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Alexander Pines

Publications and source records attributed to Alexander Pines.

At least 37 records · Page 2Linked to original sources
PubMed2005

SQUID-detected MRI at 132 microT with T1-weighted contrast established at 10 microT--300 mT.

T(1)-weighted contrast MRI with prepolarization was detected with a superconducting quantum interference device (SQUID). A spin evolution period in a variable field between prepolarization and detection enabled the measurement of T(1) in fields between 1.7 microT and 300 mT; T(1) dispersion curves of agarose gel samples over five decades in frequency were obtained. SQUID detection at 5.6 kHz drastically reduces the field homogeneity requirements compared to conventional field-cycling methods using Faraday coil detection. This allows T(1) dispersion measurements to be easily combined with MRI, so that T(1) in a wide range of fields can be used for tissue contrast. Images of gel phantoms with T(1)-weighted contrast at four different fields between 10 microT and 300 mT demonstrated dramatic contrast enhancement in low fields. A modified inversion recovery technique further enhanced the contrast by selectively suppressing the signal contribution for a specific value of the low-field T(1).

Humans↗
PubMed2005

High-resolution nuclear magnetic resonance spectroscopy of biological tissues using projected magic angle spinning.

High-resolution NMR spectra of materials subject to anisotropic broadening are usually obtained by rotating the sample about the magic angle, which is 54.7 degrees to the static magnetic field. In projected magic angle spinning (p-MAS), the sample is spun about two angles, neither of which is the magic angle. This provides a method of obtaining isotropic spectra while spinning at shallow angles. The p-MAS experiment may be used in situations where spinning the sample at the magic angle is not possible due to geometric or other constraints, allowing the choice of spinning angle to be determined by factors such as the shape of the sample, rather than by the spin physics. The application of this technique to bovine tissue samples is demonstrated as a proof of principle for future biological or medical applications.

Animals↗
PubMed2005

NMR spectroscopy in inhomogeneous B0 and B1 fields with non-linear correlation.

Resolved NMR spectra from samples in inhomogeneous B0 and B1 fields can be obtained with the so-called "ex situ" methodology, employing a train of composite or adiabatic z-rotation RF pulses to periodically refocus the inhomogeneous broadening during the detection of the time-domain signal. Earlier schemes relied on a linear correlation between the inhomogeneous B0 and B1 fields. Here the pulse length, bandwidth, and amplitude of the adiabatic pulses of the hyperbolic secant type are adjusted to improve the refocusing for a setup with non-linear correlation. The field correlation is measured using a two-dimensional nutation experiment augmented with a third dimension with varying RF carrier frequency accounting for off-resonance effects. The pulse optimization is performed with a computer algorithm using the experimentally determined field correlation and a standard adiabatic z-rotation pulse as a starting point for the iterative optimization procedure. The shape of the z-rotation RF pulse is manipulated to provide refocusing for the conditions given by the sample-, magnet-, and RF-coil geometry.

2-Propanol↗
PubMed2005

NMR in rotating magnetic fields: magic-angle field spinning.

Magic-angle sample spinning is one of the cornerstones in high-resolution NMR of solid and semisolid materials. The technique enhances spectral resolution by averaging away rank 2 anisotropic spin interactions, thereby producing isotropic-like spectra with resolved chemical shifts and scalar couplings. In principle, it should be possible to induce similar effects in a static sample if the direction of the magnetic field is varied (e.g., magic-angle rotation of the B0 field). Here we will review some recent experimental results that show progress toward this goal. Also, we will explore some alternative approaches that may enable the recovery of spectral resolution in cases where the field is rotating off the magic angle. Such a possibility could help mitigate the technical problems that render difficult the practical implementation of this method at moderately strong magnetic fields.

Anisotropy↗
PubMed2004

"Shim pulses" for NMR spectroscopy and imaging.

A way to use adiabatic radiofrequency pulses and modulated magnetic-field gradient pulses, together constituting a "shim pulse," for NMR spectroscopy and imaging is demonstrated. These pulses capitalize on phase shifts derived from probe gradient coils to compensate for nonlinear intrinsic main magnetic field homogeneity for spectroscopy, as well as for deviations from linear gradients for imaging. This approach opens up the possibility of exploiting cheaper, less-than-perfect magnets and gradient coils for NMR applications.

Journal Article↗
PubMed2004

Development of a functionalized xenon biosensor.

NMR-based biosensors that utilize laser-polarized xenon offer potential advantages beyond current sensing technologies. These advantages include the capacity to simultaneously detect multiple analytes, the applicability to in vivo spectroscopy and imaging, and the possibility of "remote" amplified detection. Here, we present a detailed NMR characterization of the binding of a biotin-derivatized caged-xenon sensor to avidin. Binding of "functionalized" xenon to avidin leads to a change in the chemical shift of the encapsulated xenon in addition to a broadening of the resonance, both of which serve as NMR markers of ligand-target interaction. A control experiment in which the biotin-binding site of avidin was blocked with native biotin showed no such spectral changes, confirming that only specific binding, rather than nonspecific contact, between avidin and functionalized xenon leads to the effects on the xenon NMR spectrum. The exchange rate of xenon (between solution and cage) and the xenon spin-lattice relaxation rate were not changed significantly upon binding. We describe two methods for enhancing the signal from functionalized xenon by exploiting the laser-polarized xenon magnetization reservoir. We also show that the xenon chemical shifts are distinct for xenon encapsulated in different diastereomeric cage molecules. This demonstrates the potential for tuning the encapsulated xenon chemical shift, which is a key requirement for being able to multiplex the biosensor.

