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Optical technique achieves label-free imaging of cells

21 Nov 2007

A new microscopy technique could enable biologists to detect cancerous cells without the use of any label to enhance optical contrast.

A team of researchers from the University of Bordeaux have produced high-contrast images of mitochondria in live cells using a new label-free, far-field optical imaging technique. Their approach could enable scientists to detect cancer cells without limitations associated with using labels. What's more, the far-field set-up also offers a non-contact method that is particularly useful for imaging live cells. (Optics Express 15 14184)

"This method has the potential to be an efficient diagnostic tool. High-quality imaging of mitochondria is of great interest for medical applications," Laurent Cognet, a researcher from the University of Bordeaux, told optics.org. "In particular, the shape, spatial distribution, and aggregation of mitochondria can be markers for identifying some myopathies or cancer cells."

The use of labels, such as fluorescent molecules, is commonly used to target particular components to gain specificity and contrast. One of the drawbacks is that such labels may interfere with cellular activity. "It is not always possible to label a cellular organelle with fluorescent tags," explained Cognet. "Using labels can also alter the biological function of organelles and labeling efficiency can vary according to experimental conditions, making quantitative experiments difficult."

Far-field imaging also allows the team to project images at distances larger than the optical wavelength. "Near-field imaging needs to get in close contact to the objects to be imaged — typically a fraction of the optical wavelength," commented Cognet. "This is not the case for far-field imaging, which is simpler and more widely used."

The team used an ultra-sensitive photothermal method that was initially developed for the detection of individual absorbing nano-objects. The technique, known as "Light Induced Scattering around a NanoAbsorber", uses a laser to increase heat in and around the sample to be imaged. The approach relies on the sample, such as mitochondria, releasing the majority of its absorbed energy as heat.

The increase in temperature around the absorber induces a time-modulated variation of refractive index around the absorber. "It is the interaction of a probe beam with this index profile that produces a scattered field, which is then detected by a microscope objective," explained Cognet.

"This method is highly original in microscopy because it is based on absorption and does not suffer from any background, even in the highly scattering and autofluroescent environment of living cells," continued Cognet. "What's more, the resolution we obtain is comparable to confocal microscopes."

The next step for the team is to understand the molecular origin of the signal, and test the method as a diagnostic tool.

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