The terms ”hot” and ”cold” areas are extensively used in nuclear medicine diagnosis interpretation. ”Hot” indicates high accumulation of radioactivity, and ”cold” indicates a photon deficiency area of low accumulation of radioactivity. In the 18F-FDG PET system, the hypermetabolic or ”hot” area indicates increases in glucose metabolism; the hypometabolic or ”cold” or photopenia area indicates decrease or absence in glucose metabolism. The ”cold” photopenia or hypometabolic area corresponds to ”Yin” and the ”hot” or hypermetababolism corresponds to ”Yang”. The concept of relativity or the concept of principal just fits the theory of opposites, which later become ”Yin” and ”Yang” in the Tai-Chi symbol of I Ching. We illustrate various examples of ”cold” and ”hot” area in nuclear medicine topography.