Image scanning microscopy with multiphoton excitation or Bessel beam illumination

Ultra-low-cost and high-fidelity NIR-II confocal laser scanning microscope with Bessel beam excitation and SiPM detection

Confocal laser scanning microscopy (CLSM) is one of the most important imaging tools in the biomedical field, and near-infrared-II (NIR-II, 900-1700nm) fluorescence imaging technology has also made fruitful research progress in deep imaging in recent years. The NIR-II based CLSM has problems such as an expensive detector and reduced image resolution caused by long wavelength excitation. Here, by simultaneously using a low-cost silicon photomultiplier (SiPM) as a detector and a Bessel beam as an excitation, we developed an ultra-low-cost and high-fidelity NIR-II confocal laser scanning microscope. The use of SiPM reduces the cost of the NIR-II fluorescence detection module in CLSM, while enabling the detection of ultra-broadband fluorescence signals spanning visible to NIR-II regions. The introduction of the Bessel beam compensates to some extent for the weakening of spatial resolution caused by the increase in the wavelength of light in the NIR region. Experimental results show that the use of the Bessel beam can improve the resolution by 12% when observing thin samples. With the increase of sample thickness, the imaging resolution of the Bessel beam at NIR-II wavelengths is better than that of the Gaussian beam at NIR-I wavelengths at the penetrable depth of the NIR-I light. At deeper depths, the imaging resolution and imaging depth of Bessel beam CLSM is superior to Gaussian beam CLSM at the same excitation power.

Extending the Depth of Focus of an Infrared Microscope Using a Binary Axicon Fabricated on Barium Fluoride

Axial resolution is one of the most important characteristics of a microscope. In all microscopes, a high axial resolution is desired in order to discriminate information efficiently along the longitudinal direction. However, when studying thick samples that do not contain laterally overlapping information, a low axial resolution is desirable, as information from multiple planes can be recorded simultaneously from a single camera shot instead of plane-by-plane mechanical refocusing. In this study, we increased the focal depth of an infrared microscope non-invasively by introducing a binary axicon fabricated on a barium fluoride substrate close to the sample. Preliminary results of imaging the thick and sparse silk fibers showed an improved focal depth with a slight decrease in lateral resolution and an increase in background noise.

Image scanning microscopy: a vectorial physical optics analysis

Image scanning microscopy (ISM) achieves resolution beyond the diffraction limit by a factor of 2. However, prior ISM research predominantly employs scalar diffraction theory, neglecting critical physical effects such as polarization, aberrations, and Stokes shift. This paper presents a comprehensive vectorial ISM point spread function (PSF) model that accounts for these phenomena. By considering the effect of polarization in emission and excitation paths, as well as aberrations and Stokes shift, our model provides a more accurate representation of ISM. We analyze the differences between scalar and vectorial theories in ISM and investigate the impact of pinhole size and aberration strength on resolution. At a numerical aperture of 1.2, the full width half maximum (FWHM) discrepancy between scalar and vectorial ISM PSFs can reach 45 nm, representing a 30% deviation from the vectorial model. Additionally, we explore multiphoton excitation in ISM and observe increased FWHM for 2-photon and 3-photon excitation compared to 1-photon excitation. The FWHM of the 2-photon excitation ISM PSF increases by 20% and the FWHM of the 3-photon excitation ISM PSF increases by 28% compared to the 1-photon excitation ISM. In addition, we found that the optimal sweep factor for 2-photon ISM is 1.22, and the optimal sweep factor of 3-photon ISM is 1.12 instead of the 2 predicted by the one-photon scalar ISM theory. Our work improves the understanding of ISM and contributes to its advancement as a high-resolution imaging technique.

Signal strength and integrated intensity in confocal and image scanning microscopy

The properties of signal strength and integrated intensity in a scanned imaging system are reviewed. These properties are especially applied to confocal imaging systems, including image scanning microscopy. The integrated intensity, equal to the image of a uniform planar (sheet) object, rather than the peak of the point spread function, is a measure of the flux in an image. Analytic expressions are presented for the intensity in the detector plane for a uniform volume object, and for the resulting background. The variation in the integrated intensity with defocus for an offset point detector is presented. This axial fingerprint is independent of any pixel reassignment. The intensity in the detector plane is shown to contain the defocus information, and simple processing of the recorded data can improve optical sectioning and background rejection.

