Biomedical Engineering Reference
In-Depth Information
9.1.3
Polarization Imaging
A polarization-gated imaging system uses polarized illumination, with an analyzer
in the detection path to separate transmitted or backscattered photons with different
polarization states, therefore providing improved structural and depth information.
A number of imaging methods based on polarization-gating techniques have been
developed for biomedical applications, such as conventional polarization imaging,
orthogonal polarized spectral imaging, and polarization ratio imaging. Polarization
imaging techniques are simple and low cost and can provide a powerful tool for
functional diagnostics and for the imaging of diseased fragments of tissues, as well
as for the detection of tumor borders that are not visible to the naked eye. This offers
the opportunity for a number of useful applications in clinical practice. However, a
limitation of this technique is that it does not provide adequate biochemical and
structural information.
9.1.4
Fluorescence Imaging
Fluorescence imaging locates diseased tissue either using autofluorescence from
endogenous fluorophores or using the fluorescence signal from a fluorescent dye.
This technique provides comprehensive and detailed probing of the structure and
dynamics of biological specimens of widely varying dimensions.
While fluorescence imaging provides a highly sensitive technique for detection
of biochemical and structural changes, it does not demonstrate a great degree
of specificity due to the presence of bacteria, blood, food particles and various
other changes that are not necessarily related to malignant transformation of tissue.
False positive detection can result from the presence of bacteria or food particles,
from detection geometry, from varying degrees of vascularization, from distribution
of fluorophores, and due to other changes not necessarily related to malignant
transformation of tissue. In addition, fluorescence imaging, by itself, lacks sufficient
surface and depth information.
9.1.5
Confocal Imaging
By placing a small aperture in front of the detector to reject scattered photons outside
of the focal point, confocal imaging increases the contrast of microscopic images,
particularly in thick specimens, and provides depth discrimination in reflectance
and fluorescence imaging. Confocal imaging utilizes the fact that scattered photons
that undergo multiple scattering can be widely spread out from the focal point.
Advantages of confocal imaging over conventional reflectance imaging include its
controllable depth of field, improved image quality, and the ability to obtain optical
sections of relatively thick specimens.
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