3D Pulse Research Articles

BIRD-HMBC: improved detection of heteronuclear long-range couplings

A new 2D pulse sequence (BIRD–HMBC) designed for the detection of heteronuclear long-range connectivities is presented. In contrast to the basic HMBC experiment and its variants, unwanted 1J(C,H) signals are sampled together with nJ(C,H) signals but are cancelled in subsequent scans, taking advantage of a BIRD pulse sequence element. As a result, sensitivities close to the HMBC with only minor losses are measured. Excellent suppression degrees for unwanted 1J(C,H) residual peaks, superior to the state-of-the-art ACCORD–HMBC experiment, are achieved for the whole range of one-bond couplings. The influence of poorly calibrated pulse angles or off-resonance effects is marginal and options such as refocusing with additional 13C broadband decoupling or the ACCORDION element, applied for improved sampling of the whole nJ(C,H) coupling range, may easily be implemented. These attractive characteristics, together with the easy set-up, make this new experiment a valuable alternative to the HMBC experiment and its known variants for standard routine applications. Copyright © 2000 John Wiley & Sons, Ltd.

Comparison of treatment plans for lower extremity arterial occlusive disease made with electrocardiography-triggered two-dimensional time-of-flight magnetic resonance angiography and digital subtraction angiography

Background: The purpose of the study was to determine whether preoperative treatment plans for patients with lower extremity ischemia can be made with electrocardiography (EKG)-triggered two-dimensional (2D) time-of-flight (TOF) magnetic resonance angiography (MRA) as accurately as digital subtraction angiography (DSA). Methods: Forty patients were prospectively evaluated with the combination of EKG-triggered 2D TOF MRA, DSA, and pulse volume recordings. Blinded reviewers graded arterial segments for disease severity. Accuracy of separate MRA- and DSA-based treatment plans was compared with the procedures performed based on all available information. Results: There was an 86% exact match between MRA- and DSA-based plans (92% MRA and 94% DSA accuracy). The MRA-based plan accurately predicted 90% of suprainguinal and 95% of infrainguinal procedures, whereas the DSA-based plan accurately predicted 100% of suprainguinal and 85% of infrainguinal procedures. Two-year primary patency was 83% for all procedures. Radiologists’ review of disease severity resulted in a mean exact correlation between studies of 81% (κ = 0.64). The agreement between radiologists interpreting the MRA was 84% (κ = 0.7) compared with 82% (κ = 0.66) for the DSA. Conclusions: MRA- and DSA-based preoperative management plans were of comparable efficacy. Significant interobserver variability was seen with the interpretations of both preoperative studies. EKG-triggered 2D TOF MRA can be used to plan arterial reconstructions; however, all patients require arterial pressure measurements prior to suprainguinal repair and confirmatory intraoperative angiography during infrainguinal revascularization.

Two-Dimensional Deuterium NMR Spectroscopy of Chiral Molecules Oriented in a Polypeptide Liquid Crystal: Applications for the Enantiomeric Analysis through Natural Abundance Deuterium NMR

We describe several two-dimensional NMR experiments to facilitate the analysis of overcrowded proton-decoupled deuterium spectra of chiral molecules oriented in poly-A-benzyl-L-glutamate liquid crystalline solutions. These 2D NMR experiments allow the correlation between the components of each quadrupolar doublet in the deuterium spectra and then their assignment on the basis of chemical shift. The 2D pulse sequences proposed in this work were developed using the Cartesian spin-operator formalism for spin I ) 1 nuclei with a small quadrupolar moment, such as deuterons in high magnetic field. The features and analytical potentialities of each pulse sequence are discussed and compared. Illustrative applications of these sequences in natural abundance deuterium NMR spectroscopy using a 9.4 T magnetic field and standard NMR equipment are reported and examined. With this technique, it is demonstrated that quantitative measurements are possible within a precision of 10%.

Probing organometallic structure and reactivity by transition metal NMR spectroscopy†

Transition metal NMR chemical shifts are readily measured by modern 1D and 2D pulse techniques and serve as a probe into electronic and steric effects of ligands and substituents in metal complexes. Quantitative correlations of metal chemical shifts with reaction rates and catalytic activities, both experimental and as a result of quantum chemical calculations, give new mechanistic insights and permit reactivity predictions and a screening of homogeneous catalysts. A variety of such examples involving the spin-1/2 nuclei 57Fe, 103Rh, and 187Os as well as the quadrupolar nuclei 51V, 53Cr, 55Mn, 59Co, and 91Zr are discussed.

