Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 199

5.6.5 Second Harmonic OCT

Second harmonic OCT incorporates the use of coherence gating of second-order
nonlinear optical response of biological tissues molecular contrast imaging of
tissue micro-structures. [ 130 , 131 ]. Optical second harmonic generation (SHG)
is the lowest-order nonlinear optical process where the second-order nonlinear
optical susceptibility is responsible for the generation of light at second harmonic
frequency. Femtosecond laser pulses were usually used to excite second harmonic
waves from biological specimens in conjunction with a reference nonlinear crystal.
Second harmonic interference fringe signals were detected and used for image
construction. Because of the strong dependence of second harmonic generation
on molecular and tissue structures, this technique offers contrast and resolution
enhancement to conventional optical coherence tomography. The second-order
nonlinear optical susceptibility is sensitive to electron configurations, molecular
structure and symmetry, local morphology, and ultra-structures; thus, SHG provides
molecular contrast for the coherence image.

5.7 Application and New Trends in OCT

5.7.1 Medical Applications

OCT has recently emerged as a promising imaging technique, mainly for medical
applications. It has a number of features that make it attractive for a broad range of
biological, clinical, and material investigations. First, the imaging can be performed
in situ and nondestructively. The image resolution of the order 10-15 misnow
routine using standard technology, and high resolution of the order of 1mmay
be achieved using state-of-the-art laser systems. High-speed, real-time imaging
is possible with more than video rate frame acquisition. OCT can be interfaced
with a wide range of imaging delivery systems and imaging probes. In comparison
with conventional optical imaging technique, the specific features of OCT such as
high resolution can be achieved independent of the beam focusing conditions and
the coherence gate, that can substantially improve the probing depth in scattering
medium, which make this technique a unique place in clinical imaging.
So far, the most successful medical application of OCT that has been identified
and developed is in the field of ophthalmology due to the low scattering found in
the optical materials of the eye [ 132 , 133 ]. OCT has provided ophthalmologists with
depth resolution in imaging the posterior and anterior of the eye previously only
achievable with histology. OCT can detect subtle changes to the eye caused during
early stage of ocular disease, such as age-related macular degeneration [ 134 , 135 ],
diabetic retinopathy [ 136 , 137 ], macular holes [ 138 , 139 ], intraocular tumors [ 140 ],
and glaucoma [ 141 - 143 ]. Other than ophthalmology, imaging studies using OCT
have been performed in a wide range of biological, medical, and surgical specialties