Biological cells and tissues studied by holographic X-ray microscopy and tomography
X-rays deeply penetrate matter and thus provide information about the functional (interior) architecture of complex samples, from biological tissues and cells to novel composite materials. However, this potential of hard x-rays in view of penetration power, high spatial resolution, quantitative contrast, and compatibility with environmental conditions has to date not been fully developed, mainly due to significant challenges in X-ray optics. With the advent of highly brilliant radiation, coherent focusing, and lensless diffractive imaging this situation has changed. We show how hard X-rays focused to nanometer spot sizes can be used for scanning as well as for full field holographic X-ray imaging of biological samples . The central challenge of inverting the coherent diffraction pattern will be discussed for holographic techniques  and ptychography [3,4].
By scanning the sample through the focused X-ray beam and recording diffraction patterns in each scan point, structural parameters can be mapped throughout the cell or histological section , yielding a 'diffraction contrast' image that can show how nanometer-sized structures can vary within the tissue. As an example, we address the sarcomeric organization in heart muscle cells (cardiomyocytes) [6,7], and show how the sarcomere organization evolves and differs between different cell types and maturation states. As a multi-scale approach, we then discuss sarcomeric structure in heart tissue sections , and then finally present phase contrast tomography reconstructions of an entire mouse heart. A similar multi-scale approach is outlined for the case of neuronal tissue .
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