Spatial localization combining projection presaturation with a two-dimensional excitation pulse.

Single-shot spatial localization of short T1 nuclei was achieved by outer volume suppression with projection presaturation followed by selective excitation of the desired volume with a two-dimensional (2D) pulse. After the projection, presaturation with a 2D pulse avoided signal contamination from magnetization regrowth in the outer volume, whereas preceding the 2D pulse with projection presaturation reduced any unwanted excitation in the outer volume caused by the 2D pulse. The improvement in outer volume suppression achieved by combining these two techniques was demonstrated by images collected after the application of projection presaturation alone, a 2D pulse alone, and the two combined.

Study of fast switching processes due to electric and magnetic fields—an NMR approach

Solid state NMR techniques have been developed to investigate dynamic molecular effects (e.g., molecular reorientations) due to simultaneously applied external electric fields on electrically sensitive materials such as liquid crystals (LC), liquid crystalline polymers (LCP) and polymeric electrets. Such effects can be observed only on relatively thin systems (10–200 μm). That means that many scans are necessary to achieve a sufficiently high signal-to-noise-ratio in the spectra (500–1000 scans). If the material is also magnetically sensitive, the electric field can be used to orient molecules in a starting orientational state and by switching-off the voltage to access fast reorientation processes in the magnetic field B 0. Until now, the behaviour of orientable molecular systems under the influence of electric fields has been investigated by means of a more or less quasistatic approach (LCP: 100 V, electrets: 1 kV) in equilibrium states. The achievable time resolution depends on the desired signal-to-noise-ratio. For the case of proton NMR this means a time resolution of about 10 min. However, very often switching processes occur on a much shorter time scale. Using conventional techniques it is impossible to observe fast (ca. 100 μs) electrically or magnetically induced reorientation processes. In this work, we present a concept to overcome the problems outlined above and to extend the area of our current in situ NMR investigations on thin electrically-switched or poled polymeric layers. The basic idea is to include synchronized electric pulses during the NMR experiment using the preparation and/or mixing periods of a 1D or 2D pulse sequence for the application of an orienting field (electric or magnetic) and to use the reversibility of the molecular switching phenomenon to achieve a sufficient signal-to-noise-ratio. The techniques extend the range of possible investigations from about 100 μs to approximately T 1 for correlated spectra (and to longer times of applied fields for uncorrelated spectra). Results are shown for a nematic LC and a nematic polymer having a similar side chain.

MR measurement and numerical simulation of steady flow in an end-to-side anastomosis model

Magnetic resonance phase contrast velocity imaging (‘MR PC velocimetry’) is a powerful tool for measuring blood velocity. While it has been demonstrated that MR PC velocimetry can accurately measure velocity profiles in simple, unidirectional flows, errors can arise when measuring the more complex, multidirectional flows of clinical importance. In this work, we have compared MR PC velocity measurements with numerical simulations of steady flow in a physiologically relevant model of an end-to-side anastomosis, which produces many of the complex flow features expected in vivo. MR PC velocity profiles, measured using both 2D and 3D pulse sequences, showed very good agreement with numerically computed profiles, although at 2D PC data showed slightly larger errors than the 3D PC data in regions of high accelerations and oblique flow, likely due to displacement artifacts. We conclude that, under ideal conditions, MR phase contrast velocimetry can measure complex flow patterns to within 15% accuracy, but that care should be taken when using 2D pulse sequences to measure such complex flows.

Investigation of Coronary Vessels in Microscopic Dimensions by Two- and Three-dimensional NMR Microscopic Imaging in the Isolated Rat Heart

