NMR Investigations of MAGnetization-Induced Non-linear Effects (IMAGINE)

Globally large efforts are dedicated to improve the sensitivity of NMR mainly via two complementary approaches: (i) increasing nuclear polarization by ever-increasing magnetic fields or more efficiently by exploiting transiently polarized species and (ii) improving the detection, in particular through the use of cold probes. However these developments entail the appearance of new phenomena related to the non-linear evolution of nuclear magnetization in liquid samples. In most cases they result from the intricate combination of (i) the non-linear coupling between the nuclear magnetization and the detection coil (radiation damping) and (ii) the enhanced contribution of long- range magnetic interactions, not averaged out by the Brownian motion (distant dipolar fields, DDFs). These effects are actually met in a wide range of other physical systems (such as Bose-Einstein condensates, superfluid 3He, or quantum entangled spin systems). The first aim of this project consists in significantly improving the understanding of these new physical phenomena and their consequences on the NMR observables: for instance deciphering the complex line shapes of nuclear spin- noise spectra acquired with cold probes, explaining the origin of multiple maser emissions, or controlling DDF- induced spin dynamics instabilities through multiple rf-pulses. Such studies will require means to increase and control both the magnetization and its cross-talk with the detection coil. To this end hyperpolarized noble gases or parahydrogen, areas of expertise of two partners, will be employed whenever possible. Use of cold probes as well as development of micro-coils will further lower the detection threshold of NMR, allowing us to induce and study novel effects. The second major aim of the project is either to find new ways to circumvent the drawbacks resulting from these non-linear effects (such as unreliable suppression of the solvent signals), or to propose new approaches to turn them into advantages. For instance, as the coupling between the magnetization and the coil tremendously enhances the detection sensitivity of spin-noise, the latter is expected to be more sensitive than classical pulse-acquisition schemes for very small numbers of spins. Also, DDFs induced by large magnetization allow new approaches in MRI and localized NMR spectroscopy, thanks to intermolecular multiple-quantum coherences between target molecules and solvent or hyperpolarized species. The consortium comprises four teams. Three belong to academic institutions and have the leadership in their respective research domains. The pivot is Partner 1, who has already worked with all the other Partners. The Austrian group (Partner 2) contributes with crucial experience in theory and practice of spin-noise phenomena on protons and provides the chemistry background required for the main-stream spectroscopic applications in high resolution NMR. Partner 3 specializes in optical pumping techniques and in low field NMR with full control of non-linear effects. The fourth Partner is a small company offering a-la-carte solutions in NMR spectroscopy and imaging; it will benefit from efficient transfers of knowledge and technologies and will provide hardware solutions to the academic partners for further enhancement of their capabilities to explore and exploit non-linear NMR. The Partners will take advantage of their complementary cultures, know-how and experimental apparatuses to treat various theoretical questions and to propose a variety of experimental applications, of different levels of risk and complexities. At the end of the project, the fundamental understanding of the phenomena arising from non-linear effects will provide a set of novel solutions for long-standing issues of NMR: new technologies to improve sensitivity and selectivity in NMR-spectroscopy and spatial resolution in MR-imaging based on scientifically sound extendable concepts of non-linear spin dynamics.

In this international (AT-FR) cooperation project, fundamental research targeted non-linear phenomena, which appear in nuclear magnetic spectroscopy and imaging, whenever a large amount of magnetization is present, mostly in liquid phase. These effects result from the distant dipolar fields (DDF) and the strong coupling between the magnetization and the detection coil, which is usually referred to as radiation damping (RD). The Austrian group focused on manifestations of non-linear phenomena involving spin noise detected NMR. For this purpose, through intense mutual exchange of results on novel experimental phenomena (mainly obtained in Austria) and new theoretical developments (mainly obtained in France) new applications could introduced. Relying on detailed theoretical work, the spin-noise detection scheme has been extended to new applications: (1) It was demonstrated that 2D NMR experiments could be successfully performed using a nuclear spin-noise detection scheme. (2) New unexpected frequency shifts under the tuning conditions leading to optimal signal detection were detected and a theoretical framework was developed that links this effect to properties of the detection circuit. Based on this observation a new theoretical model, which takes into account the electronic properties of the radio-frequency preamplifier, was derived in collaboration with the group in Saclay, and tested against extensive experimental measurements. (3) Nuclear spin-noise was shown to allow the direct detection of the secondary isotopic effects making use of a non-linear enhancement caused by multi-spin radiation damping when using cooled-coil probes. (4)Owed to the novel theoretical concept effects of small static magnetic field gradients causing spectral discontinuities, which had been discovered by the Linz group in 2009 could finally be explained on a physical basis. Finally the know-how gained could be used in new applications with new collaboration partners for monitoring dynamic nuclear polarization (DNP) and ultra-low temperature NMR. Additionally, new phenomena were observed, which allow to use spin noise detection in relaxation and diffusion studies on highly concentrated or polarized samples.In summary, the scope of NMR methodology was pushed beyond its conventional limits by exploiting non-linear effects and passive detection by nuclear spin noise.