Selective saturation and inversion of multiple resonances in high-resolution solid-state 13C experiments using slow spinning CP/MAS and tailored DANTE pulse sequences.

Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described.     

固体高分辨NMR中的DANTE实验

该文给出了旋转固体的DANTE实验的严格的理论分析,在化学位移相互作用坐标系中,应用平均Hamiltonian理论求出了自旋体系运动方程的零级近似解,从而讨论了DANTE序列在不同情况下的选择性,并给出了有关的实验结果.     

A pulse sequence for averaging the strong homonuclear scalar coupling in AB spin systems

A pulse sequence is proposed for the suppression of the strong homonuclear scalar coupling in the case of AB spin systems. The theoretical treatment is presented in terms of the average Hamiltonian theory in the case of aperiodic perturbations. The zero-order and the first-order correction terms in the full average Hamiltonian are calculated. It is shown that the chemical shift interaction for one of the spins is completely refocused and the conditions in which the interactions bilinear in spin operators are efficiently suppressed are analyzed.     

Solvent-suppression pulses. III. Design using simulated-annealing optimization with in vitro and in ivo testing

Pulses have been designed using a simulated-annealing optimization technique to overcome the problems associated with the observation of weak proton resonances in aqueous solution. As in a previous study (V. Smith, J. Kurhanewicz, and T. L. James, J. Magn. Reson. 95, 41 (1991), where optimization was accomplished using optimal control theory, pulses are tailored to provide null excitation at the water proton resonance frequency with uniform excitation and phasing of other proton resonance frequencies. The pulse duration is reasonable, easily enabling study of nuclei with spin-spin relaxation time ⩾ 10 ms. In general, effects from inhomogeneities in the stationary magnetic field and the radiofrequency field are minimal. The objectives were best achieved using a tailored 90° pulse in a Hahn spin-echo sequence with a long interpulse delay. Efficacy of some of the pulses was demonstrated with a solution containing a mixture of histidine, methionine, and threonine and in vivo with the kidney of an anesthetized rat utilizing a surface coil, in comparison with standard pulse sequences.     

Selective NMR excitation in strongly inhomogeneous magnetic fields

The NMR-MOUSE is a unilateral and mobile NMR sensor which operates with highly inhomogeneous magnetic fields. To produce a mobile NMR unit, RF excitation is sought, which can be produced with the most simple equipment, in particular nonlinear, low-power amplifiers, and to observe a free induction decay in strongly inhomogeneous fields, the excitation needs to be selective. The possibility to produce selective excitation by sequences of hard low-power radiofrequency pulses in the strongly inhomogeneous magnetic fields of the NMR-MOUSE is explored. The use of the DANTE sequence for selection of magnetization from parts of the sensitive volume was investigated for longitudinal and transverse magnetization by computer simulations and experiments. The spectra of the recorded FIDs and echo signals are in good agreement with those simulated for the excitation, which verifies the concept of the DANTE excitation. The results obtained are an important step towards a low-power operation of the NMR-MOUSE to improve its mobility.     

Chemical-shift-resolved tomography using frequency-selective excitation and suppression of specific resonances

Frequency-selective radiofrequency pulses generated by either the “soft pulse” or “DANTE” methods have been incorporated into a two-dimension chemical-shift-resolved proton tomography experiment. The utility of the approach is illustrated by observation of the image of an ethanol phantom with selective suppression of water, and by direct, selective excitation of the ethanol resonances.     

变延迟进动定制激发(DANTE)序列在工程中的优化设计

变延迟进动定制激发（Delays Alternating with Nutation for Tailored Excitation,DANTE）序列作为一种黑血预脉冲序列,通过连续施加小角度激发脉冲,以及结合散相梯度,使得流动物质和静态物质达到不同的稳态,从而抑制流动的血液.对于静态物质而言,施加DANTE序列后在图像等间隔的位置会出现暗条纹,暗条纹的宽度与梯度幅值和小单元持续时间乘积相关：乘积越大,暗纹宽度越小.对于动态物质而言,为达到较好的抑制效果,需要增加整个DANTE序列模块的准备时间,并且增大梯度幅值和小单元持续时间的乘积.因此,该方法对于梯度系统的要求较高,而实际梯度放大器（Gradient Amplifier,GPA）有一定的限额.在有限的GPA条件下,为使得DANTE序列具有更好抑制流动信号效果,本文在读出方向以及片层旋转两个方面进行了梯度优化,实现了更好的黑血效果.     

