Sharpening the focus of optical time-reversal in foggy media

Imaging in biological tissues can be likened to focusing light in fog, where optical scattering sends the ray along meandering trajectories. Barring an astronomically unlikely coincidence, it is hard to imagine that the rays would somehow spontaneously converge to create an intense spot of light. Biological tissues are essentially foggy in nature. In fact, we appear opaque because of the large amount of deviation that occurs in light’s trajectory when we direct it through tissue. Scattering has traditionally been seen as a problem that scrambles information, but it is actually a deterministic and reversible process.1–4 Therefore, there exists a wavefront solution for directing light into a scattering medium so that the field would generate a speckle-size limited focus spot. The big challenge is finding that wavefront without direct optical access to the inside of the tissue. Professor Lihong Wang’s group at Washington University at Saint Louis recently demonstrated a technique—-time-reversal of ultrasound-encoded light (TRUE)—-that uses an acoustooptic guidestar (an artificial means of correcting distortion) for optical time reversal, which alters the propagation direction and phase variation of the light beam, sending it in the exact opposite direction. The method achieves focusing in scattering tissue phantoms and ex vivo tissues.5 Wang’s group used ultrasound, since the waves are not significantly scattered in soft biological tissues, and can be focused at significant depths. A portion of the scattered light that passes through the ultrasound focus shifts in frequency, resulting in an acousto-optical ‘beacon’ within the tissue. This enables detection and time-reversal of the frequencyshifted light emanating from the beacon to achieve optical focusing at the location of the ultrasound focus. While the original demonstration of TRUE showed submillimeter-scale imaging with absorption contrast, the immediate application of this technique for fluorescence imaging remained a challenge because of the low optical gain of the Figure 1. Rendering of an ultrasound frequency-shifted optical speckle field. Color represents phase and luminance represents amplitude.