The transient processes in multi-pulse nitrogen-14 NQR

It has been demonstrated that transient processes, observed in a single crystal of NaNO₂ acted upon by pulse sequences MW-2 and MW-4 and their modifications with 180° flip angle of the pulses (Solid State Nucl. Magn. Resonance 10 (1997) 63; Sov. Phys.—JETP 88(5) (1999) 1580), which manifest themselves in the oscillating form of the NQR signals envelope, can be explained in the frames of a two-particle model. It has been proved that the nature of echo signals in the effective field of multi-pulse sequences received by the inversion of the phase of the sequence pulses or by introducing an additional 180° pulse is connected with re-focusing of accumulated digressions of the flip angle from the ideal 180° pulse. Experimental results of observing single and multiple echoes in a number of powdered nitrogenated substances in the effective field of various sequences at room temperature have been presented.     

Application of biselective refocusing soft pulses to the simplification of heteronuclear correlation spectra

The biselective spin echo technique allows the signals of coupled proton pairs to be extracted from crowded liquid state proton NMR spectra. Its use as a preparation sequence in heteronuclear chemical shift correlation experiments requires the removal of the heteronuclear coupling interaction during the biselective echo time. The discrimination between coupled and uncoupled protons signals is achieved by double quantum filtration, which delivers antiphase magnetization states. The latter are not directly compatible with the design of an HSQC-like pulse sequence. The conversion of antiphase to in-phase magnetization states by a second biselective echo sequence solves this problem. The optimization of spin echo delays is also discussed. Lastly, the article presents modified HSQC and HMBC pulses sequences in which information is obtained solely for the biselectively selected proton pairs. A peracetylated trisaccharide was used as a test molecule.     

An optimized multislice acquisition sequence for the inversion-recovery MR imaging

An optimized multislice data acquisition scheme for inversion-recovery MR imaging is proposed and experimental results are presented. In this new scheme, instead of forming a set of multislice inversion-recovery sequences in series for a given phase encoding step, 180° inversion pulses corresponding to different slices are interwoven with the spin echo data acquisition sequence in an optimal way depending on the desired inversion-recovery time. For example, between the 180° inversion RF pulse and the spin-echo imaging sequence, a number of imaging and inversion sequences are inserted with different slice combinations, i.e., long inversion-recovery time is effectively utilized for the other slice pre-inversion and data acquisition. With the optimized sequence, imaging time has been reduced by as much as a factor of four compared with the existing methods.     

Selective saturation in strongly inhomogeneous magnetic fields

The possibility to produce selective saturation by nuclear magnetic resonance (NMR) sequences of low-power radio-frequency pulses in strongly inhomogeneous magnetic fields is explored. The saturation of parts of the sensitive volume is produced by a particular pulse sequence with reduced amplitude distribution and the spectrum of the recorded signal is compared with the simulated spectrum. The spectra of the recorded free induction decays and echo signals are in good agreement with the simulated spectra of the pulse sequence, which demonstrates the effect of the selective saturation. The results obtained are an important step towards the development of new mobile and lowpower NMR equipments operating with inhomogeneous magnetic fields.     

On the Use of the Phase Memory TimeT2for the Quantitative Characterization of the Rotational Motions of Proteins in Lipid Bilayer Systems

Numerical simulations of the echo responses from a nitroxide label rigidly attached to a large protein undergoing ultraslow rotational motions in a lipid bilayer are presented. The echoes are formed by the application of Hahn, COSY, and 2D-ELDOR sequences utilizing both soft and hard microwave pulses. The simulations address the question of whether the echo responses elicited by these sequences are affected by restricted angular excursions of the long axis of the protein relative to the normal to the bilayer plane. The results indicate that all three pulse sequences yield the same quantitative motional information regardless of the nature of the microwave pulses and there is no theoretical reason for preferring one sequence above the others.     

Diffusion and relaxation effects in general stray field NMR experiments

We analyze the evolution of magnetization following any series of radiofrequency pulses in strongly inhomogeneous fields, with particular attention to diffusion and relaxation effects. When the inhomogeneity of the static magnetic field approaches or exceeds the strength of the RF field, the magnetization has contributions from different coherence pathways. The diffusion or relaxation induced decay of the signal amplitude is in general nonexponential, even if the sample has single relaxation times T(1), T(2) and a single diffusion coefficient D. In addition, the shape of the echo depends on diffusion and relaxation. It is possible to separate contributions from different coherence pathways by phase cycling of the RF pulses. The general analysis is tested on stray field measurements using two different pulse sequences. We find excellent agreement between measurements and calculations. The inversion recovery sequence is used to study the relaxation effects. We demonstrate two different approaches of data analysis to extract the relaxation time T(1). Finite pulse width effects on the timing of the echo formation are also studied. Diffusion effects are analyzed using the Carr--Purcell--Meiboom--Gill sequence. In a stray field of a constant gradient g, we find that unrestricted diffusion leads to nonexponential signal decay versus echo number N, but within experimental error the diffusion attenuation is still only a function of g(2)Dt(3)(E)N, where t(E) is the echo spacing.     

