High G-factor Research Articles

A Maximum Likelihood Approach to Parallel Imaging With Coil Sensitivity Noise

Parallel imaging is a powerful technique to speed up magnetic resonance (MR) image acquisition via multiple coils. Both the received signal of each coil and its sensitivity map, which describes its spatial response, are needed during reconstruction. Widely used schemes such as SENSE assume that sensitivity maps of the coils are noiseless while the only errors are in coil outputs. In practice, however, sensitivity maps are subject to a wide variety of errors. At first glance, sensitivity noise appears to result in an errors-in-variables problem of the kind that is typically solved using total least squares (TLSs). However, existing TLS algorithms are in general inappropriate for the specific type of block structure that arises in parallel imaging. In this paper, we take a maximum likelihood approach to the problem of parallel imaging in the presence of independent Gaussian sensitivity noise. This results in a quasi-quadratic objective function, which can be efficiently minimized. Experimental evidence suggests substantial gains over conventional SENSE, especially in nonideal imaging conditions like low signal-to-noise ratio (SNR), high g-factors and large acceleration, using sensitivity maps suffering from misalignment, ringing, and random noise.

Beyond the g‐factor limit in sensitivity encoding using joint histogram entropy

The maximum practical speed-up that can be achieved using parallel imaging methods is widely accepted to be limited by g-factor noise. An approximate expression for the g-factor noise as a function of the principal eigenvector of the inverse sensitivity matrix is derived. This formulation allows g-factor enhanced noise to be reduced by a constrained optimization procedure with joint image histogram entropy between a reference image and a SENSE image as an image quality metric. The reference image does not need to have identical resolution or contrast. The reference image may also be used for coil calibration. The limits of the method are explored using simulated and real array coil data with high g-factor using a variety of contrast and resolution combinations. The method preserves image structure, contrast, and lesions even when these were not observable in the reference data. In all cases g-factor was dramatically reduced.

G-factor analysis of protein secondary structure in solutions and thin films.

The biological activity of proteins is structure dependent. In this discussion, we describe development of g-factor analysis for characterizing the secondary structure of proteins in solutions and films. In g-factor analysis, experimental circular dichroism (CD) and UV absorbance spectra are converted to dimensionless g-factor spectra by dividing the differential absorbance of circularly polarized light (AL - AR) by the UV absorbance (A) at each wavelength. The spectra can be deconvolved and the secondary structure estimated without information on the protein molecular weight, sample concentration, or sample path length. We have refined g-factor spectral acquisition and deconvolution parameters to improve the precision and accuracy of experimental g-factor spectra and the subsequent secondary structure estimations. In general, slower scan rates and longer response times are required to obtain high quality g-factor spectra than to obtain ordinary CD spectra, particularly for data at wavelengths longer than 230 nm. The spectral acquisition and deconvolution procedures have been validated for aqueous bovine serum albumin (BSA) and poly(L-proline). The secondary structures of fibronectin and laminin in buffer solutions are predicted. When BSA and poly(L-proline) form films on quartz, their secondary structures change significantly: 13% for BSA and 32% for poly(L-proline). By contrast, the secondary structure of fibronectin is the same in solution and films. The g-factor method is an easy, rapid, accurate and precise method for determining secondary structure and structural changes in protein solutions and films. Potential applications range from proteomics and structure-based drug discovery, to the design and fabrication of biosensors, biomaterials and biofluidic devices.

Positive and Negative Charge Carriers in Doped or Photoexcited Polydithienothiophenes: A Comparative Study Using Raman, Infrared, and Electron Spin Resonance Spectroscopy

Poly(dithienothiophene)s (PDTTs), low band-gap conjugated polymers with polythiophene-like chain where an aromatic thienothiophene moiety is fused to each thiophene ring, were studied using Raman spectroscopy, photoinduced infrared absorption, as well as attenuated total reflection Fourier transform infrared (ATR-FTIR) and electron spin resonance (ESR) spectroelectrochemistry. The spectroelectrochemical studies were performed in situ during p- and n-doping (electrochemical oxidation and reduction, respectively). Raman lines of the pristine polymers are compared to infrared active vibration (IRAV) bands due to the charge carriers injected by the electrochemical doping processes or by illumination. The different π-electron distribution along the polythiophene-like chain, which determines the different band-gap sizes, also account for the different lattice relaxations and vibrational behaviors shown by these polymers. By means of in situ ESR spectroscopy, the formation of paramagnetic positive and negative charge carriers with unusually high g-factors could be proved.

