Sound approach to optical microscopy

03 Jul 2009

German scientists move well beyond the penetration limits of optical microscopy thanks to a technique that uses the sound of light.

Researchers in Munich, Germany, have broken through the barriers of modern light microscopy to create 3D images of live tissue at least 6 mm thick. The approach uses multiwavelength illumination over multiple projections to compile a single accurate image (Nature Photonics DOI:10.1038/nphoton.2009.98).

"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 millimetres to centimetres of tissue," Daniel Razansky, a researcher at the Technical University of Munich and Helmholtz Center Munich, told optics.org. "Our technique could become the method of choice for imaging cellular and subcellular processes throughout entire living tissues."

Strange as it may seem, the key to the approach adopted by Razansky and colleagues was to make light audible. In the experiment, the team illuminated zebra fish from multiple angles using flashes of laser light. The tissue of the fish had been genetically modified to include fluorescent pigments that absorb the laser light.

When the light is absorbed by the fluorescent pigment, a small increase in the local temperature results in tiny localized volume expansions. The speed at which this happens creates small shock waves, which means that the initial laser pulse in effect gives rise to an ultrasound wave that the researchers can detect with an ultrasound microphone.

The real power of the technique, however, lies in specially developed mathematical formulae that are used to analyse the resulting acoustic patterns. An attached computer exploits these formulae to evaluate and interpret the specific distortions caused by scales, muscles, bones and internal organs to generate a 3D image.

Dubbed multispectral optoacoustic tomography, or MSOT, the result is an image with a spatial resolution of better than 40 µm. And best of all, the sedated fish wakes up and recovers without harm following the procedure.

Razansky and colleagues believe that MSOT could revolutionalize biomedical research and healthcare. "In the next few years, we believe that MSOT will become available in biological labs for accelerating drug discovery, studying morphogenesis, disease progression, and treatment monitoring in living model animals such as fish, mice and rats," concluded Razansky. "But applications for medical diagnosis in humans will eventually also become available."