1. Recovery of longitudinal magnetisation following a 90° radiofrequency (RF) pulse.
Recovery of longitudinal magnetisation following a 90° radiofrequency (RF) pulse. (A) Protons aligned with B0 produce a sum vector with longitudinal magnetisation. (B) When an RF pulse is switched on longitudinal magnetisation decreases and transverse magnetisation propagates. Alternatively, it can be said that the sum magnetisation ‘tilts’ to the side. An RF pulse that abolishes longitudinal magnetisation to zero while inducing transverse magnetisation is called a 90° (saturation) pulse as the sum magnetisation vector is seen to tilt or flip 90°. Immediately following an RF pulse, protons precess in phase in the transverse plane, depicted by a single vector (arrow) in the lower circle. (C) After the 90° RF pulse, protons fall out of phase (now multiple vectors in the lower circle), transverse magnetisation decreases and longitudinal magnetisation begins to recover. During this process, the whole system continues precessing and so the sum vector takes a spiralling motion (D). Recovery of longitudinal magnetisation is termed T1 relaxation and loss of transverse magnetisation is called T2 relaxation. This figure is only reproduced in colour in the online version.
Stuart Currie et al. Postgrad Med J 2013;89:
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