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A G Webb

Publications and source records attributed to A G Webb.

At least 37 records · Page 2Linked to original sources
PubMed1998

Limited-sample NMR using solenoidal microcoils, perfluorocarbon plugs, and capillary spinning.

This study demonstrates three improvements to mass-limited NMR using solenoidal microcoils as detectors: (1) sample confinement using liquid perfluorocarbon plugs to increase the observe factor, (2) design and incorporation of a capillary spinner to improve spectral line widths, and (3) facile sample changing via the use of a capillary insert. The probe is constructed to spin a fused silica capillary of 530 microns i.d., 700 microns o.d. inside a solenoidal coil wound around a 0.8 mm i.d., 1 mm o.d. glass capillary. The smaller capillary contains the sample, and capillaries with different samples can be exchanged easily. In high-resolution limited sample microcoil NMR studies published thus far, the length of the sample plug has been 7-10 times the length of the solenoid to avoid line broadening from volume magnetic susceptibility (chi v) mismatches at both ends of the sample. This arrangement is not efficient since it places most of the sample volume outside of the coil observe volume. It is shown here that the observe factor cannot exceed 23% if the sample plug is bracketed by air, without substantial line broadening occurring. However, if the sample is bracketed by two liquid perfluorocarbon plugs, the observe factor can be increased to 70% while maintaining high spectral resolution. This is equivalent to improving the limit of detection by a factor of 3, or reducing the total data acquisition time for a given signal-to-noise by a factor of 9. It is also shown that, for the 440-nL sample plug used in this study (bracketed by the perflurocarbon plugs), sample spinning can improve the spectral resolution from 1.5 (nonspinning) to 0.6 Hz (spinning). This corresponds to a further improvement in the limit of detection of 2.5, or just over a factor of 6 decrease in data acquisition time.

Fluorocarbons↗
PubMed1998

Design of solenoidal microcoils for high-resolution 13C NMR spectroscopy.

Typical high-resolution natural-abundance 13C NMR experiments require milligrams of material. In this study, solenoidal microcoil direct and inverse detection probes have been designed to explore improvements in the limits of detection (LOD) for 13C NMR at 11.7 T. A direct-detection radio frequency probe with an observe volume (Vobs) of 1 microL has been developed with carbon observe, deuterium lock, and proton decouple channels. The nonspinning line width of a decoupled [3-13C]-L-alanine (99%) peak achieved is 1.3 Hz. The natural-abundance 13C3 sigma LOD for sucrose for a 90-min acquisition time is 18 micrograms (52 nmol). The 3 sigma LOD for [3-13C]-L-alanine (99%) is 375 ng (4.2 nmol) for an acquisition time less than 30 s. The inverse detection probe has proton observe and carbon decouple channels and a Vobs of 550 nL. The 3 sigma LOD for sucrose from a 1 D 13C decoupled HMQC spectrum is 1.6 micrograms (4.5 nmol) for a 14-min acquisition time.

Alanine↗
PubMed1998

RF microcoil design for practical NMR of mass-limited samples.

This paper addresses practical issues involved in obtaining high resolution 1H NMR spectra from samples containing less than 10 nmol. Solenoidal microcoils have been constructed to: (a) assess the effects of magnetic susceptibility mismatches at 500 MHz, (b) increase the concentration sensitivity of microcoil probes, (c) incorporate a lock channel for 2D experiments and long 1D acquisitions, and (d) assess the total amount of the sample required (with respect to the coil length) to avoid line broadening due to edge effects. Compared to previously published microcoil results, sample volumes have been increased by a factor of 20 with a concomitant decrease in the required concentration (5-20 mM). Perfluorocarbon susceptibility matching remained effective at 500 MHz, allowing acquisition of high resolution NMR spectra. A lock channel has also been successfully incorporated in microcoil probes. The limits of detection for sucrose with a 10 min acquisition time were found to be 17.8 and 34.1 pmol for the single and double resonance coils, respectively. A sample length of approximately 10 times than that of the coil was required to avoid magnetic susceptibility artifacts.

Artifacts↗
PubMed1997

A multiple echo pulse sequence for diffusion tensor imaging and its application in excised rat spinal cords.

A new imaging sequence for rapid determination of the apparent self-diffusion tensor of water was developed and tested on fixed excised rat spinal cords. To reduce the time required to determine the tensor, the sequence utilized a new single-shot approach with multiple spin echoes. An assumption of cylindrical symmetry in the sample was made, thus requiring the measurement of only four of the six unique elements of the tensor. This assumption was found experimentally to be valid, and the results obtained using the new sequence were found to be quantitatively the same as results obtained using a standard spin-echo sequence.

Animals↗
PubMed1997

Nanoliter volume, high-resolution NMR microspectroscopy using a 60-micron planar microcoil.

