Radiofrequency losses in NMR experiments on electrically conducting samples

Gadian, David G.; Robinson, F. N. H.

doi:10.1016/0022-2364(79)90023-4

Cited by 120 publications

(110 citation statements)

References 9 publications

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“…Additionally, the frequency shift, ␦f, upon loading the coil was small. Magnetic losses arise from the additional dissipation of RF power due to eddy currents induced by the RF field in the tissue (24,25). We found for this particular coil that losses from the magnetic interaction within the sample dominate.…”

Section: Dielectric and Magnetic Lossesmentioning

confidence: 79%

“…Results are summarized in Table 1. The drop in Q upon loading with distilled water, which has a high dielectric constant and a low conductivity, provides a measure of dielectric losses arising from dissipation of RF power due to displacement currents in the tissue (24,25). Because of the balanced coil design, dielectric losses remained small.…”

Section: Dielectric and Magnetic Lossesmentioning

confidence: 99%

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Reengineered helmet coil for human brain studies at 3 Tesla

Driesel

Merkle

Hetzer

et al. 2005

Concepts Magn. Reson.

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ABSTRACT:We describe the further development of a circularly polarized helmet coil for magnetic resonance imaging (MRI) of the human brain at 3 T. The coil is used in transmit and receive (transceive) mode, a useful alternative over commercially available quadrature headcoils with scanners where phased arrays are unavailable. The coil is simple to build. Its structure is based on an assembly of two coplanar dual-loop coils of the split-circle design, which are arranged in crossed fashion. Both coils circumscribe dome-like the human head. Because of the symmetry of the assembly, a high degree of circularly polarized radiofrequency (RF) is obtained within the brain. Bench and in situ measurements of the RF magnetic field, B 1 , indicated good axial homogeneity and a moderate gradient along the symmetry axis. Compared to a commercial birdcage coil, the signal-to-noise ratio was increased up to a factor of 1.7 as a result of its superior filling factor. Initial applications in healthy volunteers included anatomical MRI and functional imaging with a cognitive paradigm.

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Section: Dielectric and Magnetic Lossesmentioning

confidence: 79%

Section: Dielectric and Magnetic Lossesmentioning

confidence: 99%

Reengineered helmet coil for human brain studies at 3 Tesla

Driesel

Merkle

Hetzer

et al. 2005

Concepts Magn. Reson.

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show abstract

“…Thus, the NMR sensitivity deteriorates, because thermal ionic motion induces noise in the receiver coil (39), and the sample is heated by electrolyte friction induced by the electric component of the oscillating radiofrequency field (40). However, the sensitivity loss is insignificant at the low frequencies used here, as also indicated by the negligible difference in 90°pulse length between the H. marismortui and E. coli samples.…”

Section: Mrd Experimentsmentioning

confidence: 93%

Cell water dynamics on multiple time scales

Persson

Halle

2008

Proc. Natl. Acad. Sci. U.S.A.

