Planar Airy beam light-sheet for two-photon microscopy

Generating Airy beams through multimode fibres

Focussing light through a multimode fibre (MMF) is the basis of holographic endoscopes, which currently enable detailed imaging of deep tissue. Achieving high fidelity and purity diffraction-limited foci has been shown to be possible, when fully controlling the amplitude, phase, and two orthogonal polarisation states of the input field. Yet, generating more complex field distributions with similar performance remains to be assessed. Here, we demonstrate the generation of Airy beams through an MMF containing in excess of 90 % of the optical power delivered by the fibre. We discuss two distinct methods for generating optical landscapes: the direct field and the Fourier domain synthesis. Moreover, we showcase the flexibility of the Fourier domain synthesis to modify the generated beam.

High-throughput confocal airy beam oblique light-sheet tomography of brain-wide imaging at single-cell resolution

Brain research is an area of research characterized by its cutting-edge nature, with brain mapping constituting a crucial aspect of this field. As sequencing tools have played a crucial role in gene sequencing, brain mapping largely depends on automated, high-throughput and high-resolution imaging techniques. Over the years, the demand for high-throughput imaging has scaled exponentially with the rapid development of microscopic brain mapping. In this paper, we introduce the novel concept of confocal Airy beam into oblique light-sheet tomography named CAB-OLST. We demonstrate that this technique enables the high throughput of brain-wide imaging of long-distance axon projection for the entire mouse brain at a resolution of 0.26 μm × 0.26 μm × 1.06 μm in 58 hours. This technique represents an innovative contribution to the field of brain research by setting a new standard for high-throughput imaging techniques.

Airy beam light sheet microscopy boosted by deep learning deconvolution

Common light sheet microscopy comes with a trade-off between light sheet width defining the optical sectioning and the usable field of view arising from the divergence of the illuminating Gaussian beam. To overcome this, low-diverging Airy beams have been introduced. Airy beams, however, exhibit side lobes degrading image contrast. Here, we constructed an Airy beam light sheet microscope, and developed a deep learning image deconvolution to remove the effects of the side lobes without knowledge of the point spread function. Using a generative adversarial network and high-quality training data, we significantly enhanced image contrast and improved the performance of a bicubic upscaling. We evaluated the performance with fluorescently labeled neurons in mouse brain tissue samples. We found that deep learning-based deconvolution was about 20-fold faster than the standard approach. The combination of Airy beam light sheet microscopy and deep learning deconvolution allows imaging large volumes rapidly and with high quality.

Video-rate Raman-based metabolic imaging by Airy light-sheet illumination and photon-sparse detection

Despite its massive potential, Raman imaging represents just a modest fraction of all research and clinical microscopy to date. This is due to the ultralow Raman scattering cross-sections of most biomolecules that impose low-light or photon-sparse conditions. Bioimaging under such conditions is suboptimal, as it either results in ultralow frame rates or requires increased levels of irradiance. Here, we overcome this tradeoff by introducing Raman imaging that operates at both video rates and 1,000-fold lower irradiance than state-of-the-art methods. To accomplish this, we deployed a judicially designed Airy light-sheet microscope to efficiently image large specimen regions. Further, we implemented subphoton per pixel image acquisition and reconstruction to confront issues arising from photon sparsity at just millisecond integrations. We demonstrate the versatility of our approach by imaging a variety of samples, including the three-dimensional (3D) metabolic activity of single microbial cells and the underlying cell-to-cell variability. To image such small-scale targets, we again harnessed photon sparsity to increase magnification without a field-of-view penalty, thus, overcoming another key limitation in modern light-sheet microscopy.

Extended depth-of-field light-sheet microscopy improves imaging of large volumes at high numerical aperture

Light-sheet microscopes must compromise among field of view, optical sectioning, resolution, and detection efficiency. High-numerical-aperture (NA) detection objective lenses provide higher resolution, but their narrow depth of field inefficiently captures the fluorescence signal generated throughout the thickness of the illumination light sheet when imaging large volumes. Here, we present ExD-SPIM (extended depth-of-field selective-plane illumination microscopy), an improved light-sheet microscopy strategy that solves this limitation by extending the depth of field (DOF) of high-NA detection objectives to match the thickness of the illumination light sheet. This extension of the DOF uses a phase mask to axially stretch the point-spread function of the objective lens while largely preserving lateral resolution. This matching of the detection DOF to the illumination-sheet thickness increases the total fluorescence collection, reduces the background, and improves the overall signal-to-noise ratio (SNR), as shown by numerical simulations, imaging of bead phantoms, and imaging living animals. In comparison to conventional light sheet imaging with low-NA detection that yields equivalent DOF, the results show that ExD-SPIM increases the SNR by more than threefold and dramatically reduces the rate of photobleaching. Compared to conventional high-NA detection, ExD-SPIM improves the signal sensitivity and volumetric coverage of whole-brain activity imaging, increasing the number of detected neurons by over a third.

