Net Magnetisation, rf pulses and flip angle. a) At equilibrium, the net magnetisation, Mo is at equilibrium, aligned along the a axis. b). When an rf pulse is applied, Mo 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, Mz and Mxy. The rotating Mxy component generates the detectable MR signal. c) The maximum detectable signal amplitude after a single rf pulse occurs when Mo 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. 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.