Next, the MRI machine applies a radio frequency (RF) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body we want to examine. When the pulse is applied, the unmatched protons absorb the energy and spin again in a different direction. This is the "resonance" part of MRI. The RF pulse forces them to spin at a particular frequency, in a particular direction. The specific frequency of resonance is called the Larmour frequency and is calculated based on the particular tissue being imaged and the strength of the main magnetic field.
At approximately the same time, the three gradient magnets jump into the act. They are arranged in such a manner inside the main magnet that when they're turned on and off rapidly in a specific manner, they alter the main magnetic field on a local level. What this means is that we can pick exactly which area we want a picture of; this area is referred to as the "slice." Think of a loaf of bread with slices as thin as a few millimeters -- the slices in MRI are that precise. Slices can be taken of any part of the body in any direction, giving doctors a huge advantage over any other imaging modality. That also means that you don't have to move for the machine to get an image from a different direction -- the machine can manipulate everything with the gradient magnets.
But the machine makes a tremendous amount of noise during a scan, which sounds like a continual rapid hammering. That's due to the rising electrical current in the wires of the gradient magnets being opposed by the main magnetic field. The stronger the main field, the louder the gradient noise. In most MRI centers, you can bring a music player to drown out the racket, and patients are given earplugs.
When the RF pulse is turned off, the hydrogen protons slowly return to their natural alignment within the magnetic field and release the energy absorbed from the RF pulses. When they do this, they give off a signal that the coils pick up and send to the computer system. But how is this signal converted into a picture that means anything?