Basic Physics of MR Signal and Image Generation - Advance Image Processing in Magnetic Resonance Imaging - page 7

B 0

Precessional
motion

µ

Spin motion

FIGURE 1.2 Precession of nuclear magnetic moment about the magnetic field B .

0

, the orientation of the spin
angular momentum vector, and hence the magnetic moment, of the nucleus with
respect to the direction of

According to the quantum mechanical model

cannot be arbitrary and it is subjected to certain
restrictions. In fact, the nuclear magnetic moment can only have 2I + 1 orientations
in a magnetic field, corresponding to 2I

B

0

+ 1 allowed energy levels, so that the

proton ( H) nucleus can either align with or against the applied field. These two

orientations are similar to the two directions in which a bar magnet may orient in
a magnetic field and are consequently called the parallel (spin-up) and the anti-
parallel (spin-down) orientations. These orientations correspond to low and high

1

energy states respectively, as schematically shown in Figure 1.3 .

The difference in energy, ∆ E , between the two states is proportional to the

strength of the magnetic field B , and it is given by

0

∆ E = γ B (1.6)

0