Coherent Raman Scattering Microscopy - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 126

Fig. 6.6. Characterization of lyso-PtdCho-induced myelin swelling by CARS imag-
ing. Parallel and linear polarized pump and Stokes laser beams were focused into
the equatorial plane of axons. CARS images of normal ( A ) and swollen ( B ) myelin
sheath that wrap parallel axons recorded with vertical ( ) and horizontal ( ↔ ) exci-
tation polarization at a Raman shift of 2840 cm − 1 . C ThesymmetryaxisoftheCH 2
groups in the equatorial plane of the myelin is parallel to the vertical direction. The
images in ( B) were acquired after injecting 2 μLof10mg / mL lyso-PtdCho into the
tissue. Note the decrease of CARS intensity and the significantly reduced photose-
lection effect in the swollen region. All images were acquired at 1 . 13 s/frame (Scale
bar: 10 μm) (Image courtesy of Ji-Xin Cheng [96, 97, 116])

discrepancy in multimodal CARS imaging can be elegantly resolved by im-
plementing a spectral focusing scheme (cf. Fig. 6.2C). By using a single pair
of femtosecond pump and Stokes pulses and controlling their degree of linear
chirp, one can choose to optimize contrast in CARS imaging or enhance signals
in TPF, SHG, and THG imaging. Figure 6.7 shows a measurement of a plaque
cross section of an atherosclerotic arterial sample from a rabbit aorta, demon-
strating the capability of using this spectral focusing implementation for the
simultaneous imaging of three endogenous nonlinear signals: The CARS image
visualizes the C-H stretching vibrations at a Raman shift of 2850 cm − 1 from

lipids (Fig. 6.7A). The TPF image shows the smooth muscle elastin fiber net-
work, indicating that it is broken into short segments (Fig. 6.7B). And, the
SHG image reveals the distribution of collagen (Fig. 6.7C). As this exam-
ple demonstrates, one can routinely balance the resolution in CARS imaging

∼