Retrospective intermodality registration techniques: surface-based versus volume-based

The primary objective of this study is to perform a blinded evaluation of two groups of retrospective image registration techniques using as a gold standard a prospective, marker-based registration method, and to compare the performance of one group with the other. In order to ensure blindedness, all retrospective registrations were performed by participants who had no knowledge of the gold-standard results until after their results had been submitted. Image volumes of three modalities—X-ray Computed Tomography (CT), Magnetic Resonance (MR), and Positron Emission Tomography (PET)—were obtained from patients undergoing neurosurgery at Vanderbilt University Medical Center on whom bone-implanted fiducial markers were mounted. These volumes had all traces of the markers removed and were provided via the Internet to project collaborators outside Vanderbilt, who then performed retrospective registrations on the volumes, calculating transformations from CT to MR and/or from PET to MR. These investigators communicated their transformations again via the Internet to Vanderbilt, where the accuracy of each registration was evaluated. In this evaluation, the accuracy is measured at multiple “volumes of interest” (VOIs). Our results indicate that the volume-based techniques in this study tended to give substantially more accurate and reliable results than the surface-based ones for the CT-to-MR registration tasks and slightly more accurate results for the PET-to-MR tasks. It was also apparent that all of the registration techniques we examined have the potential to produce satisfactory results much of the time but that visual inspection is necessary to guard against large errors.