Nanoscale X-Ray Technique Devised for Imaging of Bacterial Cells

In research published December 17, 2009, in the online issue of the journal Proceedings of the [U.S.] National Academy of Sciences (PNAS), researchers reported progress in applying an approach to "lensless” X-ray microscopy, which they introduced one year ago. They have produced the first images, using this technique, of biologic cells--specifically the fascinating polyextremophile Deinococcus radiodurans.

Improved ability to see nanoscale structures in cells could provide significant insights for evolutionary biology and biotechnology. In the case of D. radiodurans, for example, it could help to answer questions about whether--or how--the structure of this organism's DNA-bearing nucleoid region accounts for its stamina against ionizing radiation. Having demonstrated the resolution, effectiveness, and reproducibility of their technique, the researchers are now working to extend it to three-dimensional (3D) imaging of biologic cells.

X-ray imaging is best known for its medical applications, such as conventional radiographs and computed tomography (CT) scans. However, the use of X-rays goes far beyond standard imaging. In particular, the very short wavelength of X-ray radiation allows various modes of microscopy that can reach the nanometer resolution. One of the major obstacles to high-resolution X-ray microscopy is the difficulty of producing high-quality X-ray lenses. To overcome these difficulties, so-called lensless microscopy methods have emerged in the last decade. A technique developed by researchers now in the biomedical physics group from the Technische Universitaet Muenchen (TUM; Munich. Germany) has shown great promise for ultra-high resolution imaging of materials and life science samples.

This imaging technique, called ptychography, was first introduced in the 1970s for electron diffraction. It consists in measuring full far-field diffraction patterns as a small illumination is scanned on a sample. While its use in electron microscopy is still limited, ptychography has gained tremendous popularity in the X-ray imaging community in the last few years, due to the development by Franz Pfeiffer, now chair of the biomedical physics group at TUM, and his team. A critical step in the development of ptychography was published by the team one year ago and published in the August 15, 2009, issue of the journal Science. The super-resolution capability of the imaging method was successfully demonstrated with a gold test structure.

Now a collaboration of the Pfeiffer group, together with researchers from the University of Gottingen (Germany) and at the Swiss Light Source (Villigen, Switzerland), has gone a step further and produced the first images of biologic cells with the same technique.

These study findings demonstrated that lensless X-ray imaging, specifically ptychography, could be used to obtain precise maps of the electron density forming a biologic sample. This type of quantitative measurement is extremely difficult with most other high-resolution techniques currently available. Moreover, biologic samples are very fragile and nearly transparent to X-rays, making this type of accurate measurement even more challenging.

The Pfeiffer group is now moving beyond this achievement and looking into ways of additionally improving the technique. In particular, the team is aiming at the next milestone: 3D imaging of biologic samples.

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Technische Universitaet Muenchen
University of Gottingen