1. INTRODUCTION
In the last decade, it has become increasingly common to use image-guided navigation systems to assist surgical procedures with medical images. Benefits have been reported for accuracy improvement, reduction of intervention time, improvement of quality of life, reduction of morbidity (and perhaps mortality), reduction of intensive care and hospital costs. Image-guided systems are used today to help the surgeon to plan the surgery and provide accurate information about anatomy and function during the intervention. Image-guided systems also enable minimally invasive interventions, since the intraoperative images can be used interactively as a guide.