Improving the Speed of Galvo-Scanners

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Abstract Optical methods provide high-resolution, non-invasive measurement of neural function, ranging from single neurons to entire populations, in the intact brain. Nevertheless, limited penetration depth, spatial scale and temporal resolution remain the main challenges for optical imaging. Laser scanning multiphoton microscopy is the main technology used for cellular-level imaging in scattering brains. Because of the point scanning nature of laser scanning microscopy, the speed of the optical scanner determines the imaging speed, even when there is sufficient signal strength for fast imaging. For inertially dependent scanners like galvanometers and resonant scanners, increasing the scan speed requires decreasing the moment of inertia of the dynamic component, while the need for high spatial resolution requires increasing the aperture of the scan mirror. The balancing of these two conflicting requirements has limited commercially available galvanometer scanners at approximately the same speed for the last 30 to 40 years. The focus of this proposal is to demonstrate a new concept that will improve the scan speed of galvanometer-based optical scanners (galvo-scanners), and increasing the imaging speed of laser scanning microscopy. The proposed new concept leverages the uniaxial nature of galvanometer scanning, i.e., a galvo-scanner scans one spatial dimension only, which leaves the orthogonal dimension free for manipulation. By using a cylindrical lens to focus the beam onto the scan mirror along the axial direction of the galvanometer (i.e., the non-scanning direction) and another cylindrical lens to recollimate the beam after scanning, we can reduce the size of the scan mirror dramatically along the non-scanning direction. The resulting reduction in mirror mass and moment of inertia will increase the scan speed without reducing the scanned field-of-view and spatial resolution. The proposed program dovetails with our effort in developing technologies for brain imaging. We will test and validate the new scanners in laser scanning 2-photon and 3-photon microscopes for in vivo recording of mouse brain activity. The successful completion of this program will create new optical scanners that will improve the imaging speed of all galvo-based laser scanning microscopes. Since the design, test and demonstration are all based on typical multiphoton microscopes, the new concept can be immediately translated to other research labs. Furthermore, we will work with commercial instrument builders to accelerate the research and adoption of the technology developed in this program.