A new way of seeing inside the body has been developed that uses normal light
Visible light is a great way of imaging and probing biological material, and it can be used in therapies such as photodynamic therapy where cancer patients are injected with a chemical that becomes poisonous only when exposed to light, minimising the side effects of chemotherapy.
The problem is that despite the fact that flesh doesn't absorb that much light, it does scatter it very very strongly, which will have noticed this if you have ever shined a torch through your hand. Instead of the light going straight through your hand, it scatters repeatedly and the whole of your hand gently glows. This makes focussing light onto something deep inside your hand - say a tumor virtually impossible.
However if light paths are all reversible - so if you put a light bulb in the tumor, worked out how the light was leaving the body, and then sent light in the opposite direction, it would focus onto the tumor, the problem is of course that putting a light bulb in your body isn't exactly non-invasive.
Xiao Xu and collegues at Washington University have managed to get around this problem - they focus ultrasound onto the area of interest which will cause it to vibrate. They then changed the frequency of the laser light slightly, in time with the ultrasound. The light then scattered into and then out of the body, but the light which passed through the ultrasound focus will have its frequency slightly shifted by the doppler effect. They then stored the pattern of the doppler shifted light in a crystal as a hologram, and fired a much more powerful laser back through the crystal producing a reversed much more powerful version of the light which left the flesh coming from the focus of the ultrasound. So this second light pulse is focused by the flesh where you wanted it.
They have also managed to image structures in light by moving the focus of the ultrasound, which allows you to get information about the colour of structures deep inside you body non-invasively. Along with the possibilities for photodynamic therapy and imaging fluorescent markers inside the body, this could be a very useful technique.