Robin L. Armstrong

Sensitivity of magnetic-resonance current-density imaging

Abstract This paper analyzes the sensitivity of magnetic-resonance current-density imaging (CDI) to random noise and systematic errors with the goal of providing a protocol for achieving a targeted noise performance. All analyses are checked with an electrolytic phantom designed to create a uniform current density or with an equivalent computer model. CDIs sensitivity properties are split between low and high current-time regimes where random noise or systematic error, respectively, dominate. Large current-time products introduce nonlinear signal loss in the magnitude image. Image alignment errors, spatial linearity, pixel size calibration, and current-dependent ringing can exceed random noise. In the low current-time limit, random noise dominates and is proportional to the inverse cube of the planar pixel dimensions. A spin-echo sequence minimizes the current-density noise for current integration times and echo times in the neighborhood of T 2 . The relation between current-density noise and the MR image resolution and signal-to-noise ratio is formulated, allowing one to predict the noise for experimental planning. The lower practical limit of sensitivity is estimated to be 0.1 A/m 2 for a 1.5 × 1.5 × 5 mm voxel.

Coupling scheme and probe damper for pulsed nuclear magnetic resonance single coil probe

A coupling scheme and a probe damper for use with a pulsed nuclear magnetic resonance single coil probe are described. The designs for both circuits incorporate the unique properties of PIN diodes. The performances of the circuits at the signal processing level are evaluated at 4.3, 15.78, and 49.0 MHz.

Proton magnetic relaxation study of molecular reorientation in NH4ReO4

Abstract Measurements of the proton T 1 , T 1 ρ , and T 1D in NH 4 ReO 4 are reported as a function of temperature over the range from 60 to 300 K. The temperature dependence of T 1 can be described in terms of the dipolar interaction between protons within an NH 4 + ion made time dependent by rapid classical reorientation at a rate which may be described by an Arrhenius relation. The activation energy for the molecular reorientation is 2.2 kcal mole −1 . Over limited regions of temperature this same model can also be used to explain the T 1 ρ and T 1D results. However, at low temperatures the T 1 ρ data suggest the importance of quantum mechanical tunneling, and at high temperatures the dipolar coupling between the proton and rhenium spins rendered time dependent by the rapid rhenium spin-lattice relaxation dominates both T 1 ρ and T 1D . Due to a strong angular dependence of the relaxation rates associated with the proton-rhenium interaction, nonexponential decays occur for a powdered sample as used in these experiments. From the T 1 ρ and T 1D results it is concluded that the rhenium spin-lattice relaxation is dominated by the quadrupolar Raman spin-phonon interaction. There is no evidence in any of the measurements for a previously postulated order-disorder phase transition in the vicinity of 200 K and involving the NH 4 + ions.

Nuclear Spin Relaxation and the Collision Frequency in Dense Gases

The relation between nuclear spin‐lattice relaxation (T1) measurements and the collision frequency in dense gases is discussed. It is shown that the density dependence of T1 in compressed H2 gas can be accounted for by assuming a hard‐sphere potential model and evaluating the Enskog correction to the collision frequency in a dense fluid. This comparison represents the first direct test of the collision frequency in a dense fluid.

Structural properties and lattice dynamics of 5d transition metal antifluorite crystals

Abstract This article contains a review of the physics of 5d transition metal antifluorite crystals with emphasis on the new insights that have resulted from the research of the past ten years. Section 2 contains a discussion of the lattice dynamics of these crystals. A rigid ion model is proposed and the necessary input data as obtained from various forms of spectroscopy considered. Force constants, normal mode frequencies and eigenvectors are deduced for the prototype crystal K2ReCl6. Nuclear magnetic resonance evidence for molecular reorientations is given. Section 3 presents a description of structural phase transitions in terms of the Landau theory. Evidence for several rotative type phase transitions is outlined; the soft modes are identified. A brief reference to central modes and cluster excitations is included. Data suggesting another type of structural transition, distortive in nature, are introduced. Section 4 provides an overview of our knowledge of the transferred hyperfine interaction and of the magnetic structure in the antiferromagnetic phase. In section 5 three topics considered peripheral to the main body of the article are mentioned briefly. These are the nature of the chemical bond, the study of mixed crystals and protonic conduction.

