Biomedical subjects

B Blümich

Publications and source records attributed to B Blümich.

At least 19 recordsLinked to original sources

PubMed2006

Multiecho sequence for velocity imaging in inhomogeneous rf fields.

The unambiguous determination of velocities with spatial resolution in a multiecho PFG NMR sequence strongly depends on the homogeneity of the B1 field. This affects, in particular, the use of surface coils that bear considerable potential for on-line flow monitoring where a fast-imaging sequence can become vital. However, even with most rf coils dedicated for imaging applications, B1 inhomogeneities are sufficiently large to generate severe problems in performing velocity-imaging experiments. In this paper, the use of a combination of different phase cycles in Carr-Purcell sequences is discussed. The suggested phase cycling scheme tolerates large flip angle imperfections arising in inhomogeneous B1 fields, and thus allows acquisition of a maximum number of echoes within a pulse train. The performance of the velocity-imaging sequence is proven by using phantom samples developing known laminar flow patterns.

Journal Article↗

PubMed2006

Single-sided sensor for high-resolution NMR spectroscopy.

The unavoidable spatial inhomogeneity of the static magnetic field generated by open sensors has precluded their use for high-resolution NMR spectroscopy. In fact, this application was deemed impossible because these field variations are usually orders of magnitude larger than those created by the microscopic structure of the molecules to be detected. Recently, chemical shift resolved NMR spectra were observed for the first time outside a portable single-sided magnet by implementing a method that exploits inhomogeneities in the rf field designed to reproduce variations of the static magnetic field. In this communication, we describe in detail the magnet system built from permanent magnets as well as the rf coil geometry used to compensate the static field variations.

Journal Article↗

PubMed2006

Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient.

A simple and fast method of measuring self-diffusion coefficients of protonated systems with a mobile single-sided NMR sensor is discussed. The NMR sensor uses a magnet geometry that generates a highly flat sensitive volume where a strong and highly uniform static magnetic field gradient is defined. Self-diffusion coefficients were measured by Hahn- and stimulated echoes detected in the presence of the uniform magnetic field gradient of the static field. To improve the sensitivity of these experiments, a Carr-Purcell-Meiboom-Gill pulse sequence was applied after the main diffusion-encoding period. By adding the echo train the experimental time was strongly shortened, allowing the measurement of complete diffusion curves in less than 1min. This method has been tested by measuring the self-diffusion coefficients D of various organic solvents and poly(dimethylsiloxane) samples with different molar masses. Diffusion coefficients were also measured for n-hexane absorbed at saturation in natural rubber with different cross-link densities. The results show a dependence on the concentration that is in good agreement with the theoretical prediction. Moreover, the stimulated-echo sequence was successfully used to measure the diffusion coefficient as a function of the evolution time in systems with restricted diffusion. This type of experiment proves the pore geometry and gives access to the surface-to-volume ratio. It was applied to measure the diffusion of water in sandstones and sheep Achilles tendon. Thanks to the strong static gradient G(0), all diffusion coefficients could be measured without having to account for relaxation during the pulse sequence.

Achilles Tendon↗

PubMed2005

Internal fluid dynamics in levitated drops by fast magnetic resonance velocimetry.

Fluid motion inside a levitated drop is determined by the interface properties. Momentum transfer through a highly mobile interface results in stationary vortex patterns inside the drop that dramatically enhance mass transfer between both phases, while immobile interfaces suppress internal dynamics. The presence of small amounts of surface-active substances can result in a partial reduction of interface mobility, the so-called rigid cap. The time dependence of internal flow patterns is presented by means of NMR velocity images of levitated drops, and is compared to fast measurements of the velocity distribution along the three orthogonal coordinates.

Journal Article↗

PubMed2005

Molecular dynamic heterogeneity of confined lipid films by 1H magnetization-exchange nuclear magnetic resonance.

The molecular dynamic heterogeneity of monolayer to submonolayer thin lecithin films confined to submicron cylindrical pores were investigated by 1H magnetization exchange nuclear magnetic resonance. In this experiment a z-magnetization gradient was generated by a double-quantum dipolar filter. The magnetization-exchange decay and buildup curves were interpreted with the help of a theoretical model based on the approximation of a one-dimensional spin-diffusion process in a three-domain morphology. The dynamic heterogeneity of the fatty acid chains and the effects of the surface area per molecule, the diameter of the pores, and the temperature were characterized with the help of local spin-diffusion coefficients. The effect of various parameters on the molecular dynamics of the mobile region of the fatty acid chains was quantified by introducing an ad hoc Gaussian distribution function of the 1H residual dipolar couplings. For the lipid films investigated in this study, the surface induced order and the geometrical confinement affect the chain dynamics of the entire molecule. Therefore, each part of the chain independently reflects the effect of surface coverage, pore size, and temperature.

