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The omega sign, also called the hand knob, is the appearance on axial magnetic resonance imaging of the brain of a knob-shaped part of the precentral gyrus that bulges backward into the central sulcus and resembles the Greek letter omega (Ω). The structure marks the motor area for the hand within the primary motor cortex, and it is used as a landmark to identify the precentral gyrus and the central sulcus on imaging.