Two-photon fluorescence microscopy (2PM)1 enables scientists in various fields including neuroscience2,3, embryology4, and oncology5 to visualize in vivo and ex vivo tissue morphology and physiology at a cellular level deep within scattering tissue. However, tissue scattering limits the maximum imaging depth of 2PM within the mouse brain to the cortical layer, and imaging subcortical structures currently requires the removal of overlying brain tissue3 or the insertion of optical probes6,7. Here we demonstrate non-invasive, high resolution, in vivo imaging of subcortical structures within an intact mouse brain using three-photon fluorescence microscopy (3PM) at a spectral excitation window of 1,700 nm. Vascular structures as well as red fluorescent protein (RFP)-labeled neurons within the mouse hippocampus are imaged. The combination of the long excitation wavelength and the higher order nonlinear excitation overcomes the limitations of 2PM, enabling biological investigations to take place at greater depth within tissue.

We compare the maximal two-photon fluorescence microscopy (TPM) imaging depth achieved with 775-nm excitation to that achieved with 1280-nm excitation through in vivo and ex vivo TPM of fluorescently-labeled blood vessels in mouse brain. We achieved high contrast imaging of blood vessels at approximately twice the depth with 1280-nm excitation as with 775-nm excitation. An imaging depth of 1 mm can be achieved in in vivo imaging of adult mouse brains at 1280 nm with approximately 1-nJ pulse energy at the sample surface. Blood flow speed measurements at a depth of 900 mum are performed.

Deep tissue in vivo two-photon fluorescence imaging of cortical vasculature in a mouse brain using 1280-nm excitation is presented. A record imaging depth of 1.6 mm in mouse cortex is achieved in vivo, approximately reaching the fundamental depth limit in scattering tissue.

Two-photon fluorescence microscopy (2PM) 1 enables scientists in various fields including neuroscience 2,3 , embryology 4 , and oncology 5 to visualize in vivo and ex vivo tissue morphology and physiology at a cellular level deep within scattering tissue. However, tissue scattering limits the maximum imaging depth of 2PM within the mouse brain to the cortical layer, and imaging subcortical structures currently requires the removal of overlying brain tissue 3 or the insertion of optical probes 6,7 . Here we demonstrate non-invasive, high resolution, in vivo imaging of subcortical structures within an intact mouse brain using three-photon fluorescence microscopy (3PM) at a spectral excitation window of 1,700 nm. Vascular structures as well as red fluorescent protein (RFP)-labeled neurons within the mouse hippocampus are imaged. The combination of the long excitation wavelength and the higher order nonlinear excitation overcomes the limitations of 2PM, enabling biological investigations to take place at greater depth within tissue.Optical imaging plays a major role in both basic biological research and clinical diagnostics, providing a non-invasive or minimally-invasive microscopic imaging capability to investigate biological tissue. Optical image acquisition through significant depths of biological tissue, however, presents a major scientific challenge since tissue is extremely heterogeneous and the strong scattering of the various tissue components has historically restricted high-resolution optical imaging to thin sections or to superficial layers. The development of 2PM has significantly extended the penetration depth of high-resolution optical imaging, particularly for in vivo applications [8][9][10][11][12] . In the last 20 years, 2PM has enabled, in many fields for the first time, direct visualization of the normal behaviour of Users may view, print, copy, download and text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#termsCorrespondence and requests for materials should be addressed to C. Xu (chris.xu@cornell.edu). † These authors contributed equally to this work * cx10@cornell.edu Author Information Reprints and permissions information is available at www.nature.com/reprints.The authors declare no competing financial interests.Supplementary Information is linked to the online version of the paper at www.nature.com/naturephotonics. Author Contributions C.X. initiated and supervised the study. N.G.H., K.W., D.K, and C.G.C. performed the experiments and data analysis. N.G.H., K.W., D.K., and C.X. contributed to the writing and editing of the manuscript. C.B.S. and C.X. contributed to the design of the experiments. F.W. and C.X. contributed to the laser source design. 3,13 . Two-photon excitation of fluorescent molecules in tissue depends on the ability of sufficient excitation light to reach the focus of the objective unscattered (i.e., ballistic excitatio...

