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  • NIR camera captures laser ablation process on teeth

Apr 2008
Michael A. Greenwood

Using a laser in the human mouth is tricky. Too much power can result in thermal damage to the surrounding tissue and, in a worst-case scenario, even result in teeth cracking.

To better understand the side effects associated with the potential use of CO2 lasers in dentistry, a team of researchers applied a real-time near-IR imaging technique that can capture with micron precision what is happening in this fragile environment while the laser does its work.

In a continuation of the group’s earlier work with NIR transillumination, researchers Cynthia L. Darling and Daniel Fried of the University of California, San Francisco, report that the technique can track the ablation process in real time.

The investigators said that, because dental enamel’s high transparency allows imaging in the NIR (1310 nm), a variety of potential side effects associated with the use of CO2 lasers in dentistry, including the formation of cracks, dehydration and thermal changes to the organic and mineral phases, were imaged with high contrast.

Although a variety of imaging techniques already are used widely in dentistry, the researchers said that the 1310-nm wavelength has a number of advantages. Healthy enamel does not scatter light at this intensity and, besides being able to record events in real time, it can image decay between the teeth and in the pits and fissures of an individual tooth.

CO2 lasers, meanwhile, offer several potential benefits for dental work, including efficient operation and high ablative efficiency. But before this laser is ready for clinical use, Darling said that safe operating procedures must be established.

To image ablation in real time, the researchers used an NIR precision zoom lens from Edmund Scientific of Barrington, N.J., in conjunction with an InGaAs focal plane array from Indigo Systems of Goleta, Calif. Illumination was provided by a 150-W fiber optic illuminator from E Licht Co. of Denver. This setup acquired 8- or 12-bit digital images.

During experiments (Figure 1), a sample tooth mounted in a holder was targeted with a CO2 laser from GSI Lumonics of Rugby, UK. The laser operated at 9.3 μm and had an incident fluence up to 45 J/cm2 and energies of 20 to 30 mJ per pulse with a pulse duration of 16 μs. Water from a spray nozzle was applied in some of the trials to limit thermal damage and to determine its effect on the imaging.

Figure 1. The setup (left) used by researchers to capture NIR images of laser ablation in real time is shown. A schematic diagram (right) shows the same arrangement, an InGaAs focal plane array with a zoom lens (A), water spray nozzle (B), laser handpiece (C), a fiber optic illuminator (D), a sample holder and a tooth section (E). Reprinted with permission of Optics Express.

The laser drilled conical holes into the sample to a depth of 1 to 2 mm and at a repetition rate of 20 Hz. Simultaneous video was acquired at a rate of 10 fps, and the progress of the hole being drilled and the peripheral changes in the opacity of the enamel were followed in real time. Thermal emission also was seen in the area surrounding the ablation, but no damage was observed.

The researchers found that ablation rates greater than 60 Hz without a water spray caused extensive cracking and thermal damage, which was captured by the NIR camera. The progress of the hole could be imaged clearly until the incision reached the junction between the enamel and the dentin.

Figure 2. Researchers used a CO2 laser to ablate a tooth’s enamel while imaging the process in real time with an NIR camera operating at 1310 nm. Courtesy of Cynthia L. Darling.

The investigators said that they intend to further develop the NIR imaging technique, with the goal of turning it into a viable tool for clinical use.

Optics Express, Feb. 18, 2008, pp. 2685-2693.

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