Methods for imaging of excised tissue specimens that obviate manual and time-consuming histology processing steps including wax-embedding, sectioning, and separate staining hold considerable promise for improving our ability to render time- and cost-efficient diagnoses. They also could potentiate the clinical adoption of machine learning tools dependent on routine production of digital histology data. A known limitation of point scanning-based ex vivo imaging technologies that precludes ready adoption for clinical use is imaging speed. We have recently described the development of an approach for multiphoton microscopy capable of imaging un-sectioned and un-embedded human tissue samples at millimeter depths with sufficient quality for primary diagnostic interpretation and at speeds more than 30-times faster than a traditional galvanometer-based system. Incorporation of a polygonal mirror combined with stage scanning in an optically efficient geometry yielded significant speed gains, beyond those possible with resonant galvanometers, and without sacrificing image quality. We hypothesized that further gains in speed, with maintenance of image quality, would be attainable by incorporating pulsed laser excitation with a repetition rate beyond the typical 80 MHz speed of standard Ti-Sapphire lasers. In this analysis we describe the use of a newly produced 250 MHz ultra-fast laser in a polygon-based microscope with stage scanning and demonstrate multiphoton human tissue imaging at speeds on par with the fastest whole slide imaging systems and with resolution, contrast, and coloration that matches physical slides but without any of the principal artifacts associated with slide scanning.
Multiphoton microscopy of cleared tissue has previously been demonstrated to generate large three-dimensional (3D) volumetric image data on entire intact mouse organs using intrinsic tissue fluorescence. This technique holds great promise for performing 3D virtual biopsies, providing unique information on tissue morphology, and guidance for subsequent traditional slicing and staining. Here, we demonstrate the use of fluorescence lifetime imaging in cleared organs for achieving molecular contrast that can reveal morphologically distinct structures, even in the absence of knowledge of the underlying molecular source. In addition, we demonstrate the power of multimodal imaging, combining multiphoton fluorescence, second harmonic generation, and lifetime imaging to reveal exceptional morphological detail in an optically cleared mouse knee.
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