This review presents a selection of advanced microscope designs employing acousto-optical deflectors (AODs). In the designs covered, AODs are used as tunable diffraction gratings to control complex illumination patterns at the Fourier plane of an objective lens. This approach allows us to generate desired illumination patterns at the focal plane of a light microscope. In simple terms, I will describe two established designs, the 3D Random-Access Multi-Photon Microscope and the Standing-Wave Super-Resolution Microscope, as well as two new schemes including the Random-Access STED Microscope and the Frequency-Encoded Multi-Beam Microscope. All instruments mentioned here were designed to overcome the throughput limitations of previously used light microscopes in experimental Neuroscience.
We developed a two-photon microscope optimized for physiologically manipulating single neurons through their postsynaptic receptors. The optical layout fulfills the stringent design criteria required for high-speed, high-resolution imaging in scattering brain tissue with minimal photodamage. We detail the practical compensation of spectral and temporal dispersion inherent in fast laser beam scanning with acousto-optic deflectors, as well as a set of biological protocols for visualizing nearly diffraction-limited structures and delivering physiological synaptic stimuli. The microscope clearly resolves dendritic spines and evokes electrophysiological transients in single neurons that are similar to endogenous responses. This system enables the study of multisynaptic integration and will assist our understanding of single neuron function and dendritic computation.
The intrinsic optical sectioning, reduced light-scattering, and reduced photodamage of multiphoton laser-scanning microscopy (MPLSM) has generated great interest for this technique in experimental Neuroscience, as it enables to study both structure and function of fine neuronal processes within living brain tissue. At present, virtually all MPLSM systems employ galvanometric beam positioning. Due to this inertia-limited approach, single-dimension line scans are employed to achieve frame rates sufficient for functional imaging. Although such line scans allow adequate sampling rates (≤1kHz), two significant drawbacks remain. First, the majority of scan time is wasted by illuminating regions of no interest, while sacrificing signal integration time at sites-of-interest. Second, the sites from which signals can be recorded are limited to those along a single line. Alternatively, acousto-optic (AO) beam positioning with high-resolution TeO2 deflectors allows inertia-free skipping between arbitrary sites within the field-of-view in <15μs. This achieves high sampling rate recording at multiple, non-adjacent sites quasi-simultaneously (1-5kHz frame rate, 12-60 sites). Such a multi-site optical recording system would greatly advance studying complex neuronal function, by enabling membrane potential or calcium transients to be observed throughout the complex geometry of neuronal dendrites. This paper presents images and functional recordings from living neurons within brain slices, acquired with AO-MPLSM. Our novel imaging system allows a user to collect structural images first and subsequently select sites of interest for fast functional imaging. To demonstrate the system’s power, we present high-speed recordings (1kHz) from >10 sites within the dendrites of pyramidal neurons in acute brain slices, at signal-to-noise ratios comparable to line-scan systems.
Fluorescence correlation spectroscopy(FCS) normally employs measured fluctuations in molecular fluorescence from within a defined observation volume created by a laser focus to extract information about dynamic processes associated with those molecules. In order to extract quantitative information about these processes, the laser focal volume must be well characterized. We show in this work, similar to others before, that a description of the focal volume in terms of vector diffraction theory allows quantitative analysis of FCS sufficient to resolve the diffusion coefficients of small molecules-Rhod 6G and Alexa 546, here-in the diffraction-limited volume of a high numerical aperture objective lens. We subsequently demonstrate the utility of a “reverse” FCS experimental paradigm in which the focal volume itself is characterized for a known molecular sample. Using a combination of wide-field and localized detection, we characterize the focal volume location and relative size as the beam collimation and correction collar are varied. We find that Gaussian beam theory can predict the focal volume location over a wide range of collimation conditions. It can also predict the relative focal volume size for converging and collimated beams, provided that spherical aberration is corrected for each collimation condition using the correction collar.
The femtoliter excitation volume of multiphoton microscopy renders all emitted photons useful in detecting fluorescence signals. We have previously demonstrated the viability of transmitting the emitted fluorescence via multimode optical fibers onto a photomultiplier tube (PMT) in a full-field arrangement. A custom MPLSM, based on a commercially available confocal microscope was developed to readily switch between the regular descanned path for confocal microscopy and our non-descanned pathways (direct detection) supporting fiber-coupled detection. We now wish to demonstrate the efficacy of fiber-based detection using a side-by-side comparison of our fiber-coupled paradigm to the traditional method of directly focusing the fluorescence, through air, onto a PMT. To effectively compare the two methods, we have incorporated a second direct detection epi-fluorescence pathway for air-coupling onto a PMT that does not affect the performance of the fiber-based MPLSM. We found that fiber-based detection compares favorably against traditional direct detection. We demonstrate the viability of fiber-based detection for high-resolution neuronal brain slice imaging.
