John Georg Seland

Background gradient suppression in pulsed gradient stimulated echo measurements.

Pulsed gradient spin echo (PGSE) experiments can be used to measure the probability distribution of molecular displacements. In homogeneous samples this reports on the molecular diffusion coefficient, and in heterogeneous samples, such as porous media and biological tissue, such measurements provide information about the samples morphology. In heterogeneous samples however background gradients are also present and prevent an accurate measurement of molecular displacements. The interference of time independent background gradients with the applied magnetic field gradients can be removed through the use of bipolar gradient pulses. However, when the background gradients are spatially non-uniform molecular diffusion introduces a temporal modulation of the background gradients. This defeats simple bipolar gradient suppression of background gradients in diffusion related measurements. Here we introduce a new method that requires the background gradients to be constant over coding intervals only. Since the coding intervals are typically at least an order of magnitude shorter than the storage time, this new method succeeds in suppressing cross-terms for a much wider range of heterogeneous samples.

Exploring the separate NMR responses from crude oil and water in rock cores

In analysis of transverse relaxation time (T2) curves in a Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system originating from measurements of oil and water in rock cores, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information to be extracted of the different components contributing to the totalT2 relaxation time curve. From the study of rock cores saturated with different amounts of crude oil and water, we show that with an optimised experimental setup it is possible to extract information from the nuclear magnetic resonance response that is not resolved by any other methods. This setup combines pulsed field gradient methods with the CPMG experiment utilizing data from both rock cores and bulk oil and water. Then it becomes feasible to separate the signals from oil and water where the two-dimensional inverse Laplace transform ordinarily seems to fail.

Combining PFG and CPMG NMR measurements for separate characterization of oil and water simultaneously present in a heterogeneous system

When analyzingT2 relaxation time curves from an ordinary Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information regarding the mobility of the components and on the type of environment experienced by each component. Compared to a standard CPMG experiment, a combination of pulsed field gradient (PFG) methods with the CPMG experiment will increase the amount of information that is obtainable from the nuclear magnetic resonance (NMR) experiment on a system of components differing significantly in molecular mobility. We propose a method for achieving separate measurements of theT2 attenuation of two components simultaneously present within a sample, and we believe it to be generally valid for any system in which the components differ significantly in molecular mobility. The two components could be oil and water in porous rock, or fat and water in a biological tissue, where a separation of theT2 attenuations for the two components will add insight to the study of the systems. In order to verify the method we made use of a sample containing a mixture of oil and water in two separate bulk phases, and compared the results with PFG-CPMG experiments performed on samples containing oil or water only, respectively. The method was applied to systems containing glass spheres immersed in water and oil, and it was possible to obtain information about the physical environment of the components which otherwise is not easily obtainable. The method presented here is therefore presumably applicable to whole rock cores or tissue samples.

Analyzing equilibrium water exchange between myocardial tissue compartments using dynamical two-dimensional correlation experiments combined with manganese-enhanced relaxography

Water compartments were identified and equilibrium water exchange was studied in excised rat myocardium enriched with intracellular manganese (Mn2+). Standard relaxographic measurements were supplemented with diffusion‐T2 and T1‐T2 correlation measurements. In nonenriched myocardium, one T1 component (800 ms) and three T2 components (32, 120, and 350 ms) were identified. The correlation measurements revealed fast‐ and slow‐diffusing water fractions with mean diffusion coefficients of 1.2 × 10−5 and 3.0 × 10−5 cm2 s−1. The two shortest T2 components, which had different diffusivities, both originated from water in intracellular compartments. A component with longer relaxation time (T1 ≈ 2200 ms; T2 ≈ 1200 ms), originating from extra‐tissue water, was also observed. The presence of this component may lead to erroneous estimations of water exchange rates from multiexponential relaxographic analyses of excised tissues. The tissue T1 value is strongly reduced with increasing enrichment of Mn2+, and eventually a second tissue T1 component emerges, indicating a shift in the equilibrium water exchange between intra‐ and extracellular compartments from the fast‐exchange limit to the slow‐exchange regime. Using a two‐site water exchange analysis, the lifetime of intracellular water, Tic, was found to be 475 ms, with a fraction, pic, of 0.71. Magn Reson Med 58:674–686, 2007.

A comparison of retrospectively self-gated magnetic resonance imaging and high-frequency echocardiography for characterization of left ventricular function in mice

Non-invasive imaging methods like echocardiography and magnetic resonance imaging (MRI) are very valuable in longitudinal follow-up studies of cardiac function in small animals. To be able to compare results from studies using different methods, and explain possible differences, it is important to know the agreement between these methods. As both self-gated high-field MRI and high-frequency echocardiography (hf-echo) M-mode are potential methods for evaluation of left ventricular (LV) function in healthy mice, our aim was to assess the agreement between these two methods. Fifteen healthy female C57BL/6J mice underwent both self-gated MRI and hf-echo during the same session of light isoflurane anaesthesia. LV dimensions were estimated offline, and agreement between the methods and reproducibility for the two methods assessed using Bland–Altman methods. In summary, hf-echo M-mode had better inter-observer repeatability than self-gated MRI for all measured parameters. Compared with hf-echo, systolic posterior wall thicknesses were significantly higher when measured by MRI, while diastolic anterior wall thicknesses were found to be significantly smaller. MRI measurements of diastolic LV diameter were also higher using MRI, resulting in larger fractional shortening values compared with the values obtained by hf-echo. In conclusion, hf-echo M-mode is easy to apply, has high temporal and spatial resolution, and good reproducibility. Self-gated MRI might be advantageous in cases of abnormal LV geometry and heterogeneous regional myocardial function, especially with improvements in spatial resolution. The moderate agreement between the methods must be taken into account when comparing studies using the two modalities.

