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And when this vector of A is larger, it will reach closer to our antenna; thus the imaginary bell at the tip of vector A will cause a louder, stronger signal in our "microphone", the antenna, than vector B. The difference in signal intensity in this experiment depends on the difference in longitudinal magnetization, and this means on the difference in T1 between the tissues. Using these two pulses, we can now differentiate tissue A from tissue B, which might have been impossible chosing only one 90° pulse or two 90° pulses that are a long time apart (after a long time, the differences in T1 between tissue A and B no longer play a role in our experiment, because after that time the tissue B with the longer T1 is back to its original state, too).
This curve is going downhill, as transversal magnetization disappears with time. And as you probably expect: there is also a time constant, describing how fast transversal magnetization vanishes, goes downhill. This time constant is the transversal relaxation time T2. How to remember what "T2" is? Easy: T2 = T x 2 = T T = Tt, and this means, it describes the "T transversal", thus the relaxation of the transversal magnetization. The resulting curve in figure 21 thus is called a T2-curve. Another term for transversal relaxation is spin-spin-relaxation, reminding us of the underlying mechanism, a spin-spin interaction.
When the lattice consists of pure liquid/water, it is difficult for the protons to get rid of their energy, as the small water molecules move too rapidly. And as the protons (which are on the higher energy level) cannot hand their energy over to the lattice quickly, they will only slowly go back to their lower energy level, their longitudinal alignment. Thus it takes a long time for the longitudinal magnetization to show up again, and this means that liquids/ water have long T1s. When the lattice consists of medium-size molecules (most body tissues can be looked at as liquids containing various sized molecules, kind of like a soup), that move and have fluctuating magnetic fields near the Larmor frequency of the precessing protons, energy can be transferred much faster, thus T1 is short.