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 B1 which represents the amplitude or the power of the impulsion and tp 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.
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.