Local Magnetic Field Gradients Research Articles

The magnetic properties and water dynamics of the red blood cell: a study by proton-NMR lineshape analysis.

The magnetic properties and water dynamics of human red blood cells were examined by analysis of the water proton spectra of suspensions of oxygenated, deoxygenated, carbon monoxide-treated, and methemoglobin-containing cells at a magnetic field strength of 7.05 T. Total lineshape analysis of spectra from deoxygenated red blood cell suspensions was performed to determine the transmembrane water exchange rate, the contribution of diffusion in local magnetic field gradients to the transverse relaxation rate, and the difference between the intra- and extracellular chemical shifts of water protons. Mathematical models are proposed to account for the dependence of the chemical shifts and linewidths of these spectra on the magnetic susceptibility or density of the cells.

NMR studies of intracellular water at 300 MHz: T2‐specific relaxation mechanisms in synchronized or EGF‐stimulated cells

Responses specific to the spin-spin relaxation time (T2) have been observed in two time-dependent studies of the intracellular water in normal and transformed Syrian hamster fetal fibroblasts. At 300-MHz (7.0 T), the spin-lattice relaxation time (T1) was insensitive to several aspects of cellular physiology that produced changes in the T2 and the apparent self-diffusion coefficient (Dapp) of intracellular water. In normal cells stimulated with epidermal growth factor (EGF), T1 was insensitive to time-dependent changes detected by T2 and Dapp. In synchronized tumor cells, T1 was insensitive to cell-cycle-dependent changes detected by T2. The strongly coupled behavior of T2 and Dapp that was observed as a function of time in EGF-stimulated cells indicates that the diffusion of intracellular water through inhomogeneous local magnetic field gradients produced effects observable in T2. Conformational changes in large intracellular macromolecular assemblies such as chromatin or the cytoskeleton may alter the magnitude and inhomogeneity of local field gradients, producing responses in T2 and Dapp only.

Proton N.M.R. studies of chemical and diffusive exchange in carbohydrate systems

A computer-simulated stochastic model is developed capable of predicting the combined effects of chemical and diffusive exchange on the transverse relaxation of spin-1/2 nuclei in a heterogeneous system. Comparison is first made with previous analytical theories for the special case of two site chemical exchange in a homogeneous system and the experimental data on several homogeneous aqueous carbohydrate systems are analysed. Results show that transverse water proton relaxation in these systems is dominated by proton exchange between water and carbohydrate hydroxyl groups. Analysis of model heterogeneous carbohydrate systems shows that in addition to chemical exchange, diffusion coefficients, particle morphology and local magnetic field gradients all have a role to play in determining the proton relaxation behaviour.

Does signal-attenuation on high-field T2-weighted MRI of the brain reflect regional cerebral iron deposition? Observations on the relationship between regional cerebral water proton T2 values and iron levels.

T2-weighted MRI shows attenuated signals from the basal ganglia, such signal attenuation being more evident at high magnetic field strengths of 1.5 tesla (T). The basal ganglia contain high levels of iron, and it has been suggested that these iron deposits lead to shortening of bulk water T2 protons via a mechanism involving diffusion of water through local magnetic field gradients generated by the iron. This mechanism generates a relaxation contribution that is proportional to the square of the applied static field B0, and if it is significant the relaxation rate 1/T2 should be strongly dependent on Bo. T2-weighted MRI would then provide a potential means of imaging regional cerebral iron levels at field strengths that are high enough for this mechanism to be important. The bulk water proton spin-spin relaxation times (T2) of samples from caudate nucleus, frontal cortex, and white matter, taken from fresh cerebral necropsy material of four subjects dying of non-neurological conditions, and one subject with Parkinson's disease have been measured. T2 values were compared with regional cerebral iron content. At high field strengths (2.35 T and 8.5 T) no significant variation in regional cerebral water proton T2 values was found; caudate, cortex and white matter had similar water proton spin-spin relaxation times in spite of the variation in their iron content. Increasing the field strength from 2.35 T to 8.5 T resulted in a generalised 50% decrease in mean regional cerebral T2 values, as opposed to the 13-fold decrease expected if T2 relaxation was dominated by a mechanism that is dependent on B02. It was thus not possible to provide evidence that iron deposition per se is responsible for the attenuated signal obtained from the basal ganglia in T2-weighted MRI.

Calcified intracranial lesions: detection with gradient-echo-acquisition rapid MR imaging.

