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 two-photon excitation volume is very small due to short Rayleigh range of focused Gaussian beam. Hence, the sampling volume is much smaller than one-photon flow cytometer, which makes it challenging to count or detect rare circulating cells in vivo. Non-diffracting beam like Bessel beams and Airy 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 them an ideal solution as a light source for scanning beam flow cytometry. The trade-off of using Airy beams rather than Gaussian beam is the fact that Airy beams have side lobes 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 2D Airy beams to form a light sheet that intersects the blood vessel. The set up can successfully detect and count flowing tumor cells in micro channel.
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