<b>Bone Isotope Scans</b>
are done to screen for bone tumors, or to get a baseline so that tumor changes can be picked up at a later time.  A radioisotope is injected into a vein, and an hour or so later, the scan is done.  Only bone cells that are very active pick up this radioisotope.  The films will show dark "hot spots" superimposed on a skeleton.  It is a screening method for bone metastases, useful when X-rays are negative.  

Hotspots can be high grade tumors, inflammation [like arthritis], or infection and would require an Xray first and then possibly an MRI follow up to get exact information.  Again, smaller tumors might be missed.  And low grade tumors may not pick up enough radioisotope to be stand out from the background count.

Bone scans are easy to read and relatively quick to do. If something shows up, then it's followed by Xrays, and possibly MRI.   If nothing shows up, then there is probably nothing there of significant size, or the tumor is low grade.

NIH/Medline Site
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Information on Imaging of the body
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