Photoacoustic method could yield better cardiovascular disease, diabetes diagnoses

June 9, 2011

Measuring ultrasound signals from molecules exposed to a fast-pulsing laser, Purdue University researchers have developed a way to diagnose cardiovascular disease and other disorders, such as diabetes.

Measuring ultrasound signals from molecules exposed to a fast-pulsing laser, Purdue University (West Lafayette, IN) researchers have developed a way to diagnose cardiovascular disease and other disorders, such as diabetes. Their 'photoacoustic' technique could be used to take precise, 3-D images of plaques lining arteries, says Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at Purdue University. It can also penetrate tissue in living patients deep enough to reveal the 3-D structure of the plaques, yielding better diagnoses, he says.

The researchers' photoacoustic technique uses nanosecond laser pulses in the near-infrared range of the spectrum. The laser generates 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 device called a transducer. The researchers are also working to miniaturize the system so that they can build an endoscope to put into blood vessels using a catheter, says Cheng, enabling them to see the exact nature of plaque formation in the walls of arteries to better quantify and diagnose cardiovascular disease.

Lihong Wang, Gene K. Beare Distinguished Professor of Biomedical Engineering at Washington University in St. Louis, was the first to use the photoacoustic imaging of blood vessels based on the absorption of light by the electrons in hemoglobin.

Researchers 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 courtesy of Han-Wei Wang and Ji-Xin Cheng)

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-infrared 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. "You do one scan and get all the cross-sections. Our initial target is cardiovascular disease, but there are other potential applications, including diabetes and neurological conditions.

The imaging reveals the presence of carbon-hydrogen bonds making up lipid molecules in arterial plaques that cause heart disease. The method also might be used to detect fat molecules in muscles to diagnose diabetes and for 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.

The researchers' findings are based on research using pig tissues in laboratory samples and also with live fruit flies. Cheng says that because you can see fat inside fly larvae, there is potential to study how obesity affects physiology in humans.

The National Institutes of Health (NIH) and the American Heart Association supported this work, which is detailed in a paper to be published in the June 17 issue (expected) of the journal Physical Review Letters.

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Posted by Lee Mather

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