Biomedical Engineering Reference
In-Depth Information
Fig. 9.3 Multimodal
imaging system combining
OCT and spectroscopic
modalities
spectroscopy may be affected by the presence of bacteria, food particles, and other
unknown subsurface structures, as well as the depth location of the fluorophores;
Raman spectroscopy cannot provide microstructural information even though it
can reveal tissue biochemical composition. Without depth-resolved anatomical
information, it may be difficult to determine the cause of the variation in spectra.
In order to detect a diseased region with higher sensitivity and specificity,
it is useful to combine OCT and spectroscopic techniques into one system for
complementary information because of the complementary nature of contrast
mechanisms. Spectroscopic data can be spatially and temporally correlated with the
tissue structure in OCT images so that the two sets of data can confirm to each other.
Depending on the configuration, the imaging points for two modalities can be
either overlapped using a dichroic mirror or separated without any combining com-
ponent. Tumlinson et al. developed a combined OCT and laser-induced fluorescence
(LIF) spectroscopic endoscope for in vivo mouse colon imaging [ 8 , 9 ]. Two systems
run in parallel fibers down to the tip of an endoscopic probe and collect the signals
from two different points simultaneously. The data can be correlated because the
relative imaging points for two modalities are fixed.
In free space, the two imaging modalities are commonly combined with a
dichroic mirror located in front of the scan mirror, as shown in Fig. 9.3 . Therefore,
the two modalities share the same objective lens and can obtain automatically
registered, complementary tissue images simultaneously. One challenge is that
the objective lens should have good correction of chromatic aberration over the
spectrum for both modalities. Barton et al. developed a free-space combined
OCT/LIF system to collect simultaneous OCT/LIF data from ex vivo tissues using
the same objective lens [ 10 ]. A longpass dichroic mirror was employed to transmit
the near-IR light for OCT imaging and reflect the UV or blue excitation light for
fluorescence spectroscopy.
The configuration in Fig. 9.3 is also applicable for endoscopy when a special
fiber, such as double-cladding fiber, is used [ 11 ]. The OCT channel uses the core
of the double-cladding fiber, while the fluorescence spectroscopy channel delivers
excitation light and collects the fluorescence signal through the large-area inner
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