Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 299

While the spectroscopic method probes the tissue noninvasively and provides
information on tissue morphology, function, and biochemical composition, it
also has some potential shortcomings. First, the spectroscopic method is a point
measurement, which means it does not provide surface detail. Second, spectra may
be difficult to interpret because they may be influenced by unknown subsurface
structures. Without depth-resolved anatomical information, it may be difficult to
determine whether variation is due to changes in tissue function, biochemical
differences, or thickness variation. For example, the loss of the fluorescence
signal either can be a true loss of the fluorescence emission or can be caused
by a thickening of overlying tissue layers. The Raman signal, typically much
weaker than the tissue's autofluorescence, is often obscured by background signal,
such as tissue autofluorescence and fluorescence from components in the light
path.

9.2 Multimodal Optical Imaging Systems and Applications

Each individual optical imaging modality is typically sensitive only to a certain
change in optical response. For instance, optical coherence tomography measures
the backward scattered light from tissue to generate a morphological structural
image. Fluorescence imaging measures fluorescence emission from the different
biochemical constituents of tissue and generates maps of the tissue biochemistry. All
imaging techniques provide valuable information of the light and tissue interaction,
yet they are not all capable of comprehensive measurement of optical responses. The
sensitivity and specificity of each single imaging modality, considered separately, is
often low for detection and diagnosis of diseases.
In order to increase the sensitivity and specificity of optical imaging techniques,
an emerging paradigm is to combine two or more optical imaging modalities,
originating from independent contrast mechanisms, for a more comprehensive, non-
destructive, and minimally invasive tissue characterization. By combining imaging
modalities with complimentary contrast mechanisms, such as coherent backscatter-
ing for OCT and incoherent fluorescence excitation for fluorescence imaging, the
multimodal imaging system can be more sensitive to tissue function and pathology
and can have more specificity in detecting abnormal tissues. Multimodal images can
be fused together to increase image contrast or can be registered together to reveal
structural and function information.
The effectiveness of a multimodal system relies on the combination of the various
imaging techniques but not on the extreme diagnostic capabilities of each technique.
While multiple imaging modalities can provide complementary information, one
modality can also guide another high-resolution, small FOV imaging modality to
obtain detailed tissue information or to identify tissue margin.
In this subsection, only a few of the promising multimodal imaging systems will
be discussed in detail.