91̽»¨

Abstract:

Various numerical techniques for alleviating crowding in two-dimensional NMR correlation spectra are investigated with a view to facilitating the measurement of spin-spin coupling constants. The following strategy for the analysis of cross sections through COSY cross peaks is developed: The active coupling constant is determined by J doubling (McIntyre and Freeman, J. Magn. Reson.96, 425, 1992). The active splitting is then removed (J deconvolution) by the maximum-entropy method. Estimates of the passive coupling constants are next obtained from the deconvolved spectrum. Finally, the values of all coupling constants are refined, if necessary, by model fitting to the original spectrum. This approach is shown to be successful for complex, congested cross peaks, whose multiplet structures are far from obvious. © 1993 by Academic Press, Inc.

Abstract:

Phase-modulated rotating-frame imaging, a technique used. for the detection of localized metabolites in vivo, is usually implemented in a less than ideal manner because of the practical constraints imposed by the human or animal sample. As a consequence, the spectra obtained by Fourier transformation contain artifacts which distort or obscure the genuine spectral features. An attractive alternative is to use the maximum-entropy method to process the data, incorporating the time-domain response of the nuclear spins predicted by the Bloch equations. Here we demonstrate this approach using simulated data, data from phantoms, and data from the human liver and chest wall. We show that artifacts in the Fourier-transform spectra arising from off resonance effects, imperfect pulse angles, and truncation can be effectively avoided by maximum-entropy processing. © 1992.

Abstract:

The relationship between NMR spectra obtained by the maximum entropy method and by conventional processing (Fourier transformation) is explored. In certain circumstances, the maximum entropy reconstruction is simply a nonlinearly amplified form of the Fourier transform spectrum and is therefore essentially worthless. More complex and interesting behavior is found under conditions more likely to be met in practice. Using simple examples, it is argued that a maximum entropy reconstruction can reveal information that could not be obtained from a single Fourier transform spectrum. © 1991.

Abstract:

It has often been asserted that the maximum entropy method can simultaneously suppress noise and enhance resolution in NMR spectra. We examine this claim using a Monte Carlo method to determine the accuracy and precision of line heights and integrals in spectra obtained by various forms of the maximum entropy method. It is shown that maximum entropy processing with lineshape deconvolution produces results that are roughly comparable to least-squares fitting of a model lineshape to the conventional Fourier transform spectrum. © 1991.