Shaped RF Pulses Research Articles

The effects of residual dipolar coupling on carnosine in proton muscle spectra.

Carnosine, an MR-visible dipeptide in human muscle, is well characterized by two peaks at ~8 and ~7 ppm from C2 and C4 imidazole protons. Like creatine and other metabolites, carnosine is subject to residual dipolar coupling in the anisotropic environment of muscle fibers, but the effects have not been studied extensively. Single-voxel TE 30-32 PRESS spectra from three different 3T studies were acquired from gastrocnemius medialis and soleus muscles in the human lower leg. In these studies, carnosine T2 values were measured, and spectra were obtained at three different foot angles. LCModel was used to fit the carnosine peaks with a basis set that was generated using shaped RF pulses and included a range of dipolar couplings affecting the C4 peak. A seven-parameter analytic expression was used to fit the CH2 doublets of creatine. It incorporated an optimized "effective TE" value to model the effect of shaped RF pulses. The fits confirm that the triplet C4 peak of carnosine is dipolar coupled to a pair of CH2 protons, with no need to include a contribution from a separate pool of freely rotating uncoupled carnosine. Moreover, the couplings experienced by carnosine C4 protons and creatine CH2 protons are strongly correlated (R2 = 0.88, P<0.001), exhibiting a similar 3cos2 θ - 1 dependence on the angle θ between fiber orientation and B0. T2 values for the singlet C2 peak of gastrocnemius carnosine are inversely proportional to the C4 dipolar coupling strength (R2 = 0.97, P < 0.001), which in turn is a function of foot orientation. This dependence indicates that careful positioning of the foot while acquiring lower leg muscle spectra is important to obtain reproducible carnosine concentrations. As proton magnetic resonance spectroscopy of carnosine is currently used to non-invasively estimate the muscle fiber typology, these results have important implications in sport science.

Generating accurate tip angles for NMR outside the rotating-wave approximation

The generation of accurate tip angles is critical for many applications of nuclear magnetic resonance. In low static field, with a linear rather than circular polarized rf field, the rotating-wave approximation may no longer hold and significant deviations from expected trajectories on the Bloch sphere can occur. For rectangular rf pulses, the effects depend strongly on the phase of the rf field and can be further compounded by transients at the start and end of the pulse. The desired terminus can be still be achieved, however, through the application of a phase-dependent Bloch–Siegert shift and appropriate consideration of pulse timings. For suitably shaped rf pulses, the Bloch–Siegert shift is largely phase independent, but its magnitude can vary significantly depending on details of the pulse shape as well as the characteristics of the rf coil circuit. We present numerical simulations and low-field NMR experiments with 1H and 3He that demonstrate several main consequences and accompanying strategies that one should consider when wanting to generate accurate tip angles outside the validity of the rotating-wave approximation and in low static field.

A hybrid numeric-analytic solution for pulsed CEST.

Chemical exchange saturation transfer (CEST) methods measure the effect of magnetization exchange between solutes and water. While CEST methods are often implemented using a train of off-resonant shaped RF pulses, they are typically analyzed as if the irradiation were continuous. This approximation does not account for exchange of rotated magnetization, unique to pulsed irradiation and exploited by chemical exchange rotation transfer methods. In this work, we derive and test an analytic solution for the steady-state water signal under pulsed irradiation by extending a previous work to include the effects of pulse shape. The solution is largely accurate at all offsets, but this accuracy diminishes at higher exchange rates and when applying pulse shapes with large root-mean-squared to mean ratios (such as multi-lobe sinc pulses).

Simulated basis sets for semi-LASER: the impact of including shaped RF pulses and magnetic field gradients

ObjectiveTo study the need for inclusion of shaped RF pulses and magnetic field gradients in simulations of basis sets for the analysis of proton MR spectra of single voxels of the brain acquired with a semi-LASER pulse sequence.Materials and methodsMRS basis sets where simulated at different echo times with hard RF pulses as well as with shaped RF pulses without or with magnetic field gradients included. The influence on metabolite concentration quantification was assessed using both phantom and in vivo measurements. For comparison, simulations and measurements were performed with the PRESS pulse sequence.ResultsThe effect of including gradients in the simulations was smaller for semi-LASER than for PRESS, however, still noticeable. The difference was larger for strongly coupled metabolites and at longer echo times. Metabolite quantification using semi-LASER was thereby less dependent on the inclusion of gradients than PRESS, which was seen in both phantom and in vivo measurements.DiscussionThe inclusion of the shaped RF pulses and magnetic field gradients in the simulation of basis sets for semi-LASER is only important for strongly coupled metabolites. If computational time is a limiting factor, simple simulations with hard RF pulses can provide almost as accurate metabolite quantification as those that include the chemical-shift related displacement.

