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Collagen is the most abundant protein in mammalian and forms various types of tissues. On ocular surface, sclera,
limbus and cornea are composed with fibril form collagen. However, unlike other connective tissues with high opacity,
cornea has extraordinary high transparency which originates from the regular arrangement of collagen fibers within
cornea. Cornea is responsible for 80% of focusing power of our vision and any corneal damage can cause severe vision
loss. The high transparency of cornea makes it difficult to probe it without invasive processes, especially stromal
structure alternations.
Collagen, however, is an effective second harmonic generator due to its non-centrosymmetric molecule structure and
can be visualized with nonlinear optical process without labeling. In addition, the deeper penetration and point like
effective volume of SHG can also provide 3-dimensional information with minimum invasion. Backward SHG imaging
has been approved effectively demonstrating structure alternation in infective keratitis, thermal damage in cornea,
corneal scar, post refractive surgery wound healing and keratoconus which is also a main complication after refractive
surgery[1-6]. In practical, backward SHG has the potentiality to be developed as clinical examination modality.
However, Han et al also demonstrated that backward SHG (BSHG) imaging provides collagen bundle information while
forward SHG (FSHG) provides more detailed, submicron fibril structure visualization within corneal stroma[7]. In
sclera, which also has type I collagen as its main composition, BSHG and FSHG imaging reveal similar morphology.
Comparing with what Legare et al demonstrated that BSHG in bulk tissue mainly originate from backscattered
FSHG[8], the huge difference between corneal BSHG and FSHG imaging originate from the high transparency of
cornea. However, only BSHG could be applied in practical. Therefore, if the correlation of BSHG and FSHG, which
reveals more architecture details, can be established, BSHG may be used in clinical examination in the future.
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