By H H Schild
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Let us perform another experiment similar to the one above, which is illustrated in figure 30. In figure 30a, we have two tissues, A and B, which have different relaxation times (tissue A has a shorter transversal as well as longitudinal relaxation time). We send in a 90° RF pulse, and wait a certain time TRlong (we will explain later why we use the term TR), and then send in a second 90° pulse. What will happen? As after the time TRlong tissue A and tissue B have regained all of their longitudinal magnetization (frame 5), the transversal magnetization after the second pulse will be the same for both tissues, as it was in frame 1.
Just put both curves together, and you can see something like a mountain with a ski slope. You first have to go uphill (T1-curve), before you jump down (T2-curve) (fig. 22). Fig. 21 If one plots transversal magnetization vs. time after the RF pulse is switched off, one gets a curve as illustrated, which is called a T2-curve. Fig. 22 Coupling of a T1- and a T2-curve resembles a mountain with a slope. It takes longer to climb a mountain than to slide or jump down, which helps to remember that T1 is normally longer than T2.
The 180° pulse in our experiment acts like a wall, from which the protons bounce back, like a mountain reflecting sound waves as echoes. This is why the resulting strong signal is also called an echo, or spin echo. After we have our signal, our spin echo, the protons lose phase coherence again, the faster ones getting ahead as we have heard. We naturally can perform the experiment again with another 180° pulse, and another and another . . If we now plot time vs. signal intensity, we get a curve like in fig.