New X-Ray Method With Low Radiation Dose Can Visualize Soft Tissue

Scientists developed a new, lower dose X-ray method that works completely different than classical X-ray radiographs that provide information about absorptive structures such as bones. Conversely, the new method is based on diffraction and can image soft tissues in 3D and high resolution.

The method was presented in a Nature article published by a German-American-Russian research team led by KIT (Karlsruhe Institute of Technology). For periods of about two hours, time-lapse sequences of cellular resolution were obtained of three dimensional reconstructions showing developing embryos of the African clawed frog (Xenopus laevis).

“X-ray diffraction enables high-resolution imaging of soft tissues,” explains Ralf Hofmann, one author of the study and physicist at KIT. “In our work, we did not only manage to resolve individual cells and parts of their structure, but we could also analyze single cell migration as well as the movement of cellular networks.”

Using X-ray diffraction, similar tissues can be distinguished by minute variations of their refractive index. However, in contrast to classical absorption imaging, this does not require any contrast agent, and X-ray dose is profoundly reduced. The method is of particular advantage when probing sensitive tissues in living organisms, such as frog embryos.

During gastrulation, germ layers are formed and organized in their proper locations. Thereby, an initially simple spherical ball of a few hundred cells turns into a complex, multilayered organism with differentiated tissues eventually turning into the nervous system, muscles, and internal organs. Quoting the renowned developmental biologist Lewis Wolpert: “it is not birth, marriage, or death, but gastrulation that is the most important event in your life.”

Jubin Kashef, zoologist and co-author, said that for the first time they could observe the process of how cells interact with each other in a living embryo and how regions void of cells form and disappear. “It is like the migration of peoples. Stimulated by the migration of individual cell groups, other cells join in. They form functional cellular networks, which adjust to their changing environment. During migration, cells specialize to form progenitor tissues of future organs, e.g. the brain or skin.”

“It is fascinating to have digital capabilities to observe and analyze these processes in an individual living frog embryo,” Hofmann and Kashef emphasize. The African clawed frog (Xenopus laevis) is one of the most important model systems of developmental biology whose study is of relevance in understanding human embryogenesis and diseases.

The novel technique -- combining latest X-ray measurement technology with advanced image analysis and developmental biology -- will soon be established at the synchrotron radiation facilities ANKA in Karlsruhe and APS in Chicago for routine use by a broad community of scientific users.

In their study, KIT researchers, which were supported by biologists from Northwestern University, used coherent X-rays from the Advanced Photon Source of Argonne National Laboratory in Chicago. Prior to this investigation, the method had been developed at ANKA. During measurement, a coherent bundle of X-rays passes the nearly spherical 1-mm frog embryo, which rotates half way around its axis within 18 seconds. By variation of the irradiation direction, information on the three-dimensional (3D) structure is acquired. As X-rays pass through different types of tissues at variable speeds, diffraction occurs.

Study:

J. Moosmann, A. Ershov, V. Altapova, T. Baumbach, M. S. Prasad, C. LaBonne, X. Xiao, J. Kashef, and R. Hofmann, “X-ray phase-contrast in vivo mictotomography probes new aspects of Xenopus gastrulation“, Nature (2013), doi:10.1038/nature12116.