Raman diagnostic tool detects eye disease

Researchers from the University of Utah (Salt Lake City, UT) have developed an argon-laser-based diagnostic tool that could eventually help physicians slow, and possibly prevent, the onset of age-related macular degeneration in some patients. The device uses low-energy resonance Raman spectroscopy to measure the levels of two macular carotenoid pigments, lutein and zeaxanthin, which are thought to protect the eye from light-induced oxidative damage and aging.

"Both lutein and zeaxanthin effectively absorb the blue region of the visible spectrum—the most damaging wavelengths of light to which the retina is routinely exposed," said Paul Bernstein, assistant professor of ophthalmology and visual sciences at the university's Moran Eye Center and one of the developers of the device. "This new test will allow us to determine whether raising a patient's macular pigment levels through diet and nutritional supplements translates into a lower risk for macular degeneration."

The noninvasive in vivo test requires the patient to look into the instrument for less than one second. The light is slightly absorbed by the eye and then reflected back to the equipment. Using a backscattering geometry and resonant molecular excitation in the visible range, the system measures the Raman peaks that originate from the single- and double-bond stretch vibration of the p-conjugated molecule's carbon backbone. The Raman signals scale linearly with carotenoid content, and the reflected light can be analyzed for a variety of carotenoids because these pigments each have a very specific signature, originating from vibrations within their molecular structure.

Raman spectroscopy has been considered unsuitable for routine measurements in living tissue because Raman signals are typically of weak intensity, according to Werner Gellermann, research professor in the university's Department of Physics and associate director of the Dixon Laser Institute. Historically this weakness has required the lasers to be used in combination with sophisticated light collection and analysis instrumentation. But Gellermann and Bernstein found that macular pigments in the eye exhibit extremely strong Raman signals when excited with blue laser energy (see figure).

"Lutein and zeaxanthin glow green when a blue laser light shines on them," Gellermann said. "Under proper conditions, this resonance amplification can be as high as a factor of 10,000, turning a weak signal into a readily measured strong signal, many times higher than background signals from other cells in the retina." The new technique can also be used with higher reliability than heterochromatic flicker photometry, the most commonly used test for measuring macular pigments in patients with significant visual loss from macular degeneration or other diseases.

Bernstein believes the noninvasive test could become as common as tests for high cholesterol and blood glucose levels. If, based on the results of this test, a physician determines a patient has low levels of macular pigment, that patient could be encouraged to increase consumption of foods or nutritional supplements containing lutein and zeaxanthin.

The system also may be appropriate for gauging a patient's susceptibility to skin cancer. In the skin, important antioxidant molecules include the carotenoids lycopene and various carotene isomers, according to Bernstein. Although the carotenoid levels in skin are about two orders of magnitude lower than in the macula and skin has a large native fluorescence level, the technique can measure carotenoid levels over a large concentration range. In addition, it is possible to monitor changes in skin carotenoid levels as a result of dietary intervention based on the test results.

Bernstein's and Gellermann's research was initially funded by the State of Utah Centers of Excellence Program, and they were recently granted a US patent on the technology. They also received a $500,000 Small Business Technology Transfer grant from the National Institutes of Health and the National Eye Institute. Based on the results of their current clinical study, they believe the device could reach the market by 2003 and they have formed a start-up company, Spectrotek, to develop the system for routine testing in eye clinics.