Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 313

9.3.1 Multimodal Imaging System Combining Polarized

Reflectance, Fluorescence, and OCT Imaging

To improve sensitivity and specificity in rapid screening and diagnosis of disease,
one potential approach is a multimodal imaging system combining polarized
reflectance, fluorescence, and OCT imaging modalities. Polarized reflectance imag-
ing provides a true-color surface image without specular reflection, fluorescence
imaging detects biochemical changes, and OCT imaging investigates subsurface
features of suspicious regions. The information provided by these three imaging
modalities is complementary and can increase sensitivity and specificity in detection
and diagnosis of diseases.
Fluorescence imaging is sensitive to early tissue transformation and is suitable
for rapid screening of a large area, but it is sensitive to the environment and it
can be difficult to quantify the data obtained. OCT imaging can provide a high-
resolution depth image, but its FOV is typically small. With this multimodal
system, one can use sensitive fluorescence imaging to locate suspicious regions
rapidly, then guide OCT imaging to investigate the subsurface feature of suspicious
regions.
Figure 9.7 shows two configurations of multimodal systems combining polar-
ized reflectance, fluorescence, and OCT imaging modalities. In Fig. 9.7 a, three
modalities share the same objective lens through the dichroic mirror, which
transmits visible light and reflects near-infrared light. The light for reflectance
and fluorescence imaging is combined by another dichroic mirror. The polarizer
or excitation filter is placed in front of the light source for the corresponding
modalities, respectively. An analyzer and emission filter can either be moved in/out
of the imaging path when taking the corresponding images or stay in the detection
path. The three-dimensional OCT image is obtained by scanning the focused beam
across the object through the scan mirror and scan lens. This configuration is suitable
when the FOV is small and high resolution is required. The requirement for the
objective lens is more demanding, considering the special needs for each modality.
For example, both depolarization effects and autofluorescence in the objective lens
should be minimized.
The configuration in Fig. 9.7 b shows an approach for large-imaging FOV. Polar-
ized reflectance imaging and fluorescence imaging share the same imaging lens, and
OCT imaging has its own objective lens. The polarized light and excitation light
illuminate the object directly without a dichroic mirror. The achievable resolution
is typically lower than that in Fig. 9.7 a because of the longer working distance.
Because the analyzer and emission filter can be placed in front of the imaging lens,
depolarization and autofluorescence in the imaging path will not degrade image
quality; therefore, conventional optical materials can be used in the imaging path.
To design such a multimodal imaging system, we need to understand how
to design each subsystem. This section will focus on the design of fluorescence
imaging and polarization imaging. The design of OCT system is discussed in
Chap. 5 .