Pulse programmer is an integrating part of NMR spectrometer, which is used to generate RF pulse and to receive echo trains signals. Taking advantage of the synchronous performance and excellent timing control capacity of FPGA, a FPGA-based pulse programmer for low field NMR spectrometer was designed to generate pulse sequences for different NMR experiments. Flexible control of the frequency, phase and amplitude of the RF pulses could be achieved with this pulse programmer. Performance of the pulse programmer was evaluated with measurements of transverse and longitudinal relaxation rates of CuSO₄ solution under uniform magnetic field (0.540 Tesla). The pulse programmer was also used to collect echo trains signals with high signal-to-noise ratio.

In recent years, low-field magnetic resonance has increasingly demonstrated its application of relaxation and diffusion measurements for the study of materials, catalysts, cement hydration, fluid transport in rocks and soil, geological prospecting, and characterization of tissue properties for medical diagnosis. In particular, the application on porous materials has benefited tremendously from the development in the multi-dimensional methods. Porous media are ubiquitous in our environment and their microstructure (μm to mm scale) is essential in determining their properties and applications. This article will summarize a few key advances in basic physics and NMR methodology, and their applications.

Polarization of hyperpolarized xenon-129 gas prepared by spin exchange optical pumping (SEOP) is affected by many operating parameters. In this study, we measured polarization of hyperpolarized xenon-129 gas on a low field (0.002 T) NMR system, and used the results of such measurements to optimize the operating parameters of SEOP. The optimal operating temperature was first determined. Then the optimal building time for isotopically enriched and naturally abundant xenon-129 gases was determined to be 15 min and 22 min, respectively. The pressure and composition of the gases in the optical pumping process were then optimized to minimize spectral shift, broadening and lineshape change of the rubidium atom absorption lines. Finally, the optimal laser wavelength was determined. In conclusion, measuring polarization of hyperpolarized xenon-129 gas with low-field NMR provides an experimental basis for optimizing operating parameters of SEOP.

Low noise pre-amplifier is an important component in the radiofrequency (RF) receiving subsystem of magnetic resonance systems. The performance of pre-amplifier determines the quality of the final images directly. Currently, low noise pre-amplifiers on the market mostly are designed for high-field magnetic resonance systems. Few such low noise pre-amplifiers were designed specifically for low-field magnetic resonance systems. Moreover, commercially-available low noise pre-amplifiers are relatively expensive, and most of the designs use a complicated two-stage amplification structure. For these reasons, a low noise pre-amplifier with only one-stage amplification structure was developed in this study for 0.5 T low-field magnetic resonance systems using the advanced design system (ADS) software. The circuit design and layout effects were presented. Experimental measurement showed that the noise figure of on-resonance frequency 21 MHz was about 0.5 dB, and a gain of 30 dB was achieved.

Low field-nuclear magnetic resonance (LF-NMR) relaxation characteristics of glyceride mixture can be used in rapid monitoring and product analysis of lipid remodeling processes. In this study, the LF-NMR relaxation properties of single or mixed glyceryl oleate with different esterification degrees were studied. The results indicated that three peaks appeared in the T₂ relaxation map of glyceryl oleate. With increasing esterification degree or temperature, both the single component and multi-component relaxation time increased, and the ratios of peak area changed as well. The higher the esterification degree was, the larger the relaxation time changed. For the binary or ternary mixed glyceryl oleate systems, with the glycerol trioleate (GTO) ratio increased to 40%, the single and multi-component relaxation time, the peak area proportion S₂₂ all increased, while the peak area proportion S₂₃ decreased. Principal components analysis (PCA) showed that the LF-NMR relaxation characteristics of ternary system had regular distribution on the score plot with changing GTO ratio and glycerol dioleate (GDO)/glycerol monooleate(GMO) ratio.

Methods to map radiofrequency (RF) field generated by an RF coil/pulse have been developed. By measuring the changes of RF field in biological tissues when an RF pulse passing through, the electrical properties of the tissue could be retrieved and used for early diagnosis of diseases such as cancer. So far, RF field mapping is mainly done for birdcage coils at high fields. Less research has been performed on phased array coils at low fields. This work studied how loading would affect the distribution of RF field in low-field MRI with permanent magnet (i.e., 17.8 MHz). Both finite element simulation and experimental results demonstrated that coil loading changed the uniformity of RF field significantly, and the distortion of RF field under the loaded condition could reflect the electrical characteristics of the loaded biological tissues.

During in-situ exploitation of the oil shale, the connectivity of the pore structure of the reservoir directly affects not only the flow behavior and heat transfer efficiency of the heat-carrying medium, but also the diffusion and flow behaviors of the oil and gas. In this study, the T₂ spectra of saturated water and bound water in samples from the Fushun oil shale were measured with low-field nuclear magnetic resonance at different final pyrolysis temperatures (23~650℃). NMR pore parameters, including T₂ cutoff value of movable fluid, bound fluid porosity, saturated fluid porosity and permeability, were analyzed. The evolution of pore connectivity of the oil shale with final pyrolysis temperature was studied quantitatively. The results demonstrated that final pyrolysis temperature affected the variations of pore connectivity and the permeability of oil shale. The increase of total porosity could be attributed mainly to the increments of movable fluid. These results indicated that increases in final pyrolysis temperature enhanced the permeability and transport of oil and gas, providing a basis for further understanding of the evolution of pore structure in oil shale in-situ pyrolysis.

Based on the ring-down signal generation principle of nuclear magnetic resonance (NMR), we presented here a design scheme of transmitting and receiving separated short dead-time radio frequency (RF) coil composed of loop-gap resonator (LGR) coil and solenoid coil in low-field NMR system, to improve transmission efficiency by optimizing tuning and matching network. We also designed a fast RF switch and driver to fulfill the requirement of the RF coil. Based on the simulation results, the short dead-time RF coil was fabricated and applied to a self-developed 9.51 MHz portable low-field NMR spectrometer. The NMR experiments showed that the dead-time was reduced to less than 10 μs, which verified the feasibility of the design scheme.