Lasers Reveal Biomarkers

Laser light can be used to detect molecules in breath that may be markers for diseases like asthma or cancer.

Although it has yet to be tested in clinical trials, a new apparatus may allow doctors to screen people for certain diseases simply by sampling their breath, according to JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado (CU-Boulder).

"This technique can give a broad picture of many different molecules in the breath all at once," said Jun Ye, a fellow of JILA and NIST who led a research team investigating the technology.

Known as optical frequency comb spectroscopy, the method is powerful enough to sort through all the molecules in human breath and sensitive enough to distinguish rare molecules that may be biomarkers for specific diseases, said Ye. CU-Boulder graduate research assistant Michael Thorpe, Ye, CU-Boulder doctoral student Matthew Kirchner and former CU graduate student David Balslev-Clausen describe the research in a paper that appeared in Optics Express, a journal published by the Optical Society of America.

University of Colorado at Boulder physics doctoral student Michael Thorpe holds a detection chamber next to a novel laser apparatus at JILA. Thorpe and JILA's Jun Ye led a new study showing how scientists can use laser light to detect faint breath molecules that may be biomarkers for disease. (Photo: JILA, NIST, University of Colorado at Boulder)

When breathing, people inhale a complex mixture of gases, including nitrogen, oxygen, carbon dioxide, water vapor and traces of other gases like carbon monoxide, nitrous oxide and methane, said Ye, an adjoint professor of physics CU-Boulder. Exhaled breath contains less oxygen, more carbon dioxide and a rich collection of more than a thousand types of other molecules, most of which are present only in trace amounts.

Just as bad breath can indicate dental problems, excess methylamine may signal liver and kidney disease, ammonia may be a sign of renal failure, elevated acetone levels can indicate diabetes and nitric oxide levels can be used to diagnose asthma, he said. When many breath molecules are detected simultaneously, highly reliable, disease-specific information can be collected. Asthma, for example, can be detected much more reliably when carbonyl sulfide, carbon monoxide and hydrogen peroxide are all detected simultaneously with nitric oxide.

Many studies have showcased the potential of optical technologies for breath analysis, but the JILA approach takes an important step toward demonstrating the full power of optics for this prospective medical application. While current breath analysis using biomarkers is a noninvasive and low-cost procedure, approaches are limited because the equipment is either not selective enough to detect a diverse set of rare biomarkers or not sensitive enough to detect particular trace amounts of molecules exhaled in human breath.

"The new technique has the potential to be low-cost, rapid and reliable, and is sensitive enough to detect a much wider array of biomarkers all at once for a diverse set of diseases," Ye said.

The optical frequency comb is a very precise laser for measuring different colors, or frequencies, of light, said Ye. Each comb line, or "tooth," is tuned to a distinct frequency of a particular molecule's vibration or rotation, and the entire comb covers a broad spectral range -- much like a rainbow of colors -- that can identify thousands of different molecules.

Laser light can detect and distinguish specific molecules because different molecules vibrate and rotate at certain distinct resonant frequencies that depend on their composition and structure, he said. He likened the concept to different radio stations broadcasting on separate radio frequencies.

Optical frequency comb spectroscopy was developed in the 1990s by Ye’s JILA colleague John L. Hall and Theodor W. Hänsch of Germany’s Max-Planck Institute (they shared the 2005 Nobel Prize in Physics with Roy J. Glauber for their invention).

In their paper, Michael Thorpe, a graduate research assistant, Ye, and their colleagues describe the novel application of this technique to breath analysis. Optical comb spectroscopy is powerful enough to sort through all the molecules in human breath, Ye said, but it is also sensitive enough to find those rarest molecules that may be markers of specific diseases. Ye's group has pioneered the application of frequency combs to spectroscopy, or the analysis of light emitted or absorbed by matter. The technique allows for many different gases to be detected all at once with high sensitivity through their interaction with light from such "combs" (demonstrated by Thorpe, Ye and colleagues in the journal Science in 2006).

To test the technology, Ye's team had several CU-Boulder volunteer students breathe into an optical cavity -- a space between two curved mirrors -- then directed sets of ultrafast laser pulses into the cavity. As the light pulses ricocheted around the cavity tens of thousands of times (covering a distance of several kilometers by the time it exited the cavity), the researchers determined which frequencies of light were absorbed, indicating which molecules -- and their quantities -- were present by the amount of light they absorbed.

The remarkable combination of a broad spectral coverage of the entire comb and a sharp spectral resolution of individual comb lines allows them to sensitively identify many different molecules, Ye said. They detected trace signatures of gases like ammonia, carbon monoxide and methane from the samples of volunteers. In one measurement, they detected carbon monoxide in a student smoker that was five times higher compared to a nonsmoking studen.

While the efficacy of this technique has yet to be evaluated in clinical trials, monitoring the breath for such biomarkers is an attractive approach to medicine because breath analysis is the ultimate noninvasive and low-cost procedure, JILA said in a statement. "Existing approaches to breath analysis are limited, because the equipment is either not selective enough to detect a diverse set of rare biomarkers, or it is not sensitive enough to detect trace amounts of the molecules exhaled in human breath. The biggest shortcoming of existing approaches is their inability to provide rapid and reliable breath measurements for many biomarkers."
,br> The new technique addresses these problems with its capability to rapidly, simultaneously, sensitively and accurately detect many breath biomarkers, JILA said. "The results can qualitatively change the field of breath analysis, realizing its real potential as a low-cost, rapid, robust, and noninvasive method for health screening."

Funding was provided by the Air Force Office of Scientific Research, Agilent Technologies Foundation, DARPA, NIST, the National Science Foundation and a CU-Boulder grant.

For more information, visit: www.colorado.edu. A podcast featuring Ye and Thorpe is available at www.colorado.edu/news/podcasts/.