Raman Spectroscopy Detects First Signs of Tooth Decay

David L. Shenkenberg

Dentists commonly use x-rays and metal picks to detect cavities, but Raman spectroscopy could detect tooth decay earlier. With low power and near-IR excitation, it could do so more safely than x-rays and without pain.

Recently, researchers discovered that both polarized and nonpolarized Raman spectroscopy can discriminate between healthy teeth and decay. The group included investigators from the Institute for Biodiagnostics of the National Research Council Canada and the University of Manitoba, both in Winnipeg, and from Dalhousie University in Halifax, Nova Scotia, Canada.

The initial studies were performed with a Raman microscope, and now the same researchers have explored the use of optical fibers for polarized Raman spectroscopy on teeth as a step toward bringing the technology to the dentist’s office. They took parallel and cross-polarized Raman spectra of tooth enamel in a single measurement.

They used this technique on 20 human premolar teeth, which had been removed for orthodontic reasons. They recorded the spectra of each tooth 10 times for 1 s each time and averaged the signal. The laser power was just 90 mW at each tooth.

They detected the 959 cm^–1 band because it was most intense. At 959 cm^–1, the cross-polarized band was weak in healthy teeth but much stronger in those with decay, whereas the parallel-polarized band mostly stayed the same (Figure 1).

Figure 1. In healthy teeth (a), researchers detected a strong parallel-polarized Raman signal (red line) at 959 cm^–1, but the cross-polarized signal (blue line) was much weaker. In teeth with decay (b), the cross-polarized signal was stronger. Images reprinted with permission of Optics Express.

To induce Raman scattering, the researchers used an 830-nm, 300-mW diode laser from Process Instruments Inc. of Salt Lake City. The laser light passed through a 0.22-NA optical fiber, a fiber collimator and a polarizing beamsplitter before reflecting off a notch filter (Figure 2).

Figure 2. Researchers are developing Raman spectroscopy for detecting tooth decay. OBJ = objective lens; N = notch filter; PBS = polarizing beamsplitter; IF = laser interference filter; FC = fiber collimator; PS = polarization scrambler.

The excitation laser light then traveled through the back aperture of a Newport Corp. objective lens with 40× magnification and a 0.65 NA. The lens focused the excitation beam onto the teeth and collected the resulting scattered photons, which reflected off the same notch filter.

The parallel and cross-polarized photons went to a polarization scrambler, which was necessary to eliminate polarization dependence of the results. The photons passed through a fiber collimator and toward each arm of a custom-made bifurcated optical fiber. Still separated by polarity, they traveled through the common arm of the fiber and were detected by two areas of the CCD chip of an Andor Technologies camera that was attached to a Horiba Jobin Yvon axial Raman spectrometer.

Among the 20 teeth, there were no false-negatives and only one false-positive, so the Raman technique had 98% specificity and 100% sensitivity. The team determined that these findings have statistical significance.

The researchers said that their next step is to make the device smaller and narrower so that it will fit comfortably in a patient’s mouth. Principal investigator Lin-P’ing Choo-Smith said that the instrument they ultimately develop may combine Raman spectroscopy with optical coherence tomography.

Optics Express, April 28, 2008, pp. 6274-6284.