Illustration of the use of the SPY System for evaluation of tissue perfusion in a mastectomy flap. The black-and-white fluorescence image (Panel A) shows large dark area of minimal fluorescence, reflecting poor perfusion, surrounding the incision; the surgeon’s pen can be seen tracing the outline of this region. Panel B is a colorized version of the same image, showing quantification of absolute fluorescence (numbers in boxes); darker colors represent areas of lower fluorescence signal. The incision prior to reconstruction is shown in Panel C; areas of poor perfusion identified by SPY are noted in blue-pen outlines superior and inferior to the incision. In this case, clinical judgment of tissue viability (including appearance of skin and presence of bleeding at tissue edge) was favored over SPY findings, and the regions of poor perfusion noted on SPY were left intact. The post-operative result (Panel D) shows necrosis superior to the incision, corresponding to the region of poor perfusion identified by SPY.