Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X5: Emerging Tomographic Algorithms: From Bending Molecules to Beating Hearts |
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Sponsoring Units: FIAP Chair: Peter Schwander, University of Wisconsin Room: Portland Ballroom 256 |
Thursday, March 18, 2010 2:30PM - 3:06PM |
X5.00001: Tomography of faint spinning objects: From molecules to viruses Invited Speaker: A new generation of powerful algorithms is poised to enable the determination of the three-dimensional structure of objects ranging from single molecules to beating hearts and breathing lungs. At one extreme, new algorithms are paving the way to atomic-level mapping of the conformations of biological molecules with femtosecond time resolution. At the other, they are driving ultra-low-dose tomography of non-stationary, faintly scattering macroscopic objects. These approaches combine concepts from information theory, graph theory, Riemannian geometry, and scattering physics to reconstruct objects at signal levels orders of magnitude below what was previously thought possible. We describe how data from a new generation of X-ray Free Electron Lasers or existing electron microscopes can be used to reconstruct the structure and conformational continuum of individual molecules, viruses, and potentially living cells. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:42PM |
X5.00002: Cryo-Electron Microscopy of Viruses Infecting Bacterium Invited Speaker: Single particle cryo-EM can yield structures of infectious bacterial viruses with and without imposed icosahedral symmetry at subnanometer resolution. Reconstructions of infectious and empty phage particles show substantial differences in the portal vertex protein complex at one of the 12 pentameric vertices in the icosahedral virus particle through which the viral genomes are packaged or released. In addition, electron cryo-tomography of viruses during infecting its bacterial host cell displayed multiple conformations of the tail fiber of the virus. Our structural observations by single particle and tomographic reconstructions suggest a mechanism whereby the viral tail fibers, upon binding to the host cell, induce a cascade of structural alterations of the portal vertex protein complex that triggers DNA release. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 4:18PM |
X5.00003: X-ray cone-beam computed tomography: principles, applications, challenges and solutions Invited Speaker: In the nineties, x-ray computed tomography, commonly referred to as CT, seemed to be on the track to become old technology, bound to be replaced by more sophisticated techniques such as magnetic resonance imaging, due in particular to the harmful effects of x-ray radiation exposure. Yet, the new century brought with it new technology that allowed a complete change in trends and re-affirmed CT as an essential tool in radiology. For instance, the popularity of CT in 2007 was such that approximately 68.7 million CT examinations were performed in the United States, which was nearly 2.5 times the number of magnetic resonance (MRI) examinations. More than that, CT has expanded beyond its conventional diagnostic role; CT is now used routinely in interventional radiology and also in radiation therapy treatment. The technology advances that allowed the revival of CT are those that made fast, accurate cone-beam data acquisition possible. Nowadays, cone-beam data acquisition allows scanning large volumes with isotropic sub-millimeter spatial resolution in a very fast time, which can be as short as 500ms for cardiac imaging. The principles of cone-beam imaging will be first reviewed. Then a discussion of its applications will be given. Old and new challenges will be presented along the way with current solutions. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:54PM |
X5.00004: High-resolution imaging with coherent X-rays Invited Speaker: In the last decade, the development of techniques commonly grouped under the name ``coherent diffractive imaging'' (CDI) has greatly expanded the means whereby spatial information can be collected using radiation. These imaging techniques, which can be described as ``microscopy without lenses'' or ``crystallography without crystals'', present a great potential for diffraction-limited imaging with short wavelength radiation. After giving an overview of the current state of the field, I will describe recent results obtained at the Swiss Light Source with a scanning method called ``ptychography''. CDI techniques typically require much more data processing than traditional microscopy methods. Special emphasis will be put on the algorithmic developments that accompany and often enable new coherent imaging schemes. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:30PM |
X5.00005: Structure recovery by new convergent beam techniques Invited Speaker: Coherent diffractive imaging (CDI) is an emerging methodology for very high resolution lensless imaging, a major application of which is the imaging of single biomolecules using X-ray free electron laser sources. Typically, the object of interest is required to be isolated and is illuminated with a planar highly spatially coherent beam. The diffraction pattern produced by the object is then measured in the far-field. The reconstruction of the object distribution is related to the diffracted field via a Fourier transform and the imaging problem can be treated as a phase recovery problem. Iterative phase-recovery techniques that use the known size of the isolated object are employed. The iterative methods have a tendency to stagnate and are very sensitive to the illumination having imperfect spatial coherence. More recently, it has been proposed that there are benefits to illuminating the object with a spherical beam. Benefits include more reliable convergence of the iterative algorithm and a greater degree of robustness to imperfect coherence. The resulting diffraction pattern is also obtained in the far-field but the diffraction pattern is described by the Fresnel formalism. In the paper I describe our experimental work on the development of Fresnel CDI. In particular I will discuss how the use of a finite expanding wave can be used to define a finite region within an otherwise infinite object and so permit the application a coherent diffractive imaging to extended objects, enabling CDI to evolve into a generally applicable and reliable form of high resolution X-ray imaging. I will also discuss our recent work on coherence measurement and show how a measurement of the coherence properties of an X-ray beam can be included into the imaging method and thereby lead to further improvements in the broad applicability of CDI. [Preview Abstract] |
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