Nuclear magnetic resonance (NMR) imaging of macroscopic coronary vessels is rapidly advancing, whereas little attention has focused on development of NMR techniques for investigation of coronary microvessels. Such techniques would be of particular importance, since conventional methods to visualize coronary microvessels have specific limitations. The aim of our study was to develop two- and three-dimensional (2D and 3D) high-resolution imaging of coronary microvessels. Quantitative analysis of vessel size was performed in tomograms and applied to evaluate the vasoconstrictor effect of endothelin 1. Angiographic imaging was performed on an 11.75-T magnet by 2D and 3D gradient-echo pulse sequences. In tomograms, the validity of this method in providing correct vessel size was tested by phantom experiments. Experiments were carried out in the isolated constant-pressure-perfused rat heart with continuous registration of coronary flow and left ventricular pressure. NMR pulse sequences were pressure-triggered in mid diastole. Four groups of hearts were studied. In group 1 (n = 20), 2D imaging perpendicular and parallel to the long axis of the heart was performed. Cross sections of vessels with diameter > 140 microns were clearly detectable. In group 2 (control, n = 5) and group 3 (n = 13), tomograms perpendicular to the long axis were obtained before and after administration of vehicle (group 2) and 200 pmol endothelin 1 bolus (group 3). Vehicle had no effect on vessel cross section. Endothelin 1, which decreased global coronary flow by 47%, reduced vessel cross section by 38 +/- 19%. A weak but, on average, significant inverse correlation between area of cross section and vessel size was found. In group 4 (n = 10), 3D imaging was performed in 7 normal hearts and 3 hearts with anterior myocardial infarction. A 3D image of the entire coronary artery tree was obtained, revealing excellent agreement with anatomic studies. In infarcted rat hearts, occlusion of the left coronary artery was demonstrated. Visualization and quantification of coronary microvessels are feasible by NMR microscopy. NMR microscopy bears the potential of becoming a powerful tool for the investigation of the coronary microcirculation.

Experimental Aspects of Multidimensional Exchange Solid-State NMR

A five-pulse sequence for two-dimensional exchange 2H NMR that results in improved processability of the obtained spectra is presented, together with the phase cycle necessary to eliminate unwanted signal contributions. The scheme of the three-dimensional Fourier transformation to yield pure-absorption-mode three-dimensional 2H NMR spectra is presented, and the off-resonance measurement of multidimensional 2H NMR experiments is discussed. Furthermore, the phase cycles of six- and eight-pulse sequences for the acquisition of undistorted three-dimensional 2H NMR spectra are derived from the phase cycles of the 2D pulse sequences. Differences in the phase cycles for spin-12 nuclei, such as 13C, and spin-1 nuclei, such as 2H, are explicitly addressed.

Assignment of 1H signals in complex overlapping spectral regions using proton-detected 2D heteronuclear shift correlation spectroscopy

Abstract The application of a heteronuclear 2D pulse sequence is demonstrated which correlates chemical shifts of carbons with directly bound protons and with remote protons belonging to the same coupled spin system by using the effect of Hartmann—Hahn cross polarization among protons. The enhanced spectral resolution obtained by spreading individual proton subsystems into the carbon domain and the option for multiplicity discrimination makes these kind of experiments most suitable for proton and carbon signal assignments of natural products as alkaloids and steroids with strongly overlapping proton resonances. With the application to a steroid we want to demonstrate that such unambiguous signal assignments are the prerequisite and the basis for detailed structural investigations using additional, more conventional NMR experiments.

Convex cost functions in blind equalization

Existing blind adaptive equalizers that use nonconvex cost functions and stochastic gradient descent suffer from lack of global convergence to an equalizer setup that removes sufficient ISI when an FIR equalizer is used. The authors impose convexity on the cost function and anchoring of the equalizer away from the all-zero setup. They establish that there exists a globally convergent blind equalization strategy for 1D pulse amplitude modulation (PAM) systems with bounded input data (discrete or continuous) even when the equalizer is truncated. The resulting cost function is a constrained l/sub 1/ norm of the joint impulse response of the channel and the equalizer. The results apply to arbitrary linear channels (provided there are no unit circle zeros) and apply regardless of the initial ISI (that is whether the eye is initially open or closed). They also show a globally convergent stochastic gradient scheme based on an implementable approximation of the l/sub 1/ cost function. >

Simultaneous measurement of one‐bond and long‐range carbon–carbon coupling constants in cembrene

AbstractA 1D pulse sequence designed for the selective detection of carbon–carbon connectivities and the measurement of the corresponding coupling constants is presented. The sequence takes advantage of the increased sensitivity and the higher accuracy obtained with multiselective excitation and 1D detection, respectively. The sequence is shown to be a valuable alternative to the popular 2D INADEQUATE experiment in cases where parts, instead of the full, carbon–carbon connectivity matrix are of interest. The sequence is especially useful for the selective measurement of the stereochemically interesting long‐range coupling constants.

Sequential proton saturation for the heteronuclear 1D NOE experiment

AbstractAn improved 1D pulse sequence for detecting heteronuclear NOEs on a qualitative level is described and demonstrated. It makes use of shaped selective pulses and takes advantage of sequential perturbation of several proton resonances during one experiment and correspondingly more efficient data accumulation schemes. The method is particularly suitable for assigning quaternary carbons and for establishing connectivities between assigned molecular fragments bound to these quaternary centres.

