High-pulse-energy multiphoton imaging of neurons and oligodendrocytes in deep murine brain with a fiber laser.

Here we demonstrate high-pulse-energy multiphoton microscopy (MPM) for intravital imaging of neurons and oligodendrocytes in the murine brain. Pulses with an order of magnitude higher energy (~ 10 nJ) were employed from a ytterbium doped fiber laser source at a 1-MHz repetition rate, as compared to the standard 80-MHz Ti:Sapphire laser. Intravital imaging was performed on mice expressing common fluorescent proteins, including green (GFP) and yellow fluorescent proteins (YFP), and TagRFPt. One fifth of the average power could be used for superior depths of MPM imaging, as compared to the Ti:Sapphire laser: A depth of ~ 860 µm was obtained by imaging the Thy1-YFP brain in vivo with 6.5 mW, and cortical myelin as deep as 400 µm ex vivo by intrinsic third-harmonic generation using 50 mW. The substantially higher pulse energy enables novel regimes of photophysics to be exploited for microscopic imaging. The limitation from higher order phototoxicity is also discussed.