Excitation conditions for quantitative determination of quadrupolar spins with one pulse or spin-echo sequences in solid-state NMR

One of the major problems concerning quadrupolar spins in solid-state NMR is their quantification. If the optimal excitation conditions with one radio-frequency pulse are widespread known now, this is not the case with the spin-echo sequences. This paper reports some theoretical predictions and their limitations concerning quantification with the echo obtained with spin-echo resonances. To realize that, first, the relative line intensity of a transition (m+1,m) is defined in order to allow the comparison of results, from different authors. Then results concerning one pulse excitation on a spinI=3/2 are summarized. The condition of short pulse excitation is generalized to higher spins using the Pauli matrices applied to the two extreme cases: hard pulse or non selective excitation, and selective excitation. Finally the same procedure has been followed for the spin-echo sequence involving twox-pulses. It was shown that the optimum conditions are: both the pulse length must be sufficiently short, and the interpulse delay should be taken as short as the duration of the FID provided the phase of the second pulse alternates without changing the receiver phase. In these conditions, the relative echo amplitude depends linearly on the first pulse length and quadratically on the second. The limitations are: the homonuclear magnetic dipolar interaction must be much smaller than the heteronuclear case which must be itself much smaller than the amplitude of the pulse. Furthermore, quantification with the echo requires the determination of the spin-spin relaxation time as well.     

The 'DANTE' experiment

The selective excitation scheme known as ‘DANTE’ emerged from a confluence of several ideas for new NMR experiments, some more fanciful than others. DANTE offers a simple and effective way to restrict excitation to a very narrow frequency band, usually that of a single resonance line. Initially applied to the study of individual proton-coupled carbon-13 spin multiplets, the method has been extended to water presaturation, relaxation measurements, and chemical exchange studies. Through the imposition of a magnetic field gradient it offers a simple method to enhance resolution by restricting the effective volume of the sample. Multiple DANTE excitation (with Hadamard encoding) can speed up multidimensional spectroscopy by orders of magnitude. Applied to magnetic resonance imaging, the DANTE sequence has been used to superimpose a rectangular grid onto a cardiac image, permitting motional distortions to be monitored in real time.     

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.     

PRAWN: mixing sequences for selective heteronuclear J cross polarization

In this work, we present a family of pulse sequences for selective heteronuclear J cross-polarization (JCP), which we have developed especially for indirect 13C imaging using JCP, for example in the CYCLCROP environment. The sequences are straightforward to implement and operate reliably. Results of an average Hamiltonian analysis are given for the basic sequence, which we term PRAWN (pulsed rotating frame transfer sequence with windows). It is shown experimentally that the pulse sequence, which operates efficiently with low RF duty cycles down to a few percent, has a useful tolerance range to absolute Hartmann-Hahn mismatch and generates coherence transfer spectra in close correspondence with the JCP average Hamiltonian. Computer simulation of the performance of the basic sequence on a heteronuclear spin-(1/2) AX system is also presented. The mismatch compensation of PRAWN may be markedly enhanced further by issuing a pi pulse to each spin halfway through the basic PRAWN train and in phase quadrature to it. A simple analysis of this modified sequence, PRAWN-pi, is given under conditions of mismatch and off-resonance irradiation.     

Tailored slice selection in solid-state MRI by DANTE under magic-echo line narrowing

We propose a method of slice selection in solid-state MRI by combining DANTE selective excitation with magic-echo (ME) line narrowing. The DANTE RF pulses applied at the ME peaks practically do not interfere with the ME line narrowing in the combined ME DANTE sequence. This allows straightforward tailoring of the slice profile simply by introducing an appropriate modulation, such as a sinc modulation, into the flip angles of the applied DANTE RF pulses. The utility of the method has been demonstrated by preliminary experiments performed on a test sample of adamantane.     

