Nuclear Magnetic Resonance (NMR) has continuously grown for the last 60 years and initiated the race for the highest (10-20 Tesla) and most homogeneous (1 ppb/cm3) magnetic fields. Magnetic Resonance Imaging (MRI) soon joined the race since sensitivity, resolution and contrast to noise ratio seemed to improve with higher magnetic fields. Magnetic Resonance projects are considered nowadays as large scale facilities, are dedicated to perform specific high-throughput tasks, require important funding (many MEuros) and are localized dedicated laboratories.
The development of prepolarization techniques (Dynamic Nuclear Polarization, Hyperpolarized gases) and highly sensitive detectors (SQUIDs, optical magnetometers etc.) have recently proved invaluable in detecting magnetic resonance at very low Larmor frequencies, and thus, in the presence of extremely low magnetic fields (microTesla) and use it to perform imaging even without a magnet! Additionally, novel concepts relying on spatially correlated magnetic fields allow nowadays for the use of inhomogeneous magnetic fields in high-resolution NMR and MRI. Finally, nuclear spin dynamics can be manipulated using average Hamiltonian theory, so that the effect of field inhomogeneities, concomitant tensor components and non-linearities is averaged to zero. Finally, in the low field regime, even the field itself can be manipulated. Anisotropic interactions in high-field are usually averaged by spinning the sample (magic angle sample spinning), and nowadays, using low fields, the old idea of magic angle field spinning can become reality.
These new developments will be reviewed, together with the basic underlying physical principles. My goal is to show that low-field NMR and MRI can offer a promising area of fundamental research with possible applications varying from portable MRI to oil well-logging and nuclear engineering.
Lab. de Structure et Dynamique par Résonance Magnétique CEA/Saclay/DSM/DRECAM/SCM/LSDRM