In 2007, over 78,000 women in the United States underwent mastectomies and many of these women required reconstructive surgery. A common type of breast reconstruction involves transplantation of a flap of tissue from another site on the body to the affected breast. Similar types of reconstructive procedures may be needed after surgery to remove skin cancer or to treat birth defects or physical defects from severe accidents. The transplanted tissue in a flap can die if its primary arteries or veins become sealed off or blocked by a blood clot, a kink in an artery or vein, or increasing pressure from local internal bleeding. After surgery, flaps can often be checked with simple tests such as assessment of color, temperature, and capacity to bleed; however, these tests may fail to detect problems before serious damage has occurred and they are not useful for flaps that lie under the skin.