Near-Infrared Light-Based Imaging Reveals Caries

Dentistry Today
Photo by Chia-Chi Charlie Chang


Photo by Chia-Chi Charlie Chang

Advances in laser and light-based imaging technologies may soon change the way dentistry is practiced, reports the National Institute for Dental and Craniofacial Research (NIDCR). For example, tooth enamel is almost transparent at longer wavelengths of light, making it possible to shine near-infrared light on a tooth to detect caries.

“You can see right into the tooth,” said Daniel Fried, PhD, a professor of preventive and restorative dental science at the University of California San Francisco School of Dentistry. “The enamel looks almost like an ice cube.” 

Light-based imaging is minimally invasive and provides a safer alternative to the ionizing radiation of x-rays, Fried said at a recent NIDCR Grand Rounds event. Fried and his colleagues also are researching laser technology that can remove tooth decay, composite fillings, bonding, and adhesives, which could mean less painful visits to the dentist, they said.

There long has been a need for more reliable methods for diagnosing tooth decay, Fried said. Most cavities form on the occlusal surfaces of teeth. Dentists visually inspect teeth for decay, which can lead to false positives and overtreatment. Even x-rays aren’t sensitive enough to detect early occlusal cavities.

“Many lesions in the mouth have been re-mineralized and… no longer need intervention,” said Fried. “Dentists have trouble telling the difference between active and arrested lesions. This new technology has the potential of differentiating them.”

Fried’s research focuses on two kinds of light-based imaging that provide a more precise picture than x-rays and therefore could help diagnose and treat tooth decay much earlier. Near-infrared imaging is sensitive enough to detect early demineralization and can screen many teeth at once. Optical coherence tomography (OCT), similar to an ultrasound, shows cross-sections and can image deep into the tooth.

“OCT [already] has changed the practice of ophthalmology,” said Fried. “It’s been very successful for retinal imaging… and it’s also very promising for dentistry.”

Able to image through composites and sealants, OCT is particularly useful for assessing lesion severity and activity, Fried said.

“If the dentist doesn’t know if [a lesion] is active or arrested,” said Fried, “with OCT, you can actually see the lesion structure, how deep it is, and if it has a definitive surface zone suggesting that remineralization has occurred.”

Tomography is especially suited for clinical trials, as it can track changes over time, Fried says. In OCT clinical trials, his lab has detected significant demineralization that wasn’t spotted visibly. He recalled that their first studies in 2010 were encumbered by slow technology. Now, the researchers have acquired a new system that uses a scanning device on a chip capable of taking entire 3-D images in a second.

“One of the most exciting things we can do with OCT is monitor the changes in lesions as we re-mineralize them,” said Fried. “With nonsurgical intervention, you can treat [the tooth] with fluoride and re-mineralize lesions… That’s important for assessing lesion activity” and whether intervention is necessary.

In a demineralized tooth, the decay reflects a lot of light and appears white against the healthy enamel, which looks dark in the near-infrared.

“We get the highest contrast  at these longer wavelengths, significantly higher than other imaging technologies,” said Fried. 

Near-infrared imaging also offers another benefit.

“Stains, which are responsible for a lot of false positives, don’t absorb at these longer wavelengths, so you can image just the demineralization without the stain,” said Fried. 

Fried notes that a recent clinical study found a dramatic difference between near-infrared imaging and x-rays. In 26 lesions seen at the near-infrared that penetrated the dentin, only one of them showed up on x-ray.

In addition to imaging, infrared technologies can help remove tooth decay too. Compact and precise infrared lasers can scan the tooth’s surface and emit tiny and fast pulses to remove decay selectively without overly impacting healthy tooth structure.

In treatment, a near-infrared image is taken, and Fried’s team has an algorithm to convert it to pixels. Then the high-speed laser scans the surface and removes decay, followed by an OCT scan that checks how much was removed.

“You can also use the near-infrared to enhance visibility of composites,” said Fried. “Dentists spend more time removing existing composites and restorations than putting in new ones. If the composite is color-matched to the tooth, it’s hard to see where it is,” and that makes it tough to remove without damaging nearby healthy enamel.

Spectral-guided ablation can be used to selectively remove composite from tooth surfaces. When a laser strikes material, some of it vaporizes, and the plume looks different when it strikes enamel versus composite, said Fried.

Using spectral-guided ablation, it’s possible to see the calcium lines of enamel to make sure the laser is only striking composite. In a current clinical study, Fried’s lab is removing small composite restorations in less than a minute. Together, the researchers say, these laser and light-based technologies could lead to earlier and better detection and intervention of tooth decay.

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