 About 15 minutes before the biopsy the physician injects a blue dye in the same manner.Then, during the biopsy, the physician visually inspects the lymph nodes for staining and uses a gamma probe or a Geiger counter to assess which lymph nodes have taken up the radionuclide.One or several nodes may take up the dye and radioactive tracer, and these nodes are designated the "sentinel lymph nodes".