Magnetization1

So, in order to rotate M away from the z direction:
The magnetic field of the RF pulse must lie somewhere in the x-y plane (this is the plane perpenicular to M)
The B1 field must stay synchronous with the magnetization by rotating at the same (Larmor) frequency as M (Fig. 5-2A).3 Otherwise, the effect of the B1 field would average out and produce no net rotation.
In this case, as B1 tips M off axis, the motion of M relative to B1 is characterized by a simple rotation around the B1 axis (Fig. 5-2B). At the Larmor frequency, the motion of M describes a cone-shaped "precession" around B1 so that M lies within the y-z plane. The precessional frequency of M around B1 is γB1 and the angle α that M rotates through is given by

where t is the duration for which the RF field of strength B1 is applied (see Fig. 5-2B). α is generally referred to as the "flip angle."