visible wavelengths using a combined imaging and ray tracing approach. We demonstrate that the method can be used with transgenic and vital dye labeling as well as with both fixed and living animals. Using a key eye lens morphogen and its inhibitor, we have measured their effects both on lens size and on refractive index. Our technique provides insights into the mechanisms involved in the development of this natural graded index micro-lens and its associated optical properties.

Precise spatio-temporal control of rapid optogenetic cell ablation with mem-KillerRed in Zebrafish

The ability to kill individual or groups of cells in vivo is important for studying cellular processes and their physiological function. Cell-specific genetically encoded photosensitizing proteins, such as KillerRed, permit spatiotemporal optogenetic ablation with low-power laser light. We report dramatically improved resolution and speed of cell targeting in the zebrafish kidney through the use of a selective plane illumination microscope (SPIM). Furthermore, through the novel incorporation of a Bessel beam into the SPIM imaging arm, we were able to improve on targeting speed and precision. The low diffraction of the Bessel beam coupled with the ability to tightly focus it through a high NA lens allowed precise, rapid targeting of subsets of cells at anatomical depth in live, developing zebrafish kidneys. We demonstrate that these specific targeting strategies significantly increase the speed of optoablation as well as fish survival.

Motion characterization scheme to minimize motion artifacts in intravital microscopy

Respiratory- and cardiac-induced motion artifacts pose a major challenge for in vivo optical imaging, limiting the temporal and spatial imaging resolution in fluorescence laser scanning microscopy. Here, we present an imaging platform developed for in vivo characterization of physiologically induced axial motion. The motion characterization system can be straightforwardly implemented on any conventional laser scanning microscope and can be used to evaluate the effectiveness of different motion stabilization schemes. This method is particularly useful to improve the design of novel tissue stabilizers and to facilitate stabilizer positioning in real time, therefore facilitating optimal tissue immobilization and minimizing motion induced artifacts.

Advances in Intravital Non-Linear Optical Imaging of the Central Nervous System in Rodents

Purpose of review: Highly coordinated cellular interactions occur in the healthy or pathologic adult rodent central nervous system (CNS). Until recently, technical challenges have restricted the analysis of these events to largely static modes of study such as immuno-fluorescence and electron microscopy on fixed tissues. The development of intravital imaging with subcellular resolution is required to probe the dynamics of these events in their natural context, the living brain. Recent findings: This review focuses on the recently developed live non-linear optical imaging modalities, the core principles involved, the identified technical challenges that limit their use and the scope of their applications. We highlight some practical applications for these modalities with a specific attention given to Experimental Autoimmune Encephalomyelitis (EAE), a rodent model of a chronic inflammatory disease of the CNS characterized by the formation of disseminated demyelinating lesions accompanied by axonal degeneration. Summary: We conclude that label-free nonlinear optical imaging combined to two photon imaging will continue to contribute richly to comprehend brain function and pathogenesis and to develop effective therapeutic strategies.

Fast Fluorescence Microscopy with Light Sheets

In light sheet microscopy, optical sectioning by selective fluorescence excitation with a sheet of light is combined with fast full-frame acquisition. This illumination scheme provides minimal photobleaching and phototoxicity. Complemented with remote focusing and multi-view acquisition, light sheet microscopy is the method of choice for acquisition of very fast biological processes, large samples, and high-throughput applications in areas such as neuroscience, plant biology, and developmental biology. This review explains why light sheet microscopes are much faster and gentler than other established fluorescence microscopy techniques. New volumetric imaging schemes and highlights of selected biological applications are also discussed.

Overview of Innovative Mouse Models for Imaging Neuroinflammation

Neuroinflammation demands a comprehensive appraisal in situ to gain in-depth knowledge on the roles of particular cells and molecules and their potential roles in therapy. Because of the lack of appropriate tools, direct visualization of cells has been poorly investigated up to the present. In this context, reporter mice expressing cell-specific fluorescent proteins, combined with multiphoton microscopy, provide a window into cellular processes in living animals. In addition, the ability to collect multiple fluorescent colors from the same sample makes in vivo microscopy uniquely useful for characterizing many parameters from the same area, supporting powerful correlative analyses. Here, we present an overview of the advantages and limitations of this approach, with the purpose of providing insight into the neuroinflammation field. We also provide a review of existing fluorescent mouse models and describe how these models have been used in studies of neuroinflammation. Finally, the potential for developing advanced genetic tools and imaging resources is discussed. © 2016 by John Wiley & Sons, Inc.