High‐Resolution Solid‐State MAS13C‐ and 1H‐NMR Spectra of Benzenoid Aromatics Adsorbed on Alumina and Silica: Successful Applications of 1D and 2D Pulse Experiments from Liquid‐State NMR

AbstractStrong line‐narrowing effects in solid‐state, magic‐angle‐spinning (MAS) 13C‐ as well as 1H‐NMR spectra of benzenoid aromatics adsorbed at alumina or silica surfAccs indicate high mobility of the organic adsorbates. Even under modest spinning rales (1 kHz), dipolar couplings are sufficiently reduced to allow scalar 13C,1H couplings to be measured. Hetero‐ and homonuclear pulse sequences known from high‐resolution NMR in liquids, like SEFT, J‐RESOLVED, DEPT, COSY, and 13C,1H shift‐correlation experiments are successfully applicable. 13C spin‐lattice relaxation limes are as short as 0.5 s (CH) and 1.1 s (Cq), and T1(1H) values are in the order of 0.3 s.

Direct time integration of Maxwell's equations in linear dispersive media with absorption for scattering and propagation of femtosecond electromagnetic pulses

The initial results for femtosecond pulse propagation and scattering interactions for a Lorentz medium obtained by a direct time integration of Maxwell's equations are reported. The computational approach provides reflection coefficients accurate to better than 6 parts in 10,000 over the frequency range of dc to 3 x 10 to the 16th Hz for a single 0.2-fs Gaussian pulse incident upon a Lorentz-medium half-space. New results for Sommerfeld and Brillouin precursors are shown and compared with previous analyses. The present approach is robust and permits 2D and 3D electromagnetic pulse propagation directly from the full-vector Maxwell's equations.

A two-dimensional technique for chemical-shift resolution in one-dimensional NMR images

Recently, we described a technique for one-dimensional NMR microscopy that uses a single field gradient perpendicular to a layered structure ( 1) . By using selective saturation, we obtained images that showed different distributions of chloroplast and nonchloroplast water in a plant leaf. Advantages of ID imaging include rapid data acquisition and automatic averaging of information. This Note reports a two-dimensional technique that combines 1D imaging with chemical-shift resolution. The high-resolution ‘H NMR spectrum of a plant leaf consists almost entirely of signals from H20. In some species, the spectrum shows multiple peaks because portions of the water are located in different magnetic environments (2). Figure I A shows the NMR spectrum of a Norway maple shade leaf (Acer. platanoides, var “Emerald Queen”). The sample, a 4 mm disk of leaf tissue positioned with its surface perpendicular to the static magnetic field (the z axis), was imaged in a Bruker AM-400 spectrometer equipped with a Bruker microimaging probe. Our 2D pulse sequence, D,--~/2-t rn / 2D2r/2-acquisition ( t2 ), resembles a standard NOESY sequence except that a z gradient is applied during D2 and f2. The sequence starts with a relaxation delay, D, . The first r/2 pulse excites transverse magnetization, encoding chemical shifts in the absence of gradients during the variable t , evolution period. The second x/ 2 pulse stores chemical-shift-modulated transverse magnetization along the z axis. A stabilization delay, D2, provides time for the z gradient to rise to full intensity; an x gradient is applied during the first millisecond of D2, and a y gradient is used during the second D2 millisecond to reduce residual transverse magnetization. After the final 7r/2 pulse, a FID is collected while the imaging z gradient is on. A 2D Fourier transform of the raw data gives a plot on which the w, dimension contains pure chemical-shift information, but on which a2 contains a combination of

31P Spectroscopic localization using pinwheel NMR excitation pulses

Spectroscopic imaging with a one-dimensional phase-encoding gradient and surface-coil reception relies on the restricted range of sensitivity of the surface coil to provide localization in the dimensions transverse to the coil axis and consequently suffers from relatively poor localization in these dimensions. A two-dimensional (2D) cylindrically selective excitation pulse with a large spectral bandwidth is presented here to remedy this problem. The gradient waveforms are derived from multiple spirals in k space which form an overall pinwheel pattern, resulting in a pulse which is much shorter than the equivalent single-spiral trajectory. Nonuniform traversal of the spirals further reduces the pulse width under conditions of gradient slew-rate limitations, yielding overall gains in bandwidth of up to about 30 compared with the equivalent single-spiral trajectory traversed at constant angular rate. The accompanying rf waveform is obtained by weighted 2D Fourier transformation of the desired sensitivity profile. A new weighting factor is introduced into the rf waveform to compensate for nonuniform sampling of k space by the pinwheel near the origin. This factor is independent of the weighting used to account for the rate of traversal of the trajectory and is applicable to 2D pulse design in general. Pulse sequences employing pinwheel excitation in conjunction with either phase-encoding or slice-selective inversion are used to produce multiple-voxel and single-voxel localization in a human heart and a phantom. Pinwheel pulses may be used to advantage on moieties with long spin-lattice relaxation times and short transverse relaxation times and are therefore ideal for applications in phosphorus (31P) NMR.

