A bone scan uses radioactive tracers to detect areas of bone injury. Evidence of abnormalities shows up either as darker "hot spots" with greater tracer uptake or as lighter, "cold spots" with little or no tracer uptake. Hot spots represent increased bone metabolism while cold spots indicate decreased bone metabolism.
The radioactivity generated in the body by the tracers is less than that of a chest x-ray and generally disappears within one to three days. The radionuclide used in horses is the same as tracers used for people, therefore they have been extensively tested for safety. Bone scan is used in subtle lameness, for horses who are lame in more than one leg or in horses whose lameness is located in the upper leg and could not be localized using nerve and joint blocks to help pinpoint areas of bone injury.
While x-rays can detect changes in bone, bone scans can detect changes in bone smaller than one billionth of a milligram, and can therefore show problems before they are visible by x-ray. Also, because the tracers are given intravenously, it is consequently distributed throughout the entire body and it is much easier and cost effective to perform a bone scan of a full body than to take x-rays of all body parts in horses who have multiple limb lameness or very subtle problems.