Previous studies demonstrated the feasibility of using 100-microns inner diameter planar spiral inductors (microcoils) as detectors in 1H nuclear magnetic resonance (NMR) microspectroscopy. However, high-resolution NMR applications were not possible due to poor spectral resolution and low signal-to-noise ratio (SNR). These limitations in performance have now been largely overcome by using a nonconductive liquid fluorocarbon (FC-43) to minimize the effects of susceptibility mismatch between materials, and by carefully optimizing the microcoil geometry for maximum SNR. In this study, liquid samples were loaded into a fused silica capillary (75-microns inner diameter, 147-microns outer diameter). The capillary was positioned 50 microns above a 3.5-turn microcoil so that approximately 1 nL of the sample was present in the sensitive region of the microcoil. The microcoil was fabricated on a gallium arsenide substrate with an inner diameter of 60 microns, an outer diameter of 200 microns, trace width of 10 microns, trace spacing of 10 microns, and trace height of 3 microns. At 5.9 T (250 MHz) in 1H-NMR microspectroscopy experiments using a spectral width of 1 kHz, 4096 sampled data points, and a recovery delay of 1 s, a SNR of 25 (per acquisition) and a spectral linewidth of less than 2 Hz were obtained from a sample of water. These results demonstrate that planar microcoils can be used for high-resolution NMR microspectroscopy. Such coils may also be suitable for localized NMR studies at the cellular level and as detectors in capillary electrophoresis or microbore liquid chromatography.

Electric Conductivity↗
PubMed1995

Monitoring pH of otitis media effusion in chinchillas using fluorescence spectroscopy.

We have developed a fiber optic fluorometer to measure fluorescent signal intensities across an epithelium barrier. As a medically relevant example, we have measured the pH of the effusion formed during Hemophilus influenzae induced otitis media infection in the chinchilla, the classical animal model for human middle ear disease. Because the choice of antibiotic used in clinical therapy is dependent on the pH of the effusion, a noninvasive method of measuring pH is highly desirable. Using the fluorescent pH probe carboxy-seminapthorhodafluor, we were able to detect pH changes of 0.15 units in the pH range around 7.0. The development and resolution of the otitis media was followed with magnetic resonance imaging to confirm the presence of the effusion formed during the infection.

Animals↗
PubMed1995

Synthesis, antimicrobial activity and in vivo fluorine NMR of a hexafluorinated derivative of tilmicosin.

A new fluorinated analog of tilmicosin was synthesized by the reductive amination of desmycosin with 3,5-bis(trifluoromethyl)piperidine. Despite an apparently small change in structure, the fluorinated analog had much less in vitro antimicrobial activity than tilmicosin and it failed to protect 3-day old chicks against a Pasteurella multocida challenge at 64 mg/kg sc. In a preliminary in vivo fluorine NMR experiment in a female Sprague-Dawley rat, a 19F NMR signal was detected in the liver one hour after ip administration of the fluorinated compound. Therefore, although this fluorinated derivative had less antimicrobial activity than tilmicosin, it may nevertheless provide a suitable model of tilmicosin for pharmacokinetic studies using in vivo fluorine NMR.

Animals↗
PubMed1992

Specific MR imaging of human lymphocytes by monoclonal antibody-guided dextran-magnetite particles.

Human lymphocytes were labeled with biotinylated anti-lymphocyte-directed monoclonal antibodies, to which streptavidin and subsequently biotinylated dextran-magnetite particles were coupled. This labeling resulted in a strong and selective negative contrast enhancement of lymphocyte suspensions at 2.0 T, caused predominantly by the specific increase of R2 with a small but significant specific increase of R1. The R1 was found to decrease with increasing field strength. The immunolabeling procedure described here may be used for the selective signal depletion of target cells in MR imaging.

Antibodies, Monoclonal↗
PubMed1991

A general formalism for the manipulation of multiple-echo data sets, and application to chemical shift editing.

A general data processing formalism is presented which can be applied to the signal intensities of successive echoes of a multiple-echo data set in order to null those species for which the signal decay function is exponential. If asymmetric echo times are introduced into the multiple-echo sequence then chemical shift images akin to those obtained from the magnitude Dixon method can be obtained in half the data acquisition time.

Animals↗
PubMed1991

Sucrose polyester: a new oral contrast agent for MRI.

Sucrose polyester (a dietary fat substitute) opacifies the entire gastrointestinal tract of the rat with oral administration in magnetic resonance imaging (MRI). These results suggest the potential use of this material as an oral contrast agent for human abdominal MRI studies.

Animals↗
PubMed1991

Sensitivity enhancement and reduction of Gibbs artifact in T2-weighted imaging using variable tip angle excitation.

Introduction of a Gaussian weighting function into the phase encoding of the spin-echo imaging method via variable tip angle excitation, with concomitant optimization of the recycle delay, is shown to increase the sensitivity of T2-weighted imaging by up to 40%. In addition, the Gibbs artifact associated with sharp boundaries is reduced considerably without the need for postprocessing of the data.

Artifacts↗
PubMed1986

Airport emergency medical services.

To obtain improved emergency medical services at the Minneapolis/St. Paul International Airport, a combination of utilization of existing fire, police, and medical services and improved technology was adopted. Exemplified by the use of EMT and an automatic cardiac defibrillator/pacer (Heart Aide), this system resulted in increased survival for victims of cardiac arrest at the Twin Cities airport. Furthermore, this system allowed for improved triage efficiency and breadth of services that complemented the already existing aircraft crash disaster program.

Aerospace Medicine↗