Spin isochromat

Fig. 5. A spin-echo pulse sequence used to determine T, . (a) A (it/2)y pulse puts Mq into the /-direction, and (b) the spin isochromats dephase with time. At a time Ij later, a it pulse is applied along the j -axis, causing the spins to rotate through it radians (c) such that they refocus along the /-aids to form an echo at time 21 (d). The decrease in magnitude of the magnetization vector between stages (a) and (d) provides a measure of (Eq. (4)). All reversible contributions to the T2 process are removed by the application of the it pulse.

On timescales of less than or equal to that of spin-spin relaxation (T2) processes occur. T2 characterises the loss of phase coherence of the individual spin isochromats within the spin ensemble comprising the total magnetisation vector M0. A spin isochromat represents a group of spins that experiences the same homogeneous magnetic field and which, therefore, behaves in the same... 

The PGSE gradient pulse pairs are stepped so as to phase-encode the spins for molecular translational motion. Because both pairs of -pulses are applied in the same direction, a spin isochromat corresponding to a set... 

Closer examination of equation Bl.14.3 reveals that, after the slice selection pulse, the spin isochromats at different positions in the gradient direction are not in phase. Rather they are rotated by i exp... 

The result is that the net magnetization along the y axis decays to 0 as the spin isochromats fan out in x -y plane. 

One way to increase the efficiency of data taking in such a case is a sequence called DEFT for driven equilibrium Fourier transform (Becker, et al. 1969). The idea of this experiment is to force the system back to equilibrium more rapidly after each FID so that it can be pulsed more frequently. In a DEFT sequence, a 90° pulse is applied to the system, and the FID is recorded. A 180° pulse is applied at a time x when the FID has disappeared due to the dephasing of the spin isochromats to form an echo at time 2x. At the... 

These echoes formed by a set of n/2 pulses differ in a s the other echoes we discussed in I.C.2. For the latter conditions, spin isochromats from all parts of the samp 354... 

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