Dynamic shim updating on the human brain

Dynamic alteration of shim settings during a multi-slice imaging experiment can improve static magnetic-field homogeneity over extended volumes. In this report, a pre-emphasized dynamic shim updating (DSU) system capable of rapidly updating all non-degenerate zeroth through second-order shims is presented and applied to high-field multi-slice imaging studies on the human brain. DSU is utilized in both non-oblique and oblique slicing geometries while updating in-plane and through-slice shims. Image-based magnetic-field maps are used to quantify homogeneity improvements and comparisons are made on a slice-specific basis between static global shimming and increasing orders of shim inclusion utilized DSU. The influence of oblique slicing geometry on DSU-utilized global homogeneity is also quantified computationally. Finally, the effect of DSU on susceptibility artifact reduction in single-shot axial-sliced EPI is analyzed using experimental acquisitions.     

Electron impact excitation of the ã (3)B(1u) electronic state in C2H4: an experimentally benchmarked system?

We report on differential and integral cross section measurements for the electron impact excitation of the lowest-lying triplet electronic state (a? (3)B(1u)) in ethylene (C(2)H(4)). The energy range of the present experiments was 9 eV-50 eV, with the angular range of the differential cross section measurements being 15°-90°. As the ground electronic state of C(2)H(4) is a (1)A(g) state, this singlet → triplet excitation process is expected to be dominated by exchange scattering. The present angular distributions are found to support that assertion. Comparison, where possible, with previous experimental results from the University of Fribourg group shows very good agreement, to within the uncertainties on the measured cross sections. Agreement with the available theories, however, is generally marginal with the theories typically overestimating the magnitude of the differential cross sections. Notwithstanding that, the shapes of the theoretical angular distributions were in fact found to be in good accord with the corresponding experimental results.     

Characterization of the Fermi surface of the organic superconductor - by measurements of Shubnikov-de Haas and angle-dependent magnetoresistance oscillations and by electronic band-structure calculations

The electronic structure of the quasi two-dimensional (2D) organic superconductor -(ET)₂SF₅CH₂CF₂SO₃ was examined by measuring Shubnikov-de Haas (SdH) and angle-dependent magnetoresistance (AMRO) oscillations and by comparing with electronic band-structure calculations. The SdH oscillation frequencies follow the angular dependence expected for a 2D Fermi surface (FS), and the observed fundamental frequency shows that the 2D FS is 5% of the first Brillouin zone in size. The AMRO data indicate that the shape of the 2D FS is significantly non-circular. The calculated electronic structure has a 2D FS in general agreement with experiment. From the temperature and angular dependence of the SdH amplitude, the cyclotron and band effective masses were estimated to be and ,where g is the conduction electron g factor and the free electron mass. The band effective mass is estimated to be from the calculated electronic band structure. Received: 3 March 1997 / Revised: 5 May 1997 / Received in final form: 5 November 1997 / Accepted: 10 November 1997     

Goniometry for Interface Studies in Transmission Electron Microscopy

A method using a double-tilt goniometer stage for determining the orientation of crystalline specimens in transmission electron microscopy (TEM) is described. The procedure involves visual identification of two zone axes, and using the corresponding goniometer settings to plot these poles stereographically. Crystal orientations and misorientations can be established rapidly, and the method is intended for use as a practical complement to more precise but time-consuming techniques. The commutative operation of double-tilt stages and their goniometric capability are also discussed. Using Rodrigues-Frank space, it is shown that the set of attainable operations for such stages is represented by points on an hyperboloidal surface.     

Electron paramagnetic resonance spectroscopic studies on the zero-valent rhodium complex [Rh(P(OPri)3)4] at X-and Q-band frequencies

The EPR spectroscopic properties of the zero-valent rhodium complex [Rh(P(OPrⁱ)₃)₄] have been examined at both X- and Q-band frequencies. The spectra show a large, essentially axial anisotropy and ligand hyperfine coupling to four equivalent ³¹P phosphorous nuclei. No resolved coupling to ¹⁰³Rh was detected. The data indicate a tetragonally compressed tetrahedral structure (D_2d) rather than the alternative C_3v geometry provisionally proposed for the related [Co(P(OMe)₃)₄] complex.