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Virtual lens improves X-ray microscope


Klaus Wakonig and Ana Diaz (left-to-right), together with other PSI researchers, have for the first time given Fourier ptychography to X-ray microscopy. Credit: Paul Scherrer Institute / Markus Fischer

With X-ray microscopes, PSI researchers look at computer chips, catalysts, small bones, or brain tissue. The short wavelength of X-ray makes visible information less than millions of times smaller than sand grains in the nanometer range (millionths of a millimeter). As with a conventional microscope, the lens is used to collect sample diffused light and to create an enlarged image in the camera. Tiny structures, however, dissipate light at very high angles. For a high resolution image, a large lens is required. "The production of such large lenses is still extremely complicated," says PSI physicist Klaus Vakonig: "When working with visible light, there are lenses that can capture very large angles of dispersion. is quite ineffective. "

The new method developed by Wakonig and his colleagues makes this problem difficult. "The final picture is as if we had measured a large lens," the researcher explains. The PSI team uses a small but effective lens, for example, it is commonly used in X-ray microscopy and is moved over the area to which the ideal lens would be. It practically creates a large lens. "In practice, we use different points with the lens and take the picture in every place," explains Wakonig. "Then we use computer algorithms to combine all the images to create one high-resolution image."

From visible light to X-rays

Usually, researchers avoid the lens moving tools away from the optical axis, as this can cause image distortion. However, as scientists in this case know the exact location of the lens and illuminate many of the closest points, they can reconstruct how light was scattered and what was the sample. The method known as Fourier ptychography has been used for microscopy in the visible region since 2013. In PSI experiments, researchers first used this principle for X-ray microscopy. "To the best of our knowledge, no successful x-ray Fourier ptychography has been reported so far," the researchers write Science Development.

The new method provides not only higher resolution, but also two complementary types of imaging information. First of all, there is a measurement of how much light is absorbed in the image object, just like any normal camera. However, the way in which light is disengaged is also fixed. Experts talk about contrast contrast and phase contrast. "Our method provides phase contrast that is otherwise difficult to obtain, virtually free," says Ana Diaz, a scientist at the PSI beam line: "It makes the image quality much better." Phase contrast even allows conclusions to be drawn on the material properties of the test sample, which are usually not possible with normal imaging techniques.

Particularly interesting for biological samples

The investigator tested in his experiments was a detector chip. In the future, the new method could be used to discover, for example, how the catalyst works by adding gas, or when and how the metal breaks under pressure.

However, this method could also better investigate tissues and cell aggregates. Researchers hope that it will create new insights into the development of diseases such as Alzheimer's or hepatitis. Diaz explains the benefits of the new method: "Biological samples do not usually have a good absorption contrast. Here, the phase contrast allows for a significant improvement in image quality." In addition, researchers believe that Fourier ptychography is softer than previous methods. "A comparison with a conventional X-ray microscope shows that the new method requires less radiation because it is more effective," says Wakonig. "It might be particularly interesting to study biological samples."

Researchers set up demonstration equipment on the Swiss light source SLS cSAXS beam line. "Currently, experiments are still quite complicated and time consuming," says Diaz. In order for the new method to work, working X-rays should be in unity: as researchers point out, they must be consistent. Such experiments currently require extensive research capabilities, such as SLS. But Wakonig is also exploring whether this method could be implemented with less coherence. If this method could be used to test samples in conventional laboratory X-ray sources, many additional applications could be opened.

The electron microscope detector reaches the recording resolution

More information:
K. Wakonig et al. Science Development February 2019 (online) DOI: 10.1126 / sciadv.aav0282

Magazine Reference:
Science Development

Paul Scherrer Institute

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