Optical imaging is a powerful tool for studying biology. Compared to other imaging methods, optical imaging has the advantage of providing molecular information through, for example, Raman scattering or fluorescence.
There are two fundamental challenges in optical imaging. One is diffraction, which limits the spatial resolution of an optical imaging system. Over the past two decades, various methods have been developed to break the diffraction limit and provide three-dimensional super-resolution images. The other challenge is scattering. Except in creatures such as jellyfish, most biological tissues are not transparent. This is mainly because of optical scattering in tissues. Despite advances in imaging technologies, tissue scattering remains a significant challenge.
Most optical imaging systems rely on an objective lens to form an optical focus or to project the image onto a camera. For either method to work, the sample being imaged must be highly transparent and the optical path length inhomogeneity within the sample must be much less than the optical wavelength (a few hundred nanometers). For tissues that are more than several hundred microns thick, scattering is a significant problem. My goal is to develop a new tool for millimeter-scale deep-tissue imaging.
In conventional laser scanning microscopes, a single-mode laser beam fills the back aperture of an objective lens and is focused on a tiny spot on the focal plane. If a highly scattering tissue is between the lens and the focal plane, the focus will be severely distorted and spread to a much greater area. What if the input beam is not single mode? A properly engineered optical beam can cancel all the distortion due to scattering and form a high-quality focus. The key is to determine the correct wavefront quickly.
A phenomenon known as phase conjugation can be used to determine the correct wavefront. Suppose a light source is placed inside the tissue. If we capture the emitted light and reverse its wavefront and propagation direction, the phase-conjugated wave can precisely propagate back toward the light source in a time-reversed fashion. In this way, we can quickly determine the correct wavefront and form an optical focus inside deep tissues. How do we first put a light source, noninvasively, at arbitrary positions inside the tissue?
I plan to use ultrasound to form a virtual light source inside the tissue. The scattering of a sound wave in tissues is much weaker than the scattering of a light wave. As such, it is straightforward to form a high-quality ultrasound focus in deep tissues. A sound wave can both modulate the refractive index and cause displacement of light scatterers. In either way, the phase of the light wave that travels through the ultrasound focus is modulated, which can shift the frequency of light. The frequency-shifted light originates from the ultrasound focus. By detecting and phase conjugating the frequency-shifted light, we can form an optical focus at the ultrasound focus.
The use of ultrasound to modulate light in deep tissues is made difficult by the low signal-to-noise ratio (SNR). Light is diffused inside tissues and only a small fraction of diffused light enters the acoustic focus. Consequently, the modulation depth, defined as the ratio of the modulated light to unmodulated light, is very low. Phase conjugation can help to change this. When ultrasound modulation is combined with phase conjugation, the light beam no longer randomly propagates inside the tissue. A large portion of light goes through the ultrasound focus, greatly improving the modulation depth and SNR. In conventional techniques based on ultrasound, the spatial resolution is limited by the bandwidth, wavelength, and numerical aperture of the ultrasound transducer. By combining ultrasound modulation with phase conjugation, I will also explore the possibility of breaking the spatial resolution limit of ultrasound.
The advance of high-resolution and high-sensitivity optical molecular imaging has revolutionized the way biological events are viewed and studied. Because many biological events happen below the surface, it is important to develop a robust, turnkey tool that biologists can use to see deeper inside tissues. My lab will work to enable optical focusing inside tissues and to provide a platform to implement fluorescence and nonlinear microscopy with high sensitivity. Besides combining ultrasound with phase conjugation, I will also explore other possibilities of achieving these goals. Janelia's interdisciplinary environment provides an opportunity to learn the experimental challenges biologists face and to work with them to solve these challenges.
As of August 17, 2010