Spin—Lattice Relaxation in Dilute Gases. I. Proton Relaxation in HCl, HBr, and HI

Proton spin—lattice relaxation times have been measured in gaseous HCl, HBr, and HI as a function of temperature and pressure in the dilute‐gas region. The observed temperature dependence is consistent with that expected for relaxation dominated by a spin—rotation interaction. Effective cross sections for the transfer of angular momentum to molecular rotation during collisions are deduced. The cross sections are large, indicating the importance of the long‐range forces between the molecular dipole moments.

Pure nuclear quadrupole resonance studies of structural phase transitions

Abstract Structural phase transitions of the tilting variety occur and transform certain cubic antifluorite and cubic perovskite structures to lower symmetry structures. This article provides a review of recent advances in our understanding of these phase transitions as revealed by pure nuclear quadrupole resonance (NQR) experiments in K2PtBr6, K2ReCl6, CsPbCl3, and K2OsCl6. In general, the results show that both NQR frequency and spin-lattice relaxation data may be analyzed to reveal the condensation of the rotary-lattice mode responsible for a particular transition. The similarity of the antifluorite and perovskite structures with respect to tilting type phase transitions is illustrated both by the behavior of the NQR data and by the results of rigid-ion model calculations. In particular, the K2PtBr6 data provide experimental confirmation of the dominance of the anharmonic Raman process as a relaxation mechanism for the bromine nuclei. The K2ReCl6 data provide an example for analysis in which the temperature dependence of the NQR frequency data is dominated by specific volume effects. The CsPbCl3 relaxation data reflect the extraordinary degree of anharmonicity present in the cubic phase. The substance K2OsCl6 provides evidence for the formation of tetragonal phase dynamic clusters in the cubic phase. In all instances the interpretation makes use of available information from other types of experiments. For example, the NQR frequency spectrum of CsPbCl3 is shown to be consistent with structural determinations from neutron scattering studies.

Spin‐Lattice Relaxation in Dilute Gases. II. 19F Relaxation in CF4 and SiF4

Fluorine spin‐lattice relaxation times have been measured in gaseous CF4 and SiF4 as a function of temperature and pressure in the dilute gas region. The observed temperature dependence is consistent with that expected for relaxation dominated by a spin‐rotation interaction. Effective cross sections for the transfer of angular momentum to molecular rotation during collisions are deduced. It is found that between one and three binary collisions are sufficient to randomize the molecular rotational angular momentum of these molecules in the temperature range studied.

Simple Bridge for Pulsed Nuclear Magnetic Resonance

An asymmetrical rf bridge for pulsed magnetic resonance experiments is described. The balancing adjustments, which can be made quickly and easily, ensure a proper impedance match to transmitter and receiver. The bridge is particularly well suited for pure nuclear quadrupole resonance experiments.

Proton and deuteron magnetic resonance study of the HD–He potential energy surface

The relaxation of hydrogen and deuterium nuclei in HD–He gas mixtures is studied both experimentally and theoretically in the temperature range 90–300 K. A rationalization is given for the temperature dependence of the proton and the deuteron relaxations in terms of the relative strengths of the proton and deuteron intramolecular couplings and the role played by those HD molecules in the ground rotational state. Using a recent ab initio potential, quantitative agreement is found between the temperature dependence of the spin–lattice relaxation time of the proton in HD, as calculated theoretically and determined experimentally. A similar comparison between the calculated and experimental temperature dependence of the spin–lattice relaxation time of the deuteron in HD gave only semiquantitative agreement. It is suggested that the difference in quantitative agreement may be attributed to the selectivity of the respective predominant relaxation mechanisms to slightly different aspects of the anisotropic compo...