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Infrared light detects tooth decay

15 Sep 2003

A dentist’s toolkit for detecting tooth decay could soon include an infrared-imaging system.

Shining infrared light with a wavelength of 1310 nm through a tooth is an effective way to spot early decay, according to a team from the University of California at San Francisco, US. The researchers also say that the so-called infrared transillumination technique has distinct advantages over traditional methods, such as X-rays. (Optics Express 11 2259)

“The principal advantages are greater sensitivity and contrast for early decay located between teeth,” research leader Daniel Fried told Optics.org. “The contrast provided by changes in light scattering in the near IR is greater than changes in tissue density needed to produce contrast in an X-ray.”

Decay begins on the surface of the tooth’s enamel and progresses into its interior dentin. Dentists ideally want to spot the decay when it is still in the enamel, but X-rays are not sensitive enough to pick it up. According to Fried, decay is usually visible on an X-ray only after it has penetrated into the dentin and at that point dentists have to drill and fill the tooth.

To simulate decay, Fried and colleagues sectioned teeth free-from-decay into slices. A hole was then drilled in the sections and filled with hydroxyapatite powder, a substance that has similar light-scattering properties to decay.

The set-up for imaging the decay consisted of a fiber-coupled halogen lamp; a pair of crossed linear polarizers and a 50 nm bandpass filter placed in front of an InGaAs focal-plane array.

The researchers say the simulated decay was clearly discernible in a 3-mm thick sample when using infrared illumination, but could not be seen using visible-light illumination. They add that there was poor contrast between the tooth and the decay on a standard X-ray image.

The team went on to shine infrared light through 30 samples between 2 and 6.75-mm thick. The researchers calculated a contrast ratio of greater than 0.35 in all cases, with the exception of the 6-mm sample.

Fried says that the high contrast of the simulated decay with the surrounding enamel indicates that there is significant potential for the infrared transillumination technique. “There are no obvious safety concerns with this technology that would prevent introduction into the clinic,” he said. “The next step is to acquire extracted teeth containing decay and to modify the system so that we can start imaging in vivo.”

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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