Biomedical Engineering Reference
In-Depth Information
Finally, plasmon-resonant gold nanoparticles in the form of spheres, shells,
cages, and rods have been utilized as strong as wavelength-specific absorbers
and scatterers [73-75]. By changing the size and structure of these nanopar-
ticles, the wavelength-dependent absorption and scattering can be tuned
throughout the near-infrared and visible wavelengths. These nanoparticles
have been used as contrast agents for OCT, and also proposed as multifunc-
tional therapeutic agents because their strong absorption properties can be
used to induce local hyperthermia in cells and tissues.
All of these agents are expected to be highly biocompatible and composed
of materials that have been previously found to be suitable for in vivo use.
These agents, with protein, iron-oxide, gold, or biocompatible molecular sur-
faces, may be functionalized with antibodies or molecules to target them to
specific molecules, cells, or tissue types and thus provide additional selectivity
that can enhance the utility of OCT as an emerging diagnostic technique.
8.9 Molecular Imaging using Optical Coherence
Tomography
The advantages of OCT, compared to other imaging techniques, are numer-
ous. In particular, OCT can provide imaging resolutions that approach those
of conventional histopathology and imaging can be performed in situ. Despite
its advantages, a serious drawback to OCT is that the linear scattering prop-
erties of pathological tissue probed by OCT are often morphologically and/or
optically similar to the scattering properties of normal tissue. For example,
although morphological differences between normal and neoplastic tissue may
be obvious at later tumor stages, it is frequently di
cult to optically detect
early-stage tumors. To improve the ability of discriminating tissue types in
this scenario, molecular imaging techniques are being developed.
Molecular imaging involves the generation of images or maps of tissue
that contain molecular-specific information. The use of exogenous contrast
agents in OCT (described above) is one example of molecular imaging, when
such agents are functionalized to target and label specific molecular structures
on cells or in tissue, such as cell-surface receptors that are over-expressed in
tumors. In many instances, it may be more desirable to detect the presence of
endogenous molecules without the addition exogenous agents that could alter
the biology or the viability of the tissue. Pump and probe techniques have been
used to detect the presence of molecules that have transient absorption states
in the tissue that are induced by an external pump beam [76]. Spectroscopic
OCT has been used to probe for variations in the oxygenation state of tissue,
recognizing the spectroscopic differences between oxy- and deoxy-hemoglobin
[77]. Both of these methods, however, rely on a limited set of molecules or
molecular features that can be detected.
A new method called Nonlinear Interferometric Vibrational Imaging
(NIVI) has been developed to achieve molecular contrast in OCT imaging by
