Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

TEI'I180 0 EchoRotatingframe./FID signalgraduallydecays with decay rate T2*After 180 0 pulse, signalreforms at same rateSpin echo peak amplitudedependson T2FIGURE 14-17. The spin echo pulse sequence starts with a 90-degree pulse and produces a freeinduction decay (FID) that decays according to T2* relaxation. After a delay time equal to TE/2,a 180-degree radiofrequency pulse inverts the spins that reestablishes phase coherence and producesan echo at a time TE. Inhomogeneities of external magnetic fields are canceled, and thepeak amplitude of the echo is determined by T2 decay. The rotating frame shows the evolutionof the echo vector in the opposite direction of the FID.the 90-degree pulse, but quickly their tight grouping at the starting line spreadsout (dephases) as they run at different speeds. After a short period, the runnersare spread out along the track, with the fastest runners in front and the slowerones in the rear. At this time (TE/2), a 180-degree pulse is applied and the runnersall instantly reverse their direction, but they keep running at the same speedas before. Immediately after the 180-degree rephasing RF pulse, the fastest runnersare the farthest behind and the slowest runners are in front of the pack.Under these conditions, the fast runners at the end of the pack will catch the slowrunners at the front of the pack as they all run past the starting line together (i.e.,at time TE). Even in a field of runners in which each runs at a markedly differentspeed from the others, they all will recross the starting line at exactly TE. TheMR signal is at a maximum (i.e., the peak of the FIO envelope) as the runners areall in phase when they cross the starting line. They can run off in the other direction,and after another time interval ofTE/2 reverse their direction and run backto the starting line. Again, after a second TE period, they will all cross the startingline (and the FIO signal will be at its third peak), then head off in the otherdirection. This process can be repeated (Fig. 14-18). The maximal echo amplitudedepends on the T2 constant and not on T2*, which is the decay constant thatincludes magnetic field inhomogeneities. Of course all MR signals depend on theproton density of the tissue sample, as well. Just before and after the peak amplitudeof the echo (centered at time TE), digital sampling and acquisition of the