Fig. 5. When the radiofrequency (RF) wave is turned off, the excited nuclei realign parallel to the static magnetic field. The excess energy released as the nuclei “relax” to a lower energy stare is emitted as an RF signal. (Dortzbach RK, Kronish JW, Gentry LR: Magnetic resonance imaging of the orbit. Part I, Physical principles. Ophthal Plast Reconstr Surg 5:153, 1989)