By measuring ultrasound signals from molecules exposed to a fast-pulsing laser, biomedical engineers at Purdue University developed an imaging technology that diagnoses multiple diseases. The new method could be used to take precise 3-D images of plaques lining arteries, said Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry. Other imaging methods that provide molecular information cannot penetrate tissue deeply enough to reveal the 3-D structure of arterial plaques; being able to do so would make better diagnoses possible. "You would have to cut a cross section of an artery to really see the three-dimensional structure of the plaque," he said. "Obviously, that can't be used for living patients." Researchers at Purdue have developed a new type of imaging technology to diagnose cardiovascular disease and other disorders by measuring ultrasound signals from chemical bonds in molecules exposed to a pulsing laser. This "vibrational photoacoustic" image shows plaque in an arterial wall. (Image: Han-Wei Wang and Ji-Xin Cheng, Purdue University) The imaging reveals the presence of carbon-hydrogen bonds making up lipid molecules in arterial plaques. The method also might be used to detect fat molecules in muscles to diagnose diabetes and other lipid-related disorders, including neurological conditions and brain trauma. The technique also reveals nitrogen-hydrogen bonds making up proteins, meaning the imaging tool also might be useful for diagnosing other diseases and to study collagen's role in scar formation. "Being able to key on specific chemical bonds is expected to open a completely new direction for the field," Cheng said. The findings of Cheng and his colleagues are detailed in a paper to be published in the June 17 issue of Physical Review Letters. The new technique uses nanosecond laser pulses in the near-IR range to generate molecular “overtone” vibrations, or wavelengths that are not absorbed by the blood. The pulsed laser causes tissue to heat and expand locally, generating pressure waves at the ultrasound frequency that can be picked up with a transducer. "We are working to miniaturize the system so that we can build an endoscope to put into blood vessels using a catheter," Cheng said. "This would enable us to see the exact nature of plaque formation in the walls of arteries to better quantify and diagnose cardiovascular disease." The Purdue researchers are the first to show that a strong photoacoustic signal can arise from the absorption of light by the chemical bonds in molecules. The near-IR laser causes enough heating to generate ultrasound but not enough to damage tissue. "You can measure the time delay between the laser and the ultrasound waves, and this gives you a precise distance, which enables you to image layers of the tissues for three-dimensional pictures," Cheng said. The approach represents a major improvement over coherent anti-Stokes Raman scattering, or CARS, which has been used by the Purdue-based lab to study 3-D plaque formation in arteries. For more information, visit: www.purdue.edu