Seventeen patients with partially calcified intracranial lesions, as documented by CT, were evaluated with MR imaging at 1.5 T. All patients were imaged with both conventional spin-echo techniques and reduced flip-angle gradient-echo-acquisition (GEA) sequences, during which a signal is acquired in the absence of a 180 degrees radiofrequency pulse. GEA parameters were implemented so that T2* effects were maximized on these scans. In all 17 patients GEA images showed marked hypointensity throughout the entire area of calcification, matching the calcified region as seen on CT. In contrast, spin-echo findings in the calcified portions of the lesions were extremely variable, precluding confident identification of calcification on these images. The depiction of regions of calcification as marked hypointensity on GEA images can be ascribed to T2* shortening from static local magnetic field gradients at interfaces of regions differing in magnetic susceptibility, a phenomenon that is well documented in vitro, when various diamagnetic solids are placed in aqueous suspension. However, we cannot exclude the possible additional role of accompanying paramagnetic ions, which sometimes are present with diamagnetic calcium salts in various intracranial calcifications. Since the hypointensity due to calcification on GEA images is not specific, noncontrast CT could be used to confirm its presence. Although this lack of specificity and the artifacts that emanate from diamagnetic susceptibility gradients at or near air-brain interfaces somewhat limit the application of GEA techniques, we suggest that rapid MR imaging using GEA sequences can consistently demonstrate intracranial calcification, and that this technique thus seems to be a useful adjunct to conventional spin-echo imaging.

Modelling of solar quiet magnetic field variations near a conductivity anomaly

Summary. A simplified model of the solar quiet-time ionospheric current system is used to calculate the induced currents in a model earth. The conductivity is assumed to be constant below a depth of about 400 km and zero above that depth. The current induced in the north-south conductivity anomaly under the Rocky Mountains is then estimated from the time-varying potential difference between points at 30 and 45 latitude at the surface of the conducting sphere. The purpose of these calculations is to investigate whether variations in the latitude of the northern hemisphere current system vortex will substantially alter the relationship between the observed magnetic field components at the Earth's surface and the local magnetic field gradient caused by the conductivity anomaly. We find that a 10 shift in the latitude of the ionospheric current focus causes a change of 6 per cent or less in the transfer function from the field components to the gradient in the total field. Thus such latitude shifts cannot explain much of the magnetic field gradient variation at periods near 24 hr that has been observed near Boulder, Colorado.

Structure and dynamics of ammonia adsorbed on graphitized carbon black. Part 4.—Nuclear magnetic resonance spectra

Proton nuclear magnetic resonance experiments on ammonia adsorbed on Graphon are described. Proton chemical shifts, spin–lattice relaxation times and linewidths have been measured as a function of temperature and coverage.The variation of the signal from adsorbed fluid ammonia has been used to measure adsorption isotherms, the isosteric heat of adsorption and melting curves of solid ammonia in contact with Graphon. All these results agree well with gravimetric and neutron scattering experiments described in three earlier papers.The chemical shift of adsorbed ammonia is large (39 p.p.m. to high field) and can be attributed to the effects of anisotropy in the susceptibility of graphite on ammonia molecules adsorbed close to the graphite basal planes. A semi-quantitative model of the effect shows that, above ≈ 195 K (the melting point of bulk solid ammonia), there can only be limited multilayer formation. As the temperature increases, the chemical shift decreases because ammonia molecules are promoted to layers further from the surface. At low temperatures the chemical shift also decreases because ammonia starts to aggregate at heterogeneities on the surface where there is no susceptibility effect. This is correlated with neutron scattering results. The two experiments taken together show the gradual transition with decreasing temperature from anisotropic fluid, through aggregates of ammonia similar to the super-cooled liquid, to bulk solid, the whole process resembling heterogeneous nucleation.The linewidths are abnormally large at all temperatures and coverages. There are two sources of the broadening, diffusion through local magnetic field gradients in the sample, and the possible restriction of the motion of the adsorbate to two dimensions. Other mechanisms which have been considered by other authors are eliminated by considering the neutron scattering results.The spin–lattice relaxation time of adsorbed ammonia is abnormally small (≈ 20 ms) and shows some evidence of a minimum corresponding to a correlation time of 2 × 10–9 s. Partial deuteration shows that the mechanism is intramolecular dipolar relaxation. Consideration of the frequency spectrum of the diffusive motions of adsorbed ammonia, known from quasielastic neutron scattering, indicates that the slow correlation time may arise from a slow rate of jumping between the molecular layers of adsorbate.