Aluminum Oxide↗

PubMed2005

Self-diffusion anisotropy of water in sheep Achilles tendon.

The principal values of the diffusion tensor of free water in the pores of sheep Achilles tendon were determined. For this purpose, the azimuthally angular dependence of the self-diffusion coefficient was measured using a radiofrequency tilt coil and pulsed-field-gradient stimulated-echo (PFGSE) NMR. Combining the PFGSE with multiple acquisitions of Hahn echoes using the Carr-Purcell-Meiboom-Gill pulse sequence reduced the measuring time. The diffusion measurements revealed two diffusion process characterized by a fast and a slow effective diffusion coefficient. A model which describes the stimulated-echo amplitude, encoded by the water diffusion and magnetization transfer, was used for evaluation of the fast diffusion coefficients. The fast diffusion process characterizes the water molecules in pores surrounding the collagen fibrils. The diffusion coefficients characterizing the fast process show a well-defined anisotropy. The principal values of the diffusion tensors were determined assuming the elongated pores to be oriented parallel to the tendon fibrils and thus the orientation distribution function of the pores followed that of the collagen fibrils. The average aspect ratio of pores was estimated from the principal values of the water diffusion tensor and is suitable to characterize quantitatively the changes in tendon morphology due to healing or aging. The methods in this investigation can also be applied to measurements of the diffusion anisotropy using ex situ NMR sensors.

Achilles Tendon↗

PubMed2005

Multispin moments edited by multiple-quantum NMR: application to elastomers.

The spin system response to the five-pulse sequence used for measurements of double-quantum and triple-quantum buildup curves is evaluated in the initial excitation/reconversion regime. The multispin dipolar network that is present also in many soft solids like elastomers was considered. It is proved rigorously that the relevant quantity for analysis of double-quantum build-up curves in the initial regime is the second van Vleck moment. The higher-order moments edited by double-quantum as well as higher-order coherences in the multiple-quantum build-up experiments are different from van Vleck moments. These results can be applied to compare (1)H residual moments edited by double-quantum and triple-quantum experiments with those measured by other NMR methods. The sensitivity of multiple-quantum coherences to the changes in the values of residual dipolar couplings for cross-linked natural rubber under uniaxial elongation is also discussed. Under such conditions (1)H second van Vleck moments were measured for different elongation ratios of a cross-linked natural rubber. Moreover, (1)H triple-quantum edited moments were also measured for the same sample under uniaxial compression. The dependence of the second van Vleck moment and the time of the maximum of the double-quantum buildup curve on the cross-link density of natural rubber measured at low magnetic field was also investigated.

Journal Article↗

PubMed2005

Velocity imaging by ex situ NMR.

A pulsed field gradient stimulated spin-echo NMR sequence is combined with imaging methods to spatially resolve velocity distributions and to measure 2D velocity maps ex situ. The implementation of these techniques in open sensors provides a powerful non-invasive tool to measure molecular displacement in a large number of applications inaccessible to conventional closed magnets. The method is implemented on an open tomograph that provides 3D spatial localization by combining slice selection in the presence of a uniform static magnetic field gradient along the depth direction with pulsed field gradients along the two lateral directions. Different pipe geometries are used to demonstrate that the sequence performs well even in the extremely inhomogeneous B0 and B1 fields of these sensors.

Journal Article↗

PubMed2005

Profiles with microscopic resolution by single-sided NMR.

A single-sided NMR sensor to produce depth profiles with microscopic spatial resolution is presented. It uses a novel permanent magnet geometry that generates a highly flat sensitive volume parallel to the scanner surface. By repositioning the sensitive slice across the object one-dimensional profiles of the sample structure can be produced with a space resolution better than 5 microm. The open geometry of the sensor results in a powerful testing tool to characterize arbitrarily sized objects in a non-destructive way.

Journal Article↗

PubMed2005

Visualizing flow vortices inside a single levitated drop.

