Reaching into astronomy's toolbox, professor Lihong Wang at Washington University in St. Louis has invented a guide star for biomedical imaging. Wang's guide star is an ultrasound beam that "tags" light passing through it. Emerging from the tissue, the tagged light, together with a reference beam, creates a hologram. When a "reading beam" is then shown back through the hologram, it acts as a time-reversal mirror, creating light waves that follow their own paths backward through the tissue, coming to a focus at their virtual source—the spot where the ultrasound is focused.
The technique, called time-reversed ultrasonically encoded (TRUE) optical focusing, thus allows the scientist to focus light to a controllable position within tissue. Wang thinks TRUE will lead to more effective light imaging, sensing, manipulation and therapy, all of which could facilitate medical research, diagnostics and therapeutics. In photothermal therapy, for example, scientists have had trouble delivering enough photons to a tumor to heat and kill the cells. So they either have to treat the tumor for a long time or use very strong light to get enough photons to the site, Wang says. But TRUE will allow them to focus light right on the tumor, ideally without losing a single tagged photon to scattering.
The new method is detailed in Nature Photonics (2011), doi:10.1038/ nphoton.2010.306.
By the way, Wang's group is responsible for this issue's cover image. For details on that work, see "Deep down and label-free," p. 40.
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