Vol 47, No 10 (2016)

Manifestations of the spin coherence transfer induced by the exchange and dipole–dipole interactions between spin probes in dilute solutions in the electron paramagnetic resonance (EPR) spectra have been studied. The perturbation theory of manifestations of the spin coherence transfer in the EPR spectra of nitroxide free radicals elaborated by one of the authors Salikhov (Applied Magnetic Resonance 38:237–256, 2010) has been generalized with allowance for the super hyperfine structure of the EPR spectra. For ¹⁴N nitroxide radicals, the total EPR spectrum was presented as a sum of three independent components in the case of slow and intermediate spin coherence transfer rates. The shapes of these components were found. The side components of the EPR spectrum contain the absorption and dispersion contributions and, as a result, have the asymmetric (mixed) shapes. These asymmetric components can be presented as J = J_absorption ± pJ_dispersion. The p value is found for the arbitrary super hyperfine structure of the spectrum. In the slow and intermediate spin coherence transfer rate regime, the parameter p is independent of the super hyperfine interactions in the nitroxide radicals, but the shapes of J_absorption and J_dispersion terms depend on the super hyperfine structure of the nitrogen components of the nitroxide EPR spectrum and on the spin coherence transfer rate. It is confirmed theoretically that a good strategy to evaluate the spin coherence transfer rate from the EPR spectra is using the dispersion contribution to the shape of the EPR spectra of nitroxide free radicals. An algorithm is suggested and tested for determining the spin coherence transfer and spin decoherence rates.

The enhancement of X-band spin–lattice and spin–spin relaxation rates for the nitroxide tempone (2,2,6,6-tetramethyl-4-oxo-piperidin-1-oxyl) in 1:1 water:glycerol by Dy³⁺, Er³⁺, Tm³⁺ or Co²⁺ was examined between 20 and 200 K. Nitroxide relaxation rates were measured by two-pulse spin echo and three-pulse inversion recovery. The impact of the rapidly relaxing metal aquo ions on 1/T₁ of the nitroxide increases in the order Co²⁺ ~ Er³⁺ < Dy³⁺ < Tm³⁺. The maximum spin–lattice relaxation enhancement occurs at about 35 K for Dy³⁺, 40 K for Er³⁺, and 60 K for Co²⁺. When the metal ion is bound to the chelator diethylenetriamine pentaacetic acid (DTPA) the maximum enhancements for Dy(DTPA)²⁻ and Er(DTPA)²⁻ shift to about 80 K. The maximum enhancement is proposed to occur when 1/T₁ for the metal ion is approximately equal to the resonance frequency for the nitroxide. Interaction with the paramagnetic metal ion causes a much larger fractional change in 1/T₁ than for 1/T₂. Below about 20 K the enhancement of nitroxide 1/T₂ increases, which is attributed to relaxation of the metal ions at rates comparable to the electron–electron dipolar coupling, expressed in frequency units.

Signal-to-noise ratio (SNR) is an important factor in magnetic resonance imaging (MRI), and it strongly depends on the structure of radio frequency (RF) coils. To obtain a high SNR and uniform image in a vertical-field MRI at 0.5 T, a four-channel received coil has been designed and optimized by establishing the relationship between coil geometry and SNR. Then, the most efficient design of coil array is optimized by the Particle Swarm Optimization (PSO) algorithm. After optimization, the coil is manufactured, where the decoupling is implemented with only inductors and operated at the permanent magnet MRI system built in our laboratory. Finally, SNR map with pixel-by-pixel manner is applied to evaluate the imaging quality, which shows the accuracy between simulated and experimental results. Furthermore, a higher SNR and a more homogeneity in the image have been achieved by the optimized coil array. Hence, this optimized design for the phased-array received coil in vertical-field MRI is verified and applicable.

In the last decade, multidimensional Laplace nuclear magnetic resonance (NMR) was widely used in laboratory and oil industry. Compared to one-dimensional NMR, multidimensional NMR measurements have advantages in fluid recognition and content calculation, etc. However, the applications are limited due to its long measurement time. The paper presents a rapid T₁–T₂ measurement pulse sequence—drive equilibrium (DE) pulse sequence which can obtain fluid T₁–T₂ distribution down to two one-dimensional measurements and the obtained correlation maps are in good consistency with the results by traditional T₁ encoding pulse sequence. It is found that the performance of DE pulse sequence is related to measurement times. If we add measurement times, DE pulse sequence will work better. The sequence can be used in time limit experiments, like downhole fluid analysis and online reaction monitoring.