The internal flow dynamics in single liquid drops, kept in place through levitation by a counterflowing continuous fluid phase in a suitably designed glass cell, is investigated by PFG NMR techniques. The positional stability of the drops was confirmed from series of one-dimensional profiles and was found to be below the spatial resolution of the experiment. Velocity distribution functions (propagators) along all three coordinates were obtained and demonstrated the long-time stability of the internal dynamics in terms of the velocity magnitudes occurring in the systems. Finally, velocity imaging was applied to visualize the internal vortex patterns in the drops either as projections onto different planes or within thin slices of selected orientations. Two different fluid systems were investigated in order to cover the principal cases of rigid and mobile interfaces. Different fast velocity imaging techniques were employed for monitoring the vastly differing velocity ranges of both cases, and the high sensitivity of the internal three-dimensional motion to the cell geometry is demonstrated.

Journal Article↗

PubMed2005

[Measuring the contrast-enhancement in the skin and subcutaneous fatty tissue with the NMR-MOUSE: a feasibility study].

PURPOSE: The NMR-MOUSE is an open and mobile sensor for measuring NMR relaxation parameters in organic matters. T1-measurements of the subcutaneous fatty tissue and the skin are reported. MATERIAL AND METHOD: For the first time, the NMR-MOUSE was employed to measure the signal recovery following saturation of the skin and the subcutaneous fatty tissue of three patients, before and after administering a contrast agent. RESULTS: Despite a low signal-to-noise ratio, changes in the relaxation behaviour of the skin could be detected. Malignant tissue exhibits faster signal recovery than scar tissue and healthy tissue, which only show a small difference. CONCLUSIONS: Changes in the relaxation behavior can be monitored with the NMR-MOUSE. Before the clinical use of the NMR-MOUSE, sensitivity, sensor mounting device, and sensor tuning must be improved. Further investigations need to be performed on a statistically relevant number of patients.

Adipose Tissue↗

PubMed2004

Complex morphology of melt-spun nylon-6 fibres investigated by 1H double-quantum-filtered NMR spin-diffusion experiments.

The complex morphology of high-speed melt-spun nylon-6 fibres hydrated with D2O was investigated using 1H double-quantum-filtered spin-diffusion NMR experiments. The magnetisation exchange from selected crystalline domains along the fibrils and interfibrils was simulated with the help of a three-dimensional solution of a spin-diffusion equation approximated by a product of one-dimensional analytical NMR signals, which correspond to a lamellar morphology. This allows to measure the sizes of crystalline and less-mobile amorphous domains along the fibrils, as well as the diameter of the fibrils and interfibril distances. A series of nylon-6 fibres with extreme values of winding speed and draw ratio was investigated. The changes detected in the domain size along the fibrils and interfibrils show the same trend in the data obtained from wide-angle X-ray diffraction and small-angle X-ray scattering.

Caprolactam↗

PubMed2004

Multi-echo imaging in highly inhomogeneous magnetic fields.

A new pulsed field gradient multi-echo imaging technique to encode position in the phase of every echo generated by a CPMG sequence in the presence of a strongly inhomogeneous static magnetic field is presented. It was applied to improve the sensitivity in an imaging experiment by adding the echo train acquired during the CPMG sequence and to spatially resolve relaxation times of inhomogeneous specimens using single-sided probes. The sequence was implemented in a new bar-magnet MOUSE equipped with a gradient coil system to apply a pulsed magnetic field with a constant gradient along one spatial coordinate. An important reduction by a factor larger than two orders of magnitude in the acquisition time was obtained compared to the previously published single-point imaging technique.

Algorithms↗

PubMed2004

3D imaging with a single-sided sensor: an open tomograph.

An open tomograph to image volume regions near the surface of large objects is described. The central achievement in getting such a tomograph to work is the design of a fast two-dimensional pure phase encoding imaging method to produce a cross-sectional image in the presence of highly inhomogeneous fields. The method takes advantage of the multi-echo acquisition in a Carr-Purcell-Meiboom-Gill (CPMG)-like sequence to significantly reduce the experimental time to obtain a 2D image or to spatially resolve relaxation times across the sensitive volume in a single imaging experiment. Depending on T(2) the imaging time can be reduced by a factor of up to two orders of magnitude compared to the one needed by the single-echo imaging technique. The complete echo train decay has been also used to produce T(2) contrast in the images and to spatially resolve the T(2) distribution of an inhomogeneous object, showing that variations of structural properties like the cross-link density of rubber samples can be distinguished by this method. The sequence has been implemented on a single-sided sensor equipped with an optimized magnet geometry and a suitable gradient coil system that provides two perpendicular pulsed gradient fields. The static magnetic field defines flat planes of constant frequency parallel to the surface of the scanner that can be selected by retuning the probe frequency to achieve slice selection into the object. Combining the slice selection obtained under the presence of the static gradient of the open magnet with the two perpendicular pulsed gradient fields, 3D spatial resolution is obtained.

