Multiple-pulse Technique Research Articles

SY1.3. Probing the pathophysiology of movement disorders with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a widely-used noninvasive brain stimulation method for research in the field of neurology, psychiatry, and rehabilitation. TMS can be used to study connectivity and therefore help to shed light on brain networks. This mode of noninvasive brain stimulation is particularly useful to study brain regions relevant to movement, as stimulating the primary motor cortex results in motor-evoked potential (MEP)s, which can be useful for interpreting the effects of various brain regions on the motor cortex. Connectivity among the primary motor cortex and other movement-relevant regions can be studied by using a paired- or multiple-pulse technique. This is possible by assessing the effects of a pre- conditioning stimulus on the conditioning-test pair or a conditioning stimulus on the test pulse. TMS can also be used in conjunction with electroencephalography (EEG), magnetoencephalography (MEG) or imaging tools such as functional magnetic resonance imaging (fMRI) or positron emission tomography (PET) to further assess TMS-induced changes. Combining these techniques can enable researchers to obtain spatial or temporal information of the effects of TMS on the brain. Movement in healthy human subjects has been studied using TMS, and information obtained from these studies has been useful in understanding which brain regions are relevant in producing movement. Furthermore, there are various TMS measures of the motor cortex that reflect different aspects of cortical excitability. Some examples are short and long intracortical inhibition (SICI and LICI), intracortical facilitation (ICF), short- and long- afferent inhibition (SAI and LAI), transcallosal inhibition, premotor cortex inhibition, etc. TMS has also been used to study movement disorders such as Parkinson’s disease, dystonia, essential tremor, and Huntington’s disease. While TMS still has limited use in the clinical realm, it has provided a wealth of data in our understanding of movement disorders. In this talk, I will give an overview of TMS studies that probe the physiology of normal human movement and also discuss those focusing on movement disorders. I will also review limitations of TMS and the findings of TMS studies to date, and briefly discuss the need for future studies.

Nonresonant Multiple-Pulse Control of Molecular Motions in Liquid

We propose the implementation of the multiple-pulse excitation for manipulation of the molecular contributions to the optically-heterodyne-detected optical-Kerr-effect. The key parameters controlling the specificity of the multiple-pulse excitation scenarios are the pulses durations, the delays between pulses, the relation between the pump pulses amplitudes and the pulses polarizations. We model the high-order optical responses and consider some principles of the scenarios construction. We show that it is possible to adjust the excitation scenario in such a way that the some responses can be removed from detected signal along with the enhancement of the interested response amplitude. The theoretical analysis and first experimental data reveal that the multiple-pulse excitation technique can be useful for the selective spectroscopy of the molecular vibrations and rotations in liquid.

Broadband Simulation of Earthquake Ground Motion by a Spectrum-Matching, Multiple-Pulse Technique

A kinematic, stochastic fault model and simulation procedure is proposed for realistic, application-oriented simulation of earthquake ground motion. An extended fault plane is discretized as a grid of point subsources. Subsource signal, in its low-frequency part, is produced by a dislocation strip sweeping the fault area. In its high-frequency part, it is defined by random local slip history, represented by a segment of pulsed noise. Subsource signals are convolved with broad-band Green functions for a layered half-space, and stacked, resulting in the ground motion at a site. Through a special choice of subsource signals, the Fourier spectrum of ground motion obeys an observation-based scaling law. Effects of the rupture velocity behavior, rise time, wavenumber spectrum for the final slip, the degree of spikiness of time functions etc can be easily analyzed. For illustration, several near-source 1994 Northridge earthquake records are simulated, and related uncertainties are estimated.

Determination of methylmercury at Nafion® coated electrodes by single and multiple pulse voltammetric techniques

The voltammetric behaviour of methylmercury at Nafion ® coated glassy carbon electrodes is examined. Cyclic voltammograms recorded at coated electrodes show a dramatic increase of peak currents with respect to bare electrodes, this effect being more evident in perchlorate supporting electrolytes rather than in chloride containing media. Experimental evidence indicates that ion-exchange preconcentration of the cation CH3Hg is the prevailing process, even if the incorporation of neutral species such as CH3HgCl by hydrophobic interactions cannot be neglected. The application and optimization of pulsed techniques like differential pulse voltammetry and Osteryoung’s square wave voltammetry, indicates the possibility to use Nafion ® coated electrodes for the determination of trace methylmercury at submicromolar levels. The analytical performances can be further improved by using, at the Nafion ® coated electrode, a multiple pulse technique named multiple square wave voltammetry. The application and optimization of this technique to the system under investigation is presented and discussed. © 1999 Elsevier Science S.A. All rights reserved.