In laser-scanning microscopy, acousto-optic (AO) deflection provides a means to quickly position a laser beam to random locations throughout the field-of-view. Compared to conventional laser-scanning using galvanometer-driven mirrors, this approach increases the frame rate and signal-to-noise ratio, and reduces time spent illuminating sites of no interest. However, random-access AO scanning has not yet been combined with multi-photon microscopy, primarily because the femtosecond laser pulses employed are subject to significant amounts of both spatial and temporal dispersion upon propagation through common AO materials. Left uncompensated, spatial dispersion reduces the microscope’s spatial resolution while temporal dispersion reduces the multi-photon excitation efficacy. In previous work, we have demonstrated, 1) the efficacy of a single diffraction grating scheme which reduces the spatial dispersion at least 3-fold throughout the field-of-view, and 2) the use of a novel stacked-prism pre-chirper for compensating the temporal dispersion of a pair of AODs using a shorter mechanical path length (2-4X) than standard prism-pair arrangements. In this work, we demonstrate for the first time the use of these compensation approaches with a custom-made large-area slow-shear TeO2 AOD specifically suited for the development of a high-resolution 2-D random-access AO scanning multi-photon laser-scanning microscope (AO-MPLSM).
The extremely small (femtoliter) excitation volume of multiphoton (MP) microscopy renders all emitted photons useful in detecting fluorescence signals. Hence, multiphoton laser scanning microscopy (MPLSM) systems can collect fluorescence through the objective (epi-fluorescence), as well as the condenser (trans-fluorescence). For maximal collection efficiency, both optical paths can be used concurrently (4π detection). Most MPLSM systems incorporate photodetectors directly in or adjacent to the epi- and trans-fluorescence optical paths of the microscope, generally photomultiplier tubes with associated optics. These arrangements are optically straightforward, but are often bulky and difficult to reconfigure. Here, we demonstrate that all fluorescence from the specimen can be efficiently coupled into two multimode optical fibers -- one each for the epi- and trans-fluorescence pathways. Fiber-coupled detection enables a modular detection paradigm where light can be routed to easily reconfigurable and interchangeable detection module(s). A novel MPLSM system was constructed, which is readily switched between the original de-scanned detection path for confocal microscopy, and the newly added pathways supporting fiber-coupled non-descanned 4π detection for MP microscopy. Sample MP images of fluorescent beads and fluorescent-labeled hippocampal neurons are presented, demonstrating the viability of fiber-coupled detection.
A compact two-photon laser-scanning microscope (TPLSM) was constructed using a diode-pumped, mode-locked Nd:YLF laser (Biolight 1000, Coherent Laser Group) and a small confocal laser scan-head (PCM2000, Nikon Bioscience). The laser emits at 1047nm and is fiber-coupled to a compact compressor unit producing a pulse-width of ~175fsec. Both the pulse compressor and confocal scan head were interfaced on a small optical breadboard that was directly attached to an upright research microscope (Eclipse E600FN, Nikon Bioscience). Two-photon fluorescence emitted from the specimen was collected into a multimode fiber and transmitted directly to an external PMT supplied with the Nikon confocal system. The modifications to the scanhead were minimal (a single mirror replacement) and did not interfere with its confocal function. The resulting system offers several advantages: compact size, turnkey operation, and the ability to translate the microscope rather than an often delicate specimen. In addition, it is possible to switch between confocal and two-photon operation, allowing for straightforward comparison. Using this compact TPLSM, we obtained structural and functional images from hippocampal neurons in living brain slices using commonly available fluorophores.
In laser scanning microscopy, acousto-optic (AO) deflection provides a means to quickly position a laser beam to random locations. Compared to conventional scanning using galvanometer-driven mirrors, this approach increases the frame rate and signal-to-noise ratio, and reduces time spent illuminating sites of no interest. However, AO scanning has not yet been applied to multi-photon microscopy, primarily, because the femtosecond laser pulses employed are subject to significant amounts of both temporal and spatial dispersion upon propagation through common AO materials. Temporal dispersion of ultrashort pulses - also known as group velocity dispersion (GVD) - is commonly compensated by the inclusion of a pre-chirper. However, because of the large GVD of AO materials, commonly used pre-chirper designs entail a rather large footprint (> 2 meters). Spatial dispersion of the scanned beam - arising from AO deflection being a wavelength-dependent diffraction process - limits the achievable spatial resolution. To address these problems, we developed: 1) a novel four-pass double-decker pre-chirper geometry that reduces the assembly's footprint by another factor of two relative to current designs, and 2) a single diffraction grating scheme for significantly reducing spatial dispersion. The presented findings enable the construction of an acousto-optic two-photon microscope (AO-TPM).
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