Scintigraphy is an imaging technique which enables one to trace the distribution, in  a patient’s body, of radioactive elements that have been introduced there in minute quantities. In the case of bone scintigraphy it is a matter of observing the skeleton via a scan of the entire body.  The tracer used (Technetium 99) has an affinity for bone.
As cancer cells have a higher rate of metabolic activity than normal ones, areas of hyperfixation reveal the presence of metastases.