When a positron leaves the atomic nucleus, after traveling along a short path (a few millimeters at most, along which it can ionize or excite other molecules or atoms), it will inevitably collide with an electron, resulting in the annihilation of both. Their combined energy will be transferred to two gamma photons (each having approxi­mately 511 keV of energy) that will fly in almost opposite directions (as described in Section 5.3.3). In this way, positron decay results in gamma radiation that can be detected easily; moreover, we can utilize the coincidence detection of the pair of photons traveling in opposite directions to identify the line of their source and thus to enable the imaging of the tracer’s distribution.