Numerically optimized band-selective pulses in SOFAST-HMQC experiments for biomolecular NMR

This work demonstrates the efficacy of numerically optimized band-selective pulses in 2D fast-pulsing NMR pulse sequences of the SOFAST-HMQC variety. In order to achieve robust band selectivity the amplitude and phase of the shaped RF pulses are modulated according to a numerically optimized function. During the pulse duration, the spin trajectories evolve along complex and often unexpected pathways. The pulses have been designed using the GRAPE algorithm and are experimentally implemented on a model protein ubiquitin (13C, 15N labeled). Signal to noise ratios of peaks have been computed and compared for the different experiments performed using both numerically optimized band-selective pulses and standard pulse shapes. The numerically optimized pulses perform better in terms of signal enhancement and phase, as compared to standard pulse shapes.

Study of dynamics in pentachlorophenol in the solid state by 1D Nuclear Quadrupole Resonance exchange spectroscopy

The paper reports studies of the dynamic process in pentachlorophenol (PCP) in the solid state by one-dimensional Nuclear Quadrupole Resonance (NQR) exchange spectroscopy employing shaped rf pulses. Pentachlorophenol exhibits intramolecular hydrogen bonding between the hydroxyl proton and one of the o-chlorines. Reorientation of the hydrogen bond with migration of the hydroxyl proton from one o-chlorine to the other is possible in a wide temperature range. In this process the two o-chlorines are exchanged; at the same time this implies the exchange of the two m-chlorines as well, which we have chosen to monitor, modeling it as a two-site exchange process. A detailed analysis of the appropriate Bloch-McConnell equation is performed to formulate the relevant kinetic matrix. The exchange pulse sequence implemented by us involves suitably modulated shaped RF pulses to achieve simultaneous on-resonance excitation of the two exchanging sites. This approach results in a clear definition of the exchange time window and requires only a short experiment time. Kinetic parameters, namely, the spin-lattice relaxation rate and the exchange rate are deduced from the experimental signal intensities. Variable temperature measurements in the range 213 K–318 K have been carried out to extract the activation parameters as well.

Tailored Excitation by Shaped RF Pulses. Optimization and Implementation of Excitation Containing a Notch

Design optimization and practical implementation of tailored π/2 pulses able to provide a flat null at a narrow band of resonance frequencies (several hundred hertz) with uniform excitation and minimum phase distortion of alt other resonance frequencies (±5000 Hz) are described. The self-refocusing pulses are capable of being programmed into typical commercial NMR spectrometers and possess a practical duration (1-3 ms). A conjugate gradient method is employed in shape-design optimization. For practical implementation on a 500 MHz spectrometer, the optimized pulses are approximated by a train of width-modulated DANTE pulses. Implementation of one such shaped DANTE pulse, termed a NODE-1 pulse, on a spectrometer and application to a DNA oligomer are demonstrated via a one-dimensional proton NMR spectrum of the DNA duplex in H 2O with suppression of the water proton resonance and flat excitation of all frequencies including the imino proton resonances. Further application to a two-dimensional nuclear Overhauser effect spectrum of the DNA duplex in deuterium oxide solution with null excitation at frequencies corresponding to H2′ and H2″ resonances is illustrated.

A selective excitation/B0 gradient technique for high-resolution 1H NMR studies of metabolites via zero-quantum coherence and polarization transfer.

A new method for selective observation of scalar coupled metabolites by either zero-quantum coherence transfer or polarization transfer with concurrent water suppression in a single acquisition was developed. Gaussian shaped RF pulses were used to selectively generate multiple-quantum and zero-quantum coherence in the metabolite of interest, single quantum (including water) and double quantum coherences were then dephased under the influence of a B0 field gradient and the surviving zero-quantum coherence was converted to observable metabolite signal. The duration of the gradient application and the frequency and angle of the final selective read pulse determined whether a polarization transfer or a coherence transfer signal was observed. Water suppression factors of around 8000 were achieved which allowed operation of the receiver at high gain levels resulting in greatly improved signal to noise in the metabolite spectra. The CH3 and CH resonances of lactate in a mouse brain homogenate were selectively edited and the method was also applied to selective editing of ethanol.

Homonuclear coherence transfer experiments using shaped RF pulses having a tailored phase profile

Homonuclear coherence transfer experiments using shaped RF pulses having a tailored phase profile

A novel analysis of selective excitation and phase reversal

The Bloch equations are reanalyzed in a spherical coordinate system for shaped RF pulses applied in the presence of a static magnetic field gradient. In such a system the behavior of large angle-selective RF pulses can be better compared to small angle nutations and in particular the performance of 180° pulsed use for spin inversion and phase reversal can be better described. It is found that the major problem using conventional 180° sinc-shaped pulses is the phase change that arises when the magnetization passes through the transverse plane. Alterations in the magnetic field gradient compensate for such a phase change and thereby improve the performance of selective pulses for both spin inversion and phase reversal.