Dual k-space and image-space post-processing for field-cycling MRI under low magnetic field stability and homogeneity conditions.

We present a method to correct artifacts typically present in images acquired in field-cycled MRI experiments under poor magnetic field spatial-homogeneity and time-stability conditions. The proposed method was tested in both simulated and experimental data. The experiments were performed using a fast field-cycling MRI relaxometer of own design, based on a current-driven variable-geometry electromagnet. Current instability-induced artifacts in the images were mitigated through a phase correction array resulted from entropy and background minimization. Image distortions due to magnetic field inhomogeneity were compensated through two different approaches, involving a previous determination of the magnetic field homogeneity-map, or an experimental protocol where two images are acquired with inverted readout gradient polarity. Results show that images acquired at extreme conditions can be successfully improved, thus strengthening the possibilities for both low-cost MRI devices and faster field-switched MRI systems.

NMR relaxometry analysis of molecular degradation in internal combustion engine lubricants.

A set of experimental techniques headed by proton fast field-cycling nuclear magnetic resonance (1 HFFC-NMR) were used to analyze the effects of degradation of lubricant oil used in an internal combustion engine (ICE). Its relaxometric, spectroscopic, and rheological properties were evaluated and interpreted in terms of changes in the chemical structure and the involved molecular dynamics. In order to better understand the relaxometric behavior, chemical changes induced by heat were investigated for selected n-alkanes, as model-systems due to their structural simplicity. Fourier transform infrared (FTIR) spectroscopy, viscosity measurements, and foaming were used to contrast NMR relaxometry experiments. Main observed changes associated with oil degradation can be attributed to molecular oxidation, fragmentation, and ramification. As an outstanding feature of this work, we show that the relaxometric analysis can be done without any special treatment of the sample, allowing results in less than 10 min.

A fast field-cycling MRI relaxometer for physical contrasts design and pre-clinical studies in small animals.

We present a fast field-cycling NMR relaxometer with added magnetic resonance imaging capabilities. The instrument operates at a maximum proton Larmor frequency of 5 MHz for a sample volume of 35 mL. The magnetic field homogeneity across the sample is 1400 ppm. The main field is generated with a notch-coil electromagnet of own design, fed with a current whose stability is 220 ppm. We show that images of reasonable quality can still be produced under such conditions. The machine is being designed for concept testing of the involved instrumentation and specific contrast agents aimed for field-cycling magnetic resonance imaging applications. The general performance of the prototype was tested through localized relaxometry experiments, T1-dispersion weighted images, temperature maps and T1-weighted images at different magnetic field intensities. We introduce the concept of positive and negative contrast depending on the use of pre-polarized or non-polarized sequences. The system is being improved for pre-clinical studies in small animals.

Dynamical regimes of lipids in additivated liposomes with enhanced elasticity: A field-cycling NMR relaxometry approach.

We study the molecular dynamics of lipids in binary large unilamellar liposomes suspended in D2O composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or soy phosphatidylcholine (SPC) additivated with different percentiles of sodium deoxycholate (SDC). We use the fast field-cycling proton NMR relaxometry technique over a wide timescale and at diverse temperatures. A model previously validated in different formulations is here employed for the relaxometric analysis of elastic vesicles. A new dynamical regime is observed for the first time in additivated DMPC and additivated/non-additivated SPC liposomes. This surprising feature is discussed in terms of vesicle shape fluctuations, enhanced elasticity and lipid & additive diffusion within the membrane. The continuum elastic theory is revisited for a better understanding of recent experiments and those here presented. We address the point of deformability measurements across rigid permeable barriers versus measurements of the bending elastic modulus in free-standing vesicles.

Longitudinal gradient coils with enhanced radial uniformity in restricted diameter: Single-current and multiple-current approaches.

An important requirement for a gradient coil is that the uniformity of the generated magnetic field gradient should be maximal within the active volume of the coil. For a cylindrical geometry, the radial uniformity of the gradient turns critic, particularly in cases where the gradient-unit has to be designed to fit into the inner bore of a compact magnet of reduced dimensions, like those typically used in fast-field-cycling NMR. In this paper we present two practical solutions aimed to fulfill this requirement. We propose a matrix-inversion optimization algorithm based on the Biot-Savart law, that using a proper cost function, allows maximizing the uniformity of the gradient and power efficiency. The used methodology and the simulation code were validated in a single-current design, by comparing the computer simulated field map with the experimental data measured in a real prototype. After comparing the obtained results with the target field approach, a multiple-element coil driven by independent current sources is discussed, and a real prototype evaluated. Opposed equispaced independent windings are connected in pairs conforming an arrangement of independent anti-Helmholtz units. This last coil seizes 80% of its radial dimension with a gradient uniformity better than 5%. The design also provides an adaptable region of uniformity along with adjustable coil efficiency.

Measurement of the bending elastic modulus in unilamellar vesicles membranes by fast field cycling NMR relaxometry.

