Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 333

depth, size, and boundary information. However, the OCT system has a small FOV
and is not practical for large-scale imaging for in vivo dental applications. The com-
mon limitation of fluorescence imaging and OCT imaging is that both techniques
cannot identify calculus and stains with enough sensitivity and specificity.
In order to quickly detect dental decay with high sensitivity and specificity, it
is advantageous to combine polarized reflectance, fluorescence, and OCT imag-
ing techniques. Fluorescence and OCT images are distinct and complementary,
with fluorescence providing information about biochemical composition and OCT
providing information about tooth structural information. The polarized illumina-
tion method is used in reflectance imaging to remove specular reflection. With the
help of reflectance imaging, calculus, stain, and amalgam can be clearly identified.
Another advantage of this multimodal imaging system is that fluorescence and
reflectance imaging can quickly screen an entire tooth and identify the suspicious
regions, then OCT imaging can be performed to obtain detailed structural infor-
mation. Thus, this multimodal imaging system may achieve higher sensitivity and
specificity in dental caries detection than any single modality alone.

9.4.2 Light Source

To design an effective fluorescence imaging system, it is necessary to select
the spectrum matching the peak of the fluorescence excitation spectrum of the
tooth. Figure 9.12 a is the excitation spectrum of a carious region with fluorescence
detected at 700 nm. The most efficient fluorescence excitation is around 400 nm
[ 53 ]. The fluorescence emission spectrum of the tooth with excitation light at
400 nm is shown in Fig. 9.12 b. It is very clear that there is a significant decrease
of fluorescence as tooth decay develops. Therefore, a light source with peak
wavelength of around 400 nm or a broadband light source with a bandpass filter
whose central wavelength is around 400 nm is the choice for fluorescence imaging
for caries detection. The UV LED with peak wavelength around 400 nm is optimal to
excite tooth fluorescence. This source has narrow spectrum bandwidth, light weight,
instant on and off, small footprint, and long lifetime. However, there are tails in the
emitted spectrum and those tails may be inside the emission spectrum of the sample.
Therefore, a bandpass or shortpass filter is often needed to block the tails within the
emission spectrum.
The main function of polarized reflectance imaging is to provide the dental
practitioner with a clinical view of the tooth surface, as well as white spots of
early decay. A true color rendition of the tooth is important in examining tooth
condition. Therefore, a visible spectrum light source is needed. White LEDs are
excellent for this application because they are compact and can be turned on/off
instantly so that the reflectance imaging mode and fluorescence imaging mode can
be switched quickly.
For OCT imaging, it is necessary to select a wavelength that can penetrate as
deep into the enamel as possible, which helps to map the depth of tooth decay. As