Multi-photon microscopy without scanning for faster than video-rate fluorescence imaging of live cells

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During the last twenty years, there has been an explosion in new microscopy techniques which exploit the high peak intensities from laser sources for excitation of fluorescent dyes used as markers in live cells. These methods, which are based on nonlinear optics, offer several advantages for the biologist over more traditional imaging techniques. These include imaging of deeper tissue thanks to longer excitation wavelengths, avoidance of damaging short-wavelengths, and an overall reduction in photo-bleaching. However, it has been generally accepted that these nonlinear microscopy methods must use a laser focused to a tiny spot which is then scanned around the specimen. This limits the capture rate of information to around 1 frame/second. This is a major limitation to the method for studying live cells, since rapid and important changes in the intra-cellular biochemistry are often missed.A few methods for increasing the imaging speed of nonlinear microscopy have been demonstrated, but only one is commercially available (which is essential when the technology is to be used in a biology research laboratory). This technique involves splitting a single high-intensity laser beam into up to 64 lower intensity 'beamlets' which are then scanned around the specimen, but this unfortunately can result in a 'patchwork quilt' effect which introduces unwanted artifacts into the images and can render interpretation and analysis difficult.To provide the advantages of nonlinear microscopy but at fast capture speeds, we propose to capitalize on innovations in sensor technology and use a less well-focused laser beam, which will illuminate the full image field. This 'wide-field' method is known to biologists, but in a linear (single-photon) rather than nonlinear (two-photon) approach, and therefore is a simple adaptation to existing instrumentation that is familiar to the end-user. The key difference in our technology over a conventional fluorescence microscope will be the light source, which we will change from a light-emitting diode to a high peak intensity laser (which we already have in our laboratory). We will also use small modifications to the microscope and add a sensitive scientific camera detector. Our calculations show that nonlinear excitation of fluorescence is possible at capture speeds of up to 100 frames/second. We will test this new technology with non-biological specimens initially, and then apply the method to two different cell types to study both fast and slow calcium signalling events. If we are successful, this technology is almost certain to change how cell biologists obtain images of their specimens which, in turn, will likely have a long-term impact on pharmacology and the development of new medicines.