Non-iterative 2D rational approximation

The rational approximation of two-dimensional (2D) digital filters in the spatial domain is examined. Non-iterative methods are presented for finding 2D recursive digital filters to approximate, in some sense, a desired 2D pulse response over a finite area. The methods require only linear calculations and they are quite straight forward. Two examples are given to illustrate the utility of the presented techniques.

Suppression or differentiation of solvent resonance by a combination of DEFT with a two-dimensional sequence

A new pulse sequence is described which combines the driven equilibrium Fourier transform (DEFT) and 2D pulse scheme, and allows simultaneous suppression of the solvent peak (H 2O/D 2O) in 1D and 2D experiments without perturbing the nearby solute signals. To overcome the sensitivity problem, and to allow for 2D experiments in a reasonably short time, a steady-state condition was set up by adjusting τ 1 (pulse delay) and D (recycle delay) to be 2–4 times the longest spin-lattice relaxation time T 1 (peptide) such that M z (solvent) was nulled prior to the 2D pulse sequence. Longitudinal and transverse relaxation rates of the solvent were analyzed in the steady state, and a direct calculation of T 1 (solvent) was performed using an equation derived in the present work. This novel pulse sequence is also extended to 2D NOE experiments with effective solvent suppression. The method is demonstrated in a variety of 1D and 2D experiments on the cyclic delta opioid receptor selective peptide ▪.

Anatomic and functional evaluation of pure and associated mitral stenoses using the echo-Doppler scanner technique

Twelve normal subjects, 18 control patients and 25 patients with mitral valve lesions (MVL), including 10 pure stenoses (MS), and 15 associated regurgitations (MS + MR) were investigated using a 2-dimensional (2D) pulse echo Doppler procedure, the latter group before open heart surgery. The Doppler signal output consisted of an audio-signal and of a graphic display including an analogue flow velocity trace and a frequency spectrum (Time Interval Histogram). The investigation was two-fold. (1) Diagnosis and assessment of severity of MVL comparing 2D echo and Doppler reliability. The main criteria for diagnosis relied, firstly for the 2D pulsed echo technique on the determination of the planimetric mitral valve area (MVA) for MS and on the finding of an increased annulus diameter for MR, and secondly for the Doppler technique, on the detection of diastolic (MS) and systolic (MR) anomalies of the mitral flow velocity patterns. (2) 2D echo-cardiographic assessment of the mitral apparatus (valves, annulus, subvalvular apparatus), studied on the basis of quantitative and qualitative data. Independently performed correlations with catheterization, angiographic and surgical data showed that a positive diagnosis of MS was obtained in all cases, both with 2D echo and Doppler techniques with a specificity of 92% for the former and 96% for the latter. The assessment of the severity of lesions was satisfactory in 88% of cases for the Doppler, and in 80% of cases for the 2D echo technique with a linear correlative coefficient at 0.83 for the value of MVA measured at catheterization and echocardiography. For MR, a net advantage in diagnosis was found for the Doppler technique (sensitivity of 93%, specificity of 92%) as compared with the 2D echo technique (sensitivity of 33%, specificity of 82%), and also for the assessment of the severity (60% for 2D echo and 85% for Doppler). Furthermore, Doppler assessment of the site of regurgitation and of the direction of the regurgitant jet was in agreement with all the cases also submitted to invasive investigations. Quantitative data obtained by the 2D echo technique for the assessment of the annulus diameter, and of the quality of mitral valve tissue (pliable, fibrous, calcified) or subvalvular apparatus (discrete, moderate, severe alterations), significantly differentiated normals from patients, but no clear-cut separation appeared for patients between close types of alterations. A better assessment was achieved from qualitative data for mitral valve tissue (76% of cases), and subvalvular apparatus (84% of cases).