ACOSTA RODOLFO HECTOR
Congresos y reuniones científicas
Título:
Spin echo formation in presence of Fast Stochastic Dynamics
Autor/es:
P.P. ZÄNKER; J. SCHMIDT; J. SCHMIEDESKAMP; R. H. ACOSTA; H.W. SPIESS
Lugar:
Pacific Grove, EEUU
Reunión:
Congreso; 49th ENC - Experimental Nuclear Magnetic Resonance Conference; 2008
Resumen:
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 [4].
The experiments were realized by using hyperpolarized (HP) 3He (provided by the Physics Department of the University
of Mainz [5]) 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 [6]). 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.
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 [4].
The experiments were realized by using hyperpolarized (HP) 3He (provided by the Physics Department of the University
of Mainz [5]) 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 [6]). 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.
