Biological Soft X-Ray Tomography
Abstract: Much like medical computed tomography scans reveal an atomical structures in the body, soft X-ray tomography (SXT) visualizes and quantifies the organization of subcellular structures within a cell. In SXT, the specimen is illuminated with X-ray photons from within a region of the spectrum known as the "water window" (284-543 eV). Water window X-ray photons are absorbed an order of magnitude more strongly by carbon- and nitrogen-containing organic material than by water. Consequently, variation in biomolecule composition and concentration gives rise to quantitative, high-contrast images of intact, fully hydrated cells. Attenuation of soft X-rays as they pass through the specimen adheres to the Beer-Lambert Law and is, therefore, a function of chemical composition and concentration of organic material, yielding unique quantitative linear absorption coefficient measurements for cellular components. To localize molecules without perturbing cell structures, we use correlated cryogenic fluorescence tomography (CFT). This multimodal approach — imaging the same cell using both CFT and SXT — allows localization of labeled molecules directly in the context of a high-resolution three-dimensional tomographic reconstruction of the cell in the near-native state.