Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 295

9.1 Biomedical Optical Imaging Techniques:

Advantages and Limitations

A number of biomedical optical imaging and spectroscopic techniques have been
developed and commercialized for fundamental research and clinical applications.
Each of these utilizes different contrast mechanisms and has unique features with
encouraging results, but each also has some limitations. Assessment of image data
is often complicated by the fact that a number of factors are involved in disease
progression, each of which may affect the optical properties of tissue and influence
the measured signal.

9.1.1 Transillumination Imaging

Transillumination imaging utilizes spatial variation of absorption and scattering in
tissue. It is often used in wide field of view (FOV) mode and can be very effective
to detect a target deep inside the tissue, which may be difficult to be detected using
other methods.
While transillumination imaging is effective in detecting internal structures and
features, such as lesions, deep inside the tissue, there are several drawbacks to this
method. First, the transmitted image cannot provide depth information; secondly,
the transmitted image does not provide detailed information on tissue surface; and
thirdly, image resolution is often low because the light-carrying information comes
from inside the tissue and undergoes some scattering. In addition, it is not always
possible to apply transillumination imaging methods when the tissue of interest is
relatively thick.

9.1.2 Reflectance Imaging

Reflectance imaging collects and analyzes both backscattered and specularly
reflected photons without filtering. It is routinely used in the clinic for direct visual
inspection, usually in lowresolution, large FOV mode. Reflectance imaging can also
be used in highresolution, small FOV mode and can provide detailed information
about the tissue surface.
Reflectance images generally show surface texture well; however, reflectance
imaging is limited to detecting lesions based on gross morphological changes
and has relatively low sensitivity in detecting changes in tissue. In addition, the
detected light often contains light that is specularly reflected from the tissue surface.
Unfortunately, specularly reflected light often saturates the detector and obscures
tissue surface. With reflectance imaging, it can be difficult to determine the margin
of disease with precision when the tumor extends into the submucosa because most
of the photons collected by the imaging system come from the superficial layers of
tissue.