Even field, as the name suggests, is to make the strength of the magnetic field uniform.
Why do I need leveling?
First, review the basic theory of MR: under the premise of a certain magnetic field direction, the application of a radio frequency field perpendicular to the direction of the main magnetic field will deflect hydrogen protons, and the recovery process after the deflection will produce useful MR Signals. This deflection is actually caused by the word "resonance" in our magnetic resonance.
What is resonance?
Basically, they have the same frequency. In order to generate magnetic resonance, the Larmor precession frequency of the hydrogen proton under the main magnetic field (mentioned earlier) needs to be equal to the frequency of the radio frequency field.
Suppose that if the magnetic field is not uniform/different in size within a certain range, then the Larmor precession frequency of the hydrogen protons contained in the exact same mass within this range will not be the same. So when the substance receives a certain frequency of the RF field, some areas of the hydrogen proton can not produce resonance/deflection, can not be deflected, can not produce MR Signal, then the final result may be: a piece of the same uniform material, but produced a different image. This is, of course, unacceptable.
Again, let's assume that we talked about the stratification of the gradient field. So if the main magnetic field is not uniform, the layer selection will be disturbed by the signal generated by deflecting hydrogen protons at the same field strength nearby.