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From light to sound to 3-D images

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Charles T. Troy,

Scientists have long used light to scrutinize thin sections of tissue for ascertaining whether they are diseased, or for investigating cell function. However, the penetration limits of light range between one-half and 1 mm of tissue. In thicker layers, light is diffused so strongly that all useful details are obscured.

Now, professor Vasilis Ntziachristos, director of the Institute of Biological and Medical Imaging of the Helmholtz Zentrum München German Research Center for Environmental Health, has broken through this barrier and rendered three-dimensional images through at least 6 mm of tissue, allowing whole-body visualization of adult zebra fish.

To accomplish this, Ntziachristos and his team made light audible. They illuminated the fish from multiple angles using flashes of laser light that were absorbed by fluorescent pigments in the tissue of the genetically modified fish. When the pigments absorbed the light, local temperature increased slightly, which in turn resulted 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.

Light and ultrasound can be used to visualize the red fluorescent spinal column of a live fish: Multispectral optoacoustic tomography, or MSOT, allows the investigation of subcellular processes in live organisms. Image courtesy of Helmholtz Zentrum München/TU München, montage.

The real power of the technique, however, lies in specially developed mathematical formulas used to analyze the resulting acoustic patterns. A computer evaluates and interprets the distortions caused by scales, muscles, bones and internal organs and generates a three-dimensional image.

The result of this “multispectral opto-acoustic tomography” is an image with a spatial resolution of better than 40 μm.

Dr. Daniel Razansky, who played a pivotal role in developing the method, said, “This 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.”

Ntziachristos is convinced that “multispectral optoacoustic tomography can truly revolutionize biomedical research, drug discovery and health care. Since multispectral optoacoustic tomography allows optical and fluorescence imaging of tissue to a depth of several centimeters, it could become the method of choice for imaging cellular and subcellular processes throughout entire living tissues.”

Photonics Spectra
Aug 2009
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
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