Biomedical Engineering Reference
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Fig. 6.3
The Hahn Spin Echo experiment. (
a
) Pulse sequence with Free Induction Decay and signal echo. (Adapted from Wikipedia.) (
b
) Concept of spin echo
refocussing in the rotating frame of reference. (
A
) The net magnetization vector
M
(
green vector
), initially aligned with the Z-axis under the influence of
B
0
.
(
B
)90
◦
RF pulse tilts (
orange arrow
)
M
on to the transverse plane. (
C
)and(
D
) Spins dephasing due to local field inhomogeneities. Slow spins fall back, while
the fast spins move ahead in the transverse plane. (
E
) 180
◦
RF pulse (
orange arrow
) flips the spins around such that now the slow spins are ahead and the fast
spins are behind. (
F
) Refocussing begins as the fast spins begin to catch up the slow spins from behind. The echo begins to form. (
G
) The spins are completely
refocussed. This is the centre of the echo and the signal is free of the effects of field inhomogeneities (Drawing by A.G. Filler, image source Wikipedia)
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