The rotating coordinates or rotating frame
It is convenient to figure only the magnetic momentum of the excess of the nuclei in their fundamental states (Fig.3)
M is the macroscopic magnetization as a result of the individual nuclear moment . The xy plane does not contain any magnetization component. In addition to the coordinate system K(x,y,z), another system K'(x,y,z) rotates around the z axis with the angular speed thus . In this figure, is a virtual ﬁeld Bf opposite to B0, which is generated by the only relative rotation from one versus the other of the two coordinate systems K and K'. This means that the vector is static in the system K' which is called rotating frame, it happens when has the same sign and the same value than .