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Biomedical subjects

M Howells

Publications and source records attributed to M Howells.

12 recordsLinked to original sources
PubMed2005

Damped and thermal motion of laser-aligned hydrated macromolecule beams for diffraction.

We consider a monodispersed Rayleigh droplet beam of water droplets doped with proteins. An intense infrared laser is used to align these droplets. The arrangement has been proposed for electron- and x-ray-diffraction studies of proteins which are difficult to crystallize. This paper considers the effect of thermal fluctuations on the angular spread of alignment in thermal equilibrium, and relaxation phenomena, particularly the damping of oscillations excited as the molecules enter the field. The possibility of adiabatic alignment is also considered. We find that damping times in a high-pressure gas cell as used in x-ray-diffraction experiments are short compared with the time taken for molecules to traverse the beam and that a suitably shaped field might be used for electron-diffraction experiments in vacuum to provide adiabatic alignment, thus obviating the need for a damping gas cell.

Chemistry, Physical↗
PubMed2004

Phasing diffuse scattering. Application of the SIR2002 algorithm to the non-crystallographic phase problem.

A new phasing algorithm has been used to determine the phases of diffuse elastic X-ray scattering from a non-periodic array of gold balls of 50 nm diameter. Two-dimensional real-space images, showing the charge-density distribution of the balls, have been reconstructed at 50 nm resolution from transmission diffraction patterns recorded at 550 eV energy. The reconstructed image fits well with a scanning-electron-microscope (SEM) image of the same sample. The algorithm, which uses only the density modification portion of the SIR2002 program, is compared with the results obtained via the Gerchberg-Saxton-Fienup HiO algorithm. The new algorithm requires no knowledge of the object's boundary and proceeds from low to high resolution. In this way, the relationship between density modification in crystallography and the HiO algorithm used in signal and image processing is elucidated.

Journal Article↗
PubMed2004

Coherence and sampling requirements for diffractive imaging.

Coherent Diffractive Imaging (CDI) allows images to be reconstructed from diffraction patterns by solving the non-crystallographic phase problem for isolated nanostructures. We show that the Shannon sampling of diffraction intensities needed in CDI requires a coherence width about twice the lateral dimensions of the object, and that the linear number of detector pixels fixes the energy spread needed in the beam. The Shannon sampling, defined by the transform of the periodically repeated autocorrelation of the object, is related to Bragg scattering from an equivalent crystal, and shown to be consistent with the sampling of Young's fringes established by scattering from extreme points in the object. The results are relevant to the design of diffraction cameras for CDI and plans for femotosecond X-ray diffraction from individual proteins.

Models, Theoretical↗
PubMed2003

Experimental lensless soft-X-ray imaging using iterative algorithms: phasing diffuse scattering.

Images of randomly placed two-dimensional arrays of gold balls have been reconstructed from their soft-X-ray transmission diffraction patterns. An iterative hybrid input-output (HiO) algorithm was used to solve the phase problem for the continuous distribution of diffuse X-ray scattering. Knowledge of the approximate size of the clusters was required. The images compare well with scanning electron microscope (SEM) images of the same sample. The use of micrometre-sized silicon nitride window supports is suggested, and absorption filters have been used to allow collection of low spatial frequencies often obscured by a beam stop. This method of phasing diffuse scattering may have application to scattering from individual inorganic nanostructures or single macromolecules.

Journal Article↗
PubMed2002

Phase recovery and lensless imaging by iterative methods in optical, X-ray and electron diffraction.

Thomas Young's quantitative analysis of interference effects provided the confidence needed to revive the wave theory of light, and firmly established the concept of phase in optics. Phase plays a similarly fundamental role in matter-wave interferometry, for which the field-emission electron microscope provides ideal instrumentation. The wave-particle duality is vividly demonstrated by experimental 'Young's fringes' using coherent electron beams under conditions in which the flight time is less than the time between particle emission. A brief historical review is given of electron interferometry and holography, including the Aharonov-Bohm effect and the electron Sagnac interferometer. The simultaneous development of phase-contrast imaging at subnanometre spatial resolution has greatly deepened our understanding of atomic processes in biology, materials science and condensed-matter physics, while electron holography has become a routine tool for the mapping of electrostatic and magnetic fields in materials on a nanometre scale. The encoding of phase information in scattered farfield intensities is discussed, and non-interferometric, non-crystallographic methods for phase retrieval are reviewed in relationship to electron holography. Examples of phase measurement and diffraction-limited imaging using the hybrid input-output iterative algorithm are given, including simulations for soft X-ray imaging, and new experimental results for coherent electron and visible-light scattering. Image reconstruction is demonstrated from experimental electron and visible-light Fraunhofer diffraction patterns. The prospects this provides for lensless imaging using particles for which no lenses exist (such as neutrons, condensates, coherent atom beams and X-rays) are discussed. These new interactions can be expected to provide new information, perhaps, for example, in biology, with the advantage of less damage to samples.

Algorithms↗
PubMed2001

Lensless imaging: a workshop on "new approaches to the phase problem for non-periodic objects.".

Over the past two decades, theoretical tools and algorithms have been developed which, under not very restrictive conditions, allow the reconstruction of images from diffraction patterns of non-periodic objects. These methods promise lensless imaging for any radiation, free of aberrations, with wavelength-limited resolution. Recent experimental successes prompted an interdisciplinary international workshop on this topic at the Lawrence Berkeley National Laboratory, Berkeley, CA, USA, on May 17-19 2001, supported by the DOE, LBL and the Advanced Light Source. Our aim was to review the field, and to stimulate communication between the Signal Recovery, Coherent Optics, X-ray, Electron Microscopy and Applied Mathematics communities. The results are summarized in this paper and on the web. A second workshop is planned for 2003.

Journal Article↗
PubMed2000

Illumination for coherent soft X-ray applications: the new X1A beamline at the NSLS.

The X1A soft X-ray undulator beamline at the NSLS has been rebuilt to serve two microscopy stations operating simultaneously. Separate spherical-grating monochromators provide the resolving power required for XANES spectroscopy at the C, N and O absorption edges. The exit slits are fixed and define the coherent source for the experiments. The optical design and the operational performance are described.

Journal Article↗
PubMed1987

X-ray holograms at improved resolution: a study of zymogen granules.

X-ray holography offers the possibility of three-dimensional microscopy with resolution higher than that of the light microscope and with contrast based on x-ray edges. In principle, the method is especially advantageous for biological samples if x-rays in the wavelength region between the carbon and oxygen K edges are used. However, until now the achieved resolution has not exceeded that of the light microscope because of the poor coherence properties of the x-ray sources and the low resolution of the detectors that were available. With a recently developed x-ray source based on an undulator on an electron storage ring, and high resolution x-ray resist, a hologram has been recorded at about 400-angstrom resolution. The experiment utilized x-rays with wavelengths of 24.7 angstroms and required a 1-hour exposure of the pancreatic zymogen granules under study.

Animals↗