Nonresonant multiple spin echoes

An ultra-broadband low-frequency magnetic resonance system

MR probes commonly employ resonant circuits for efficient RF transmission and low-noise reception. These circuits are narrow-band analog devices that are inflexible for broadband and multi-frequency operation at low Larmor frequencies. We have addressed this issue by developing an ultra-broadband MR probe that operates in the 0.1-3MHz frequency range without using conventional resonant circuits for either transmission or reception. This "non-resonant" approach significantly simplifies the probe circuit and allows robust operation without probe tuning while retaining efficient power transmission and low-noise reception. We also demonstrate the utility of the technique through a variety of NMR and NQR experiments in this frequency range.

Low-frequency NMR with a non-resonant circuit

Nuclear magnetic resonance typically utilizes a tuned resonance circuit with impedance matching to transmit power and receive signal. The efficiency of such a tuned coil is often described in terms of the coil quality factor, Q. However, in field experiments such as in well-logging, the circuit Q can vary dramatically throughout the depth of the wellbore due to temperature or fluid salinity variations. Such variance can result in erroneous setting of NMR circuit parameters (tuning and matching) and subsequent errors in measurements. This paper investigates the use of a non-resonant transmitter to reduce the circuit sensitivity on Q and demonstrates that such circuits can be efficient in delivering power and current to the coil. We also describe a tuned receiver circuit whose resonant frequency can be controlled digitally. Experimental results show that a range of common NMR experiments can be performed with our circuits.

Ex situ NMR in highly homogeneous fields: 1H spectroscopy

Portable single-sided nuclear magnetic resonance (NMR) magnets used for nondestructive studies of large samples are believed to generate inherently inhomogeneous magnetic fields. We demonstrated experimentally that the field of an open magnet can be shimmed to high homogeneity in a large volume external to the sensor. This technique allowed us to measure localized high-resolution proton spectra outside a portable open magnet with a spectral resolution of 0.25 part per million. The generation of these experimental conditions also simplifies the implementation of such powerful methodologies as multidimensional NMR spectroscopy and imaging.

Encoding of diffusion and T1 in the CPMG echo shape: single-shot D and T1 measurements in grossly inhomogeneous fields

We present a new approach of NMR measurements in the presence of grossly inhomogeneous fields where information is encoded in the echo shape of CPMG trains. The method is based on sequences that consist of an initial encoding sequence that generates echoes with contributions from at least two different coherence pathways that are then both refocused many times by a long string of closely spaced identical pulses. The generated echoes quickly assume an asymptotic shape that encodes the information of interest. High signal-to-noise ratios can be achieved by averaging the large number of echoes. We demonstrate this approach with different implementations of the measurements of longitudinal relaxation time, T(1), and diffusion coefficient, D. It is shown that the method can be used for novel single-shot measurements.

Time-of-flight flow imaging using NMR remote detection

A time-of-flight imaging technique is introduced to visualize fluid flow and dispersion through porous media using NMR. As the fluid flows through a sample, the nuclear spin magnetization is modulated by rf pulses and magnetic field gradients to encode the spatial coordinates of the fluid. When the fluid leaves the sample, its magnetization is recorded by a second rf coil. This scheme not only facilitates a time-dependent imaging of fluid flow, it also allows a separate optimization of encoding and detection subsystems to enhance overall sensitivity. The technique is demonstrated by imaging gas flow through a porous rock.

High-resolution NMR of static samples by rotation of the magnetic field

Mechanical rotation of a sample at 54.7 degrees with respect to the static magnetic field, so-called magic-angle spinning (MAS), is currently a routine procedure in nuclear magnetic resonance (NMR). The technique enhances the spectral resolution by averaging away anisotropic spin interactions thereby producing isotropic-like spectra with resolved chemical shifts and scalar couplings. It should be possible to induce similar effects in a static sample if the direction of the magnetic field is varied, e.g., magic-angle rotation of the B0 field (B0-MAS). Here, this principle is experimentally demonstrated in a static sample of solid hyperpolarized xenon at approximately 3.4 mT. By extension to moderately high fields, it is possible to foresee interesting applications in situations where physical manipulation of the sample is inconvenient or impossible. Such situations are expected to arise in many cases from materials to biomedicine and are particularly relevant to the novel approach of ex situ NMR spectroscopy and imaging.

BlochLib: a fast NMR C++ tool kit

Computational power, speed, and algorithmic complexity are increasing at a continuing rate. As a result, scientific simulations continue to investigate more and more complex systems. Nuclear magnetic resonance (NMR) is no exception. NMR theory and language is extremely well developed, that simulations have become a standard by which experiments are measured. Nowadays, complex computations can be performed on normal workstations and workstation clusters. Basic numerical operations have also become extremely optimized and new computer language paradigms have become implemented. Currently there exists no complete NMR tool kit which uses these newer techniques. This paper describes such a tool kit, BlochLib. BlochLib is designed to be the next generation of NMR simulation packages; however, the basic techniques implemented are applicable to almost any problem. BlochLib enables the user to simulate almost any NMR idea both experimental or theoretical in nature. Both classical and quantum mechanical techniques are included and demonstrated, as well as several powerful user interface tools. The total tool kit and documentation can be found at http://waugh.cchem.berkeley.edu/blochlib/.