185

165

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Water-biomolecule interactions have been extensively studied in dilute solutions, crystals, and rehydrated powders, but none of these model systems may capture the behavior of water in the highly organized intracellular milieu. Because of the experimental difficulty of selectively probing the structure and dynamics of water in intact cells, radically different views about the properties of cell water have proliferated. To resolve this long-standing controversy, we have measured the 2 H spin relaxation rate in living bacteria cultured in D2O. The relaxation data, acquired in a wide magnetic field range (0.2 mT-12 T) and analyzed in a modelindependent way, reveal water dynamics on a wide range of time scales. Contradicting the view that a substantial fraction of cell water is strongly perturbed, we find that Ϸ85% of cell water in Escherichia coli and in the extreme halophile Haloarcula marismortui has bulk-like dynamics. The remaining Ϸ15% of cell water interacts directly with biomolecular surfaces and is motionally retarded by a factor 15 ؎ 3 on average, corresponding to a rotational correlation time of 27 ps. This dynamic perturbation is three times larger than for small monomeric proteins in solution, a difference we attribute to secluded surface hydration sites in supramolecular assemblies. The relaxation data also show that a small fraction (Ϸ0.1%) of cell water exchanges from buried hydration sites on the microsecond time scale, consistent with the current understanding of protein hydration in solutions and crystals. biomolecular hydration ͉ buried water molecules ͉ Escherichia coli ͉ Haloarcula marismortui ͉ in vivo NMR W ater, the ubiquitous biosolvent, mediates or modulates the intermolecular forces that govern the self-assembly of biological cells, it controls the rates of substrate diffusion and conformational transitions, and it participates in molecular recognition and enzyme catalysis (1-4). It is therefore imperative to characterize and understand any differences between cell water and bulk water. Biopolymers and other solutes make up one-third of the mass of a typical cell, so this difference could be substantial. The few experimental techniques that can monitor the molecular properties of water in vivo have suffered from interpretational ambiguities, allowing widely discordant views about cell water structure and dynamics to coexist for a long time (5-7). NMR spectroscopy can provide information about cell water via the spin relaxation times of the dominant water-1 H signal (8, 9). In fact, tissue-specific variations in water relaxation times provided the impetus for developing magnetic resonance imaging (10). Unfortunately, the interpretation of water-1 H relaxation data from biological samples is confounded by crossrelaxation, intermolecular paramagnetic couplings, and protonexchange modulation of the nuclear shielding (11,12). Here, we circumvent these complications by measuring the relaxation rate of the longitudinal water-2 H magnetization from cells cultured in D 2 O. We have chosen to study...

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“…An NMR solenoidal coil utilized in MAS probes for solids is usually a multituned coil, which is intended to transmit and receive at largely different resonance frequencies for protons (high frequency) and at resonance frequencies for 13 C and/or 15 N (low frequencies) simultaneously in order to maintain a high sample filling factor. However, the RF magnetic field distributions obtainable in single-coil multiply tuned probes strongly depend on the frequency, because a solenoidal coil with given geometric parameters (in particular, the radius and pitch length) responds quite differently for widely varying frequencies because of the frequency dependency of the wave compression factor.…”

Section: Discussionmentioning

confidence: 99%

“…We know that a characteristic spatial translation p exists, which is the turn to turn distance (pitch length) for the helix; that is, upon translation of p in the z direction the longitudinal field components coincide up to a phase factor exp(Ϫj 0 ), where the phase lag per helix turn is defined as 0 ϭ ␤ e p [13] Further, for the helix we have to require a similar periodic boundary condition for a rotation with angle 2z 1 /p in the transverse plane (r, ) and a simultaneous translation by z 1 in the longitudinal direction:…”

Section: Periodic Boundary Conditions For Field Components In Helicesmentioning

confidence: 99%

Electromagnetic wave compression and radio frequency homogeneity in NMR solenoidal coils: Computational approach

Engelke

2002

Concepts Magn. Reson.

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ABSTRACT:The homogeneity of the radio frequency (RF) magnetic field in the region of the NMR sample is prerequisite for good performance in many NMR experiments. When calculating such fields, the quasistationary approximation is valid for wavelengths that are large compared to the physical dimensions of the NMR coil, but it no longer applies for the geometrical dimensions of NMR coils at high frequencies. The present study investigates electromagnetic fields in solenoidal coils. Considering the electromagnetic field derivable from first principles allows us to systematically study the effects that originate from the finiteness of the wavelength compared to the physical size of solenoidal coils employed in NMR probes. There are three major effects that occur: the wavelength in the coil is generally different from the wavelength in free space, even for an absent sample; the dielectric properties of the NMR sample have an additional influence on the electromagnetic field distribution inside the coil; and the circuit surrounding the coil leads to a wave reflection that affects the RF magnetic field distribution originating from the superposition of forward and backward traveling waves. The results allow the derivation of principal conclusions for the characterization of NMR solenoidal coils.

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Radiofrequency losses in NMR experiments on electrically conducting samples

Cited by 120 publications

References 9 publications

Reengineered helmet coil for human brain studies at 3 Tesla

Reengineered helmet coil for human brain studies at 3 Tesla

Cell water dynamics on multiple time scales

Electromagnetic wave compression and radio frequency homogeneity in NMR solenoidal coils: Computational approach

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