The uniform field of magnetic resonance system technique

Why do we need homogenization?
First of all, the basic theory of MR is reviewed: under the premise of a certain magnetic field direction, the application of RF field perpendicular to the direction of the main magnetic field will deflect the hydrogen proton, and the recovery process after the deflection will produce useful MR signals. This deflection, in fact, is caused by the word resonance in our magnetic resonance.

What is resonance?
Basically it's the same frequency. In order to generate an MRI, then, the Lamore precession frequency of the hydrogen proton in the main magnetic field (as discussed earlier) is equal to the frequency of the radio frequency field.

For example, if the magnetic field is not uniform over a certain range, then the Larmor precession frequencies of hydrogen protons are not the same in an identical piece of material within that range. Then when the substance receives a certain frequency of the RF field, some regions of hydrogen protons can not resonance/deflection, cannot not deflection can not produce MR signal, then the final result may be: the same homogeneous material, but produce different light and dark image. This is, of course, unacceptable.
Suppose, again, that we talked about the role of layer selection in gradient fields. So if the main magnetic field is not uniform, the layer selection will be disturbed by the signal generated by the deflected hydrogen protons under the same field intensity nearby.

The emission frequency of the radiofrequency field can be easily controlled in real time, but the frequency of the magnetic field, which is affected by the innate structure of the magnet, as we'll see below, is not very uniform over a certain range, so we need the operation of the uniform field to make the magnetic field as uniform as possible.
The uniformity of magnetic field is an important criterion to measure the quality of MR equipment.

Recall the structure of the superconducting magnet, in the center of the superconducting electromagnet hole, without any constraints, the magnetic induction line is not always maintained parallel distribution, and the formation of a magnet diverges at both ends, the middle approximately parallel distribution state

A silicon steel sheet with diamagnetic material is added at a specific position inside the magnet hole to attract the magnetic induction lines to move in the desired direction, so as to maintain the horizontal distribution of magnetic induction lines.
The process of passive leveling field is very complicated. A custom leveling field stand and magnetic field detector are placed in the magnet hole