In what may prove to be a profound advance, researchers have used a combination of light and ultrasound to visualize fluorescent proteins that are several centimeters below the surface of living tissue. In the past, even modern technologies have failed to produce high-resolution fluorescence images from this depth because of the strong scattering of light. Lead author Dr. Daniel Razansky, who played a pivotal role in developing the new method, said that it "opens the door to a whole new universe of research. For the first time, biologists will be able to optically follow the development of organs, cellular function, and genetic expression through several millimeters to centimeters of tissue.” To achieve its feat, the research team made light “audible.” They illuminated adult zebra fish from multiple angles using flashes of laser light that are absorbed by fluorescent pigments in the tissue of the genetically modified fish. The fluorescent pigments absorb the light, a process that causes slight local increases in temperature, which in turn result in tiny local volume expansions. This happens very quickly and creates small shock waves. In effect, the short laser pulse gives rise to an ultrasound wave that the researchers pick up with an ultrasound microphone. The real power of the technique, however, lies in specially developed mathematical formulas used to analyze the resulting acoustic patterns. An attached computer uses these formulas to evaluate and interpret the specific distortions caused by scales, muscles, bones, and internal organs to generate a three-dimensional image. The researchers performed similar successful experiments with fruit fly pupae. The research team included scientists from the Helmholtz Center Munich, Massachusetts General Hospital, and Harvard Medical School.
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