Nuclear medicine is a medical specialty
involving the application of radioactive substances in the diagnosis and
treatment of disease. Nuclear medicine imaging, in a sense, is "radiology
done inside out" or "endoradiology" because it records radiation
emitting from within the body rather than radiation that is generated by
external sources like X-rays. In addition, nuclear medicine scans differ from
radiology, as the emphasis is not on imaging anatomy, but on the function. For
such reason, it is called a physiological imaging modality. Single photon
emission computed tomography (SPECT) and positron emission tomography (PET)
scans are the two most common imaging modalities in nuclear medicine.