n electromagnetic laws a nucleus spining around its own axis with an electric charge will cause a small magnetic moment (composed of spin and charge) (Donald W et al. 2010). The rotation (spin) of the nucleus is perpendicular to the axis of the nucleus magnetic field. In short, vector (Figure 1) is used to describe the magnitude (amplitude) and the direction of the magnetic moment (Westbrook, Roth & Talbot 2005).
When the atomic mass number is odd an example is Na 23. Another example is 7Li. It has seven atomic mass number 3 protons and 4 neutrons and thus will give rise to an MR signal. The spin (I) for 23Na and 7Li will be 23⁄2 and 3/2, respectively.
For example, 14N has seven protons and seven neutrons (Z=7 and A=14). The spin (I) for 14N will be However, if the atomic number and atomic mass number are even, then there is no magnetic moment and thus no MR signal (for instance, 114Cd is composed of 48 protons plus 66 neutrons, Z=48 and A=114) (BROWN & SEMELKA 1999; Donald W et al. 2010).
It is important to know that the vectors of the MR active nuclei in the absence of the main magnetic field will spin randomly; thus, the random directions of the nuclei will affect the magnitude of the net magnetic moment. The probability of the net magnetic moment approaching zero is when there are many nuclei aligned in different directions
If RF pulse is applied at the resonance, then the protons can absorb that energy at the quantum level, a single proton jumps to higher energy state. At the macro or classical level, to an observer in the external laboratory frame of reference, the magnetization vector spiral down towards the xy plane.
Now to see the behavior of the rotating frame when a RF pulse is applied from external source ,we keep this thing in minds that pulses are often labeled by their tip angle which can be any value of angle but most of the times the 90o and 180o. The tip angle is dependent on both the magnitude of the externally applied magnetic field and