Improvement of resolution in solid state NMR spectra with J-decoupling: an analysis of lineshape contributions in uniformly 13C-enriched amino acids and proteins

In magic angle spinning (MAS) NMR spectra of highly and uniformly 13C,15N-enriched amino acids and proteins, homo-nuclear coupling interactions contribute significantly to the 13C linewidths, particularly for moderate applied magnetic field strengths and sample spinning frequencies. In this work, we attempted to dissect, analyze, and control the contributions of J-coupling and residual homo-nuclear dipolar coupling interactions to the linewidths of uniformly 13C,15N-enriched crystalline alanine; these studies were carried out at 9.4 T using a range of spinning frequencies from 5 to 15 kHz. The anisotropic second-order dipolar shifts and the J-splittings are comparable in their contribution to the linewidths, but behave very differently in terms of experimental protocols for line narrowing. In contrast to the J-coupling interactions, the second-order dipolar broadening cannot be refocused using selective pulses on the passively coupled spin. We carried out experiments to remove or refocus the 13C J-coupling interactions (omega1 J-decoupling) using a selective DANTE pulse in the center of the indirect evolution period. Inversion profiles and bandwidths of selective DANTE pulses acting on transverse magnetization, in the regime of moderate spinning frequencies, were characterized computationally and experimentally. A dramatic improvement in the resolution of the 2D spectrum was achieved when this decoupling protocol was employed.     

Improvement of resolution in solid state NMR spectra with J-decoupling: an analysis of lineshape contributions in uniformly C-enriched amino acids and proteins

In magic angle spinning (MAS) NMR spectra of highly and uniformly 13C,15N-enriched amino acids and proteins, homo-nuclear coupling interactions contribute significantly to the 13C linewidths, particularly for moderate applied magnetic field strengths and sample spinning frequencies. In this work, we attempted to dissect, analyze, and control the contributions of J-coupling and residual homo-nuclear dipolar coupling interactions to the linewidths of uniformly 13C,15N-enriched crystalline alanine; these studies were carried out at 9.4 T using a range of spinning frequencies from 5 to 15 kHz. The anisotropic second-order dipolar shifts and the J-splittings are comparable in their contribution to the linewidths, but behave very differently in terms of experimental protocols for line narrowing. In contrast to the J-coupling interactions, the second-order dipolar broadening cannot be refocused using selective pulses on the passively coupled spin. We carried out experiments to remove or refocus the 13C J-coupling interactions (omega1 J-decoupling) using a selective DANTE pulse in the center of the indirect evolution period. Inversion profiles and bandwidths of selective DANTE pulses acting on transverse magnetization, in the regime of moderate spinning frequencies, were characterized computationally and experimentally. A dramatic improvement in the resolution of the 2D spectrum was achieved when this decoupling protocol was employed.     

Recoupling of heteronuclear dipolar interactions with rotational-echo double-resonance at high magic-angle spinning frequencies

Heteronuclear dipolar recoupling with rotational-echo double-resonance (REDOR) is investigated in the rapid magic-angle spinning regime, where radiofrequency irradiation occupies a significant fraction of the rotor period (10-60%). We demonstrate, in two model (13)C-(15)N spin systems, [1-(13)C, (15)N] and [2-(13)C, (15)N]glycine, that REDOR DeltaS/S(0) curves acquired at high MAS rates and relatively low recoupling fields are nearly identical to the DeltaS/S(0) curve expected for REDOR with ideal delta-function pulses. The only noticeable effect of the finite pi pulse length on the recoupling is a minor scaling of the dipolar oscillation frequency. Experimental results are explained using both numerical calculations and average Hamiltonian theory, which is used to derive analytical expressions for evolution under REDOR recoupling sequences with different pi pulse phasing schemes. For xy-4 and extensions thereof, finite pulses scale only the dipolar oscillation frequency by a well-defined factor. For other phasing schemes (e.g., xx-4 and xx-4) both the frequency and amplitude of the oscillation are expected to change.     

