Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 306

also provides depth-sensitive information because each modality usually operates
over a different spectrum. The typical wavelength for fluorescence spectroscopy
is in the UV region, diffuse reflectance spectroscopy is in the visible region, and
Raman spectroscopy is in the near-IR region. The penetration depth of light in
tissues increases with wavelength. Therefore, fluorescence spectroscopy measures
information at the most superficial level, diffuse reflectance spectroscopy provides
information at an intermediate depth, and Raman spectroscopy gathers information
from deeper within the tissue.
Typically, one light source, or a broadband light source with wavelength selection
filters, is needed for each spectroscopic technique. In free space, the light from each
light source is combined together with a dichroic mirror and is focused onto the
tissue through the objective lens. The light from the tissue is collected through
the same objective lens and is directed to the detector for each modality. In fiber-
optic multimodal spectroscopy, the light from each light source is coupled into
a multimode illumination fiber, and the light from the tissue is collected by the
detection fibers. Each spectroscopic technique has a set of fibers to collect the
light from the tissue and direct the light to the corresponding detector. Typically,
an imaging lens is used to focus the light from the illumination fiber to the tissue
and to couple the light from the tissue to the detection fibers. As an example,
Fig. 9.4 shows a multimodal spectroscopy approach developed by Feld et al. to
acquire reflectance, fluorescence, and Raman spectra through a fiber-optic probe
for a more complete biochemical and morphological information that can be used
in detecting and diagnosing disease more accurately [ 18 - 20 ]. This system contains
three light sources: a Xe flash lamp with a wavelength selection filter for diffuse
reflectance spectroscopy, a 377-nm N 2 laser for fluorescence spectroscopy, and an
830-nm diode laser for Raman spectroscopy. All three light sources are configured
to be externally triggerable for data collection. The light from three light sources
is coupled into a multimode fiber for illumination and then focused onto the
tissue through the imaging lens. A set of fibers collect and send Raman light to
a spectrograph after passing an 830-nm notch filter; the rest of the fibers collect
reflectance or fluorescence light from the tissue and couple them to a spectrometer.
This system demonstrated the ability to detect several morphologic features of
vulnerable atherosclerotic plaques, including a thin fibrous cap, a large necrotic
core, and an accumulation of superficial foam cells. It has the potential to serve
as a robust, catheter-based clinical diagnostic technique.
Various combinations of spectroscopic techniques have been explored. Tunnel
et al. developed a reflectance spectrofluorometer for clinical studies in the oral cav-
ity, the uterine cervix, and the gastrointestinal tract. This approach combines diffuse
reflectance spectroscopy, light scattering spectroscopy, and intrinsic fluorescence
spectroscopy to provide biochemical, structural, and morphological information
[ 21 ]. Rajaram et al. combined two modalities, namely, diffuse reflectance and
intrinsic fluorescence spectroscopy, to obtain complementary information on tissue
morphology, function, and biochemical composition for the spectral diagnosis of
melanoma and nonmelanoma skin cancers [ 22 ]. Volynskaya demonstrated the com-
bination of diffuse reflectance spectroscopy and intrinsic fluorescence spectroscopy