Pulse sequence optimization for T2-weighted MR imaging of the brain

The authors implemented bipolar velocity compensated pulse techniques for T2-weighted MR imaging of the brain. Signal-to-noise (S/N) and image quality was compared for pulse sequences with standard and optimized RF pulses, low and regular bandwidth versions and cardiac triggering. Images from bipolar velocity compensated sequences allowed better visualization of vessels and basilar cisterns and improved image quality relative to standard sequences without velocity compensation. The implementation of optimized RF pulses with bipolar sequences resulted in further improvement in image quality. Single echo sequences consistently had improved image quality and signal-to-noise relative to the second echo of a double echo sequence. Low bandwidth bipolar sequences with extended sampling period had 30% higher S/N, but at the cost of slight loss in edge definition. The highest image quality was obtained with the bipolar, optimized RF, single echo sequence. Using this technique contiguous high quality image slices could be obtained with velocity compensation. The addition of cardiac triggering to bipolar sequences resulted in slight improvement in image quality, but this difference was marginal and probably rarely necessary for MR imaging of the brain.     

The nuclear spin-echo signals under combined action of magnetic field and RF pulses

The effect of a pulse magnetic field on formation of the single-pulse echo signal in some magnetic materials has been studied. The signals of nuclear-spin echo formed by the combined action of radio frequency and magnetic pulses have been observed. The duration of the magnetic pulse was only limited by the length of the radio frequency pulse. The echo signal formed by the two magnetic pulses, the edges of a long magnetic pulse, and other situations that allowed us to reproduce the analogue of multipulse action, were observed as well.     

Optimizing tissue contrast in magnetic resonance imaging

Magnetic resonance imaging demands that tissue contrast and signal-to-noise advantages be sought in each component of the imaging system. One component of magnetic resonance imaging in which contrast and signal-to-noise ratios are easily manipulated is in the choice of pulse sequences and interpulse delay times. This article provides a general method for determining the best choices of interpulse delay times in pulse sequences and applies that method to saturation recovery, inversion recovery, and spin-echo sequences. Saturation recovery and inversion recovery sequences with rephasing pulses, and tissues with unequal hydrogen densities are considered. Optimization of pulse sequences is carried out for the two distinct cases of (a) a fixed number of sequence repetitions and (b) a fixed total imaging time. Analytic expressions are derived or approximate expressions are provided for the interpulse delay times that optimize contrast-to-noise ratios in each pulse sequence. The acceptable range of interpulse delay times to obtain reasonable contrast using each pulse sequence is discussed.     

Two-frequency multiple-pulse sequences in nitrogen-14 NQR

The theoretical investigation of the multiple-pulse sequences in which the radio-frequency pulses are applied simultaneously at two transitions are presented. The time-independent solution of the quantum Liouville equation for the average Hamiltonian of inhomogeneous broadening in the sequence is obtained by the projection operator technique. This solution predicts the multiple echo signals on the third transition that is not irradiated by radio-frequency pulses.     

Constant time steady gradient NMR diffusometry using the secondary stimulated echo

A pulse sequence producing a second stimulated echo is suggested for the compensation of relaxation and residual dipolar interaction effects in steady gradient spin echo diffusometry. Steady field gradients of considerable strength exist in the fringe field of NMR magnets, for instance. While the absolute echo time of the second stimulated echo is kept constant throughout the experiment, the interval between the first two radiofrequency pulses is augmented leading to a modulation of the amplitude of that second stimulated echo by self-diffusion only. The unique feature of this technique is that it is of a single-scan/single-echo-signal nature. That is, no reference signals neither of the same pulse sequence nor of separate experiments are needed. The new method was tested with poly(ethylene oxide) melts and proved to provide reliable data for (time dependent) self-diffusion coefficients down to the physical limit (D approximately 10(-15)m(2)/s) when flip-flop spin diffusion starts to become effective.     

Compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) in multiplicity-edited HSQC

Use of adiabatic pulses in broadband inversion and decoupling is well known. Replacement of the rectangular pi pulses in the INEPT and rev-INEPT parts of the HSQC and gHSQC experiments with adiabatic pulses substantially improves the sensitivity of these experiments. However, modulation of cross peak intensity in multiplicity-edited HSQC or gHSQC experiments can be quite severe. These modulations arise during the multiplicity-editing periods due to the inefficient refocusing of the spin-echo caused by the mismatch of the echo delay with the one-bond coupling constant. These modulations (which we call echo modulations) are field strength (and hence spectral width) independent. Use of adiabatic pulses with the inversion sweep synchronized to the 1H-13C coupling constant range typically observed in a 13C spectrum will provide substantial improvement in sensitivity. The inversion profile problems associated with rectangular pi pulses can be moderately compensated by composite pulse schemes and these schemes could prove to be reasonable alternatives to adiabatic pulses. However, the adiabatic sweep provides a unique method to compensate the echo modulations for multiplicity-edited experiments. The origin and the compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) method to minimize these modulations is described.     

