Variable rotation composite pulses for high resolution nuclear magnetic resonance using inhomogeneous magnetic and radiofrequency fields

Design of a three-loop asymmetric coil producing a homogeneous radiofrequency B1 field gradient along the axis of a vertical sample tube

A coil system generating a vertical radio-frequency (rf) field gradient (B₁ gradient) has been built for surrounding, in a horizontal magnet, a vertical sample (object) of axial symmetry. The system comprises three coaxial loops with an overall shape either spherical or ellipsoidal. The geometry has been theoretically and experimentally devised for producing a very uniform gradient (cancellation of B₁ derivatives from second order up to sixth order) in the central region where a vertical receiver/transmitter coil is installed. The latter is of the saddle-shaped type and is geometrically and electrically decoupled from the gradient coil system. This receiver/transmitter coil not only ensures an optimal signal reception but, in addition, is able to deliver perfectly homogeneous rf hard pulses which are mandatory in most NMR experiments. In its present design, the system delivers a uniform gradient in a limited region but could be extended at will. Its main advantages over static field gradients (B₀ gradients) appear clearly in the case of very short transverse relaxation times. This property has been emphasized in the case of experiments leading to the measurement of diffusion coefficients. Also, this system would be suitable for chemical shift imaging (CSI) experiments as confirmed by a preliminary test experiment.

Reimagining magnetic resonance instrumentation using open maker tools and hardware as protocol

Over the course of its history, the field of nuclear magnetic resonance spectroscopy has been characterized by alternating periods of intensive instrumentation development and rapid expansion into new chemical application areas. NMR is now both a mainstay of routine analysis for laboratories at all levels of education and research. On the other hand, new instrumentation and methodological advances promise expanded functionality in the future. At the core of this success is a community fundamentally dedicated to sharing ideas and collaborative advancements, as exemplified by the extensive remixing and repurposing of pulse sequences. Recent progress in modularity, automation, and 3D printing have reignited the tinkering spirit and demonstrate great promise to mature into a maker space that will enable similarly facile sharing of new applications and broader access to magnetic resonance.

High-resolution 2D NMR spectra in inhomogeneous fields based on intermolecular multiple-quantum coherences with efficient acquisition schemes

High-resolution 2D NMR spectra in inhomogeneous fields can be achieved by the use of intermolecular multiple-quantum coherences and shearing reconstruction of 3D data. However, the long acquisition time of 3D spectral data is generally unbearable for in vivo applications. To overcome this problem, two pulse sequences dubbed as iDH-COSY and iDH-JRES were proposed in this paper. Although 3D acquisition is still required for the new sequences, the high-resolution 2D spectra can be obtained with a relatively short scanning time utilizing the manipulation of indirect evolution period and sparse sampling. The intermolecular multiple-quantum coherence treatment combined with the raising and lowering operators was applied to derive analytical signal expressions for the new sequences. And the experimental observations agree with the theoretical predictions. Our results show that the new sequences possess bright perspective in the applications on in vivo localized NMR spectroscopy.

Diffusion damping during adiabatic z-rotation pulses for NMR spectroscopy in inhomogeneous magnetic fields

High-resolution nuclear magnetic resonance spectra from samples located in inhomogeneous static and radio frequency magnetic fields can be obtained by applying a train of z-rotation radio frequency pulses to repeatedly refocus the inhomogeneous broadening during signal detection. z-rotation pulses based on an adiabatic double passage are effective over wide bandwidths using a limited amount of radio frequency power at the expense of being time consuming and, consequently, sensitive to motion of the spin bearing molecules. The signal damping resulting from molecular self-diffusion during the pulse was studied experimentally and using Brownian dynamics simulations. The results show that the analytical expression for diffusion damping during a double spin echo is a reasonable approximation for the signal decay during an adiabatic z-rotation pulse. Methods to alleviate the effects of diffusion are discussed.

Single-sided sensor for high-resolution NMR spectroscopy

The unavoidable spatial inhomogeneity of the static magnetic field generated by open sensors has precluded their use for high-resolution NMR spectroscopy. In fact, this application was deemed impossible because these field variations are usually orders of magnitude larger than those created by the microscopic structure of the molecules to be detected. Recently, chemical shift resolved NMR spectra were observed for the first time outside a portable single-sided magnet by implementing a method that exploits inhomogeneities in the rf field designed to reproduce variations of the static magnetic field. In this communication, we describe in detail the magnet system built from permanent magnets as well as the rf coil geometry used to compensate the static field variations.

NMR spectroscopy in inhomogeneous B0 and B1 fields with non-linear correlation

Resolved NMR spectra from samples in inhomogeneous B0 and B1 fields can be obtained with the so-called "ex situ" methodology, employing a train of composite or adiabatic z-rotation RF pulses to periodically refocus the inhomogeneous broadening during the detection of the time-domain signal. Earlier schemes relied on a linear correlation between the inhomogeneous B0 and B1 fields. Here the pulse length, bandwidth, and amplitude of the adiabatic pulses of the hyperbolic secant type are adjusted to improve the refocusing for a setup with non-linear correlation. The field correlation is measured using a two-dimensional nutation experiment augmented with a third dimension with varying RF carrier frequency accounting for off-resonance effects. The pulse optimization is performed with a computer algorithm using the experimentally determined field correlation and a standard adiabatic z-rotation pulse as a starting point for the iterative optimization procedure. The shape of the z-rotation RF pulse is manipulated to provide refocusing for the conditions given by the sample-, magnet-, and RF-coil geometry.

"Shim pulses" for NMR spectroscopy and imaging

A way to use adiabatic radiofrequency pulses and modulated magnetic-field gradient pulses, together constituting a "shim pulse," for NMR spectroscopy and imaging is demonstrated. These pulses capitalize on phase shifts derived from probe gradient coils to compensate for nonlinear intrinsic main magnetic field homogeneity for spectroscopy, as well as for deviations from linear gradients for imaging. This approach opens up the possibility of exploiting cheaper, less-than-perfect magnets and gradient coils for NMR applications.

Three-dimensional phase-encoded chemical shift MRI in the presence of inhomogeneous fields

A pulse sequence consisting of an excitation pulse and two adiabatic full-passage pulses with scaled relative peak amplitudes is combined with phase encoding to recover chemical shift information within 3D images in a 1D inhomogeneous static magnetic field with a matched rf field gradient. The results are discussed in the context of ex situ magnetic resonance and imaging. The future directions of our research in implementing the ex situ technique in a real one-sided system are also discussed.

Self-diffusion measurements with chemical shift resolution in inhomogeneous magnetic fields

A methodology for chemical shift resolved molecular self-diffusion measurements in time-independent static and radiofrequency field gradients is demonstrated. Diffusion encoding is provided by a stimulated echo sequence with additional z-storage that allows for a change of diffusion time without affecting the relaxation weighting. The signal is acquired stroboscopically between the pulses of a train of adiabatic double passages that induces a z-rotation counteracting the phase spread resulting from precession in the inhomogeneous static field, as demonstrated in recent approaches to the goal of high-resolution "ex situ" NMR. Simulations of the pulse sequence show that the acquired signal results from the desired coherence pathway. Successful demonstrations of the experiment were performed on a mixture of water and isopropanol.