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Since we have been talking about energies above, let us first note that the energies of gamma-rays emitted from a radioactive source are always distinct. For example 99mTc emits gamma-rays which all have an energy of 140 keV and 51Cr emits gamma-rays which have an energy of 320 keV.
Gamma-rays have many modes of interaction with matter. Those which have little or no relevance to nuclear medicine imaging are:
and will not be described here.
Those which are very important to nuclear medicine imaging, are the Photoelectric Effect and the Compton Effect. We will consider each of these in turn below. Note that the effects described here are also of relevance to the interaction of X-rays with matter since as we have noted before X-rays and gamma-rays are essentially the same entities. So the treatment below is also of relevance to radiography.
