The long-term objective of this research is to design and develop a dedicated, imaging guided, minimally invasive, percutaneous axillary sentinel lymph node biopsy system for staging of breast cancer. While axillary lymph node dissection remains the gold standard of care for staging of breast cancer, over 70% of women with small breast tumors who undergo axillary dissection are found to have negative axillary nodes. The proposed minimally invasive biopsy system could spare the morbidity of axillary node dissection for over 32,000 women who fall into this category each year in the U.S. In Phase I, we established the technical feasibility of accurately identifying and localizing sentinel nodes. In Phase II, we will demonstrate the clinical feasibility of safely extracting intact sentinel nodes through a very small incision in the axilla, and compare the clinical efficacy of the proposed system to axillary dissection as a staging tool. The proposed system integrates spatially correlated ultrasound and nuclear medicine imaging technologies, and computer driven, specialized biopsy devices, to provide a standardized platform for minimally invasive percutaneous sentinel node biopsy. We believe that this computerized platform will lead to more widespread use of sentinel node biopsy for clinically appropriate patients.