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

Spatial fidelity is a paramount issue in image guided neurosurgery. Until recently, three-dimensional computed tomography (3D CT) has been the primary modality because it provides fast volume capture with pixel level (1 mm) accuracy. While three-dimensional magnetic resonance (3D MR) images provide superior anatomic information, published image capture protocols are time consuming and result in scanner- and object-induced magnetic field inhomogeneities which raise inaccuracy above pixel size. Using available scanner calibration software, a volumetric algorithm to correct for object-based geometric 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 fiducial markers, mean registration error dropped to 0.62 mm (subpixel accuracy). Three dimensional object-induced error maps present highest 3D MR spatial infidelity at the tissue interfaces (skin/air, scalp/skull) where frameless stereotactic fiducial markers are commonly applied. The algorithm produced similar results in two patient 3D MR-scans.