Tomographic phase microscopy

Seeing inside cells usually requires the use of stains, fluorescent markers, or other contrast agents, and the cells themselves typically have to be fixed, or killed. But now Michael Feld and colleagues at MIT have developed a technique that allows three-dimensional mapping of unperturbed, live cells and tissues in their native state. In the new method, the sample is placed in one arm of an interferometer, and the 2D interference pattern is recorded while the angle of the light through the sample changes and the frequency of the reference beam is modulated. From the resulting quantitative phase images, the team reconstructs a 3D map of the refractive index using algorithms similar to those used in medical x-ray computed tomography (CT) scans. In less than 10 seconds, the MIT researchers could record 100 2D phase images and obtain a 3D tomogram, like this one of a cervical-cancer cell, with a resolution of about 0.5 μm. The technique can resolve internal cell structures, including parts in the nucleus, as well as various structures in tissues and multicellular organisms. (W. Choi et al., Nat. Methods 4 , 717, 2007 http://dx.doi.org/10.1038/nmeth1078 .)