Automated spherical aberration correction in scanning confocal microscopy

Pixel dislocation correction method for a laser confocal scanning microscope through the nonlinear triangular wave driving mode and square wave index reconstruction

The reciprocate scanning scheme of two-dimensional galvanometers is widely used in laser confocal scanning microscopes with high speed. However, the equal interval acquisition of an analog digital acquisition card (AD) and the unequal change of the galvanometer's scanning speed will cause the dislocation of pixels and distortion of the reconstructed image. Meanwhile, the movement properties of the galvanometers in the edge of the scanning area are complex, which will increase the difficulty of segmenting the collected one-dimensional data stream the AD collected into row data of a two-dimensional reconstructed image. Therefore, how to timely and accurately segment the one-dimensional data stream the AD collected into the row data of two-dimensional reconstructed image is not only the key to solve image distortion of a laser confocal scanning microscope with high speed but also the prerequisite to improve the accuracy of row data dislocation correction. A driving mode based on the nonlinear triangular wave and a dislocation-correcting method using a square wave index are proposed. Namely, on the basis of the galvanometer's scanning analysis, the equation of a nonlinear triangular wave driving voltage is established, and the switching frequency of the Y-galvanometer's driving voltage is obtained by calculating the collected switching frequency of the X-galvanometer; thus, the uniformity of the galvanometer's scanning trajectories is secured. Finally, the row segmentation flag pulse is first introduced into the one-dimensional data stream the AD collected, and the square wave index is used to segment the collected data, which means the one-dimensional data stream can be segmented timely and accurately via hardware method. Meanwhile, the pixel dislocation can be corrected. The experimental result shows that, compared with the Nikon A1R+ confocal microscope, the proposed method can effectively correct the pixel dislocation, and the position coincidence error is less than 0.7%. The proposed method will be helpful to improve the image quality of a laser confocal scanning microscope with high speed.

Variable immersion microscopy with a high numerical aperture

Three-dimensional (3D) optical microscopy with a high numerical aperture (NA) remains challenging for thick biological specimens owing to aberrations arising from interface refractions. We developed a variable immersion lens (VIL) to passively minimize these aberrations. A VIL is a high-NA concentric meniscus lens and was used in combination with an aberration-corrected high-NA reflecting objective (TORA-FUJI mirror). Wave-optics simulation at a wavelength of 488 nm showed that a VIL microscope enables diffraction-limited 1.2-NA imaging in water (refractive index of 1.34) at a depth of 0.3 mm by minimizing aberrations due to refraction of a sample interface. Another aberration due to the refractive index mismatching between a mounting medium, and an object can also be corrected by the VIL system, because various fluids with different refractive indices can be used as mounting media for the VIL. As a result of correcting the two aberrations at the same time, we experimentally demonstrated that a 6 µm diameter fluorescent bead can be imaged to the true dimensions in 3D.

Comparison of Multiscale Imaging Methods for Brain Research

A major challenge in neuroscience is how to study structural alterations in the brain. Even small changes in synaptic composition could have severe outcomes for body functions. Many neuropathological diseases are attributable to disorganization of particular synaptic proteins. Yet, to detect and comprehensively describe and evaluate such often rather subtle deviations from the normal physiological status in a detailed and quantitative manner is very challenging. Here, we have compared side-by-side several commercially available light microscopes for their suitability in visualizing synaptic components in larger parts of the brain at low resolution, at extended resolution as well as at super-resolution. Microscopic technologies included stereo, widefield, deconvolution, confocal, and super-resolution set-ups. We also analyzed the impact of adaptive optics, a motorized objective correction collar and CUDA graphics card technology on imaging quality and acquisition speed. Our observations evaluate a basic set of techniques, which allow for multi-color brain imaging from centimeter to nanometer scales. The comparative multi-modal strategy we established can be used as a guide for researchers to select the most appropriate light microscopy method in addressing specific questions in brain research, and we also give insights into recent developments such as optical aberration corrections.

Q&A: Array tomography

Array tomography encompasses light and electron microscopy modalities that offer unparalleled opportunities to explore three-dimensional cellular architectures in extremely fine structural and molecular detail. Fluorescence array tomography achieves much higher resolution and molecular multiplexing than most other fluorescence microscopy methods, while electron array tomography can capture three-dimensional ultrastructure much more easily and rapidly than traditional serial-section electron microscopy methods. A correlative fluorescence/electron microscopy mode of array tomography furthermore offers a unique capacity to merge the molecular discrimination strengths of multichannel fluorescence microscopy with the ultrastructural imaging strengths of electron microscopy. This essay samples the first decade of array tomography, highlighting applications in neuroscience.

Adaptive optical versus spherical aberration corrections for in vivo brain imaging

Adjusting the objective correction collar is a widely used approach to correct spherical aberrations (SA) in optical microscopy. In this work, we characterized and compared its performance with adaptive optics in the context of in vivo brain imaging with two-photon fluorescence microscopy. We found that the presence of sample tilt had a deleterious effect on the performance of SA-only correction. At large tilt angles, adjusting the correction collar even worsened image quality. In contrast, adaptive optical correction always recovered optimal imaging performance regardless of sample tilt. The extent of improvement with adaptive optics was dependent on object size, with smaller objects having larger relative gains in signal intensity and image sharpness. These observations translate into a superior performance of adaptive optics for structural and functional brain imaging applications in vivo, as we confirmed experimentally.

Recovering fluorophore concentration profiles from confocal images near lateral refractive index step changes

Optical aberrations due to refractive index mismatches occur in various types of microscopy due to refractive differences between the sample and the immersion fluid or within the sample. We study the effects of lateral refractive index differences by fluorescence confocal laser scanning microscopy due to glass or polydimethylsiloxane cuboids and glass cylinders immersed in aqueous fluorescent solution, thereby mimicking realistic imaging situations in the proximity of these materials. The reduction in fluorescence intensity near the embedded objects was found to depend on the geometry and the refractive index difference between the object and the surrounding solution. The observed fluorescence intensity gradients do not reflect the fluorophore concentration in the solution. It is suggested to apply a Gaussian fit or smoothing to the observed fluorescence intensity gradient and use this as a basis to recover the fluorophore concentration in the proximity of the refractive index step change. The method requires that the reference and sample objects have the same geometry and refractive index. The best results were obtained when the sample objects were also used for reference since small differences such as uneven surfaces will result in a different extent of aberration.