Multifocal multiphoton microscopy based on a spatial light modulator

Wavefront correction in two-photon microscopy with a multi-actuator adaptive lens

A multi-actuator adaptive lens (AL) was incorporated into a multi-photon (MP) microscope to improve the quality of images of thick samples. Through a hill-climbing procedure the AL corrected for the specimen-induced aberrations enhancing MP images. The final images hardly differed when two different metrics were used, although the sets of Zernike coefficients were not identical. The optimized MP images acquired with the AL were also compared with those obtained with a liquid-crystal-on-silicon spatial light modulator. Results have shown that both devices lead to similar images, which corroborates the usefulness of this AL for MP imaging.

New microscopy technique based on position localization of scattering particles

We introduce the Holographic - Single Scatterer Localization Microscopy in which we combine dynamical laser speckle illumination with centroid localization of backscattered light spots in order to localize isolated scattering particles. The reconstructed centroid images show very accurate particle localization, with precision much better than the width of diffraction-limited image of the particles recorded by the CCD. Furthermore, the method provides an improved resolution in distinguishing two very close scattering objects compared to the standard laser scanning techniques and can be assimilated to a confocal technique in the ability of light background rejection in three-dimensional disposition of scattering objects. The illumination is controlled via a digital holography setup based on the use of a spatial light modulator. This allows not only a high level of versatility in the illumination patterns, but also the remarkable characteristics of absence of moving mechanical parts, typical of the laser scanning techniques, and the possibility of strongly miniaturizing the setup.

Correction of spherical aberration in multi-focal multiphoton microscopy with spatial light modulator

We demonstrate that high-quality images of the deep regions of a thick sample can be obtained from its surface by multi-focal multiphoton microscopy (MMM). The MMM system incorporates a spatial light modulator to separate the excitation beam into a multi-focal excitation beam and modulate the pre-distortion wavefront to correct spherical aberration (SA) caused by a refractive index mismatch between the immersion medium and the biological sample. When fluorescent beads in transparent epoxy resin were observed using four SA-corrected focal beams, the fluorescence signal of the obtained images was ~52 times higher than that obtained without SA correction until a depth of ~1100 μm, similar to the result for single-focal multiphoton microscopy (SMM). The MMM scanning time was four times less than that for SMM, and MMM showed an improved fluorescence intensity and depth resolution for an image of blood vessels in the brain of a mouse stained with a fluorescent dye.

Stable and flexible multiple spot pattern generation using LCOS spatial light modulator

The LCOS spatial light modulator (LCOS-SLM) can generate desired multiple spot patterns (MSPs) via the application of suitable computer-generated-holograms (CGHs), but the MSP intensity distribution varies because ambient temperature affects the phase modulation characteristic and causes wavefront distortion. To generate high-optical-quality MSPs we use our hardware-compensated (with a Peltier system to even out phase modulation) and software-corrected (via multiplication of the CGH by temperature correction coefficients) LCOS-SLMs. Experimental results with a 14 × 14 MSP generation show that the hardware-compensated LCOS-SLM provides stable MSPs between 9 to 32 °C. The software-corrected LCOS-SLM provides uniform spots over twice the temperature range obtained with conventional SLM method. We confirm that our methods are highly efficient for use in two-photon excitation microscopy application such as multifocal mulitphoton microscopy.

Temporal resolution in fluorescence imaging

Temporal resolution is a key factor for imaging rapidly occurring events in biology. In this feature article, I investigate an approximate estimate for determining the temporal resolution limit. The condition that led to this limit is, the time taken by the ensemble (99.9%) of excited molecules to relax to ground state, assuming all the emitted photons are detected. In a simplistic three-level system, the temporal resolution is, ≈3τ p , where τ p = (log e 10)/(kf + knr ) and, kf and knr are respectively the radiative and non-radiative emission rates. This further assumes the ideal condition that, the quantum efficiency of the detector is unity and there are no other loses. We discuss few state-of-art microscopy techniques that are capable of high temporal resolution. This includes techniques such as multifocal multiphoton microscopy (MMM), multifocal plane microscopy, multiple excitation spot optical microscopy (MESO), multiplane microscopy and multiple light-sheet microscopy (MLSM).

Pulse splitter-based nonlinear microscopy for live-cardiomyocyte imaging

Second harmonic generation (SHG) microscopy is a new imaging technique used in sarcomeric-addition studies. However, during the early stage of cell culture in which sarcomeric additions occur, the neonatal cardiomyocytes that we have been working with are very sensitive to photodamage, the resulting high rate of cell death prevents systematic study of sarcomeric addition using a conventional SHG system. To address this challenge, we introduced use of the pulse-splitter system developed by Na Ji et al. in our two photon excitation fluorescence (TPEF) and SHG hybrid microscope. The system dramatically reduced photodamage to neonatal cardiomyocytes in early stages of culture, greatly increasing cell viability. Thus continuous imaging of live cardiomyocytes was achieved with a stronger laser and for a longer period than has been reported in the literature. The pulse splitter-based TPEF-SHG microscope constructed in this study was demonstrated to be an ideal imaging system for sarcomeric addition-related investigations of neonatal cardiomyocytes in early stages of culture.

An adaptive approach for uniform scanning in multifocal multiphoton microscopy with a spatial light modulator

We propose high-quality generation of uniform multiple fluorescence spots (MFS) with a spatial light modulator (SLM) and demonstrate uniform laser scanning in multifocal multiphoton microscopy (MMM). The MFS excitation method iteratively updates a computer-generated hologram (CGH) using correction coefficients to improve the fluorescence intensity distribution in a dye solution whose consistency is uniform. This simple correction method can be applied for calibration of the MMM before observation of living tissue. We experimentally demonstrate an improvement of the uniformity of a 10 × 10 grid of MFS by using a dye solution. After the calibration, we performed laser scanning with two-photon excitation to observe fluorescent polystyrene beads, as well as the gastric gland of a guinea pig specimen.

Addressable, large-field second harmonic generation microscopy based on 2D acousto-optical deflector and spatial light modulator

We present an addressable, large-field second harmonic generation microscope by combining a 2D acousto-optical deflector with a spatial light modulator. The SLM shapes an incoming mode-locked, near-infrared Ti:Sapphire laser beam into a multifocus array, which can be rapidly scanned by changing the incident angle of the laser beam using a 2D acousto-optical deflector. Compared to the single-beam-scan technique, the multifocus array scan can increase the scanning rate and the field-of-view size with the multi-region imaging ability.

RECENT PROGRESS IN MULTIFOCAL MULTIPHOTON MICROSCOPY

Multifocal multiphoton microscopy (MMM) has recently become an important tool in biomedicine for performing three-dimensional fast fluorescence imaging. Using various beamsplitting techniques, MMM splits the near-infrared laser beam into multiple beamlets and produces a multifocal array on the sample for parallel multiphoton excitation and then records fluorescence signal from all foci simultaneously with an area array detector, which significantly improves the imaging speed of multiphoton microscopy and allows for high efficiency in use of the excitation light. In this paper, we discuss the features of several MMM setups using different beamsplitting devices, including a Nipkow spinning disk, a microlens array, a set of beamsplitting mirrors, or a diffractive optical element (DOE). In particular, we present our recent work on the development of an MMM using a spatial light modulator (SLM).