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Nuclear magnetic resonance studies of paramagnetic relaxation enhancement at high magnetic fields : methods and applications

Authors
  • Nasser Din, Rami
Publication Date
Dec 14, 2023
Source
HAL-Descartes
Keywords
Language
English
License
Unknown
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Abstract

Nuclear Magnetic Resonance (NMR) is an analytical technique widely used in research fields such as chemistry, physics, and biomedicine. One of its most prominent applications is Magnetic Resonance Imaging (MRI) for both, medical and technical investigations. In this context, Paramagnetic Relaxation Enhancement (PRE) is an important research area, particularly with the increasing use of high magnetic fields in MRI. PRE changes the MRI image contrast by speeding up the nuclear spin relaxation of typically proton spins of water via their magnetic interactions with electron spins of added paramagnetic compounds. Therefore, compounds exhibiting PRE are used as contrast agents in MRI. PRE depends on the concentration of the paramagnetic compounds in the solvent. The PRE of a compound at a given field is measured by its relaxivity, defined as the enhancement of the relaxation rate of the water proton normalized by its concentration. Despite many years of research on PRE, there are still interesting questions about the microscopic mechanisms.In this context, it is important to measure the field dependence of PRE up to the highest possible magnetic field. However, most PRE studies end up at frequencies below 800 MHz (18.8 T). This frequency range can be extended beyond the current superconducting magnet limit of 28.2 T by employing resistive magnets.The availability of resistive magnets at the LNCMI laboratory in Grenoble opens the way for PRE studies up to 33 T (1.4 GHz) and beyond. However, the limited field quality of resistive magnets, field inhomogeneity and fluctuations make NMR experiments a technical challenge. For this reason, during this thesis, we extensively worked on the development of NMR instrumentation and methods to overcome the disadvantage of resistive magnets including a wideband NMR setup up to 1.4 GHz for microliter sample volume, single scan NMR, and tailored data analysis routines.The validity of these methods was confirmed by performing PRE studies on aqua-solutions of lanthanides that have been extensively investigated since the early days of NMR. Much of their PRE behavior is known, which makes them ideal benchmark samples for the comparison between PRE studies performed on superconducting and resistive magnets. They provide information on the quality of the resistive magnet data and the identification of potential systematic errors. In addition, the extension of the PRE field range up to 1.4 GHz of these relatively simple lanthanide complexes is expected to provide important information for the analysis of more complex compounds.Moreover, the trend towards future high-field MRI applications requires the development of new, effective contrast agents due to the reduced relaxivity of conventional PRE compounds based on Gd(III) at high fields. One of the strategies is to investigate other lanthanide-based complexes. Therefore, new lanthanide-based complexes like paramagnetic polyoxometalates (PM-POMs) have been synthesized, and investigated over a wide range of frequencies from 20 MHz up to 1.4 GHz using commercial permanent and superconducting magnets, and non-standard NMR in the resistive magnets for frequencies above 800 MHz. They indicate that these compounds are potential candidates for contrast agents, especially at high magnetic fields.

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