Design parameters for Airy beams in light-sheet microscopy

We derive analytical expressions for the length, thickness, and curvature of an Airy light sheet in terms of basic parameters of the cubic phase and the paraxially defined focusing optics that form the beam. The length and thickness are defined analogously to the Rayleigh range and beam waist of a Gaussian beam, hence providing a direct and quantitative comparison between the two beam types. The analytical results are confirmed via numerical Fresnel propagation simulations and discussed within the context of light-sheet microscopy, providing a comprehensive guide for the design of the illumination unit.

Airy-like beam-based light-sheet microscopy with improved FOV for zebrafish intracerebral hemorrhage

Airy light-sheet microscopy is rapidly gaining importance for imaging intact biological specimens because of the rapid speed, high resolution, and wide field nature of the imaging method. However, the depth of field (DOF) of the detection objective imposes limitations on the modulation transfer function (MTF) of the light sheet, which in turn affects the size of the field of view (FOV). Here we present an optimized phase modulation model, based on 'Airy-like' beam family, to stretch the curved lobes, which brings a wider FOV while maintaining high resolution. In addition, we further develop a planar 'Airy-like' light-sheet by two-photon excitation which can avoid the deconvolution process. We validated the new imaging method by performing a real-time monitoring of the dynamic process of cerebral hemorrhage in zebrafish larva. The proposed Airy-like beam-based light-sheet microscopy has great potential to be applied to the precise screening of cerebral hemorrhage-related drugs to help precision medicine in the future.

Axial resolution enhancement for planar Airy beam light-sheet microscopy via the complementary beam subtraction method

Airy beam light-sheet illumination can extend the field of view (FOV) of light-sheet fluorescence microscopy due to the unique propagation properties of non-diffraction and self-acceleration. However, the side lobes create undesirable out-of-focus background, leading to poor axial resolution and low image contrast. Here, we propose an Airy complementary beam subtraction (ACBS) method to improve the axial resolution while keeping the extended FOV. By scanning the optimized designed complementary beam that has two main lobes (TML), the generated complementary light-sheet has almost identical intensity distribution to that of the planar Airy light-sheet except for the central lobe. Subtraction of the two images acquired by double exposure respectively using the planar Airy light-sheet and the planar TML light-sheet can effectively suppress the influence of the out-of-focus background. The axial resolution improves from ∼4µm to 1.2 µm. The imaging performance was demonstrated by imaging specimens of aspergillus conidiophores and GFP labeled mouse brain section. The results show that the ACBS method enables the Airy beam light-sheet fluorescence microscopy to obtain better imaging quality.

Maximizing content across scales: Moving multimodal microscopy and mesoscopy toward molecular imaging

Molecular imaging aims to depict the molecules in living patients. However, because this aim is still far beyond reach, patchworks of different solutions need to be used to tackle this overarching goal. From the vast toolbox of imaging techniques, we focus on those recent advances in optical microscopy that image molecules and cells at the submicron to centimeter scale. Mesoscopic imaging covers the "imaging gap" between techniques such as confocal microscopy and magnetic resonance imagingthat image entire live samples but with limited resolution. Microscopy focuses on the cellular level; mesoscopy visualizes the organization of molecules and cells into tissues and organs. The correlation between these techniques allows us to combine disciplines ranging from whole body imaging to basic research of model systems. We review current developments focused on improving microscopic and mesoscopic imaging technologies and on hardware and software that push the current sensitivity and resolution boundaries.

Unconventional circularly polarized Airy light-sheet spinner tweezers

Standard circularly polarized Airy light-sheets are synthesized by combining two dephased TE and TM wave fields, polarized in the transverse directions of wave propagation, respectively. Somewhat counterintuitively, the present analysis theoretically demonstrates the existence of unconventional circularly polarized Airy light-sheets, where one of the individual dephased wave fields is polarized along the direction of wave propagation. The vector angular spectrum decomposition method in conjunction with the Lorenz gauge condition and Maxwell's equations allow adequate determination of the Cartesian components of the incident radiated electric field components. Subsequently, the Cartesian components of the optical time-averaged radiation force and torque can be determined and computed. The example of a subwavelength light-absorptive (lossy) dielectric sphere is considered based upon the dipole approximation method. The results demonstrate the emergence of negative force components, suggesting retrograde motion and spinning reversal depending on the polarization of the Airy light-sheet and its transverse scale and attenuation parameter. The results are important in the design of light-sheet spinner tweezers and applications involving optical switching and particle manipulation and rotation.

