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Additional details for
Anna Tribble (Chemistry)
"Coherence Pathways for CPMG and PIETA NMR Experiments"
Kentucky Academy of Sciences Annual Meeting, November 14, 2013
The use of Carr-Purcell-Meiboom-Gill (CPMG) echo sequences in NMR spectroscopy has grown substantially in the last ten years. In particular CPMG is used for imaging, wideline spectra, relaxation dispersion, and well-logging/core analysis. In all of these experiments the sequence is assumed to generate a series of spin-echoes that decay in time according to a transverse relaxation rate. With the discovery of the Phase Incremented Echo Train Acquisition (PIETA) approach we are now able to analyze the coherence transfer pathways occurring within a CPMG echo train. We have shown that when RF offset effects are large, the CPMG sequence leads to significant numbers of stimulated echoes instead of the spin-echoes. The CPMG echoes follow a coherence pathway that spends considerable time in a zero-coherence state that decays with the longitudinal relaxation time rather than the transverse relaxation time. This means that in most of these applications where the results depend on the echo decay dimension, the offsets can lead to seemingly unpredictable results. PIETA offers a partial solution to the problem by guaranteeing a coherence pathway, though it still has significant signal loss due to offset effects.
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