Validation of object-induced MR distortion correction for frameless stereotactic neurosurgery

Spatial fidelity is a paramount issue in image guided neurosurgery. Until r ecently, three-dimensional computed tomography (3D CT) has been the primary modality because it provides fast volume capture with pixel level (1 mm) a ccuracy. While three-dimensional magnetic resonance (3D MR) images provide superior anatomic information, published image capture protocols are time c onsuming and result in scanner- and object-induced magnetic field inhomogen eities which raise inaccuracy above pixel size, Using available scanner cal ibration software, a volumetric algorithm to correct for object-based geome tric distortion, and a Fast Low Angle SHot (FLASH) 3D MR-scan protocol, we were able to reduce mean CT to MR skin-adhesed fiducial marker registration error from 1.36 to 1.09 mm, After dropping the worst one or two of six fid ucial markers, mean registration error dropped to 0.62 mm (subpixel accurac y), Three dimensional object-induced error maps present highest 3D MR spati al infidelity at the tissue interfaces (skin/air, scalp/skull) where framel ess stereotactic fiducial markers are commonly applied. The algorithm produ ced similar results in two patient 3D MR-scans.