SWIR imaging shows promise for detecting active tooth decay

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When combined with air drying, shortwave-infrared can be used to detect active dental cavities. This is possible because active cavities are porous and hold more water, which affects the infrared measurements around the affected area as the tooth dries. (Image: Chang et al.)

Researchers have been experimenting with shortwave-infrared (SWIR) and thermal imaging in discerning active tooth decay.

While modern dentistry has many solutions for treating caries (cavities) of many shapes and sizes, the restorative materials used do not always bond well to the surrounding healthy tooth structure. Microscopic leaks may form, allowing fluids and bacterial acids to penetrate beneath the restoration. This can lead to the formation of secondary caries that appear and grow around a previously restored cavity.

According to Dr Nai-Yuan Chang, from the Department of Preventive and Restorative Dental Sciences at University of California, San Francisco, dentists now spend more time replacing failed restorations than placing new ones due to the maladaptation of bonding materials.

In a recent study published in the Journal of Biomedical Optics, Chang’s research team have therefore evaluated emerging imaging modalities for discerning active tooth decay. 

“The traditional methods relying on tactile sensation via a dental explorer and visual inspection based on texture and colour are highly subjective and unreliable,” said Chang. “However, there is currently no established dental imaging technology that can provide diagnostic information with high specificity and sensitivity when assessing dental decay activity.”

Chang and his colleagues have therefore explored whether SWIR and thermal imaging can be combined with air drying to accurately diagnose the activity of a secondary caries lesion.

The idea underlying both these methods is that active lesions are more porous than healthy teeth, and these pores hold water. In the SWIR-based approach, one can indirectly detect active lesions by observing changes in the SWIR reflectivity as the tooth dries out. On the other hand, the thermal imaging-based approach relies on the fact that the temperature changes in active lesions during air drying are different from that in healthy teeth, owing to the water trapped in the pores of the lesion.

In their work, the team acquired 63 human tooth samples from oral surgeons and analysed 109 suspected secondary lesions in them using both SWIR and thermal imaging. In addition to these methods, the researchers also observed the samples using optical coherence tomography (OCT), a more sophisticated technique that uses near-infrared light to create high resolution 3D images. To determine whether SWIR and thermal imaging were indeed useful for detecting active lesions, the results of these methods were compared with those obtained via OCT.

Overall, SWIR proved superior to thermal imaging and performed better in most circumstances. The SWIR permeability measurements were well correlated with the thickness of the transparent surface layer (TSL) of lesions measured via OCT. The team found that the highly mineralised TSL was thickest when a lesion had been fully arrested and needed no further intervention. According to the OCT results, a TSL thicker than 70µm was a potential indication that a lesion was no longer active.

The findings of this study could help pave the way to a new era of diagnostic imaging in dentistry, according to Chang. “Our work provides further developmental milestones towards meeting the need for better diagnostic and easily operable clinical devices,” he concluded.

Image: atdigit/shutterstock.com

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