Avidin↗
PubMed2004

Three-dimensional phase-encoded chemical shift MRI in the presence of inhomogeneous fields.

A pulse sequence consisting of an excitation pulse and two adiabatic full-passage pulses with scaled relative peak amplitudes is combined with phase encoding to recover chemical shift information within 3D images in a 1D inhomogeneous static magnetic field with a matched rf field gradient. The results are discussed in the context of ex situ magnetic resonance and imaging. The future directions of our research in implementing the ex situ technique in a real one-sided system are also discussed.

Journal Article↗
PubMed2004

Microtesla MRI with a superconducting quantum interference device.

MRI scanners enable fast, noninvasive, and high-resolution imaging of organs and soft tissue. The images are reconstructed from NMR signals generated by nuclear spins that precess in a static magnetic field B0 in the presence of magnetic field gradients. Most clinical MRI scanners operate at a magnetic field B0 = 1.5 T, corresponding to a proton resonance frequency of 64 MHz. Because these systems rely on large superconducting magnets, they are costly and demanding of infrastructure. On the other hand, low-field imagers have the potential to be less expensive, less confining, and more mobile. The major obstacle is the intrinsically low sensitivity of the low-field NMR experiment. Here, we show that prepolarization of the nuclear spins and detection with a superconducting quantum interference device (SQUID) yield a signal that is independent of B0, allowing acquisition of high-resolution MRIs in microtesla fields. Reduction of the strength of the measurement field eliminates inhomogeneous broadening of the NMR lines, resulting in enhanced signal-to-noise ratio and spatial resolution for a fixed strength of the magnetic field gradients used to encode the image. We present high-resolution images of phantoms and other samples and T1-weighted contrast images acquired in highly inhomogeneous magnetic fields of 132 microT; here, T1 is the spin-lattice relaxation time. These techniques could readily be adapted to existing multichannel SQUID systems used for magnetic source imaging of brain signals. Further potential applications include low-cost systems for tumor screening and imaging peripheral regions of the body.

Capsicum↗
PubMed2004

Isotropic-liquid crystalline phase diagram of a CdSe nanorod solution.

We report the isotropic-liquid crystalline phase diagram of 3.0 nm x 60 nm CdSe nanorods dispersed in anhydrous cyclohexane. The coexistence concentrations of both phases are found to be lower and the biphasic region wider than the results predicted by the hard rod model, indicating that the attractive interaction between the nanorods may be important in the formation of the liquid crystalline phase in this system.

Journal Article↗
PubMed2004

Rational reduction of periodic propagators for off-period observations.

Many common solid-state nuclear magnetic resonance problems take advantage of the periodicity of the underlying Hamiltonian to simplify the computation of an observation. Most of the time-domain methods used, however, require the time step between observations to be some integer or reciprocal-integer multiple of the period, thereby restricting the observation bandwidth. Calculations of off-period observations are usually reduced to brute force direct methods resulting in many demanding matrix multiplications. For large spin systems, the matrix multiplication becomes the limiting step. A simple method that can dramatically reduce the number of matrix multiplications required to calculate the time evolution when the observation time step is some rational fraction of the period of the Hamiltonian is presented. The algorithm implements two different optimization routines. One uses pattern matching and additional memory storage, while the other recursively generates the propagators via time shifting. The net result is a significant speed improvement for some types of time-domain calculations.

Algorithms↗
PubMed2004

Remotely detected high-field MRI of porous samples.

Remote detection of NMR is a novel technique in which an NMR-active sensor surveys an environment of interest and retains memory of that environment to be recovered at a later time in a different location. The NMR or MRI information about the sensor nucleus is encoded and stored as spin polarization at the first location and subsequently moved to a different physical location for optimized detection. A dedicated probe incorporating two separate radio frequency (RF)-circuits was built for this purpose. The encoding solenoid coil was large enough to fit around the bulky sample matrix, while the smaller detection solenoid coil had not only a higher quality factor, but also an enhanced filling factor since the coil volume comprised purely the sensor nuclei. We obtained two-dimensional (2D) void space images of two model porous samples with resolution less than 1.4 mm2. The remotely reconstructed images demonstrate the ability to determine fine structure with image quality superior to their directly detected counterparts and show the great potential of NMR remote detection for imaging applications that suffer from low sensitivity due to low concentrations and filling factor.

Environmental Monitoring↗
PubMed2004

Self-diffusion measurements with chemical shift resolution in inhomogeneous magnetic fields.

