Worldwide experience with the DIEP flap has established its pre-eminent role in breast reconstruction after mastectomy. CT angiographic vascular imaging has enhanced the planning of reconstruction, allowing mapping of perforator patterns to increase the predictability of surgery. This study extends the role of perforator mapping, using measurement of perforator lumen diameters and flap weights to calculate an index that reliably predicts the amount of tissue which will survive. We call this the Flap Viability Index.