Targeted laser ablation of the zebrafish larval heart induces models of heart block, valvular regurgitation, and outflow tract obstruction

Mammalian models of cardiac disease have provided unique and important insights into human disease but have become increasingly challenging to produce. The zebrafish could provide inexpensive high-throughput models of cardiac injury and repair. We used a highly targeted laser, synchronized to fire at specific phases of the cardiac cycle, to induce regional injury to the ventricle, atrioventricular (AV) cushion, and bulbus arteriosus (BA). We assessed the impact of laser injury on hearts of zebrafish early larvae at 72 h postfertilization, to different regions, recording the effects on ejection fraction (EF), heart rate (HR), and blood flow at 2 and 24 h postinjury (hpi). Laser injury to the apex, midzone, and outflow regions of the ventricle resulted in reductions of the ventricle EF at 2 hpi with full recovery of function by 24 hpi. Laser injury to the ventricle, close to the AV cushion, was more likely to cause bradycardia and atrial-ventricular dysfunction, suggestive of an electrical conduction block. At 2 hpi, direct injury to the AV cushion resulted in marked regurgitation of blood from the ventricle to the atrium. Laser injury to the BA caused temporary outflow tract obstruction with cessation of ventricle contraction and circulation. Despite such damage, 80% of embryos showed complete recovery of the HR and function within 24 h of laser injury. Precision laser injury to key structures in the zebrafish developing heart provides a range of potentially useful models of hemodynamic overload, injury, and repair.

Dynamic structure and protein expression of the live embryonic heart captured by 2-photon light sheet microscopy and retrospective registration

We present an imaging and image reconstruction pipeline that captures the dynamic three-dimensional beating motion of the live embryonic zebrafish heart at subcellular resolution. Live, intact zebrafish embryos were imaged using 2-photon light sheet microscopy, which offers deep and fast imaging at 70 frames per second, and the individual optical sections were assembled into a full 4D reconstruction of the beating heart using an optimized retrospective image registration algorithm. This imaging and reconstruction platform permitted us to visualize protein expression patterns at endogenous concentrations in zebrafish gene trap lines.

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.

Motion compensation for in vivo subcellular optical microscopy

In this review, we focus on the impact of tissue motion on attempting to conduct subcellular resolution optical microscopy, in vivo. Our position is that tissue motion is one of the major barriers in conducting these studies along with light induced damage, optical probe loading as well as absorbing and scattering effects on the excitation point spread function and collection of emitted light. Recent developments in the speed of image acquisition have reached the limit, in most cases, where the signal from a subcellular voxel limits the speed and not the scanning rate of the microscope. Different schemes for compensating for tissue displacements due to rigid body and deformation are presented from tissue restriction, gating, adaptive gating and active tissue tracking. We argue that methods that minimally impact the natural physiological motion of the tissue are desirable because the major reason to perform in vivo studies is to evaluate normal physiological functions. Towards this goal, active tracking using the optical imaging data itself to monitor tissue displacement and either prospectively or retrospectively correct for the motion without affecting physiological processes is desirable. Critical for this development was the implementation of near real time image processing in conjunction with the control of the microscope imaging parameters. Clearly, the continuing development of methods of motion compensation as well as significant technological solutions to the other barriers to tissue subcellular optical imaging in vivo, including optical aberrations and overall signal-to-noise ratio, will make major contributions to the understanding of cell biology within the body.

Automatic zebrafish heartbeat detection and analysis for zebrafish embryos

A fully automatic detection and analysis method of heartbeats in videos of nonfixed and nonanesthetized zebrafish embryos is presented. This method reduces the manual workload and time needed for preparation and imaging of the zebrafish embryos, as well as for evaluating heartbeat parameters such as frequency, beat-to-beat intervals, and arrhythmicity. The method is validated by a comparison of the results from automatic and manual detection of the heart rates of wild-type zebrafish embryos 36-120 h postfertilization and of embryonic hearts with bradycardia and pauses in the cardiac contraction.

Laser-targeted ablation of the zebrafish embryonic ventricle: a novel model of cardiac injury and repair

Background

While the adult zebrafish (Danio rerio) heart demonstrates a remarkable capacity for self-renewal following apical resection little is known about the response to injury in the embryonic heart.

