I have seen some seemingly differing statements in various sources about the dynamics of annihilation when the positron has some kinetic energy and is travelling through a medium. Two sources I've read state or imply that a positron in motion through matter must first lose it's kinetic energy through ionization or bremsstrahlung before it can annihilate with an electron, and even then, only through an intermediate positronium, and will therefore only annihilate with the signature 511 kev photons. Is all this true? Another source (the Wiki on annihilation) seems to say that annihilations can take place involving higher energy photons or even heavier particles if the kinetic energies are high enough. So, what really happens with an energetic positron in matter? I'm especially interested in the case of positrons emitted in radioactive decay and the physics behind PET scanning. Thanks.