The elastic properties of lipid membranes can be conveniently characterized through the bending elastic modulus κ. Elasticity directly affects the deformability of a membrane, morphological and shape transitions, fusion, lipid-protein interactions, etc. It is also a critical property for the formulation of ultradeformable liposomes, and of interest for the design of theranostic liposomes for efficient drug delivery systems and/or different imaging contrast agents. Measurements of κ in liposome membranes have been made using the fast field cycling nuclear magnetic relaxometry technique. We analyze the capability of the technique to provide a consistent value of the measured quantity under certain limiting conditions. Relaxation dispersions were measured acquiring a minimal quantity of points, within a reduced Larmor frequency range and, under inferior experimental conditions (in the presence of magnetic field in-homogeneity and lower power supply stability). A simplified model is discussed, showing practical advantages when fitting the data within the reduced frequency range. Experiments are contrasted with standard measurements performed in a state-of-the-art relaxometer. The methodology was tested in samples of 1,2-dimyristoyl-sn-glycero-3-phosphocholine with different percentiles of cholesterol. We observe a tendency to a decrease in κ with increasing temperature, and a tendency to increase with the cholesterol percentile.

The effect of cholesterol on membrane dynamics on different timescales in lipid bilayers from fast field-cycling NMR relaxometry studies of unilamellar vesicles.

The general applicability of fast field-cycling nuclear magnetic resonance relaxometry in the study of dynamics in lipid bilayers is demonstrated through analysis of binary unilamellar liposomes composed of 1,2-dioleoyl-sn-glycero-3-posphocholine (DOPC) and cholesterol. We extend an evidence-based method to simulating the NMR relaxation response, previously validated for single-component membranes, to evaluate the effect of the sterol molecule on local ordering and dynamics over multiple timescales. The relaxometric results are found to be most consistent with the partitioning of the lipid molecules into affected and unaffected portions, rather than a single averaged phase. Our analysis suggests that up to 25 mol%, each cholesterol molecule orders three DOPC molecules, providing experimental backup to the findings of many molecular dynamics studies. A methodology is established for studying dynamics on multiple timescales in unilamellar membranes of more complex compositions.

Regular structures in 5CB liquid crystals under the joint action of ac and dc voltages.

A nematic liquid crystal with high, positive dielectric anisotropy (5CB) has been studied under the influence of the combined action of a dc and an ac electric field. Broad frequency, voltage, and cell thickness ranges were considered. Pattern morphologies were identified; the thresholds and critical wave numbers were measured and analyzed as a function of frequency, dc-to-ac voltage ratio, and thickness. The current-voltage characteristics were simultaneously detected.

Temperature and size-dependence of membrane molecular dynamics in unilamellar vesicles by fast field-cycling NMR relaxometry.

New methods to study dynamics in lipid bilayers are of interest particularly where they may bridge the gap between conventional experimental techniques and molecular dynamics simulations. Fast field cycling nuclear magnetic resonance relaxometry can provide valuable information as it is sensitive to dynamic processes that occur over a broad time scale. By analysis of data recorded for large unilamellar liposomes composed of 1,2-dimyristoyl-sn-glycero-3-posphocholine (DMPC) or 1,2-dioleoyl-sn-glycero-3-posphocholine (DOPC) at different temperatures and sizes, we validate an evidence-based approach to studying dynamics by relaxometry. Specifically, the number and form of the spectral density contributions from a range of dynamic processes are determined. This success of the approach strongly suggests its general applicability for the study of dynamics in membranes of more complex composition and for parameterizing molecular dynamics simulations.

Interpretation of molecular dynamics on different time scales in unilamellar vesicles using field-cycling NMR relaxometry.

Fast field-cycling (FFC) and rotating-frame nuclear magnetic resonance relaxometry were used to study molecular and collective dynamics in unilamellar liposome systems. Relaxation data for liposomes of diameter about 100 nm composed of 1,2-dimyristoyl-sn-glycero-3-posphocholine (DMPC) or 1,2-dioleoyl-sn-glycero-3-posphocholine (DOPC) were obtained. The Larmor frequency dependence of the spin-lattice relaxation rates was interpreted in terms of clearly defined relaxation mechanisms associated with the underlying molecular dynamics. The physical parameters obtained from the analysis are consistent with values available in the literature obtained from a range of experimental techniques. This work establishes the potential of our approach to study dynamics in liposomal samples of more complex lipid composition.

Fixed lock-time relaxation dispersion in the rotating frame.

The spin-lattice relaxation dispersion may be probed in the laboratory frame through field-cycling NMR relaxometry. The experiment, as usually done, has the basic weakness that the low frequency end of the measured dispersion can be blurred by the presence of local fields. An understanding of the nature of such local fields was found to be essential to the interpretation of the dispersion profile. In this work, an attempt was made to determine the extent to which specific information can be obtained from a rotating frame experiment. The technique consists in the study of the NMR signal dispersion at a fixed spin-lock time, as a function of the radio frequency field intensity. Within this scheme, a strong dispersion can be attributed to the presence of a non-zero magnetic field component along the laboratory-frame Zeeman-axis in the rotating-frame. At on-resonance condition, this component is exclusively due to the presence of local fields as projected on that axis.