Complete Dipolar Decoupling of13C and Its Use in Two-Dimensional Double-Quantum Solid-State NMR for Determining Polymer Conformations

A multiple-pulse technique for complete dipolar decoupling of directly bonded13C-labeled sites is described. It achieves significant spectral simplifications in a recently introduced two-dimensional double-quantum solid-state NMR experiment for determining torsion angles. Both homonuclear and heteronuclear dipolar couplings are removed by combining a13C multiple-pulse sequence with continuous-wave irradiation on the protons. The13C sequence has a fundamental 10-pulse cycle which is a significantly modified magic-sandwich-echo sequence. The crucial heteronuclear decoupling is achieved by breaking the 360° “inner” pulses in the magic sandwich into 90° pulses and spacing them by1H 360° pulse lengths. Spectral artifacts typical of multiple-pulse sequences are eliminated by phase shifts between cycles. In contrast to many other multiple-pulse decoupling sequences, the long window in the cycle is the dwell time and can be longer than the inverse dipolar coupling, which makes the sequence practical for direct detection even with long pulse ring-down times. A modification of the sequence to scale the chemical shift and increase the effective spectral width is also presented. The 1D and double-quantum 2D experiments are demonstrated on polyethylene with 4%13C–13C spin pairs. The potential of this approach for distinguishing segmental conformations is illustrated by spectral simulations of the two-dimensional ridge patterns that correlate double-quantum and single-quantum chemical-shift anisotropies.

Coherent reduction of dipolar interactions in molecules dissolved in liquid-crystal solvents using a new multiple-pulse technique during acquisition

A new pulse sequence, MET 6, synchronized to the sampling of the magnetization is proposed with special application to molecules in liquid crystals. It acts coherently on the dipolar interactions to produce a coherent reduction of the couplings in a way which compares to the variable angle sample spinning technique. The theory is developed in terms of average Hamiltonian theory and a full simulation of the sequence is presented together with an experimental study of its effects on an A 2 spin system dissolved in a nematic solvent. An analysis of the propagation of pulse imperfections is also given. A theoretical and experimental comparison is made between METE and other common multiple-pulse techniques used to average dipolar interactions in solids.

Perspectives in high-resolution solid-state nuclear magnetic resonance, with emphasis on combined rotation and multiple-pulse spectroscopy

Recent trends of general interest in high-resolution NMR of solids are mentioned, and some examples briefly discussed. Stress is laid on the use of the multiple-pulse technique, particularly when combined with magic-angle spinning to give CRAMPS. Two studies of proton CRAMPS are described, one concerning hydrogen bonding in carboxylic acids and the other involving phosphonic acid derivatives. The relationship between proton isotropic chemical shifts and hydrogen-bond distances is explored in some detail. A clear correlation is found.

A radar technique for high range resolution incoherent scatter autocorrelation function measurements utilizing the full average power of klystron radars

Incoherent scatter radar measurements with high range resolution have been made at Arecibo Observatory using a new technique, called the coded long pulse technique. This technique is similar to the multiple pulse technique in its uses, and in the way in which random clutter from unwanted ranges sets a lower limit on the noise level. It differs in that the full average power can be obtained from a radar that uses klystrons in the output stage. A description of the new technique is given, and the two techniques are compared. Some data showing the range variation of the HF induced enhanced plasma line are presented, and the superiority of the new technique for most E region measurements is demonstrated. Computational algorithms are discussed, and it is shown that in many cases the amount of computation required is not significantly more than is required by the multiple pulse technique.

Application of High Resolution Solid-State NMR to Surface Science

The principles and methods of high resolution solid-state NMR spectroscopy are explained briefly, followed by a review of recent reports especially on surface science studied by the technique. Two methods are used to obtain high resdution spectra, depending on the magnitude of homonuclear dipolar broadening. In the rare spin system, e.g., 13C, 29Si, and 15N, the line broadening is mostly due to heteronuclear dipolar coupling and chemical shift anisotropy, The techniques of magic angle spinning of the sample, dipolar decoupling, and cross polarization are effective to obtain high resolution spectra. As for abundant spins such as 1H, homonuclear dipolar broadening is usually dominant, which is suppressed by the multiple pulse technique. Structures and dynamics of surface species have been investigated by measuring high resolution spectra of 13C, 29Si, 1H, 31P, and 19F nuclei; usefulness of the technique is discussed.

