Fig. 4. A radiofrequency (RF) wave applied perpendicular to the static magnetic field is depicted by the undulating arrow directed downward. This RF energy is absorbed by the nuclei, causine them to till out of parallel alignment to an antiparallel orientation and to “resonate” at a higher energy level. (Dortzbach RK. Kronish JW, Gentry LR: Magnetic resonance imaging of the orbit. Part I. Physical principles. Ophthal Plast Reconstr Surg 5:153, 1989)