Pure-phase selective excitation in fast-relaxing systems

Selective pulses have been used frequently for small molecules. However, their application to proteins and other macromolecules has been limited. The long duration of shaped-selective pulses and the short T(2) relaxation times in proteins often prohibited the use of highly selective pulses especially on larger biomolecules. A very selective excitation can be obtained within a short time by using the selective excitation sequence presented in this paper. Instead of using a shaped low-intensity radiofrequency pulse, a cluster of hard 90 degrees pulses, delays of free precession, and pulsed field gradients can be used to selectively excite a narrow chemical shift range within a relatively short time. Thereby, off-resonance magnetization, which is allowed to evolve freely during the free precession intervals, is destroyed by the gradient pulses. Off-resonance excitation artifacts can be removed by random variation of the interpulse delays. This leads to an excitation profile with selectivity as well as phase and relaxation behavior superior to that of commonly used shaped-selective pulses. Since the evolution of scalar coupling is inherently suppressed during the double-selective excitation of two different scalar-coupled nuclei, the presented pulse cluster is especially suited for simultaneous highly selective excitation of N-H and C-H fragments. Experimental examples are demonstrated on hen egg white lysozyme (14 kD) and the bacterial antidote ParD (19 kD).     

Selective Excitation of Protons Directly Bonded to Carbon-13

New pulse sequences for the selective excitation and editing of NMR spectra of protons directly bonded to carbon-13 in CH, CH₂, and CH₃ groups are described. Experimental results as well as theoretical calculations demonstrate that these sequences have excitation profiles of about ±0.5J_CH or less and require no pulse shaping or special hardware to implement. After the isotope-directed excitation of the protons has been generated, the proton magnetization may be relayed to other protons via an isotropic mixing sequence to yield 1D HOHAHA subspectra or 2D COSY subspectra. Examples of applications to compounds containing carbon-13 either at natural abundance or with enrichment are shown.     

Homonuclear zero-quantum recoupling in fast magic-angle spinning nuclear magnetic resonance

Solid-state magic-angle-spinning NMR pulse sequences which implement zero-quantum homonuclear dipolar recoupling are designed with the assistance of symmetry theory. The pulse sequences are compensated on a short time scale by the use of composite pulses and on a longer time scale by the use of supercycles. (13)C dipolar recoupling is demonstrated in powdered organic solids at high spinning frequencies. The new sequences are compared to existing pulse sequences by means of numerical simulations. Experimental two-dimensional magnetization exchange spectra are shown for [U-(13)C]-L-tyrosine.     

Selection of pulse sequences producing maximum tissue contrast in magnetic resonance imaging

The importance of spin density [N(H)] and spin-lattice (T₁) and spin-spin (T₂) relaxation in the characterization of tissue by nuclear magnetic resonance (NMR) is clearly recognized. This work considers which optimized pulse sequences provide the best tissue discrimination between a given pair of tissues. The effects of tissue spin density and machine-imposed minimum rephasing echo times (TE_MIN) for achieving maximum signal tissue contrast are discussed. A long TE_MIN sacrifices T₁-dependent contrast in saturation recovery (SR) and inversion recovery (IR) pulse sequences so that spin-echo (SE) becomes the optimum sequence to provide tissue contrast, due to T₂ relaxation. Pulse sequences providing superior performance may be selected based on spin density and T₁ and T₂ ratios for a given pair of tissues. Selection of the preferred pulse sequence and interpulse delay times to produce maximum tissue contrast is strongly dependent on knowledge of tissue spin densities as well as T₁ and T₂ characteristics. As the spin density ratio increases, IR replaces SR as the preferred sequence and SE replaces IR and SR as the pulse sequence providing superior contrast. To select the optimal pulse sequence and interpulse delay times, an accurate knowledge of tissue spin density, T₁ and T₂ must be known for each tissue.     

C–H dipolar recoupling under very fast magic-angle spinning using virtual pulses

A new solid-state NMR pulse sequence for recoupling ¹³C–¹H dipolar interactions under magic-angle spinning is proposed, which works under a spinning speed of a few to several tens kilohertz. The sequence is composed of two different frequency switched Lee–Goldburg sequences, and the modulation of the spin part of the ¹³C–¹H dipolar interaction is introduced by a virtual pulse sequence consisting of unitary operators connecting the rotating frame and the tilted rotating frame. When the cycle time of the spinning is equal to or twice the cycle time of the sequence, the ¹³C–¹H dipolar interactions can be recoupled. The sequence is insensitive to experimental imperfections such as rf inhomogeneity or frequency offset, and the resulting lineshape can be represented by a simple analytical equation based on the zeroth-order average Hamiltonian. Experimental results for [2-¹³C] -valine·HCl are reported.