DIAGNOSTIC IMAGING: IR laser probe detects tooth defects earlier

Dentists may soon be able to search for cavities using a painless diode-laser-based technique that can detect cracks or defects at a very early stage and thus help prevent tooth decay. A newly developed laser device that uses thermal radiation and light waves could replace x-rays as the diagnostic standard in dental care.

“Using this technique, we can see all the way to the pulp-more than 5 mm inside a tooth,” said Andreas Mandelis of the University of Toronto’s Centre for Advanced Diffusion Wave Studies (Toronto, Ont., Canada). “It can reveal suspicious regions invisible to the naked eye below the surface of the tooth and spot lesions as small as 50 µm in between teeth-one of the most difficult places to spot cavities.”

Mandelis, an expert in the use of thermophotonics to detect defects in metals, semiconductors, and other crystal structures, realized seven years ago that subsurface defects in tooth enamel, such as demineralization, could be detected using the same approach. Loss of mineral content is a precursor to cavity development. In its early stages, dental enamel demineralization is very difficult to detect with conventional x-rays or visual examination (minimum detection level is 30% demineralization). If early mineral loss can be detected, dentists will have different intervention technologies that can be used before resorting to filling cavities.

In addition, high-resolution techniques usually require destruction of the tooth. Most new optical-detection technologies image teeth using methods such as optical-coherence tomography or light-induced fluorescence. More-conventional imaging techniques don’t do the trick because light scatters too much in teeth. Mandelis, codeveloper of the device, along with his dentist, Stephen Abrams, are researching diagnostic alternatives and have formed a start-up company called Quantum Dental Technologies. Their research was funded by the Ontario Centres for Excellence.

The two began investigating biothermophotonic inspection for depth profilometry of the teeth by means of scanning at certain frequencies and obtaining thermal signatures. Using a laser diode, the researchers directed near-IR light at different frequencies toward human teeth. Upon penetrating a tooth, the light slightly heated it, generating thermal-IR radiation that revealed cavities. Higher frequencies worked best to reveal defects near the surface of a tooth, while lower frequencies uncovered problems deep below the enamel.

Nonintrusive, noncontacting frequency-domain photothermal radiometry (FD-PTR or PTR) and frequency-domain luminescence (FD-LUM or LUM) have been used with 659 and 830 nm diode-laser sources to assess pits and fissures on the surfaces of human teeth. The researchers examined 52 human teeth with simultaneous measurements of PTR and LUM and compared them to conventional diagnostic methods including continuous luminescence, visual inspection, and radiographs. PTR and LUM, as a combined technique, could be a reliable tool to diagnose early pit and fissure caries and could provide detailed information about deep lesions. The 830 nm laser probe detected deeper subsurface lesions than did the 659 nm probe, says Mandelis.

Different pulse rates allow exploration of different depths under the enamel. A lower rate of 5 Hz allows the researchers to conduct a deep probe because it gives the heat enough time to radiate back to the surface; a rate of 1000 Hz probes to a relatively shallow depth. Throughout the whole process, the temperature of the tooth rises by 2°C. The system discovers very early lesions in enamel about 80% of the time; the sensitivity is 33% higher than laser fluorescence and 2.8 times higher than a radiograph.

Quantum’s first clinical prototype is likely to be introduced during the summer of 2008. The technique may have other applications in detecting skin and subdermal cancers.