Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 174

5.4.4 Selection of OCT Wavelength

In OCT, the characteristics of the optical source determine the general performance
of the system. For example, the shot coherence length of the optical source deter-
mines the axial resolution, and the nominal wavelength determines the achievable
penetration depth of the system. The broader the linewidth of the source, the
smaller the coherence length l c (Eq. 5.23 ) of the source is. There are four main
criteria that need to be considered when choosing a light source for OCT. These
parameters are wavelength, bandwidth, single transverse mode power, and stability.
The depth of penetration in OCT is limited by both scattering and absorption of
the sample. The absorption window for most biological tissues between 0.65 and
1.4 m has a local minimum in the absorption of water and blood in the tissues.
Absorption attenuates the light in the imaging subject and is wavelength dependent.
Because more than 80% of biological tissue consists of water, the absorption of
tissue is mostly attributed to the water. If we analyze the water absorption spectrum
showninFig. 5.7 , we can find a window with lower absorption, termed “therapeutic
windows” for biomedical optics, existing around 600-800 nm. Absorption by
melanin, as well as the scattering from structure protein such as collagen and elastin,
exhibits a nearly monotonic decrease with increasing wavelength over this region.
These optical properties of tissues have been investigated to some degree both
experimentally and by a number of analytical and numerical models. Therefore,
the selection of wavelength is a very critical parameter in OCT.
Most of the conventional OCT systems utilize the near-infrared to far-infrared
region of the optical spectrum, which ranges from 700 to 1,600 nm. The exact
choice of the central wavelength depends on the prospective applications and is
mostly determined by the absorption and scattering properties of the medium under
investigation, though the expected resolution must also be taken into account.
In general, imaging at shorter wavelengths around 800 nm delivers higher axial
resolutions; however, the depth of penetration is considerably degraded due to
increased scattering at shorter wavelength. In the 700-1,600-nm region, scattering
decreases monotonically with wavelength; in particular, the scattering behavior of
light in biological specimen shows significant degradation with longer wavelength.
Thus, OCT systems operating at central wavelengths around 1,300 nm show
an enhanced penetration depth due to less tissue scattering and absorption of
endogenous chromophores like hemoglobin and melanin [ 21 , 22 ]. Recently, OCT
imaging in the 1,050-1,060-nm wavelength region is getting much attention in
the field of ophthalmology, where imaging the deeper layers of retina is very
crucial for diagnosing retinal pathologies [ 23 - 27 ]. In ocular imaging, the highly
scattering and absorbing retinal pigment epithelium reduces the penetration depth
for wavelengths at the 800-nm region commonly used today in retinal scanning
OCT system, while the application of long wavelength in retinal imaging is limited
by the water absorption in the relative long path through the vitreous humor, which
contains >95% of water. However, in the 1,050-1,060-nm region, a suitable