Abstract 

Much like medical CT scans reveal anatomical structures in the body, soft x-ray tomography (SXT) visualizes and quantifies the organization of sub-cellular 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 – 543eV). '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 without the need to use contrast-enhancing agents. Cells imaged with SXT are, therefore, highly representative of the cell in its native, functional state. Absorption of soft x-rays adheres to the Beer-Lambert Law and is, therefore, a function of the chemical composition and concentration of organic material, yielding unique quantitative Linear Absorption Coefficient (LAC) measurements for cellular components. LAC values are enormously powerful in terms of quantifying alterations in cell structures during events such as cell differentiation, progression or etiology of disease states, genetic manipulation, and application of exogenous agents. Recently, high numerical aperture cryogenic fluorescence tomography (CFT) has been used for correlated imaging studies. This multi-modal approach - imaging the same cell using both CFT and SXT - allows localization of labeled molecules directly in the context of a high-resolution 3-D tomographic reconstruction of the cell. I will show examples of data collected using these imaging technologies developed at the National Center for X-ray Tomography, an NIGMS-NIH supported Biomedical Technology Research Resource.