KEYWORDS: Signal to noise ratio, Particles, Point spread functions, Algorithm development, 3D image processing, Super resolution, Microscopy, Electroluminescence, Microscopes, Reconstruction algorithms
Choosing the localization algorithm in superresolution microscopy is an important factor in determining the resolution in such a modality. The point spread function (PSF) in defocused images has ring structures that can be used to localize the three-dimensional (3-D) position of single particles by calculating the ring center (x and y) and radius (z). As there is no well-developed mathematical model for a defocused PSF, it is difficult to perform a fitting-based algorithm in such images. A particle localization algorithm based on radial symmetry and ellipse fitting is developed to localize the centers and radii of defocused PSFs. Our method can localize the 3-D position of a fluorophore within 20-nm precision in three-dimensions in a range of 40 μm in z-dimension from defocused two-dimensional (2-D) images.
Flow cytometry is an important technique in biomedical discovery for cell counting, cell sorting and biomarker detection. In vivo flow cytometers, based on one-photon or two-photon excited fluorescence, have been developed for more than a decade. One drawback of laser beam scanning two-photon flow cytometer is that the two-photon excitation volume is fairly small due to the short Rayleigh range of a focused Gaussian beam. Hence, the sampling volume is much smaller than one-photon flow cytometry, which makes it challenging to count or detect rare circulating cells in vivo. Bessel beams have narrow intensity profiles with an effective spot size (FWHM) as small as several wavelengths, making them comparable to Gaussian beams. More significantly, the theoretical depth of field (propagation distance without diffraction) can be infinite, making it an ideal solution as a light source for scanning beam flow cytometry. The trade-off of using Bessel beams rather than a Gaussian beam is the fact that Bessel beams have small concentric side rings that contribute to background noise. Two-photon excitation can reduce this noise, as the excitation efficiency is proportional to intensity squared. Therefore, we developed a two-photon flow cytometer using scanned Bessel beams to form a light sheet that intersects the micro fluidic channel.
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