## The alpha impulsion angle and the position of the M vector in a rotating coordinate system

Impulsion angle
du vecteur M (Fig 5) and position of the M vector (Fig. 5) is given by the following relation
with B_{1} which represents the amplitude or the power of the impulsion and t_{p} the width or duration of the impulsion. It is possible to make these two values vary together in such a way that particularly interesting rotation angles appear. One of these is the angle
=90°.

In this case all the magnetization is orientated in the plane xy and the signal reaches its maximum of
intensity^{[1]}.

An other angle allows the inversion of the M vector by the application of an impulsion or = 180°

In this experiment, the vector M is orientated in –z. Practically, one uses the angle = 45° in order to get a good compromise between measurement time/quality of the response.