Animation 3.2 (The Larmor Grand Prix)

This is an animation of Figure 3.4. It represents three cars sitting on the start line of race around a circular racetrack. There is a sports car, a family car, and a tractor. At the beginning of the race, these three vehicles are at the same place at the same time - in other words, they are in phase. This is analogous to the resonant RF pulse in a pulse sequence that puts spins into phase. When the cars see the green light, they speed off around the track, but, of course, the sports car travels faster than the family car, and the family car travels faster than the tractor. After a short period of time, they are out of phase with one another. This is analogous to dephasing due to inhomogeneities in the field. The next step is to apply a 180° RF pulse. When this is applied, it is similar to telling the cars to turn through 180° and face the way they came, in other words, face back to the start line. Assuming they travel back to the start line at the same speed they traveled out in the first place, they will pass the start line at the same time. The cars are, therefore, in phase again, and this is analogous to the spin echo in which the spins are all back in phase.