A study of effects of different echo processing on diffusion spectra measured by the CPMG sequence in a constant gradient

Modulated gradient spin echo (MGSE) sequences are designed to measure diffusion spectrum by applying gradients that cause oscillation of the spin dephasing function and thus echo attenuation from which the diffusion spectrum at the frequency of the dephasing oscillation is calculated. In this study, the CPMG sequence performed in a constant gradient (constant gradient MGSE sequence) is analyzed. It is shown that the sequence produces a train of spin echoes that decays with the echo index multi exponentially on the account of contributions from numerous coherence pathways. Therefore, its analysis by a mono-exponential decay model yields an incorrect diffusion spectrurn (D) over tilde(nu). It is also shown that by zero frequency filtering the echo signals are composed effectively only of the direct coherence pathway. The direct coherence pathway decays mono-exponentially with the echo index and its analysis by the mono-exponential decay model yields a correct diffusion spectrum D(nu). Furthermore, our experimental results as well as preliminary theoretical analysis indicate that the difference between the two diffusion spectra (D) over tilde(nu) - D(nu) is approximately equal for samples of identical external shape and the same diffusing liquid. This was demonstrated with constant gradient MGSE experiments on two different samples of an identical size, one containing bulk water and the other containing water in a porous material. The relation was found efficient in measuring diffusion spectra in samples with low SNR. (C) 2017 Elsevier Inc. All rights reserved.