Why is the signal of water low in T 1 weighted term and high in T2, while the signal of fat is high in T 1T2?

First of all, you should know that the principle of magnetic resonance is to put the human body in a strong external magnetic field and emit a radio frequency pulse with the same frequency as the proton precession (in layman's terms, give the proton a force to make it rotate) to produce magnetic resonance.

In human terms, give H nucleus a power, and when it is absorbed and released, record the release time, speed and other parameters.

But there are two ways for H 1 to absorb the energy of that RF pulse: 1. Move in the opposite direction; 2. move in the same direction.

So there are two ways to release energy: 1: longitudinal relaxation t1; 2. Transverse relaxation T2; (corresponding to 1 and 2 above)

Then look at the figure below and understand that M0 is the signal value.