What is the role of magnetism …

Magnets, in medicine, are used to nuclear magnetic resonance (MRI) to diagnose abnormal human tissues and distinguish diseases. This is our familiar magnetic resonance imaging technique. The principle is that the nucleus has a positive electricity, and the spin motion is carried out. Normally, the arrangement of the spin axis of the nucleus is irregular, but when it is placed in an external magnetic field, the orientation of the spin space transits from disorder to order. The magnetization vector of the spin system is gradually added from zero. When the system reaches equilibrium, the magnetization reaches the stationary value. Suppose that the nuclear spin system is affected by the external effect at the moment, such as the radiofrequency excitation of the nucleus at a certain frequency can cause the resonance effect. MRI After the radiofrequency pulse is broken, the nucleus of the spin system can not adhere to this situation. It will return to the position of the original in the magnetic field and release the small energy into the radio signal. The characteristic of NMR is that no signal occurs in the active liquid. It is called activity effect or activity blank effect. So the blood vessels are gray and white, and the blood is black without