Resolution Enhancement in In Vivo NMR Spectroscopy

Abstract

In vivo NMR spectroscopy requires adequate spectral, spatial, and temporal resolution. Current methodology provides numerous efficient methods to optimize all three kinds of resolution. The achievable spatial and temporal resolution mainly depend on the experimental setup including the object that is studied, the magnetic field strength, and the hardware used for signal detection. Spectral resolution is a much more sensitive parameter. While the maximum resolution is also limited by the experimental setup, small and apparently unimportant influences can dramatically deteriorate spectral resolution. This chapter shortly reviews current methodology and limits in spatial and temporal resolution in in vivo NMR spectroscopy. Dipolar fields causing inhomogeneity of the magnetic field, which often leads to severe line broadening, are discussed as major nuisance to spectral resolution. Several methods to avoid or refocus line broadening are discussed. Shimming and susceptibility matching are methods that reduce field inhomogeneities in the sample. Two-dimensional spectroscopy can provide resolution of frequency differences that are smaller than the actual line widths. Two techniques are discussed that use physical mechanisms to actively refocus line broadening. Magic angle spinning averages out dipolar interactions, while distant dipolar field (DDF) spectroscopy, also termed intermolecular zero-quantum coherence (iZQC) spectroscopy, uses the local nature of the DDF to locally refocus magnetization. The origin of the DDF is discussed in detail and a pictorial explanation of signal refocusing is given. Current DDF spectroscopy methods and their in vivo applications are summarized.