Equipment Design↗

PubMed2004

Time resolved spectroscopic NMR imaging using hyperpolarized 129Xe.

We have visualized the melting and dissolution processes of xenon (Xe) ice into different solvents using the methods of nuclear magnetic resonance (NMR) spectroscopy, imaging, and time resolved spectroscopic imaging by means of hyperpolarized 129Xe. Starting from the initial condition of a hyperpolarized solid Xe layer frozen on top of an ethanol (ethanol/water) ice block we measured the Xe phase transitions as a function of time and temperature. In the pure ethanol sample, pieces of Xe ice first fall through the viscous ethanol to the bottom of the sample tube and then form a thin layer of liquid Xe/ethanol. The xenon atoms are trapped in this liquid layer up to room temperature and keep their magnetization over a time period of 11 min. In the ethanol/water mixture (80 vol%/20%), most of the polarized Xe liquid first stays on top of the ethanol/water ice block and then starts to penetrate into the pores and cracks of the ethanol/water ice block. In the final stage, nearly all the Xe polarization is in the gas phase above the liquid and trapped inside the pores. NMR spectra of homogeneous samples of pure ethanol containing thermally polarized Xe and the spectroscopic images of the melting process show that very high concentrations of hyperpolarized Xe (about half of the density of liquid Xe) can be stored or delivered in pure ethanol.

Complex Mixtures↗

PubMed2004

Enhanced sensitivity to residual dipolar couplings of elastomers by higher-order multiple-quantum NMR.

The homonuclear and heteronuclear residual dipolar couplings in elastomers reflect changes in the cross-link density, temperature, the uniaxial and biaxial extension or compression as well as the presence of penetrant molecules. It is shown theoretically that for an isolated methyl group the relative changes in the intensity of the homonuclear double-quantum buildup curves in the initial time regime due to variation of the residual dipolar coupling strength is less sensitive than the changes in the triple-quantum filtered NMR signal when considering the same excitation/reconversion time. For a quadrupolar nucleus with spin I=2 the sensitivity enhancement was simulated for four-quantum, triple-quantum, and double-quantum buildup curves. In this case the four-quantum build-up curve shows the highest sensitivity to changes of spin couplings. This enhanced sensitivity to the residual dipolar couplings was tested experimentally by measuring 1H double-quantum, triple-quantum, and four-quantum buildup curves of differently cross-linked natural rubber samples. In the initial excitation/reconversion time regime, where the residual dipolar couplings can be measured model free, the relative changes in the intensity of the four-quantum buildup curves are about five times higher than those of the double-quantum coherences. For the first time proton four-quantum coherences were recorded for cross-linked elastomers.

Algorithms↗

PubMed2004

Monitoring degradation in paper: non-invasive analysis by unilateral NMR. Part II.

High quality paper samples have been oxidized with a specific oxidant to reproduce one of the possible causes of the aging of paper. All samples have been characterized by 13C CP-MAS NMR spectroscopy. The artificial aging of paper has been monitored using a standard NMR relaxometer and the results have been compared with the corresponding data obtained using an unilateral NMR relaxometer. Experimental values obtained with both techniques are in agreement, demonstrating that unilateral NMR relaxometric measurements constitute a suitable non-invasive method for assessing the degradation process of cellulose-based materials. The sensitivity of the non-invasive NMR method allows the detection of degradation even at a very early stage. Effects due to the sample volume and to the penetration depth have been investigated.

Journal Article↗

PubMed2004

Velocity distributions remotely measured with a single-sided NMR sensor.

The pulsed field gradient nuclear magnetic resonance (PFG NMR) method has proved to be a powerful non-invasive technique to measure molecular displacement in various systems. It has been largely implemented with conventional NMR magnets where the volume for housing the flow setup is restricted. In this work we present the first approach to measure velocity distributions ex situ implementing a pulsed field gradient sequence on a single-sided NMR sensor. The open geometry of these sensors provides access to NMR measurements of a large number of applications previously excluded by the geometry of conventional closed magnets. Both, the distortions to the displacement encoding observed when implementing a PFG sequence in the presence of strongly inhomogeneous B0 and B1 fields, and the performance of the modifications proposed to eliminate these distortions are shown by means of numerical simulations. An alternating stimulated spin-echo PFG sequence implemented to remotely measure velocity distributions was combined with a multi-echo acquisition scheme to significantly increase the sensitivity of the method. The technique was implemented to measure the velocity propagator in a fluid undergoing laminar flow and good agreement with the theoretical result is observed.

Journal Article↗