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Mxy Spreads Out
Via T2 Relaxation

When the spins are first tilted down to the XY plane, they are all in phase. Think of a playground with a million swings. If all of the children are going up and down together, at exactly the same rate, then they are swinging in phase. Assuming that all the children are pumping their legs with the same force and frequency, then they will stay in phase. But if one child stops pumping for a few seconds and another child pumps a little harder or a little faster, then they will start to get out of synch with everyone else. The same type of thing happens to the spins. For reasons that we will go into soon, some protons spin a little faster while others spin a little slower. Very quickly, they get out of phase relative to some reference, (usually the spins at the center of the magnet.)

T2 Decay &
Loss of Phase

Question: What's the result of this loss of phase or dephasing?
Answer: The signal fades away or decays.
As another analogy, think of a room filled with a million (or 6 million billion) people, all of them whispering "Mary had a little lamb". As long as they are all speaking in phase with each other, you hear a very loud MARY HAD A LITTLE LAMB. But as usually happens in a room full of people where those at one end can't hear those at the other, some people start speaking a little faster and others a little slower. Soon the words start to become harder to make out until eventually all you hear is some low, background noise. The same happens to the spins and the resulting NMR signal.



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