Spatial Localization with Modified Fourier Series Windows

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Abstract
A modified Fourier Series Window (FSW) method is introduced that provides a simple, reasonably accurate, solution to spatial localization for layers in nuclear magnetic resonance (NMR) spectroscopy. This method was developed because signal originating from spins immediately adjacent to the coil plane can leak into standard FSW localized spectra obtained from more distal layers. The B1 profile of the surface coil and a priori sample knowledge were used to generate modified FSW coefficients that largely compensate for contamination from proximal layers. These modified coefficients were used to acquire spatially localized spectra from a three-layered phantom containing inorganic phosphate, phosphocreatine, and pyrophosphate. Spatially localized spectra also were obtained from the open-chest canine myocardium. The modified Fourier Series Window approach was validated using a quality assurance phantom. This method was then applied to the transmural analysis of 13C-containing metabolites in the in vivo canine myocardium during 3-(13)C sodium pyruvate infusion (n = 10). In vivo NMR spectra were characterized by resonances corresponding to the C2, C3, and C4 carbons of glutamic acid and to endogenous triacyglycerides and surface lipids. A transmural trend in metabolites could be observed under these conditions. This trend, however, was likely to result from the interference of surface lipids in the epicardial layer. The authors demonstrate that the modified Fourier Series Window method can be applied in situations where signal-to-noise considerations limit the applicability of more sophisticated spatial localization methods. In addition, the authors report a slight gradient in the endogenous triacylglyceride resonance, which is likely to originate from the presence of surface lipids.