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Biomedical subjects

R Pahl

Publications and source records attributed to R Pahl.

9 recordsLinked to original sources
PubMed2005

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb.

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

Journal Article↗
PubMed2005

Atomic-scale visualization of inertial dynamics.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

Journal Article↗
PubMed2005

Clocking femtosecond X rays.

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

Journal Article↗
PubMed1994

X-ray Applications with Glass-Capillary Optics.

Glass capillaries have unique properties for guiding X-rays in experiments with micrometer precision. Design considerations of such optics are presented for X-ray applications involving macromolecular crystallography, tomography and high-pressure experiments at the Cornell High Energy Synchrotron Source. The authors propose that crystallography with protein crystals is feasible on a 50 mum or smaller scale using capillary optics along with a cold gas stream and precision rotation stages. For computed tomography experiments, capillary optics can produce X-ray beams on a submicrometer scale. The distribution of X-rays passing through the sample can then be blown up in size with a secondary capillary optic to match the ~10 mum pixel size of CCD detectors. For high-pressure experiments in diamond-anvil cells, mono- and polycapillary optics may provide 1-50 mum diameter beams for diffraction or X-ray absorption fine-structure applications.

Journal Article↗
PubMed1994

3D computed X-ray tomography of human cancellous bone at 8 microns spatial and 10(-4) energy resolution.

Human cancellous bone was imaged and its absorptive density accurately measured in three dimensions (3D), nondestructively and at high spatial resolution by means of computerized microtomography (microCT). Essential for achieving the resolution and accuracy was the use of monoenergetic synchrotron radiation (SR) which avoided beam hardening effects, secured excellent contrast conditions including the option of energy-modulated contrast, and yet provided high intensity. To verify the resolution, we selected objects of approximately 8 micron size that could be observed on tomograms and correlated them in a unique manner to their counter images seen in histological sections prepared from the same specimen volume. Thus we have shown that the resolution expected from the voxel size of 8 microns used in the microCT process is in effect also attained in our results. In achieving the present results no X-ray-optical magnification was used. From microCT studies of composites (Bonse et al., X-ray tomographic microscopy (XTM) applied to carbon-fibre composites. In: Materlik G, ed. HASYLAB Jahresbericht 1990. Hamburg: DESY, 1990; 567-568) we know that by including X-ray magnification a resolution below 2 microns is obtained. Therefore, with foreseeable development of our microCT method, the 3D and nondestructive investigation of structures in mineralized bone on the 2 micron level is feasible. For example, it should be possible to study tomographically the 3D distribution and amount of osteoclastic resorption in the surrounding bone structure.

Aged↗
PubMed1994

Dopamine-dependent diastolic dysfunction in moderate hypothermia.

We designed an experimental animal study to study the effects of dopamine (DA) on diastolic function in hypothermia. DA was applied at five incremental infusion rates in 6 sheep during normothermia and moderate hypothermia (29 degrees C). Left ventricular end-diastolic pressure (LVEDP) was increased during hypothermia as compared with normothermia at all doses of DA. Contraction and relaxation velocity were changed only slightly during hypothermia; during normothermia, both velocities were markedly increased. The pronounced hemodynamic effect observed during hypothermia was further intensified by occurrence of aftercontractions, which disappeared at very high DA doses. These paradoxic results were considered the result of hypothermia-induced reduction in active transport mechanisms responsible for regulation of the cytoplasmic CA2+ concentration. The generally reduced inotropic effect of DA, the risk of paradoxic reactions, and the occurrence of aftercontractions must be taken into account when emergency drugs are administered clinically during hypothermia.

Animals↗