Photoacoustic imaging: fatty deposits visualized by infrared light and ultrasound

By bombarding tissue with an infrared laser, scientists can make plaque buildup emit ultrasound images. The technique would make noninvasive early detection of atherosclerosis possible.
Written by Janet Fang, Contributor

Using infrared light and sound, scientists have discovered a clever way to spot the silent killer atherosclerosis, fatty buildup on artery walls. ScienceNOW reports.

Because the disease causes few symptoms in its early stages, people are hit with heart attacks and strokes seemingly out of nowhere. To detect it early, doctors need to spot fat inside the walls of arteries.

  1. They shine infrared radiation from a laser onto soft tissue.
  2. The laser passes through the tissue until it hits specific chemical bonds.
  3. That causes the chemical bond to vibrate like a spring. Different chemical bonds absorb radiation at different wavelengths, giving doctors a clue about what’s going on inside tissue samples.

Here, a team led by Ji-Xin Cheng of Purdue University added on to the existing detection procedures:

  1. Infrared-induced vibration of chemical bonds is naturally damped by surrounding tissue.
  2. And that energy is converted to heat, which leads to the rapid expansion of the tissue.
  3. That sends a pressure wave traveling outward in all directions to the surface of the sample, and that pressure is emitted as ultrasound.
  4. Using detectors to pick up the ultrasound emission, the team can work out where the expansion took place, creating detailed images of fatty deposits from the irradiated tissue.

This is called photoacoustic imaging.

This technique isn’t entirely new, but it wasn’t clear that molecular vibrations could be detected in this way. In the past, absorption of visible light by hemoglobin has been used to image blood flow around skin cancer.

The researchers tested their idea by imaging arteries from pigs with varying degrees of atherosclerosis. They bombarded the samples with infrared radiation at a wavelength likely to be absorbed by the carbon-hydrogen bonds in the fatty deposits lining atherosclerotic arteries.

Measuring the ultrasound emission, they created images of the deposits in a noninvasive way with minimal interference from the surrounding 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 says. "You do one scan and get all the cross sections.”

The technique could probe about 7mm into the body, and the applications could be broad. By choosing different infrared wavelengths, researchers can excite different bonds and study a variety of processes in soft tissue, such as scar formation and the changes to the central nervous system that occur in diseases such as Alzheimer's and multiple sclerosis.

The study was published in Physical Review Letters last week.

Via ScienceNOW.

Image: Purdue University Weldon School of Biomedical Engineering / Han-Wei Wang and Ji-Xin Cheng

This post was originally published on Smartplanet.com

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