Emerging Dental Applications of Raman Spectroscopy - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 271

clinical assessment using Raman spectroscopy. A dental examination needs
to be carried out within a reasonably short period of time (10 min) which
precludes painstaking point-by-point measurements of the Raman scattered
light from surfaces of teeth. A multimodal strategy that employs a rapid
screening tool to guide the Raman measurements has been advocated [42].
One such approach relies on optical coherence tomography (OCT) to rapidly
image the surface of the tooth to search for areas of demineralization. An in-
depth discussion of the principles and operation of OCT is outside the scope
of this chapter and interested readers are directed to several articles on the
topic [61, 62]. In brief, OCT is a non-invasive technique that provides high-
resolution depth imaging of near-surface tissue structures. This technique is
based on the coherent cross-correlation detection of the interference fringe in-
tensity of backscattered light. Similar to ultrasound in operation but offering
an order of magnitude greater spatial resolution, OCT provides morphological
images with 10-20 2-4 mm [61]. This technology

has been applied to image retinal tissue of glaucoma patients [63, 64] and is
being developed for cancer detection [65, 66] and vulnerable atherosclerotic
plaque assessment [67]. OCT imaging is sensitive to refractive index changes
within the sample. Light scattered from boundaries or interfaces with differ-
ing refractive indices is detected in the OCT setup. The technique is capable
of making thousands of measurements in a second, particularly when using a
swept-source OCT configuration [68]. Thus the method is capable of scanning
a tooth with reasonably high resolution in a few seconds.
Figure 11.7 displays representative OCT images acquired at 1310 nm from
a region of sound enamel and compared to a region at the site of a carious
lesion. From these depth images, we observe that OCT imaging nondestruc-
tively provides morphological information of the tooth with regions of the
crown and root well delineated. The OCT backscattered light signal is intense
at the air-enamel interface and dies out rapidly with depth within healthy
sound enamel (Fig. 11.7B). In contrast, there is subsurface light backscatter-
ing within carious enamel sites (Fig. 11.7C). The porous carious enamel allows
for increased scattering which is detected by the OCT system. A comparison
of the OCT image with light photomicrographs generated from destructive
sectioning methods indicate that the shape of the underlying lesions match
those observed with OCT imaging (Fig. 11.7C inset). From OCT imaging,
it is possible to obtain information regarding the lesion depth. Preliminary
studies examining a group ( n =15) of OCT images at 850 nm matched with
light photomicrographs indicate that there is a linear correlation between the
lesion depths obtained from these two methods with r =0 . 91 [69]. In addition,
we have derived a parameter, the optical attenuation coecient, that fits the
decaying A-scan signal of the OCT backscattered intensity and found that
it is a useful parameter for distinguishing sound enamel from carious enamel
[70, 71]. Our initial findings based on more than 500 measurements from 21
teeth indicate that the attenuation of the OCT signal is more pronounced in
sound enamel than in carious lesions. OCT attenuation coecient at 850 nm

μ m resolution to depths of ∼