Magnets are used to generate a uniform static magnetic field (also called main magnetic field, short for magnetic field). The development trend of main magnet is the opening of low magnetic field intensity and the improvement of high magnetic field intensity. The open permanent magnet MRI equipment with low magnetic field intensity has a magnetic field intensity of 0.4T, and its structure is single-column or double-column asymmetric. Weight 10~ 13 tons, and the opening rate is over 75%. The advantage of open magnetic resonance equipment is that it can eliminate claustrophobia of patients. The magnetic field intensity of superconducting magnetic resonance equipment has developed from the traditional 1.5T to 3~4T, and there is a trend to 7~8T. The consumption of liquid helium in superconducting magnetic resonance equipment has been greatly reduced. With the further development of materials science, high temperature superconducting magnets may appear in the future. The influence of magnetic field intensity on MRI images is as follows: ① In terms of signal-to-noise ratio, as shown in figure 13- 1, the higher the magnetic field intensity, the greater the signal intensity and the higher the signal-to-noise ratio (but not linear). ② High magnetic field intensity and short scanning time. ③ In terms of image contrast, the value of tissue T 1 increases with the increase of magnetic field intensity, as shown in figure 13-2, and the relaxation time of T 1 is prolonged. When TR is a fixed value, the contrast of T 1 image will decrease, resulting in the degradation of T 1 image quality. However, the magnetic field intensity has little effect on the quality of T2 image.