ESPCI web site
Study and realization of a Nuclear Magnetic Resonance at low magnetic field device in an open and noisy environment
This work is focused on the study and the implementation of an experimental device for Nuclear Magnetic
Resonance (NMR), under weak magnetic fields in an unshielded and open environment. It takes place within a broader context of instrumental physics, such as materials spectroscopy, medical imaging and diagnostic assistance. NMR is, for this purpose, a powerful technique to probe the heart of the matter, study its
structure and molecular properties in a non-destructive and non-invasive way.
In this work, an experimental device for low field NMR application has been studied. Using low magnetic field offers the possibility to use much simpler and less expensive magnets, as well as limits eddy currents in conductive materials and promotes the differences between biological tissues in Magnetic Resonance
Imaging application. However, at low field, signals are very weak due to the strong dependence of the NMR signal on magnetic field strength when using conventional detection coil. To reduce this dependence and to obtain a sufficient signal detectability, a SQUID operating at low critical temperature is used. The SQUID is
combined with different types of gradiometer, whose purpose is to promote the sample signal from signals produced by distant spurious sources. By using analytical approach based on reciprocity principle, it was also shown that a planar configuration is more advantageous than axial configuration. The measurement system comprises also a superconducting switch, controlled by laser pulses via an optical fiber, which has been completely designed and developed during this thesis. It is used to protect the SQUID during the sample magnetization, to avoid dead time during signal acquisition phase and to control precisely all NMR sequences without adding much noise to the measurement device.
The final experimental device was used to measure NMR signals from water sample, in a fully open environment under the influence of different types of low frequency noises (50 Hz and its harmonics, transformers, cars moving, elevators...). For this purpose, low field NMR electronic was performed for acquiring signal and driving process. In addition, analog and digital signal processing methods were carried out. Hydrogen protons are pre-polarized under about 100 mT, while the relaxation is measured at very small field around 50 μT in the presence of the earth magnetic field giving frequencies around a few kilohertz.
Keywords: Magnetic sensor, Sensitivity map, Gradiometer, Pick-up coil, Nuclear Magnetic Resonance,
Magnetic Resonance Imaging, Low Field, Eddy current, SQUID.