T2 Sequance

water have long T1s.

When the lattice consists of medium-size molecules (mos t 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. This can again be illustrated by our sandwich and race car example: (see page 17) handing over sandwiches (i.e . energy ) from one car (proton) to the other (lattice ) is easy and efficient, when both move with the same speed. With a difference in speeds, the energy transfer will be less efficient .

Why does fat have a short T1? The carbon bonds at the ends of the fatty acids have frequencies near the Larmor frequency, thus resulting in effective energy transfer . And why is T1 longer in stronger magnetic fields? It is easy to imagine that in a stronger magnetic field it takes more energy for the protons to align against it. Thus these protons have more energy to hand down to the lattice, and this takes longer than handing down just a small amout of energy. Even though it may seem logical, this is the wrong explanation. As we heard in the beginning , the precession frequency depends on magnetic field strength, a relationship described by the Larmor equation. If we have a stronger magnetic field , then the protons precess faster . And when they precess faster , they have more problems handing down their energy to a lattice with more slowly fluctuating magnetic fields .

What influences T 2? T2-relaxation comes about when protons get out of phase, which - as we already know - has two causes: inhomogeneities of the external magnetic field , and inhomogeneities of the local magnetic fields within the tissues (see page 29). As water molecules move around very fast, their local magnetic fields fluctuate fast , and thus kind of average each other out, so there are no big net differences in internal magnetic fields from place to place. And if there are no big differences in magnetic field strength inside of a tissue, the protons stay in step for a long time, and so T2 is longer .

With impure liquids, e.g . those containing some larger molecules, there are bigger variations in the local magnetic fields. The larger molecules do not move around as fast , so their loca l magnetic fields do not cancel each other out as much. These large r differences in local magnetic fields consequently cause larger differences in precession frequencies, thus protons get out of phase faster , T2 is shorter. This can be illustrated by the following example: imagine that you drive down a street with many pot holes. When you drive slow (which is equal to the surroundings moving slow and you standing still), you will be jumping up and down in your car with each pot hole. The differences in the surroundings (the magnetic field variations) influence you considerably . When you drive very fast (which is the same as if the surroundings move very fast), you do not feel the single pot holes anymore. Before they have a major effect on you, you are already back on the normal street level; thus their effect is averaged out , you are much less influenced by differences in the surroundings (the variations in magnetic field strength) . What does all this have to do with what we want to know? All these processes influence how your MR picture will finally look!

A brief review might be advisable • T1 is longer than T 2 • T1 varies with the magnetic field strength ; i t is longer in stronger magnetic field s • water ha s a long T1, fa t has a short T1 • T2 o f water is longer than the T2 of impure liquids containing larger molecules .

Now let us perform an experiment Look a t figure 25 , where you see two protons , precessing around the z-axis. I hope that you recall, that the z-axis indicates the direction of a magnetic field line (see page 11) . Instead of only these two protons, in reality there may be 12 pointing up and 10 pointing down , o r 102 up and 10 0 down - there shall only be two more protons pointing up. As we know , these are the ones that have a net magnetic effect because their effects are no t cancelled out .

Now let us send in an RF pulse, which has just the correct strength and duration , that one of the two protons picks up energy, to go into the higher state o f energy (points down / walks on its hands).