A multi-dimensional experiment for characterization of pore structure heterogeneity using NMR

In a liquid saturated porous sample the spatial inhomogeneous internal magnetic field in general depends on the strength of the static magnetic field, the differences in magnetic susceptibilities, but also on the geometry of the porous network. To thoroughly investigate how the internal field can be used to determine various properties of the porous structure, we present a novel multi-dimensional NMR experiment that enables us to measure several dynamic correlations in one experiment, and where all of the correlations involve the internal magnetic field and its dependence on the geometry of the porous network. (Correlations: internal gradient - pore size, internal gradient - magnetic susceptibility difference, internal gradient - longitudinal relaxation, longitudinal relaxation - magnetic susceptibility difference.) It is always a spatial average of the internal magnetic field, or one of the related properties, that is measured, which is important to take into consideration when analyzing the obtained results. We demonstrate how these correlations can be an indicator for pore structure heterogeneity, and focus in particular on how the effect from spatial averaging can be evaluated and taken into account in the different cases.

Characterising oil and water in porous media using decay due to diffusion in the internal field

In the method Decay due to Diffusion in the Internal Field (DDIF), the diffusion behaviour of water molecules in the internal magnetic field makes it possible to determine a distribution of pore sizes in a sample. The DDIF experiment can also be extended to a DDIF-Carr-Purcell-Meiboom-Gill (DDIF-CPMG) experiment to measure correlations between the pore size and the transverse relaxation time, T2. In this study we have for the first time applied the DDIF experiment and the DDIF-CPMG experiment to porous materials saturated with both water and oil. Because of the large difference in diffusion rates between water and oil molecules, the DDIF experiment will act as a filter for the signal from oil, and we are left with the DDIF-signal from water only. This has been verified in model systems consisting of glass beads immersed in separate layers of water and oil, and in a sandstone sample saturated with water and oil. The results show that the DDIF and DDIF-CPMG experiments enable the determination of the confining geometry of the water phase, and how this geometry is correlated to T2. Data obtained in the sandstone sample saturated with water and oil also show that with the exception of the smallest pores there is no clear correlation between pore size and the relaxation time of water.

Dynamic water changes in excised rat myocardium assessed by continuous distribution of T1 and T2

Ischemic changes in excised rat myocardium were followed by series of T1 or T2 measurements from 1 to 60 min after isolated perfusion cessation, and the influence of manganese enhancement was investigated. An inverse Laplace transformation (ILT) of T1 or T2 data was used to resolve the number, time constants, and fractions of tissue water components in a continuous distribution. For T1 distributions, one single tissue component ∼900 ms was significantly shortened and dispersed by manganese enhancement (25 and 200 μM MnCl2). For T2 distributions, three tissue components (∼30, ∼100, and ∼350 ms) were obtained initially. The two shortest components merged after ∼10 min to one component (∼40 ms). Both T1 and T2 tissue components became shorter with time. In particular, the T2 distribution dynamics might be compatible with complex sequential changes in tissue water fractions during ischemia. Magn Reson Med 58:442–447, 2007.

Measurements of diffusion in porous polyethylene powder using PFGSTE NMR.

Pulsed field gradient stimulated echo (PFGSTE) nuclear magnetic resonance (NMR) has been applied to study the diffusion of toluene in porous semicrystalline polyethylene powder. A pulse sequence with unequal bipolar gradients that reduces effects from internal gradients was used. The effects of different diffusion regimens and restricted diffusion in this system were indicated by use of a simple two-component analysis of the diffusion data.

Investigating structure-dependent diffusion in hydrogels using spatially resolved NMR spectroscopy

HYPOTHESIS Incorporation of the drug-loaded surfactant micelles into polymer hydrogels is a common method used to achieve controlled drug delivery. The characterization of the diffusion processes in drug delivery systems is critical in order to tune the drug loading and release. EXPERIMENTS We present a simple and efficient NMR protocol to investigate the transport of the surfactant molecules in hydrogels on micro- and macroscale under non-equilibrium conditions. Our experimental protocol is based on a combination of 1H 1D NMR chemical shift imaging and slice-selective diffusion experiments, which enables determination of the mutual and self-diffusion coefficients of the surfactant in the non-equilibrium hydrogel-based system within the same short time frame. FINDINGS Our results show that the self-diffusion coefficient of the positively charged surfactant in the hydrogel (Dsgel) decreases with the increasing surfactant concentration until it reaches a plateau value of 6.6±0.5×10-11m2s-1. The surfactant self-diffusion in the solution (Dssln) remains constant over the experiment with an average value of 6.7±0.3×10-11m2s-1. The surfactant mutual diffusion coefficient obtained from 1D chemical shift imaging in this hydrogel system (Dm) is 7.7±0.5×10-11m2s-1. Correlation of the localized Ds to the 1D chemical shift images gives insight into the structure-dependent diffusional behavior of surfactant molecules in the hydrogel. This NMR protocol will be of great value in studies of concentration dependent structures on the interfaces between two immiscible liquids.