Heteronuclear and homonuclear radio-frequency-driven recoupling

The radio-frequency-driven recoupling (RFDR) pulse sequence is used in magic-angle spinning (MAS) NMR to recouple homonuclear dipolar interactions. Here we show simultaneous recoupling of both the heteronuclear and homonuclear dipolar interactions by applying RFDR pulses on two channels. We demonstrate the method, called HETeronuclear RFDR (HET-RFDR), on microcrystalline SH3 samples at 10 and 55.555 kHz MAS. Numerical simulations of both HET-RFDR and standard RFDR sequences allow for better understanding of the influence of offsets and paths of magnetization transfers for both HET-RFDR and RFDR experiments, as well as the crucial role of XY phase cycling.

Heteronuclear and Homonuclear Radio Frequency Driven Recoupling

The Radio Frequency Driven Recoupling (RFDR) pulse sequence is used in magic-angle spinning (MAS) NMR to recouple homonuclear dipolar interactions. Here we show simultaneous recoupling of both the heteronuclear and homonuclear dipolar interactions by applying RFDR pulses on two channels. We demonstrate the method, called HETeronuclear RFDR (HET-RFDR) on microcrystalline SH3 samples at 10 kHz and 55.555 kHz MAS. Numerical simulations of both HET-RFDR and standard RFDR sequences allow better understanding of the influence of offsets, paths of magnetization transfers for both HET-RFDR and RFDR experiments as well as the crucial role of XY phase cycling.

Heteronuclear and Homonuclear Finite Pulse Radio Frequency Driven Recoupling

Homonuclear finite-pulse radio frequency driven recoupling (fp-RFDR) has been broadly used in multi-dimensional magic-angle spinning (MAS) solid-state NMR experiments over the past 20 years. The theoretical and the simulated descriptions of this method were presented during that time, resulting in an understanding of the influence of chemical shift offset, finite pulse effects, and dipolar truncation. Here we present an operator analysis of both heteronuclear and homonuclear fp-RFDR. By numerical simulation, we show which operators are involved in the longitudinal exchange for both heteronuclear and the well-known homonuclear sequences. This results in a better understanding of the influence of phase cycling of the fp-RFDR pulses, which is typically a variant of XY cycling. We investigate the heteronuclear and homonuclear fp-RFDR signals and evolution of the operators through the fp-RFDR block. We show the convergence of the evolutions of the heteronuclear and homonuclear fp-RFDR signals at even numbers of rotor periods and completely different evolution between them. We demonstrate heteronuclear 1H- 13C and 1H-15N fp-RFDR magnetization transfer using a microcrystalline SH3 sample at 100 kHz MAS.

A Modified Phase Cycling Method for Complex-Valued MRI Reconstruction

The phase cycling method is a state-of-the-art method to reconstruct complex-valued MR image. However, when it follows practical two-dimensional (2D) subsampling Cartesian acquisition which is only enforcing random sampling in the phase-encoding direction, a number of artifacts in magnitude appear. A modified approach is proposed to remove these artifacts under practical MRI subsampling, by adding one-dimensional total variation (TV) regularization into the phase cycling method to “pre-process” the magnitude component before its update. Furthermore, an operation used in SFISTA is employed to update the magnitude and phase images for better solutions. The results of the experiments show the ability of the proposed method to eliminate the ring artifacts and improve the magnitude reconstruction.