Spin echoes and intermolecular double-quantum coherences in gases in the fast diffusion regime

P.P. ZÄNKER; J. SCHMIDT; R. H. ACOSTA; L. AGULLES-PEDRÓS; J. SCHMIEDESKAMP; H.W. SPIESS

Aachen, Alemania

Congreso; 9th International Conference of Magnetic Resonance Microscopy; 2007

The spin echo (SE) phenomenon, which was first discovered by Hahn in 1950 [1], provides the basis of numerous pulse sequences in modern NMR and MRI. The reduction of the SE signal due to spin motion has already been subject of many theoretical and experimental studies in liquids [2,3]. In gases, however, translational motion can lead to significant frequency shifts on much shorter timescales, even shorter than the pulse distance. This regime of very fast diffusion in the gas phase has not been studied yet, although it is of special interest for example in the field of gas MRI. Here, for the first time, a detailed theoretical and experimental examination of the SE time-signal under the fast motion in the gas phase is presented.   The experiments were realized by using hyperpolarized (HP) 3He (provided by the Physics Department of the University of Mainz [4]) and a self-constructed gas mixing setup, which allows a precise variation of the diffusion coefficient by admixing HP 3He with heavier buffer gases at different pressures. The influence of fast motion on the echo formation was found to be much more dramatic than in the regimes considered so far. It results in a change of the SE signal in terms of amplitude, shape and echo position (so-called pseudo SE effect [5]). These effects, which arise from the fast translational motion of the spins on the timescale of the experiment, were described by an extension of the Stejskal-Tanner equation to arbitrary times for linear gradients. The more general case of non-linear gradients, where an analytical solution cannot be easily obtained, was simulated using computer calculations.   While on the one hand the understanding of this phenomenon is of general theoretical interest, it is, on the other hand, a prerequisite for different applications, e.g. imaging sequences using SEs in MRI of HP gases, for which diffusion suppression used to be one of the major obstacles. Additionally, the developed setup and the awareness of fast diffusion effects on echo signals allowed for the detection of intermolecular double-quantum coherences in the gas phase, which is presented in detail in a second contribution (Acosta et al.).   In conclusion, we have shown that the well-known formalism to describe the SE signal can and has to be extended to include fast motional effects, e.g. when HP gases are used.   -------- [1]        E. L. Hahn. Physical Review, 80:580–594, 1950. [2]        H. Y. Carr and E. M. Purcell. Physical Review, 94(3):630–638, 1954. [3]        E. O. Stejskal and J. E. Tanner. Journal of Chemical Physics, 42(1):288–292, 1965. [4]        E. W. Otten. Europhysics News, 35(1), 2004. [5]        J. Collignon, H. Sillescu, and H. W. Spiess. Colloid & Polymer Science, 259:220–226, 1981.   E-mail: zaenker@mpip-mainz.mpg.de