Theory of artificial line-narrowing in NQR spectroscopy by various radiofrequency pulse-sequences

The theory of articifial narrowing of NQR lines is investigated by computing the response, to a number of multiple-pulse sequences, of an ensemble of spin- 3 2 nuclei whose ‘pure quadrupole resonance’ line is broadened by dipolar couplings and by a spread of electric field gradients due to static imperfections in the crystal lattice. It is concluded that a multiple-pulse technique for (partial) suppression of the dipolar coupling followed by a spin-echo method for eliminating the inhomogeneous broadening should lead to enhanced sensitivity and resolution in NQR.

K39 Spin-Lattice Relaxation Time in KH2PO4 Measured by a Technique Based on Nuclear-Double-Resonance

The method introduced by McArthur, Hahn and Walstedt for measuring the spin-lattice relaxation time of nuclei whose NMR signals are too weak to be observed by conventional NMR techniques, is modified, and the multiple-pulse spin-locking technique is jointly used to improve the signal-to-noise ratio. The spin-lattice relaxation time of K 39 in KH 2 PO 4 was first measured and determined to be 3.7±0.3 sec at 288 K and 110±10 sec at 77 K.

Spin diffusion effects in partially spin–lattice relaxed NMR spectra of solid block copolymers

Room-temperature proton NMR measurements of 65 wt % poly(dimethylsiloxane) (DMS) and 35 wt % bisphenol-A polycarbonate (BPAC) block copolymers with number average DMS block lengths ranging from 20 to 100 monomer units have shown effects which can be explained in terms of a gradient of net spin population along the DMS blocks during recovery from a nonselective 180° spin inverting pulse. This gradient occurs because the spin–lattice relaxation of the BPAC component is faster than that of the DMS component and spin diffusion takes place along the DMS blocks to the BPAC, causing spins at the centers of the DMS blocks to relax slower than those at the ends. High-resolution solid NMR spectra were obtained with a multiple-pulse technique, which permitted independent measurements of the DMS and BPAC relaxations. Unusual line shapes in partially relaxed broadline spectra were interpreted as being caused by enhanced motional narrowing of the protons at the centers of the DMS blocks in addition to the gradient of net spin population along the DMS blocks. The results of a spin diffusion model, which was developed to aid interpretation of the experimental results, were in quantiative agreement with the data. This model fits the data only if the diffusion process is limited by diffusion along the length of the DMS blocks and not by a single diffusion barrier between the DMS and BPAC blocks.

Proton magnetic shielding and susceptibility effects in single crystals of ferrocene

The proton NMR in single crystals of ferrocene has been studied by multiple pulse techniques at room temperature. In crystals of natural shape with plane faces the angular dependence of the resonance frequency due to the anisotropy of the shielding and due to the bulk susceptibility was found to be of about equal size, making analysis of the data practically impossible. By using a single crystal sphere the shape dependent part of the susceptibility contribution could be eliminated. In addition, the resolution obtained by the multiple pulse technique is considerably higher for spheres than for non-spherical crystals. As the ferrocene molecules rotate rapidly about their fivefold axes at room temperature, the shielding tensor σ must be axially symmetric. Having this in mind, the data could be analyzed to yield both the shielding tensor with Δσ = σ ‖ - σ ⊥ = −6.5 ± 0.1 ppm, σ iso = −4.2 ± 0.5 ppm from a spherical sample of TMS and the anisotropy of the susceptibility Δ x = 30 × 10 −6 cgs units.

Sensitivity Enhancement of a Pulsed NMR Spectrometer

A sensitive pulsed NMR spectrometer for solids based on a multiple-pulse technique is described. In the conventional pulse method transient signals are accumulated for a certain time when signal-to-noise ratio must be improved; however most of the time spent for measurement is wasted because T1≫T2. Since the multiple-pulse technique prolongs the decay time of transverse magnetization, one can accumulate extra signals and enhance the sensitivity. Under the optimum condition the enhancement factor is (T1/T2)1/2 for a given measuring time. One can take full advantage of low temperatures to increase the sensitivity. Typical spectra taken for aluminum metal show sideband peaks. The line width is made narrow if a high-resolution multiple-pulse technique is used. The signals were accumulated in a modified boxcar integrator. Several applications are discussed.