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Ultrafast science

Interferometry images living cells in 3D

14 Aug 2007

Researchers in the US have invented a new interferometry technique that has produced the first 3D images of a living cell without having to alter the cell beforehand. Created by physicist Michael Feld and colleagues at the Massachusetts Institute of Technology, the technique involves shining a laser beam through a biological sample at different angles to record a 3D image with sub-micrometre resolution (Nature Methods advance online publication).

Inside a cell

Tiny biological samples must normally be prepared before they can be viewed in 3D. Cells, for example, often have their inner components highlighted with fluorescent dyes. But such modifications can disrupt a cell’s normal functions, limiting the possibilities for analysis.

Feld and colleagues have done away with such preparations, and instead use the optical properties of the cell in its natural state to generate a 3D image. First, a laser beam is split into two: one beam goes through the sample while the other bypasses it. The beams are then recombined and shone onto a digital camera where they produce an interference pattern.

From this pattern the US team deduce the phase difference between the two beams, which changes according to the refractive index of the material that the sample beam passes through. By mapping this refractive index, a 2D image of the cell’s interior is generated.

To get a 3D image the researchers must place a mirror in front of the sample and rotate it incrementally with a galvanometer – a device that converts a small current into a mechanical motion. For each rotation, which alters the angle of the laser beam through the sample, they record an interference pattern.

Feld and colleagues demonstrated their technique, called tomographic phase microscopy, by imaging a cervical cancer cell (See Inside a cell). For the first time, an unaltered cell’s detailed 3D structure with elements such as the nucleus can be seen.

“Accomplishing this has been my dream, and a goal of our laboratory, for several years,” said Feld. “For the first time the functional activities of living cells can be studied in their native state.”

The resolution currently stands at about 0.5 µm, but the group says it should be able to improve it to 0.15 µm or less. They expect that it will complement electron microscopy, which can probe as small as 10 nm but requires samples to be either frozen or coated in a layer of conductive material.

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