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Small nuclear magnetic system architecture diagram
Magnetic field uniformity is very important for MRI. This article briefly introduces the shimming mode of small nuclear magnetic.
The homogeneity of a magnetic field refers to the identity of a magnetic field within a certain volume limit, that is, whether the number of magnetic lines of force passing through a unit area is the same. In a magnetic resonance system, uniformity is measured in parts per million (ppm) of the main magnetic field as a unit of deviation. For different main magnetic field sizes, the unit of deviation is also different. For example, for a 1.0T magnetic resonance, the conversion of 1 ppm deviation unit to the H proton frequency is calculated to be about 42 Hz.
The uniformity of the main magnetic field directly affects the length of the T2* relaxation time of the tissue. When the uniformity of the main magnetic field is worse, that is, the shorter the T2*, the faster the relaxation, and the shorter the tail of the FID signal is reflected from the magnetic resonance signal. When the main magnetic field uniformity is higher, that is, the longer T2*, the slower the relaxation, that is, the longer the tail of the FID signal. Theoretically, when the main magnetic field is absolutely uniform, T2*=T2, the FID is attenuated by the tissue's inherent 2 relaxation.
Influence of main magnetic field inhomogeneity on tissue T2
When collecting signals, the small nuclear magnetic device adjusts the parallelism of the two magnetic poles by observing the attenuation speed of the nuclear magnetic resonance FID signal on the display (ie, the length of the FID tail), thereby achieving the purpose of adjusting the uniformity of the main magnetic field. The longer the tail of the FID signal, the slower the FID attenuation envelope, the higher the magnetic field uniformity.
The main magnetic field uniformity of the permanent magnet type nuclear magnetic resonance imager is related to the parallelism between the magnetic poles. Therefore, we can directly adjust the parallelism of the two magnetic poles to achieve the purpose of shimming; the uniformity of the magnetic field achieved by adjusting the parallelism of the magnetic poles The need for MRI cannot be fully met, so other methods of shimming are required, including passive shimming and active shimming.
Passive shimming is to attach a small magnetic piece or a magnetic steel piece to the inner and outer surfaces of the magnetic pole, and the magnetic field uniformity is adjusted by a small magnetic piece or a magnetic steel piece to adjust the magnetic field uniformity. The uniformity between the magnetic poles is better in the experimental device. Therefore, passive shimming is not used. In the following physical map, the small magnetic piece is the passive shimming mode shimming.
The active shimming mode mainly corrects the non-uniformity of the main magnetic field by applying a magnetic field around the coil according to the energized coil and applying a small magnetic field generated by applying a suitable current to the coils in different directions. 
Permanent magnet field average physical map
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Introduction to the field of small magnetic resonance imager magnets - medical imaging nuclear magnetic resonance teaching experiment
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