Search results for "MR"
showing 10 items of 3001 documents
Sensitivity of noble gas NMR parameters to the heterocyclic ring proximity. Density functional theory studies of Ne–furan and Ar–furan complexes
2014
Theoretical modeling of noble gas interaction with furan as a simple heterocyclic ring was performed. The structures of neon–furan and argon–furan complexes were calculated at the MP2, M06-2X, CAM-B3LYP, APFD, and VSXC levels of theory using 6-311++G** basis set. The predicted 21Ne and 39Ar NMR chemical shifts for the Ne–furan and Ar–furan complexes calculated with pcS-3 and aug-pcS-3 basis sets were sensitive to the presence of the aromatic furan ring. Our results indicate a higher sensitivity of the neon and argon NMR probes than the previously reported 3He NMR spectroscopic parameters in studies of small heterocyclic rings containing the oxygen atom.
Coligand Effects on the Field-Induced Double Slow Magnetic Relaxation in Six-Coordinate Cobalt(II) Single-Ion Magnets (SIMs) with Positive Magnetic A…
2019
Two mononuclear cobalt(II) compounds of formula [Co(dmphen)2(OOCPh)]ClO4·1/2H2O·1/2CH3OH (1) and [Co(dmbipy)2(OOCPh)]ClO4 (2) (dmphen = 2,9-dimethyl-1,10-phenanthroline, dmbipy = 6,6'-dimethyl-2,2'-bipyridine and HOOCPh = benzoic acid) are prepared and magnetostructurally investigated. Each cobalt(II) ion is six-coordinate with a distorted octahedral CoN4O2 environment. The complex cations are interlinked leading to supramolecular chains (1) and pairs (2) that grow along the crystallographic c-axis with racemic mixtures of (Δ,Λ)-Co units. FIRMS allowed us to directly measure the zero-field splitting between the two lowest Kramers doublets, which led to axial anisotropy values of 58.3 cm-1 ≤…
Role of vitreous matrix on the optical activity of Ge-doped silica
2003
Abstract We report an experimental study on the relationship between the optical activity of Ge-oxygen deficient centers and dynamic properties and conformational heterogeneity of vitreous matrix in silica. We focus our attention on the absorption band at ∼5.2 eV (B 2β ) and on the two related emissions at ∼4.2 eV (α E ) and at ∼3.1 eV (β). From the temperature dependence of B 2β band we estimate a mean energy value of 26 meV for local vibrational modes coupled to the electronic transition, suggesting that the chromophore and its surrounding have access to low frequency dynamics. From the thermal behavior of the two emissions we distinguish the two competitive relaxation processes from the …
Competitive relaxation processes of oxygen deficient centers in silica
2003
Physical review / B 67, 033202 (2003). doi:10.1103/PhysRevB.67.033202
Characterization of water mobility in dry and wetted roasted coffee using low-field proton nuclear magnetic resonance
2007
Abstract Roasted and ground coffee was studied by low-field 1 H nuclear magnetic resonance at various water contents and temperatures. The spin–spin relaxation times ( T 2 ) were measured with single pulse free induction decay (FID) and Carr–Purcell–Meiboom–Gill (CPMG) sequences. Four relaxing components were distinguished: the solid population was observed with FID sequence at T 2s ∼9 μs; the other three populations, measured with the CPMG sequence, corresponded to an apolar phase, the coffee oil, and two polar phases. The two polar populations, observed at T 2m ∼6 ms and ∼27 ms (for coffee with 50% water content at 90 °C) were attributed to water in cell wall polymers and in water filling…
Fuzzy C-Means Segmentation on Brain MR Slices Corrupted by RF-Inhomogeneity
2007
Brain MR Images corrupted by RF-Inhomogeneity exhibit brightness variations in such a way that a standard Fuzzy C-Means (fcm) segmentation algorithm fails. As a consequence, modified versions of the algorithm can be found in literature, which take into account the artifact. In this work we show that the application of a suitable pre-processing algorithm, already presented by the authors, followed by a standard fcm segmentation achieves good results also. The experimental results ones are compared with those obtained using SPM5, which can be considered the state of the art algorithm oriented to brain segmentation and bias removal.
SAN plot: A graphical representation of the signal, noise, and artifacts content of spectra
2019
The signal-to-noise ratio is an important property of NMR spectra. It allows to compare the sensitivity of experiments, the performance of hardware, etc. Its measurement is usually done in a rudimentary manner involving manual operation of selecting separately a region of the spectrum with signal and noise, respectively, applying some operation and returning the signal-to-noise ratio. We introduce here a simple method based on the analysis of the distribution of point intensities in one- and two-dimensional spectra. The signal/artifact/noise plots, (SAN plots) allows one to present in a graphical manner qualitative and quantitative information about spectra. It will be shown that besides me…
Illumination Correction on MR Images
2006
Objective. An important artifact corrupting Magnetic Resonance Images is the rf inhomogeneity, also called bias artifact. This anomaly produces an abnormal illumination fluctuation on the image, due to variations of the device magnetic field. This artifact is particularly strong on images acquired with a device specialized on upper and lower limbs due to their coil configuration. A method based on homomorphic filtering aimed to suppress this artifact was proposed by Guillemaud. This filter has two faults: it doesnt provide an indication about the cutoff frequency (cf) and introduces another illumination artifact on the edges of the foreground. This work is an improvement to this method because i…
Volumetric Bias Correction
2007
This paper presents a method to suppress the bias artifact, also known as RF-inhomogeneity, in Magnetic Resonance Imaging (MRI). This artifact produces illumination variations due to magnetic field fluctuations of the device. In the latest years many works have been devoted to face this problem. In this work we present the 3D version of a new approach to bias correction, which is called Exponential Entropy Driven Homomorphic Unsharp Masking (E2D-HUM). This technique has been already presented by some of the authors for the 2D case only. The description of the whole method is detailed, and some experimental results are reported.
History of NAMRI and NAMRC
2019
History of NAMRI and NAMRC