Methods

Injury to the beating zebrafish embryo heart was induced by laser using a transgenic zebrafish expressing cardiomyocyte specific green fluorescent protein. Changes in ejection fraction (EF), heart rate (HR), and caudal vein blood flow (CVBF) assessed by video capture techniques were assessed at 2, 24 and 48 h post-laser. Change in total and mitotic ventricular cardiomyocyte number following laser injury was also assessed by counting respectively DAPI (VCt) and Phospho-histone H3 (VCm) positive nuclei in isolated hearts using confocal microscopy.

Results

Laser injury to the ventricle resulted in bradycardia and mild bleeding into the pericardium. At 2 h post-laser injury, there was a significant reduction in cardiac performance in lasered-hearts compared with controls (HR 117 ± 11 vs 167 ± 9 bpm, p ≤ 0.001; EF 14.1 ± 1.8 vs 20.1 ± 1.3%, p ≤ 0.001; CVBF 103 ± 15 vs 316 ± 13μms^− 1, p ≤ 0.001, respectively). Isolated hearts showed a significant reduction in VCt at 2 h post-laser compared to controls (195 ± 15 vs 238 ± 15, p ≤ 0.05). Histology showed necrosis and apoptosis (TUNEL assay) at the site of laser injury. At 24 h post-laser cardiac performance and VCt had recovered fully to control levels. Pretreatment with the cell-cycle inhibitor, aphidicolin, significantly inhibited functional recovery of the ventricle accompanied by a significant inhibition of cardiomyocyte proliferation.

Conclusions

Laser-targeted injury of the zebrafish embryonic heart is a novel and reproducible model of cardiac injury and repair suitable for pharmacological and molecular studies.

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.

High-resolution 3D optical microscopy inside the beating zebrafish heart using prospective optical gating

3D fluorescence imaging is a fundamental tool in the study of functional and developmental biology, but effective imaging is particularly difficult in moving structures such as the beating heart. We have developed a non-invasive real-time optical gating system that is able to exploit the periodic nature of the motion to acquire high resolution 3D images of the normally-beating zebrafish heart without any unnecessary exposure of the sample to harmful excitation light. In order for the image stack to be artefact-free, it is essential to use a synchronization source that is invariant as the sample is scanned in 3D. We therefore describe a scheme whereby fluorescence image slices are scanned through the sample while a brightfield camera sharing the same objective lens is maintained at a fixed focus, with correction of sample drift also included. This enables us to maintain, throughout an extended 3D volume, the same standard of synchronization we have previously demonstrated in and near a single 2D plane. Thus we are able image the complete beating zebrafish heart exactly as if the heart had been artificially stopped, but sidestepping this undesirable interference with the heart and instead allowing the heart to beat as normal.

Quantitative in vivo imaging of embryonic development: opportunities and challenges

Animal models are critically important for a mechanistic understanding of embryonic morphogenesis. For decades, visualizing these rapid and complex multidimensional events has relied on projection images and thin section reconstructions. While much insight has been gained, fixed tissue specimens offer limited information on dynamic processes that are essential for tissue assembly and organ patterning. Quantitative imaging is required to unlock the important basic science and clinically relevant secrets that remain hidden. Recent advances in live imaging technology have enabled quantitative longitudinal analysis of embryonic morphogenesis at multiple length and time scales. Four different imaging modalities are currently being used to monitor embryonic morphogenesis: optical, ultrasound, magnetic resonance imaging (MRI), and micro-computed tomography (micro-CT). Each has its advantages and limitations with respect to spatial resolution, depth of field, scanning speed, and tissue contrast. In addition, new processing tools have been developed to enhance live imaging capabilities. In this review, we analyze each type of imaging source and its use in quantitative study of embryonic morphogenesis in small animal models. We describe the physics behind their function, identify some examples in which the modality has revealed new quantitative insights, and then conclude with a discussion of new research directions with live imaging.

3D adaptive optics in a light sheet microscope

We report on a single plane illumination microscope (SPIM) incorporating adaptive optics in the imaging arm. We show how aberrations can occur from the sample mounting tube and quantify the aberrations both experimentally and computationally. A wavefront sensorless approach was taken to imaging a green fluorescent protein (GFP) labelled transgenic zebrafish. We show improvements in image quality whilst recording a 3D "z-stack" and show how the aberrations come from varying depths in the fish.