A methodology for chemical shift resolved molecular self-diffusion measurements in time-independent static and radiofrequency field gradients is demonstrated. Diffusion encoding is provided by a stimulated echo sequence with additional z-storage that allows for a change of diffusion time without affecting the relaxation weighting. The signal is acquired stroboscopically between the pulses of a train of adiabatic double passages that induces a z-rotation counteracting the phase spread resulting from precession in the inhomogeneous static field, as demonstrated in recent approaches to the goal of high-resolution "ex situ" NMR. Simulations of the pulse sequence show that the acquired signal results from the desired coherence pathway. Successful demonstrations of the experiment were performed on a mixture of water and isopropanol.

Journal Article↗
PubMed2004

High-resolution NMR of static samples by rotation of the magnetic field.

Mechanical rotation of a sample at 54.7 degrees with respect to the static magnetic field, so-called magic-angle spinning (MAS), is currently a routine procedure in nuclear magnetic resonance (NMR). The technique enhances the spectral resolution by averaging away anisotropic spin interactions thereby producing isotropic-like spectra with resolved chemical shifts and scalar couplings. It should be possible to induce similar effects in a static sample if the direction of the magnetic field is varied, e.g., magic-angle rotation of the B0 field (B0-MAS). Here, this principle is experimentally demonstrated in a static sample of solid hyperpolarized xenon at approximately 3.4 mT. By extension to moderately high fields, it is possible to foresee interesting applications in situations where physical manipulation of the sample is inconvenient or impossible. Such situations are expected to arise in many cases from materials to biomedicine and are particularly relevant to the novel approach of ex situ NMR spectroscopy and imaging.

Electromagnetic Fields↗
PubMed2003

Amplification of xenon NMR and MRI by remote detection.

A technique is proposed in which an NMR spectrum or MRI is encoded and stored as spin polarization and is then moved to a different physical location to be detected. Remote detection allows the separate optimization of the encoding and detection steps, permitting the independent choice of experimental conditions and excitation and detection methodologies. In the initial experimental demonstration of this technique, we show that taking dilute 129Xe from a porous sample placed inside a large encoding coil and concentrating it into a smaller detection coil can amplify NMR signal. In general, the study of NMR active molecules at low concentration that have low physical filling factor is facilitated by remote detection. In the second experimental demonstration, MRI information encoded in a very low-field magnet (4-7 mT) is transferred to a high-field magnet (4.2 T) to be detected under optimized conditions. Furthermore, remote detection allows the utilization of ultrasensitive optical or superconducting quantum interference device detection techniques, which broadens the horizon of NMR experimentation.

Lasers↗
PubMed2003

Using switched angle spinning to simplify NMR spectra of strongly oriented samples.

This contribution describes a method that manipulates the alignment director of a liquid crystalline sample to obtain anisotropic magnetic interaction parameters, such as dipolar coupling, in an oriented liquid crystalline sample. By changing the axis of rotation with respect to the applied magnetic field in a spinning liquid crystalline sample, the dipolar couplings present in a normally complex strong coupling spectrum are scaled to a simple weak coupling spectrum. This simplified weak coupling spectrum is then correlated with the isotropic chemical shift in a switched angle spinning (SAS) two-dimensional (2D) experiment. This dipolar-isotropic 2D correlation was also observed for the case where the couplings are scaled to a degree where the spectrum approaches strong coupling. The SAS 2D correlation of C(6)F(5)Cl in the nematic liquid crystal I52 was obtained by first evolving at an angle close to the magic angle (54.7 degrees ) and then directly detecting at the magic angle. The SAS method provides a 2D correlation where the weak coupling pairs are revealed as cross-peaks in the indirect dimension separated by the isotropic chemical shifts in the direct dimension. Additionally, by using a more complex SAS method which involves three changes of the spinning axis, the solidlike spinning sideband patterns were correlated with the isotropic chemical shifts in a 2D experiment. These techniques are expected to enhance the interpretation and assignment of anisotropic magnetic interactions including dipolar couplings for molecules dissolved in oriented liquid crystalline phases.

Journal Article↗
PubMed2003

High-resolution NMR correlation spectra of disordered solids.

We show how high-resolution NMR spectra can be obtained for solids for which the spectra are normally broadened due to structural disorder. The method relies on correlations in the chemical shifts between pairs of coupled spins. It is found experimentally that there are strong correlations in the chemical shifts between neighboring spins in both phosphorus-31 and carbon-13 spectra. These correlations can be exploited not only to provide resolution in two-dimensional spectra, but also to yield "chains" of correlated chemical shifts, constituting a valuable new source of structural information for disordered materials.

Journal Article↗
PubMed2003

Applications of laser-polarized 129Xe to biomolecular assays.

The chemical shift sensitivity and significant signal enhancement afforded by laser-polarized 129Xe have motivated the application of 129Xe NMR to biological imaging and spectroscopy. Recent research done by our group has used laser-polarized 129Xe in biomolecular assays that detect ligand-binding events and distinguish protein conformations. The successful application of unfunctionalized and functionalized 129Xe NMR to in vitro biomolecular assays suggests the potential future use of a functionalized xenon biosensor for in vivo imaging.

Humans↗