Here t p is the duration for which B 1 is applied, also called the pulse length. The pulses are labeled by the rotation of M 0 which they cause and the direction from which they are coming from, since the microwaves can be phase-shifted from the x-axis on to the y-axis. For example, a +y π/2 pulse means that a B 1 field, which has been 90 degrees phase-shifted out of the +x into the +y direction, has rotated M 0 by a tip angle of π/2, hence the magnetization would end up along the –x-axis. That means the end position of the magnetization vector M 0 depends on the length, the magnitude and direction of the microwave pulse B 1. In order to understand how the sample emits microwaves after the intense microwave pulse we need to go back to the laboratory frame. In the rotating frame and on resonance the magnetization appeared to be stationary along the x or y-axis after the pulse. In the laboratory frame it becomes a rotating magnetization in the x-y plane at the Larmor frequency. This rotation generates a signal which is maximized if the magnetization vector is exactly in the xy-plane. This microwave signal generated by the rotating magnetization vector is called free induction decay (FID).