Surface registration for use in interactive image-guided liver surgery

Liver surgery is difficult because of limited external landmarks, significant vascularity and inexact definition of intrahepatic anatomy. Intraoperative ultrasound (IOUS) has been widely used in an attempt to overcome these difficulties, but is limited by its two-dimensional nature, interuser variability and image obliteration with ablative or resectional techniques. Because the anatomy of the liver and intra-operative removal of hepatic ligaments make intrinsic or extrinsic point-based registration impractical, we have implemented a surface registration technique to localize physical points for liver phantoms and anatomically placed targets within the liver on CT images. Liver phantoms were created from anatomically correct molds with "tumors" imbedded within the substance of the liver. Helical CT scans were performed with 3 mm slices. Using an optically active position sensor, the surface of the liver was digitized according to anatomical segments. A surface registration was performed and RMS errors of the locations of internal tumors are presented as verification. An initial point based marker registration was performed and considered as the standard for error measurement. Errors for surface-registration were 2.9 mm for the entire surface and 2.8 mm for embedded targets. This is an initial study considering the use of surface registration for the purpose of physical to image registration in the area of liver surgery.