1. A method for registering an intraoperative three dimensional ultrasound image with a pre-operative intensity magnetic resonance image comprising gradient information, comprising the steps of: identifying an anatomical boundary from said gradient information; producing a filtered magnetic resonance image by processing said intensity magnetic image with an image filter; and registering said ultrasound image with said filtered magnetic resonance image; wherein said filtered magnetic resonance image simulates the appearance of an ultrasound image.