|
|
|
|
|
|
|
|
Spin Echo Formation
|
|
|
|
 |
|
|
Here we present the principle of spin echo formation in the rotating frame: (a) At time t = Ø, immediately after a 90° RF pulse, Mø points along the Y' axis. (b) A time of TE/2 is allowed to elapse while the spins dephase (T2* mechanisms). At t = TE/2, a 180° RF pulse is given which flips the dephased vectors about the X' axis. (c) Another TE/2 time is allowed to pass while the vectors rephase. (d) At t = TE, the vectors have rephased and an echo of opposite sign forms.
(The astute observer will note that the arrows drawn to represent the 180° rotation about the X' axis go above the X'Y' plane, not below it. This is true only if a -180° pulse is what is actually used. Whether a positive or negative 180° is used, or one that rotates about the Y' axis instead of the X' axis, the end result is the same.)
|
|
|
|
|
|
|
|
|
|
|
|
Spin Echoes and the T2 Envelope
|
|
|
|
 |
|
|
As described above, a 180° pulse can be used to reverse the T2* dephasing process and thereby produce a spin echo. As soon as the spins all come back into phase at the echo time, they immediately start to go out of phase again. A second 180° pulse will generate a second echo. This process can be repeated many times, producing many echos, as long as the pure T2 decay mechanisms have left some signal to work with. We will come back to this topic when we discuss Fast Spin Echo (FSE) imaging.
THIS IS THE END OF CHAPTER 2!
|
|
