25 research outputs found
Quantifying MRI frequency shifts due to structures with anisotropic magnetic susceptibility using pyrolytic graphite sheet
Magnetic susceptibility is an important source of contrast in magnetic resonance imaging (MRI), with spatial variations in the susceptibility of tissue affecting both the magnitude and phase of the measured signals. This contrast has generally been interpreted by assuming that tissues have isotropic magnetic susceptibility, but recent work has shown that the anisotropic magnetic susceptibility of ordered biological tissues, such as myelinated nerves and cardiac muscle fibers, gives rise to unexpected image contrast. This behavior occurs because the pattern of field variation generated by microstructural elements formed from material of anisotropic susceptibility can be very different from that predicted by modelling the effects in terms of isotropic susceptibility. In MR images of tissue, such elements are manifested at a sub-voxel length-scale, so the patterns of field variation that they generate cannot be directly visualized. Here, we used pyrolytic graphite sheet which has a large magnetic susceptibility anisotropy to form structures of known geometry with sizes large enough that the pattern of field variation could be mapped directly using MRI. This allowed direct validation of theoretical expressions describing the pattern of field variation from anisotropic structures with biologically relevant shapes (slabs, spherical shells and cylindrical shells)
Properties of Microelectromagnet Mirrors as Reflectors of Cold Rb Atoms
Cryogenically cooled microelectromagnet mirrors were used to reflect a cloud
of free-falling laser-cooled 85Rb atoms at normal incidence. The mirrors
consisted of microfabricated current-carrying Au wires in a periodic serpentine
pattern on a sapphire substrate. The fluorescence from the atomic cloud was
imaged after it had bounced off a mirror. The transverse width of the cloud
reached a local minimum at an optimal current corresponding to minimum mirror
roughness. A distinct increase in roughness was found for mirror configurations
with even versus odd number of lines. These observations confirm theoretical
predictions.Comment: Physical Review A, in print; 11 pages, 4 figure
Mental health leadership and patient access to care: a public–private initiative in South Africa
Improving the specularity of magnetic mirrors for atoms
An array of anti-parallel current-carrying wires creates an inhomogeneous
magnetic field capable of reflecting neutral atoms. We present analytical
and numerical analyses of the magnetic field produced by such an array, and
describe methods for reducing the resulting rms angular
deviation from specular reflection to less than 0.1 mrad. Careful choice of cross-sectional wire
profiles is shown to dramatically improve specularity of reflection close to the
surface. Additionally, we find that the specularity depends on whether the
number of wires in the mirror is even or odd, and that there exists an
optimal turning height above the surface that maximizes the specularity of
reflection from the mirror
Microfabricated magnetic waveguides for neutral atoms
We describe how tightly confining magnetic waveguides for atoms
can be created with microfabricated or nanofabricated wires. Rubidium atoms
guided in the devices we have fabricated would have a transverse mode
energy spacing of K. We discuss the creation of a single-mode
waveguide for atom interferometry whose depth is comparable to
magneto-optical trap (MOT) temperatures. We also discuss the application of
microfabricated waveguides to low-dimensional systems of quantum degenerate
gases, and show that confinement can be strong enough to observe
fermionization in a strongly interacting bosonic ensemble