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, Mz and Mxy. The rotating Mxy 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 Mxy 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]