Astronomers have a neat trick they sometimes use to compensate for the turbulence of the atmosphere that blurs images made by ground-based telescopes. They create an artificial star called a guide star and use its twinkling to compensate for the atmospheric turbulence.
Lihong Wang, PhD, the Gene K. Beare Distinguished Professor of Biomedical Engineering at Washington University in St. Louis, has invented a guide star for biomedical rather than celestial imaging, a breakthrough that promises game-changing improvements in biomedical imaging and light therapy.
Wang's guide star is an ultrasound beam that "tags" light that passes through it. When it emerges 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.
http://www.physorg.com/news/2011-02-star-scientists-deep-human-tissue.html