Two-photon fluorescence microscopy (2PM) 1 enables scientists in various fields including neuroscience 2,3 , embryology 4 , and oncology 5 to visualize in vivo and ex vivo tissue morphology and physiology at a cellular level deep within scattering tissue. However, tissue scattering limits the maximum imaging depth of 2PM within the mouse brain to the cortical layer, and imaging subcortical structures currently requires the removal of overlying brain tissue 3 or the insertion of optical probes 6,7 . Here we demonstrate non-invasive, high resolution, in vivo imaging of subcortical structures within an intact mouse brain using three-photon fluorescence microscopy (3PM) at a spectral excitation window of 1,700 nm. Vascular structures as well as red fluorescent protein (RFP)-labeled neurons within the mouse hippocampus are imaged. The combination of the long excitation wavelength and the higher order nonlinear excitation overcomes the limitations of 2PM, enabling biological investigations to take place at greater depth within tissue.Optical imaging plays a major role in both basic biological research and clinical diagnostics, providing a non-invasive or minimally-invasive microscopic imaging capability to investigate biological tissue. Optical image acquisition through significant depths of biological tissue, however, presents a major scientific challenge since tissue is extremely heterogeneous and the strong scattering of the various tissue components has historically restricted high-resolution optical imaging to thin sections or to superficial layers. The development of 2PM has significantly extended the penetration depth of high-resolution optical imaging, particularly for in vivo applications [8][9][10][11][12] . In the last 20 years, 2PM has enabled, in many fields for the first time, direct visualization of the normal behaviour of Users may view, print, copy, download and text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#termsCorrespondence and requests for materials should be addressed to C. Xu (chris.xu@cornell.edu). † These authors contributed equally to this work * cx10@cornell.edu Author Information Reprints and permissions information is available at www.nature.com/reprints.The authors declare no competing financial interests.Supplementary Information is linked to the online version of the paper at www.nature.com/naturephotonics. Author Contributions C.X. initiated and supervised the study. N.G.H., K.W., D.K, and C.G.C. performed the experiments and data analysis. N.G.H., K.W., D.K., and C.X. contributed to the writing and editing of the manuscript. C.B.S. and C.X. contributed to the design of the experiments. F.W. and C.X. contributed to the laser source design. 3,13 . Two-photon excitation of fluorescent molecules in tissue depends on the ability of sufficient excitation light to reach the focus of the objective unscattered (i.e., ballistic excitatio...

We present a compact and flexible endoscope (3-mm outer diameter, 4-cm rigid length) that utilizes a miniaturized resonant/nonresonant fiber raster scanner and a multielement gradient-index lens assembly for two-photon excited intrinsic fluorescence and second-harmonic generation imaging of biological tissues. The miniaturized raster scanner is fabricated by mounting a commercial double-clad optical fiber (DCF) onto two piezo bimorphs that are aligned such that their bending axes are perpendicular to each other. Fast lateral scanning of the laser illumination at 4.1 frames∕s (512 lines per frame) is achieved by simultaneously driving the DCF cantilever at its resonant frequency in one dimension and nonresonantly in the orthogonal axis. The implementation of a DCF into the scanner enables simultaneous delivery of the femtosecond pulsed 800-nm excitation source and epi-collection of the signal. Our device is able to achieve a field-of-view (FOV xy ) of 110 μm by 110 μm with a highly uniform pixel dwell time. The lateral and axial resolutions for two-photon imaging are 0.8 and 10 μm, respectively. The endoscope's imaging capabilities were demonstrated by imaging ex vivo mouse tissue through the collection of intrinsic fluorescence and second-harmonic signal without the need for staining. The results presented here indicate that our device can be applied in the future to perform minimally invasive in vivo optical biopsies for medical diagnostics.nonlinear optical endoscopy | real-time optical diagnosis | scanning fiber endoscopy | microendoscopy | endogenous fluorescence M ultiphoton imaging techniques such as two-photon fluorescence (TPF) and second-harmonic generation (SHG) microscopy hold great promise for the future of medical diagnosis because of their potential to replace surgical biopsies with minimally invasive optical diagnosis of tissue health (1-7). In a clinical setting, these diagnostic techniques will be capable of acquiring real-time, high-resolution, in vivo images without the need for contrast agents. However, a challenge in translating these beneficial imaging technologies into the clinic lies in successfully miniaturizing bulky tabletop microscope components into a compact probe without degrading the overall imaging performance of the system. These microendoscopes would not only have the potential to be used as diagnostic tools capable of early cancer detection, but could also be used for such applications as photodynamic therapy and microsurgery (8,9).A number of groups have demonstrated miniaturized instruments capable of confocal, optical coherence tomography (OCT), TPF, and SHG imaging (10-25). The primary constituents of these devices are typically a miniaturized scanning mechanism and lens assembly that is encapsulated in a protective housing with dimensions suitable for minimally invasive procedures (i.e., a probe outer diameter on the order of a few millimeters with a rigid length of several centimeters). Within these microendoscopes, various distal miniaturized scanners have been demonstr...

