238,428 research outputs found
Quantitative Susceptibility Mapping: Contrast Mechanisms and Clinical Applications.
Quantitative susceptibility mapping (QSM) is a recently developed MRI technique for quantifying the spatial distribution of magnetic susceptibility within biological tissues. It first uses the frequency shift in the MRI signal to map the magnetic field profile within the tissue. The resulting field map is then used to determine the spatial distribution of the underlying magnetic susceptibility by solving an inverse problem. The solution is achieved by deconvolving the field map with a dipole field, under the assumption that the magnetic field is a result of the superposition of the dipole fields generated by all voxels and that each voxel has its unique magnetic susceptibility. QSM provides improved contrast to noise ratio for certain tissues and structures compared to its magnitude counterpart. More importantly, magnetic susceptibility is a direct reflection of the molecular composition and cellular architecture of the tissue. Consequently, by quantifying magnetic susceptibility, QSM is becoming a quantitative imaging approach for characterizing normal and pathological tissue properties. This article reviews the mechanism generating susceptibility contrast within tissues and some associated applications
Finite-Temperature Scaling of Magnetic Susceptibility and Geometric Phase in the XY Spin Chain
We study the magnetic susceptibility of 1D quantum XY model, and show that
when the temperature approaches zero, the magnetic susceptibility exhibits the
finite-temperature scaling behavior. This scaling behavior of the magnetic
susceptibility in 1D quantum XY model, due to the quantum-classical mapping,
can be easily experimentally tested. Furthermore, the universality in the
critical properties of the magnetic susceptibility in quantum XY model is
verified. Our study also reveals the close relation between the magnetic
susceptibility and the geometric phase in some spin systems, where the quantum
phase transitions are driven by an external magnetic field.Comment: 6 pages, 4 figures, get accepted for publication by J. Phys. A: Math.
Theo
Thermodynamic and magnetic properties of the layered triangular magnet NaNiO2
We report muon-spin rotation, heat capacity, magnetization, and ac magnetic
susceptibility measurements of the layered spin-1/2 antiferromagnet NaNiO2.
These show the onset of long-range magnetic order below T_N = 19.5K. Rapid muon
depolarization persisting to about 5K above T_N is consistent with the presence
of short-range magnetic order. The temperature and frequency dependence of the
ac susceptibility suggests that magnetic clusters persist above 25K in the
paramagnetic state and that their volume fraction decreases with increasing
temperature. A frequency dependent peak in the ac magnetic susceptibility at
T_sf = 3K is observed, consistent with a slowing of spin fluctuations at this
temperature. A partial magnetic phase diagram is deduced.Comment: 4 pages, 4 figure
Phenomenological theory of a scalar electronic order: application to skutterudite PrFe4P12
By phenomenological Landau analysis, it is shown that a scalar order
parameter with the point-group symmetry explains most properties
associated with the phase transition in PrFeP at 6.5 K. The
scalar-order model reproduces magnetic and elastic properties in
PrFeP consistently such as (i) the anomaly of the magnetic
susceptibility and elastic constant at the transition temperature, (ii)
anisotropy of the magnetic susceptibility in the presence of uniaxial pressure,
and (iii) the anomaly in the elastic constant in magnetic field. An Ehrenfest
relation is derived which relates the anomaly of the magnetic susceptibility to
that of the elastic constant at the transition.Comment: 16 pages, 9 figure
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