Solvent suppression using phase-modulated binomial-like sequences and applications to diffusion measurements

Two phase-modulated binomial-like pi pulses have been developed by simultaneously optimizing pulse durations and phases. In combination with excitation sculpting, both of the new binomial-like sequences outperform the well-known 3-9-19 sequence in selectivity and inversion width. The new sequences provide similar selectivity and inversion width to the W5 sequence but with significantly shorter sequence durations. When used in PGSTE-WATERGATE, they afford highly selective solvent suppression in diffusion experiments.     

Properties of the HYSCORE spin echo signal

In hyperfine sublevel correlation spectroscopy (HYSCORE), the finite duration of the microwave pulses leads to an incomplete inversion of the electron spin magnetization by the third pulse, which results in a significant admixture of stimulated ESEEM to HYSCORE ESEEM. This virtually unavoidable contribution of stimulated ESEEM seriously hampers the analysis of the modulation amplitudes in HYSCORE. In this work, we analyze the properties of the spin echo signals contributing to the composite HYSCORE signal. Based on this analysis, we propose the strategies of HYSCORE data acquisition and processing that allow one to practically eliminate the contribution of the stimulated echo and make the HYSCORE ESEEM analyzable in quantitative terms.     

A simple scheme for the design of solvent-suppression pulses

Excitation sculpting was first introduced as a way to efficiently suppress solvent signals. It requires a pulse sequence that acts as a null pulse at the solvent-resonance frequency and as an inversion pulse everywhere else. In this article, it is shown that such a goal can be achieved starting with "top-hat" inversion shaped pulses such as I-BURP-2 or gaussian cascade G3. The result is a Globally Antisymmetric Selective Pulse, or GASP. Numerical optimization was used to extend the performance of such pulses. Multifrequency signal suppression was shown to be possible through application of successive excitation sculpting modules.     

Magnetic resonance imaging of cortical bone with ultrashort TE pulse sequences

PurposeNormal adult cortical bone has a very short T₂ and characteristically produces no signal with pulse sequence echo times (TEs) routinely used in clinical practice. We wished to determine whether it was possible to use ultrashort TE (UTE) pulse sequences to detect signal from cortical bone in human subjects and use this signal to characterise this tissue.Subjects and MethodsSeven volunteers and 10 patients were examined using ultrashort TE pulse sequences (TE=0.07 or 0.08 ms). Short and long inversion as well as fat suppression pulses were used as preparation pulses. Later echo images were also obtained as well as difference images produced by subtracting a later echo image from a first echo image. Saturation pulses were used for T₁ measurement and sequences with progressively increasing TEs for T₂* measurement. Intravenous gadodiamide was administered to four subjects.ResultsSignal in cortical bone was detected with UTE sequences in children, normal adults and patients. This signal was usually made more obvious by subtracting a later echo image from the first provided that the signal-to-noise ratio was sufficiently high.Normal mean adult T₁s ranged from 140 to 260 ms, and mean T₂*s ranged from 0.42 to 0.50 ms. T₁ increased significantly with age (P<.01).Increased signal was observed after contrast enhancement in the normal volunteer and the three patients to whom it was administered.Reduction in signal from short T₂ components was seen in acute fractures, and increase in signal in these components was seen with new bone formation after fracture malunion. In a case of osteoporosis, bone cross-sectional area and signal level appeared reduced.ConclusionSignal can be detected from normal and abnormal cortical bone with UTE pulse sequences, and this can be used to measure its T₁ and T₂* as well as observe contrast enhancement. Difference images are of value in increasing the conspicuity of cortical bone and observing abnormalities in disease.     

Choice of a modulating sequence for code separation of channels in the optical memory based on photon echo

The possibility of applying of phase-coded sequences in optical-memory systems based on photon echo is considered. Comparative analysis of the characteristics of the efficiency of using noise-like phase-coded signals as reference and reading excitation pulses is performed. Choice of the optimal code sequence is substantiated.     

Sequence parameters of double spin-echo sequences affect quantification of citrate

Quantification of citrate by localized ¹H spectroscopy is usually performed using the water signal as reference, but the signal behavior of the J-coupled AB spin system of citrate after multipulse excitation is not as trivial as for uncoupled substances. The influence of the timing scheme of double spin-echo sequences and of the spatial flip angle distribution of (nonideal) refocusing pulses was analyzed systematically for the citrate resonances. Both single echo times of the double spin-echo sequence were varied between 20 ms and 250 ms in theoretical and experimental approaches. Relatively long total echo times (TE > 120 ms) provide high selectivity to citrate signals, since signals from triglycerides at 2.6 ppm are markedly reduced. Asymmetrical timing schemes of the double spin-echo sequence with one short single echo time of 20 ms and one longer single echo time of about 120 ms result in high integral signal from the central lines of citrate, whereas symmetrical timing leads to high sensitivity for total echo times TE near 100 ms. The integral citrate signals in spectra with relatively long echo times (TE > 120 ms) were found to depend markedly on the type of the refocusing pulses, affecting quantitative citrate measurements in vitro and in vivo.