TRANSITION PROBABILITIES AND DISSOCIATION ENERGY OF ASTROPHYSICAL MOLECULE CoH

The Franck-Condon (FC) factors (transition probabilities) and r-centroids have been eval- uated by the reliable numerical integration procedure for the bands of the A 3 � 4 → X 3 � 4 system of astrophysical molecule CoH, using a suitable potential. The dissociation energy D 0 0 = 2.5 ± 0.05 eV for the electronic ground state of CoH has been estimated by fitting Hulburt-Hirschfelder function to the experimental potential energy curve, using the correlation coefficient. The knowledge of vibrational transition probabilities is required to explain the in- tensity distribution in a molecular band system. To a good approximation, Franck- Condon (FC) factors are proportional to these transition probabilities. Accurate values of FC factors and related quantities are essential to obtain radiative lifetime and vibrational temperature for the astrophysical molecules. The dissociation energy of a diatomic molecule is the most direct measure of the strength of the bond, and therefore, of the stability of the chemical combin- ation between the constituent atoms, that is, the very existence of the molecule in any environment. Astrophysicists, chemists and spectroscopists are therefore concerned with the determination of reliable values of dissociation energies of diatomic molecules. Wohl (1971) reported the presence of CoH molecule in the umbrae of sunspots. The (0,0) bands of P, Q and R branches were identified and it appears that there are many other unidentified band features in the observed solar spectra. Wohl points out that the identifications are hampered by the absence of accurate laboratory and molecular data. Further the sunspots are dynamic phenomena with a variety of flows and changes in the magnetic field (Gokhale and Zwaan, 1972; Ruedi et al., 1999). In the umbral spectrum, therefore, Zeeman lines (having high Lande g-factor) are useful for magnetic field estimates and the non-Zeeman lines are

Skyrmions and composite fermions in the limit of vanishing Zeeman energy

We describe the properties of a strongly interacting two-dimensional (2D) electron gas in high magnetic fields whose properties can be described in terms of the formation of composite fermions at fractional Landau level occupancy. For a Landau level occupancy the excitations correspond to spin waves for high g-factors and magnetic fields, but when the g-factor is reduced close to zero by the use of hydrostatic pressure there is strong evidence for the formation of skyrmions. Studies of composite fermions in the limit of vanishing Zeeman energy also suggest that skyrmionic excitations can occur for composite fermions.

Skyrmions and composite fermions for g*=0

We describe the properties of the strongly interacting 2-D electron gas in high magnetic fields whose properties can be described in terms of the formation of composite fermions at fractional Landau level occupancy. For a Landau level occupancy ν=1 the excitations correspond to spin waves for high g-factors and magnetic fields, but when the g-factor is reduced close to zero by the use of hydrostatic pressure, there is strong evidence for the formation of Skyrmions. Studies of the composite fermions in the limit of vanishing Zeeman energy also suggest that skyrmionic excitations can occur for composite fermions.

Nuclear spin polarization in InSb detected by spin-flip Raman gain spectroscopy

The conduction electron spin resonance (CESR) in n-type InSb is shifted by the spin polarization of the nuclear and electron spins (Overhauser shift). The authors measured this shift by optically detecting the magnetic CESR using coherent spin-flip Raman scattering of two CO lasers in the infrared region. This method is very useful for studying the hyperfine interaction in narrow-gap semiconductors with high effective g-factors. By using a combined CESR/NMR technique, the effective g-factor in InSb was determined at different magnetic fields with the high precision available from NMR experiments. The Overhauser shift is directly proportional to the probability mod Psi (0) mod 2 of finding an electron at the nuclear site of the 115In or the 121Sb isotope, respectively. In thermal equilibrium, when the spin polarization of the nuclei is known, measurements of the Overhauser shift directly yield information on mod Psi (0) mod 2 of the conduction electrons at the respective nuclear site. Observing the shift while either building up dynamic nuclear polarization or studying the relaxation of the nuclei from high spin polarization to thermal equilibrium yields information on the nuclear spin-lattice relaxation time T1 of each of the isotopes.

Approximate analytic expressions for integrating ESR spectra of randomly oriented particles having high g-factor tenso anisotropy

Approximate analytic expressions for integrating ESR spectra of randomly oriented particles having high g-factor tenso anisotropy

High g-factors and Landau level crossing in the valence band of narrow gap semiconductors

When the Kildal-Bodnar [1–3] model of cadmium arsenide is developed for the valence bands, it is found that, for a range of energy below the band maximum, the spin splitting of Landau states is larger than the separation between Landau levels. The same holds for a variety of inverted-gap materials of Kane type [4–5] (with three-dimensional symmetry), as well as Kildal type (with axial symmetry). This could have interesting consequences for interband optical properties of Cd 3As 2 as well as for other p-type materials.

Structural and physical properties of Cr 3± xSe 4 ( x ⩽ 0.20)

The structural, electrical, and magnetic properties of Cr 3± x Se 4 ( x ⩽ 0.20) compounds are examined. The crystal structure of Cr 2.80Se 4, monoclinic, distorted NiAs type, is determined on a single crystal ( R = 2.9%). Physical properties are influenced by the effects of nonstoichiometry. It leads to a separate discussion for compounds Cr 3+ x Se 4 (0 ⩽ x ⩽ 0.20), metallic, antiferromagnetic, with a high g-factor value on the Cr 2+ site, and Cr 3− x Se 4 (0 < x ⩽ 0.20), poorly metallic and metamagnetic. The results are interpreted on the basis of structural considerations.