Novel Methods for High Resolution NMR in Inhomogeneous Fields

在这项拟议的工作中，我们计划开发用于非均匀静态和射频领域的高分辨率核磁共振波谱的新方法。高分辨率核磁共振实验需要极其均匀的磁场来获得原子核的化学位移和J耦合信息，因为这些相互作用比与外场B0的主要塞曼耦合小一个数量级。实现这种同质性并不是一件容易的事，特别是在相对较大的数量上。这通常是以核磁共振系统的价格和核磁共振磁体所需的尺寸为代价的。当处理空间上不均匀的组织或使用远程核磁共振排列时，这种均质性是不可能实现的。在这里，我们建议发展非常通用的方法，基于时变的3D梯度和空间上非选择性的射频脉冲来了解非均匀磁场的分布，然后校正由于这些不均匀而由自旋获得的相位。通过这种修正，可以提取原子核的化学位移信息，从而获得高分辨率的光谱。这些方法有可能取代廉价磁体中的专用垫片线圈，用于传统核磁共振或成像实验的专门应用。所提出的方法还将促进流动核磁共振和异地核磁共振方法学的发展，应用于材料科学、化学工程和地球科学，以及在工艺、产品和质量控制方面。作为拟议工作的第二个主要组成部分，我们计划开发基于最优控制和傅立叶合成方法的新型脉冲序列，这些方法对自旋的拉莫色散和射频不均匀具有很强的鲁棒性，并且比用于相同目的的传统设计要短得多。随着高场高达21特斯拉的核磁共振光谱学变得司空见惯，出现了一些新的应用，提出了新的脉冲设计挑战。这些应用包括宽带激励或反转脉冲的设计、频带选择性激励、当要覆盖的带宽与可用的射频功率相当或远大于可用的射频功率的情况下的去耦合脉冲序列。在这些设置中的自然选择是使用绝热脉冲序列，该绝热脉冲序列往往非常长，并且在存在大的驰豫速率时不是所希望的。这使得开发比传统方法短得多并提供类似补偿性能的脉冲序列设计是合乎需要的。在这里，我们提出了研究此类设计的方法。这项拟议的工作还将发展基于这些新方法的蛋白质核磁共振光谱的灵敏度增强实验。

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.