Tunable depth of focus with modified complex amplitude modulation of an optical field

Bessel beams have nondiffraction and self-healing properties in the propagation direction and are widely used in particle optical manipulation and optical microscopy. Bessel beams can be generated by axicons or spatial light modulators, which can produce a zero-order or high-order Bessel beam with different parameters depending on the specific application. The modulation of Bessel beams achieved in the spatial spectrum domain by optimization algorithms has a low light energy utilization rate due to the small effective modulation region. We propose a Bessel-like beam phase generation algorithm based on an improved iterative optimization algorithm directly in the spatial domain to achieve a tunable modulation of the beam's length and the axial center position. The optimization time is reduced from minutes to seconds relative to the genetic algorithm, providing a new means of modulation for different applications in various fields.

Saturated-excitation image scanning microscopy

Image scanning microscopy (ISM) overcomes the trade-off between spatial resolution and signal volume in confocal microscopy by rearranging the signal distribution on a two-dimensional detector array to achieve a spatial resolution close to the theoretical limit achievable by infinitesimal pinhole detection without sacrificing the detected signal intensity. In this paper, we improved the spatial resolution of ISM in three dimensions by exploiting saturated excitation (SAX) of fluorescence. We theoretically investigated the imaging properties of ISM, when the fluorescence signals are nonlinearly induced by SAX, and show combined SAX-ISM fluorescence imaging to demonstrate the improvement of the spatial resolution in three dimensions. In addition, we confirmed that the SNR of SAX-ISM imaging of fluorescent beads and biological samples, which is one of the challenges in conventional SAX microscopy, was improved.

ISM-assisted tomographic STED microscopy

Stimulated emission depletion (STED) microscopy theoretically provides unlimited resolution. However, in practice the achievable resolution in biological samples is essentially limited by photobleaching. One method which overcomes this problem is tomographic STED (tomoSTED) microscopy. In tomoSTED microscopy, one-dimensional depletion patterns facing in different directions are successively applied in order to acquire a highly-resolved image in two dimensions. In this context, the number of addressed directions depends on the desired angular homogeneity of the point spread function or the optical transfer function and thus on the resolution increase as compared to diffraction-limited imaging. At a reasonable angular homogeneity the light dose and thus bleaching can be reduced, as compared to conventional STED microscopy. Here, we propose and demonstrate for the first time, to our knowledge, that the number of required depletion pattern orientations can be reduced by combining tomoSTED microscopy with the concept of image scanning microscopy (ISM). With our realization of an ISM-tomoSTED microscope, we show that approximately a factor of 2 lower number of orientations are required to achieve the same resolution and image quality as in tomoSTED microscopy.

Improving Multiphoton Microscopy by Combining Spherical Aberration Patterns and Variable Axicons

Multiphoton (MP) microscopy is a well-established method for the non-invasive imaging of biological tissues. However, its optical sectioning capabilities are reduced due to specimen-induced aberrations. Both the manipulation of spherical aberration (SA) and the use of axicons have been reported to be useful techniques to bypass this limitation. We propose the combination of SA patterns and variable axicons to further improve the quality of MP microscopy images. This approach provides enhanced images at different depth locations whose quality is better than those corresponding to the use of SA or axicons separately. Thus, the procedure proposed herein facilitates the visualization of details and increases the depth observable at high resolution.

The Development of Microscopy for Super-Resolution: Confocal Microscopy, and Image Scanning Microscopy

Optical methods of super-resolution microscopy, such as confocal microscopy, structured illumination, nonlinear microscopy, and image scanning microscopy are reviewed. These methods avoid strong invasive interaction with a sample, allowing the observation of delicate biological samples. The meaning of resolution and the basic principles and different approaches to superresolution are discussed.

Pixel reassignment in image scanning microscopy with a doughnut beam: example of maximum likelihood restoration

In image scanning microscopy, the pinhole of a confocal microscope is replaced by a detector array. The point spread function for each detector element can be interpreted as the probability density function of the signal, the peak giving the most likely origin. This thus allows a form of maximum likelihood restoration, and compensation for aberrations, with similarities to adaptive optics. As an example of an aberration, we investigate theoretically and experimentally illumination with a vortex doughnut beam. After reassignment and summation over the detector array, the point spread function is compact, and the resolution and signal level higher than in a conventional microscope.

Structured illumination microscopy and image scanning microscopy: a review and comparison of imaging properties

Structured illumination microscopy and image scanning microscopy are two microscopical tech- niques, rapidly increasing in practical application, that can result in improvement in transverse spatial resolution, and/or improvement in axial imaging performance. The history and principles of these techniques are reviewed, and the imaging properties of the two methods compared. This article is part of the Theo Murphy meeting issue 'Super-resolution structured illumination microscopy (part 1)'.