Multiphoton microscopy (MPM) is widely used for optical sectioning deep in scattering tissue, in vivo [1][2]. Phosphorescence lifetime imaging microscopy (PLIM) [3] is a powerful technique for obtaining biologically relevant chemical information through Förster resonance energy transfer and phosphorescence quenching [4][5]. Point-measurement PLIM [6] of phosphorescence quenching probes has recently provided oxygen partial pressure measurements in small rodent brain vasculature identified by high-resolution MPM [7,8]. However, the maximum fluorescence generation rate, which is inversely proportional to the phosphorescence lifetime, fundamentally limits PLIM pixel rates. Here we experimentally demonstrate a parallel-excitation/parallel collection MPM-PLIM system that increases pixel rate by a factor of 100 compared with conventional configurations while simultaneously acquiring lifetime and intensity images at depth in vivo. Full-frame three-dimensional in vivo PLIM imaging of phosphorescent quenching dye is presented for the first time and defines a new platform for biological and medical imaging.Users may view, print, copy, download and text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms * Corresponding Authors: Scott Howard < showard@nd.edu>, Chris Xu < cx10@cornell.edu>. AUTHOR CONTRIBUTIONS S.H. coordinated the project, designed and built the microscope, designed and fabricated the linear spatial light modulator, wrote control and image processing software, performed simulations, wrote analysis algorithms, analyzed data, preformed animal preparation and surgery, prepared dye and calibrations, and wrote the paper. A.S. greatly assisted with microscope design and assembly, performed simulations and analysis, wrote analysis algorithms, significantly contributed to the content of the paper, and performed experiments verifying pixel rate increase. N.H. and D.K. assisted in animal preparation and surgery for imaging. C.X. initiated the project, significantly contributed to the design of M4 and experimental design, and greatly contributed to the theoretical and experimental discussions. All authors contributed to manuscript editing. HHS Public Access Author Manuscript Author ManuscriptAuthor Manuscript Author ManuscriptCurrent technologies for overcoming the fundamental pixel rate limitation of serialacquisition MPM require parallel excitation and imaging a sample onto multi-element detectors (typically CCD) [9][10]. While satisfactory for thin tissue slices or non-scattering samples, thick scattering samples typically encountered in in vivo applications cause crosstalk between excited pixels when imaged onto a detector array, resulting in smeared images [11]. State-of-the-art fast fluorescence lifetime imaging microscopy systems utilize parallel excitation (e.g. LED arrays or pulsed diode excitation) or collection (e.g. gated CCD, single photon avalanche dio...

Two-photon fluorescence microscopy (2PM) 1 enables scientists in various fields including neuroscience 2,3 , embryology 4 , and oncology 5 to visualize in vivo and ex vivo tissue morphology and physiology at a cellular level deep within scattering tissue. However, tissue scattering limits the maximum imaging depth of 2PM within the mouse brain to the cortical layer, and imaging subcortical structures currently requires the removal of overlying brain tissue 3 or the insertion of optical probes 6,7 . Here we demonstrate non-invasive, high resolution, in vivo imaging of subcortical structures within an intact mouse brain using three-photon fluorescence microscopy (3PM) at a spectral excitation window of 1,700 nm. Vascular structures as well as red fluorescent protein (RFP)-labeled neurons within the mouse hippocampus are imaged. The combination of the long excitation wavelength and the higher order nonlinear excitation overcomes the limitations of 2PM, enabling biological investigations to take place at greater depth within tissue.Optical imaging plays a major role in both basic biological research and clinical diagnostics, providing a non-invasive or minimally-invasive microscopic imaging capability to investigate biological tissue. Optical image acquisition through significant depths of biological tissue, however, presents a major scientific challenge since tissue is extremely heterogeneous and the strong scattering of the various tissue components has historically restricted high-resolution optical imaging to thin sections or to superficial layers. The development of 2PM has significantly extended the penetration depth of high-resolution optical imaging, particularly for in vivo applications [8][9][10][11][12] . In the last 20 years, 2PM has enabled, in many fields for the first time, direct visualization of the normal behaviour of Users may view, print, copy, download and text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#termsCorrespondence and requests for materials should be addressed to C. Xu (chris.xu@cornell.edu). † These authors contributed equally to this work * cx10@cornell.edu Author Information Reprints and permissions information is available at www.nature.com/reprints.The authors declare no competing financial interests.Supplementary Information is linked to the online version of the paper at www.nature.com/naturephotonics. Author Contributions C.X. initiated and supervised the study. N.G.H., K.W., D.K, and C.G.C. performed the experiments and data analysis. N.G.H., K.W., D.K., and C.X. contributed to the writing and editing of the manuscript. C.B.S. and C.X. contributed to the design of the experiments. F.W. and C.X. contributed to the laser source design. 3,13 . Two-photon excitation of fluorescent molecules in tissue depends on the ability of sufficient excitation light to reach the focus of the objective unscattered (i.e., ballistic excitatio...