Novel Methods for High Resolution NMR in Inhomogeneous Fields

• Applicant: Professor Dr. Steffen Glaser
• Subject Area: Analytical Chemistry
• Term: from 2007 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 49580800

In the proposed work, we plan to develop novel methods for high resolution NMR spectroscopy in inhomogeneous static and radio-frequency fields. High resolution NMR experiments require extremely homogeneous magnetic fields to obtain chemical shift and J coupling information of nuclei, as these interactions are orders of magnitude smaller than the main Zeeman coupling to the external field B0. Achieving such homogeneity is not trivial, especially over relatively large volumes. This usually comes at the cost both in price of NMR systems and in size demanded by NMR magnets. Such homogeneities are not achievable when dealing with spatially heterogeneous tissues or employing remote NMR arrangements. Here we propose to develop very general methods based on time varying 3D gradients and spatially non-selective radio-frequency pulses for learning the distribution of inhomogeneous magnetic fields and then correcting for the phase acquired by the spins due to these inhomogeneities. With this correction, the chemical shift information of the nuclei can be extracted and high resolution spectra can be obtained. These methods can potentially replace dedicated shim coils in inexpensive magnets for specialized applications of traditional NMR or imaging experiments. The proposed methods will also advance the methodology of mobile and ex-situ NMR with applications to materials science, chemical engineering and geosciences, and in process, product and quality control. As a second major component of the proposed work, we plan to develop novel pulse sequences based on methods of optimal control and Fourier synthesis which are robust to Larmor dispersion of spins and rf-inhomogeneities and are significantly shorter than conventional designs used for the same purpose. As NMR spectroscopy at high fields up to 21 Tesla is becoming routine, there are emerging applications that pose new pulse design challenges. These applications include design of broadband excitation or inversion pulses, band selective excitations, decoupling pulse sequences in situations when the bandwidth to be covered is comparable or much larger than the rf power available. A natural choice in these settings is the use of adiabatic pulse sequences which tend to be very long and are not desirable in the presence of large relaxation rates. This makes it desirable to develop pulse sequence designs that are significantly shorter than the conventional methods and offer comparable compensation performance. Here we propose methodologies for the study of such designs. The proposed work will also develop sensitivity enhanced experiments in protein NMR spectroscopy based on these new methods.

• DFG Programme: Research Grants
• International Connection: USA
• Major Instrumentation: Imaging equipment
• Instrumentation Group: 1740 Hochauflösende NMR-Spektrometer
• Participating Person: Professor Dr. Navin Khaneja