“Vibrational photoacoustic” imaging technology developed at Purdue University aims to diagnose disorders such as cardiovascular disease by producing precise, three-dimensional images of structures such as plaques.1 The researchers are the first to show that absorption of light by the chemical bonds in molecules can produce a strong photoacoustic signal. The researchers say that the ability to key in on specific chemical bonds may open up a completely new direction for the biomedical field.
The approach works by using near-infrared (NIR) nanosecond laser pulses to generate molecular “overtone” vibrations (which are not absorbed by blood) that heat and expand tissue locally and in turn produce ultrasound waves. “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,” said Ji-Xin Cheng, associate professor of biomedical engineering and chemistry, who led the research along with doctoral student Han-Wei Wang. “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,” Cheng explained.
Vibrational acoustics enables detection of carbon-hydrogen bonds that comprise lipid molecules in arterial plaques that cause heart disease. It also shows promise for detecting fat molecules in muscles that would indicate diabetes, and for other lipid-related disorders such as neurological conditions and brain trauma. Because it also reveals nitrogen-hydrogen bonds that make up proteins, the approach might be useful for diagnosing additional diseases as well, and to study collagen’s role in scar formation.
The new method offers deeper tissue penetration than other molecular-information methods, according to the researchers—and in particular represents an improvement over coherent anti-Stokes Raman scattering (CARS), which the Purdue team has also used to study 3-D plaque formation in arteries.
The scientists are currently working to reduce the size of the system, with the goal of devising an endoscope for imaging the interiors of blood vessels. The researchers’ findings are based on research with pig tissues in laboratory samples and also with live fruit flies. The fact that they were able to observe fat inside fly larvae indicates that they may be able to study how obesity affects physiology in humans.
1. H.-W. Wang et al., Phys. Rev. Lett. 106, 259901 (2011).