The Vectors of the Magnetic Moment - Essay Example

Both of these entities play a significant role in producing MRI active nuclei having active magnetic moment property. In electromagnetic laws, a nucleus spinning 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 nuclear 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 numbers, 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). However, the spin (I) for 14N will be, However, if the atomic number and atomic mass number are even. 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 an RF pulse is applied at the resonance, then the protons can absorb that energy at the quantum level, a single proton jumps to a higher energy state. To an observer in the external laboratory frame of reference at the macro or classical level, the magnetization vector spiral down towards the XY plane. Now to see the behaviour of the rotating frame when an RF pulse is applied from an external source, we keep this thing in mind that pulses are often labelled by their tip angle, which can be any value of angle, but most of the time, the 90o and 180o.

The tip angle is dependent on both the magnitude of the externally applied magnetic field and the length of the pulse.