fig6

Figure 6. Equilibrium state. M₀ is aligned along the z-axis (A); when a rf pulse is applied, M₀ makes an angle with the z-axis and rotates around the axis in the direction of the curved arrow. The magnetization can be split into two components, M_z and M_xy. The rotating M_xy component generates the detectable NMR signal (B); the maximum detectable signal amplitude after a single rf pulse occurs when M₀ lies entirely in the plane of the x- and y-axes as this gives the largest M_xy component. This pulse has a 90° flip angle and is referred to as a 90° rf pulse or saturation pulse (C); a 180° rf refocusing pulse is usually applied while there is transverse magnetization already rotating in the xy-plane and is used to instantaneously flip the transverse component of magnetization through 180° about an axis also rotating in the xy-plane (D); a 180° inversion pulse is usually applied at equilibrium and is used to rotate the net magnetization through 180° from the positive to the negative z-axis. This is also known as a magnetization preparation pulse (E)^[81]