Figure 2

Net Magnetisation, rf pulses and flip angle. a) At equilibrium, the net magnetisation, M_o is at equilibrium, aligned along the a axis. b). When an rf pulse is applied, M_o makes an angle with the z-axis, known as the flip angle, and rotates around the axis in the direction of the curved arrow. At any instant the magnetisation can be split into two components, M_z and M_xy. The rotating M_xy component generates the detectable MR signal. c) The maximum detectable signal amplitude after a single rf pulse occurs when M_o 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. d) A 180° rf refocusing pulse is usually applied while there is transverse magnetisation already rotating in the xy plane and is used to instantaneously flip the transverse component of magnetisation through 180° about an axis also rotating in the xy plane. e) A 180° inversion pulse is usually applied at equilibrium and is used to rotates the net magnetisation through 180° from the positive to the negative z axis. This is also know as a magnetisation preparation pulse and is used is the preparation scheme for black blood imaging techniques.