A gentle X-ray imaging method for small living specimens

German researchers have developed a new system for X-ray imaging, which is suitable for both living specimens and sensitive materials. Described in the journal Optica, the system records images of micrometre resolution at a minimum radiation dose, and has already been tested on living parasitic wasps.

While conventional X-ray images of soft tissue are of low contrast, phase contrast methods produce far better image contrasts at a reduced radiation dose. With higher resolution, however, gentle imaging becomes increasingly difficult, as a higher dose is required. Moreover, the efficiency of the usually applied high-resolution detectors decreases, as a result of which radiation exposure is further increased. So far, high-resolution X-ray phase contrast imaging of living biological specimens has been possible for a few seconds to minutes only, before severe damage is caused by the radiation.

Researchers from Karlsruhe Institute of Technology (KIT) have now developed a method that uses radiation more efficiently and produces images of micrometre resolution. The method is suited for both living specimens and sensitive materials and opens up new opportunities in biology, biomedicine and materials sciences. The new system combines X-ray phase contrast with a so-called Bragg magnifier and a photon-counting detector.

“Instead of converting the X-ray image into an image with visible light and enlarging it afterwards, we enlarge it directly,” said KIT doctoral researcher Rebecca Spiecker. “Thanks to this approach, we can use highly efficient large-area detectors.”

The researchers use a photon-counting detector with a pixel size of 55 µm. Before, the X-ray image of the specimen is enlarged with a so-called Bragg magnifier, as a result of which the resolution of the specimen proper reaches about 1 µm. The Bragg magnifier consists of two perfect silicon crystals, whose enlarging effect results from asymmetric diffraction in the silicon crystal lattice. Another advantage of the Bragg magnifier is the very good optical image transmission, which allows for the nearly loss-free reproduction of all spatial frequencies up to the resolution limit.

Thanks to the combination of propagation-based X-ray phase contrast with a Bragg magnifier and a photon-counting detector, all of which are optimised for an X-ray energy of 30 kiloelectron-volts (keV), the method reaches about the maximum possible dose efficiency for X-ray phase contrast. This allows for far longer observation times of small living organisms with micrometre resolution.

Together with scientists from all over Germany, the researchers demonstrated the method in a pilot study of small parasitic wasps. For more than 30 minutes, they observed the wasps in their host eggs and how they emerged from them.

“The method is also suited for biomedical applications, an example being the gentle three-dimensional histological investigation of biopsy samples,” Spiecker said. The researchers now plan to further improve the setup, to enlarge the field of view, and to increase mechanical stability for even longer measurements.

Image shows a parasitic Trichogramma wasp that developed in the egg of a grain moth and emerges from it. Image credit: Rebecca Spiecker, KIT.