High-speed NMR Research Articles

High-speed NMR imaging of capillary action in thin nontransparent porous media.

An improved high-speed NMR profiling method is introduced that enables us to measure capillary action in thin, nontransparent porous media. Liquid profiles can be measured as fast as 10 ms with a spatial resolution of 14.5μm. Capillary absorption of microliter-sized droplets into nontransparent nylon-6,6 porous membranes was studied. In nylon-6,6 sharp fronts obeying a one-dimensional Darcy model were observed. In paper the uptake did not follow this model: retardation in uptake, broader fluid fronts, and swelling was observed. These measurements nicely demonstrate the possibilities and versatility of the developed method.

High-speed NMR spectroscopy

High-speed NMR spectroscopy

Combined High-Speed NMR Imaging of Perfusion and Microscopic Coronary Conductance Vessels in the Isolated Rat Heart

Noninvasive characterization of microcirculation at the level of both coronary conductance and resistance vessels is of major importance for the understanding of microvascular adaptive processes in the heart. The objective of this study was to determine simultaneously myocardial perfusion and microvessel diameters in the myocardium by magnetic resonance (MR) imaging within the same heart. A MR imaging method is presented which combines high-resolution perfusion measurement (140 × 140 μm2) by spin labeling with flow-weighted MR microscopy of coronary microvessels (φ > 140 μm). We determined changes in myocardial perfusion and vessel diameters of isolated beating rat hearts (n = 10) at rest and during administration of nitroglycerin (0.5 mg/min). Alterations in perfusion were validated by microsphere measurements. Under the influence of nitroglycerin an increase in perfusion (+2.51 ± 0.4 ml · min−1 · g−1, mean ± SEM) and vessel diameters (+14.22 ± 1.92%) could be observed. Endocardial perfusion revealed a modest enhanced susceptibility to nitroglycerin in comparison to epicardial perfusion. Analysis of vessels according to their diameters showed no significant differences. MR imaging allows the noninvasive and simultaneous determination of conducting arteries and smaller resistance vessels in one and the same beating rat heart. Due to an excellent spatial resolution of these methods, transmural characterization of both parameters at rest and during vasodilation is feasible.

Development of a High-speed NMR Technique for Sensing Maturity of Avocados

This study demonstrates the feasibility of using an NMR technique for high-speed on-line sensing of maturity of avocados. Results of our previous studies indicate that it is possible to use high-speed, single-pulse NMR techniques to evaluate quality of fruits and vegetables. The single-pulse free induction decay (FID) spectrum measurement technique was used successfully to evaluate maturity of avocados and sugar content of fresh prunes, and was found to have desired features for high-speed sensing of fruit quality. In this study we successfully used a specially designed conveyor belt to acquire FID spectra of avocados while they were moving at speeds up to 250 mm/s. The oil/water resonance peak ratio, obtained from the spectrum, correlates very well (r2 = 0.98) with the dry weight of the fruit.

4961054 Gradient current speed-up circuit for high-speed NMR imaging system

4961054 Gradient current speed-up circuit for high-speed NMR imaging system

Multi-mode resonant gradient coil circuit for ultra high speed NMR imaging

A multi-mode resonant gradient circuit is described for use in ultra high speed imaging techniques like echo-planar and echo-volumar imaging. The circuit behaves overall in a series resonant manner, but at a fixed number of discrete frequencies. By choosing the number of resonant modes, the circuit can be used to generate approximations to a square wave or a trapezoidal waveform. It it shown that trapezoidal waveforms of a given rise time may be described by a more convergent series expansion involving fewer harmonic components. Because the circuit exploits energy conserving principles, much faster current rise times can be achieved with a given driver amplifier and gradient coil than can be achieved by direct coil drive. Expressions are given for the choice of passive circuit components required to accommodate all odd harmonics up to seventh order.

4830012 High speed NMR imaging method and apparatus

4830012 High speed NMR imaging method and apparatus

4830012 High speed nmr imaging method and apparatus : Stephen J Riederer assigned to Duke University

Deriving individual tree information from discrete return, small footprint LiDAR data may improve forest aboveground biomass estimates, and provide tree-level information that is important in many ecological studies. Several crown delineation algorithms have been developed to extract individual tree information from LiDAR point clouds or rasterized canopy height models (CHM), but many of these algorithms have difficulty discriminating between overlapping crowns, and also may fail to detect understory trees. Our approach uses a watershed-based delineation of a CHM, which is subsequently refined using the LiDAR point cloud. Individual tree detection was validated with stem mapped field data from the Smithsonian Environmental Research Center (SERC), Maryland, and on a plot and stand level through comparisons of stem density and basal area to delineated metrics at both SERC and a study area in the Sierra Nevada, California. For individual tree detection, the algorithm correctly identified 70% of dominant trees, 58% of codominant trees, 35% of intermediate trees and 21% of suppressed trees at SERC. The algorithm had difficulty distinguishing between crowns of small, dense understory trees of approximately the same height. Delineated crown volume alone explained 53% and 84% of the variability in basal area at the SERC and Sierra Nevada sites, respectively. The algorithm produced crown area distributions comparable to diameter at breast height (DBH) size class distributions observed in the field in both study sites. The algorithm detected understory crowns better in the conifer-dominated Sierra Nevada site than in the closed-canopy deciduous site in Maryland. The ability for the algorithm to reproduce both accurate tree size distributions and individual crown geometries in two dissimilar and complex forests suggests great promise for applicability to a wide range of forest systems.

4830012 High speed NMR imaging method and apparatus : Stephen J Riederer assigned to Duke University

4830012 High speed NMR imaging method and apparatus : Stephen J Riederer assigned to Duke University

A method of measuring field-gradient modulation shapes. Application to high-speed NMR spectroscopic imaging

A method is proposed for measuring waveforms of a time-varying field gradient used for high-speed NMR imaging. It utilises a self-encoding gradient which calibrates the gradient modulation to be measured. Results are obtained in the form of a time integral of the gradient waveform, which can be used directly for Tropper's image reconstruction. Spectroscopic one-dimensional imaging experiments have been performed using rectangular and cosine gradient modulationsl. It has been demonstrated that, in both cases, significant improvement can be achieved by making use of actually measured gradient modulations for the reconstruction. It has also been shown that in addition to Tropper's reconstruction, appropriate deconvolution is necessary to obtain images with less distortion in the case of cosine modulation.

A new phase coding method using a slice selection gradient for high speed flow velocity measurement in NMR tomography

A new phase coding method using a selection gradient for high speed NMR flow velocity measurement is introduced and discussed. To establish a phase-velocity relationship of flow under the slice selection gradient and spin-echo RF pulse, the Bloch equation was numerically solved under the assumption that only one directional flow exists, i.e. in the direction of slice selection. Details of the numerical solution of the Bloch equation and techniques related to the numerical computations are also given. Finally, using the numerical calculation, high speed flow velocity measurement was attempted and found to be in good agreement with other complementary controlled measurements.

Active magnetic screening of coils for static and time-dependent magnetic field generation in NMR imaging

A new method is introduced for screening or reducing extraneous magnetic fields outside coil structures. The method utilises a set of current-carrying conductors or a discrete wire array to simulate the induced surface currents which occur in superconducting magnetic screens or which occur in high conductivity thick metal screens when placed around coils producing time-dependent fields or field gradients. The active screens have the advantage that they work at any frequency including DC. Field gradient coil structures utilising the principle can be compact and of low inductance. Strong external fields can be made arbitrarily low thus making it feasible to generate large rapidly switched gradients within and in close proximity to a superconductive magnet. Such gradients can be especially useful in the implementation of high speed NMR imaging methods.