Application of Airy beam light sheet microscopy to examine early neurodevelopmental structures in 3D hiPSC-derived human cortical spheroids

BACKGROUND

The inability to observe relevant biological processes in vivo significantly restricts human neurodevelopmental research. Advances in appropriate in vitro model systems, including patient-specific human brain organoids and human cortical spheroids (hCSs), offer a pragmatic solution to this issue. In particular, hCSs are an accessible method for generating homogenous organoids of dorsal telencephalic fate, which recapitulate key aspects of human corticogenesis, including the formation of neural rosettes-in vitro correlates of the neural tube. These neurogenic niches give rise to neural progenitors that subsequently differentiate into neurons. Studies differentiating induced pluripotent stem cells (hiPSCs) in 2D have linked atypical formation of neural rosettes with neurodevelopmental disorders such as autism spectrum conditions. Thus far, however, conventional methods of tissue preparation in this field limit the ability to image these structures in three-dimensions within intact hCS or other 3D preparations. To overcome this limitation, we have sought to optimise a methodological approach to process hCSs to maximise the utility of a novel Airy-beam light sheet microscope (ALSM) to acquire high resolution volumetric images of internal structures within hCS representative of early developmental time points.

RESULTS

Conventional approaches to imaging hCS by confocal microscopy were limited in their ability to image effectively into intact spheroids. Conversely, volumetric acquisition by ALSM offered superior imaging through intact, non-clarified, in vitro tissues, in both speed and resolution when compared to conventional confocal imaging systems. Furthermore, optimised immunohistochemistry and optical clearing of hCSs afforded improved imaging at depth. This permitted visualization of the morphology of the inner lumen of neural rosettes.

CONCLUSION

We present an optimized methodology that takes advantage of an ALSM system that can rapidly image intact 3D brain organoids at high resolution while retaining a large field of view. This imaging modality can be applied to both non-cleared and cleared in vitro human brain spheroids derived from hiPSCs for precise examination of their internal 3D structures. This process represents a rapid, highly efficient method to examine and quantify in 3D the formation of key structures required for the coordination of neurodevelopmental processes in both health and disease states. We posit that this approach would facilitate investigation of human neurodevelopmental processes in vitro.

Investigation of the extended focusing capability of the spherical aberration to enlarge the field of view in light-sheet fluorescence microscopy

In light-sheet fluorescence microscopy (LSFM), using Gaussian beams for light-sheet generation results in a trade-off between the thickness and the field of view (FOV). Here we present a theoretical analysis of using spherical aberration to enlarge the FOV while keeping the light-sheet thickness small. Such spherical aberration can arise when focusing beams through an interface between materials of mismatched refractive indices. The depth-of-focus extension of the Gaussian beam is achieved when using air objectives to focus light into the samples dipped in the immersion medium with a higher refractive index. By scanning this elongated beam, a thin light sheet with a wide FOV can be used for LSFM imaging. Meanwhile, the accompanied sidelobes with the spherical aberrated light sheet, which are mainly distributed in the rear part of the light sheet, are also discussed. Simulation results show that an extended FOV of 64.4µm is possible for an objective lens of NA=0.3, which is about 5 times that of the unaberrated case. For such an extended FOV, a comparatively thin thickness of 1.38µm as well as the first sidelobe about 11.1% of the peak intensity in the center are also demonstrated.

Fast in vivo multiphoton light-sheet microscopy with optimal pulse frequency

Improving the imaging speed of multiphoton microscopy is an active research field. Among recent strategies, light-sheet illumination holds distinctive advantages for achieving fast imaging in vivo. However, photoperturbation in multiphoton light-sheet microscopy remains poorly investigated. We show here that the heart beat rate of zebrafish embryos is a sensitive probe of linear and nonlinear photoperturbations. By analyzing its behavior with respect to laser power, pulse frequency and wavelength, we derive guidelines to find the best balance between signal and photoperturbation. We then demonstrate one order-of-magnitude signal enhancement over previous implementations by optimizing the laser pulse frequency. These results open new opportunities for fast live tissue imaging.