File:RF Pulse Oblique (expand the description below for a detailed explanation).webm

English: Now that we understand that magnetic fields make protons precess, how we can measure the signal that results, and how those signals decay, we can start investigating ways to utilize this. Since the proton only makes measurable magnetic fields when it is precessing, it would be useful to be able to force it away from equilibrium and make it precess. At rest, most protons have rotated to align with the magnetic field. We need to push them away from this so they can precess back to equilibrium and make a measurable signal as they do.

Unfortunately, as with gyroscopes, the very thing that makes a measurable signal -- the precessing -- makes the proton wobble in an awkward way if we add another constant magnetic field. We want to be able to quickly and precisely rotate the proton away from the Z-axis to whatever angle we want. The solution is to expose the proton to a magnetic field, called a radiofrequency (RF) pulse. This pulse must alternate at the same frequency as the proton is precessing, the Larmor frequency.

The result, as shown in the graphic, is that the proton "precesses while precessing," and perfectly rotates down to the XY-plane. If we continued the pulse, it would continue to rotate down to the negative Z-axis and back up again. Using this technique, we can rotate the proton to the exact angle away from the Z-axis we desire. Once the RF pulse is stopped, the proton will precess back up to the +Z-axis.