NMR Relaxation times of 3He adsorbed on graphite below 4.2 K

Pulsed NMR measurements were performed at 10 and 20 MHz on thin 3He films on graphitic surfaces in the temperature range between 0.35 and 4.2 K. Most of the measurements reported here have been obtained with basal-plane oriented graphite (Grafoil) outgassed at 130 ‡C, but in some experiments graphitized carbon black powder (Sterling FT), vacuum-baked at 1100 ‡C, was used for comparison. The 3He coverages examined range from 0.1 to 80 monolayers on Grafoil and from 0.3 to 1.0 monolayers on Sterling FT. Measurements have also been made on 3He-4He films on Grafoil with one-monolayer quantity of 3He mixed with various amounts of 4He. The measured free-induction decay time T 2 ′, the spin-echo decay time T 2 ‡, and the spin-lattice relaxation time T 1suggest that the observed relaxation phenomena in 3He are largely governed by the 3He-substrate interaction. In the case of 3He on Grafoil, T 2 ‡ is much longer than T 2 ′, evidently as a result of large local magnetic field gradients and significant lateral mobility of 3He on this substrate. These results, in conjunction with a simple model of spin diffusion in restricted geometries, lead to values of the diffusion coefficient D sof about 2.1 × 10−5 cm2/sec at 4.2 K and 7.0 × 10−6 cm2/sec at 1.2 K for one-monolayer coverages ; these values of D sare indicative of a nonlocalized 3He system. To verify the model used, the restricted diffusion analysis was applied to room-temperature measurements on C6F6 in Grafoil ; the values of the diffusion coefficient obtained in this way are in good agreement with diffusion rates measured directly in bulk liquid C6F6 at room temperature. In the case of 3He on Sterling FT, the restricted diffusion analysis of the T 2 ‡ data gives D s\t`t2 × 10−8 cm2/sec for 0.6 monolayer of 3He; this value of D ssuggests a relatively localized system. The measured T 2 ′ is found to decrease monotonically with coverage and there is no evidence of any phase transition in 3He films for coverages up to one monolayer.

Bubble-domain analogy in thin magnetic films with in-plane magnetization

The dynamical behaviour of an in-plane magnetized stripe-domain in a thin film strip is shown to be strikingly similar to that of a bubble-domain in a single-crystalline platelet. This is supported by theoretical as well as experimental evidence. In particular, several mechanisms of propagating a stripe domain by means of a local magnetic field gradient are discussed. However, from the physical point of view the flat and the true bubble differ substantially in two important respects: in the former case, (i) the wall energy plays actually no role - at least in the first approximation, and (ii) no bias field is needed to stabilize the domains (except for very narrow film strips in which a bias field must be used to stabilize the specimen's initial state of magnetic saturation). The above-mentioned similarity applies as well to the multi-domain case in a film strip, though in this case the magnetostatic stabilizing forces are obviously smaller.

Nuclear Magnetic Resonance Transverse Relaxation Times of Water Protons in Skeletal Muscle

The observation of the spin-echo decay in a long time domain has revealed that there exist at least three different fractions of non- (or slowly) exchanging water in the rat gastrocnemius muscle. These fractions of water are characterized with different nuclear magnetic resonance (NMR) relaxation times and are identified with the different parts of tissue water. The water associated with the macromolecules was found to be approximately 8% of the total tissue water and not to exchange rapidly with the rest of the intracellular water. The transverse relaxation time (T(2)) of the myoplasm is 45 ms which is roughly a 40-fold reduction from that of a dilute electrolyte solution. This fraction of water accounts for 82% of the tissue water. The reduced relaxation time is shown neither to be caused by fast exchange between the hydration and myoplasmic water nor by the diffusion of water across the local magnetic field gradients which arise from the heterogeneity in the sample. About 10% of the tissue water was resolved to be associated with the extracellular space, the relaxation time of which is approximately four times that of the myoplasm. Mathematical treatments of the proposed mechanisms which may be responsible for the reduction of tissue water relaxation times are given in this paper. The results of our study are consistent with the notion that the structure and/or motions of all or part of the cellular water are affected by the macromolecular interface and this causes a change in the NMR relaxation rates.

Pulsed NMR study of water mobility in muscle and brain tissue

Pulsed and wide-line nuclear magnetic resonance (NMR) measuremnts were been carried out on water in rat muscle and brain tissue. No evidence was found, either from nuclear spin relaxation time measurements or from wide-line NMR measurements of 2H in 2H 2O in tissue, for the existence of several non- (or slowly) exchanging fractions of water with differing molecular mobilities. It was found that diffusion of water molecules across local magnetic field gradient, which arisse because of the sample heterogeneity resulta in anomalously short values of T 2 as dertermined from the free induction decay or the two pulse (π/2, π) method. The Carr-Purcell method of T 2 measurement yields values of T 2 which reflect molecular mobility. Diffusion coefficients of water in brain and muscle tissue were measured by the field gradient spin-echo NMR technique. The results are interpreted in terms of the self-diffusion of bulk liquid-like water, with the molecules experiencing physical barriers to diffusion ( e.g. confined to cavities). The average distance between water molecules and the barriers is < 10 μm. It is concluded that the rotational reorientation rate for water in brain is greater than in muscle tissue, while the translational diffusion is more restricted in brain than in muscle tissue.