title: Effects of off-resonance spins on the performance of the modulated gradient spin echo sequence.
abstract: Translational molecular dynamics in various materials can also be studied by diffusion spectra. These can be measured by a constant gradient variant of the modulated gradient spin echo (MGSE) sequence which is composed of a CPMG RF pulse train superimposed to a constant magnetic field gradient. The application of the RF train makes the effective gradient oscillating thus enabling measurements of diffusion spectra in a wide range of frequencies. However, seemingly straightforward implementation of the MGSE sequence proved to be complicated and can give overestimated results for diffusion if not interpreted correctly. In this study, unrestricted diffusion in water and other characteristic materials was analyzed by the MGSE sequence in the frequency range 50-3000Hz using a 6T/m diffusion probe. First, it was shown that the MGSE echo train acquired from the entire sample decays faster than the train acquired only from a narrow band at zero frequency of the sample. Then, it was shown that the decay rate is dependent on the band's off-resonance characterized by the ratio Δω0/ω1 and that with higher off-resonances the decay is faster. The faster decay therefore corresponds to a higher diffusion coefficient if the diffusion is calculated using standard Stejskal-Tanner formula. The result can be explained by complex coherence pathways contributing to the MGSE echo signals when |Δω0|/ω1>0. In a magnetic field gradient, all the pathways are more diffusion attenuated than the direct coherence pathway and therefore decay faster, which leads to an overestimation of the diffusion coefficient. A solution to this problem was found in an efficient off-resonance signal reduction by using only zero frequency filtered MGSE echo train signals.
authors.affiliation: Jožef Stefan Institute, Jamova 39, Ljubljana 1000, Slovenia; Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, Ljubljana 1000, Slovenia. Electronic address: igor.sersa@ijs.si.
authors.forename: Igor
authors.initials: I
authors.lastname: Serša
authors.affiliation: Jožef Stefan Institute, Jamova 39, Ljubljana 1000, Slovenia; Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, Ljubljana 1000, Slovenia.
authors.forename: Franci
authors.initials: F
authors.lastname: Bajd
authors.affiliation: Jožef Stefan Institute, Jamova 39, Ljubljana 1000, Slovenia; Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, Ljubljana 1000, Slovenia.
authors.forename: Aleš
authors.initials: A
authors.lastname: Mohorič
keywords: CPMG sequence; Coherence pathways; Diffusion spectrum; Modulated gradients; Signal attenuation