Evaluation of the spatial accuracy of magnetic resonance imaging-based stereotactic target localization for gamma knife radiosurgery of functional disorders

PURPOSE: This study was undertaken to determine the impact of geometric dis tortions on the spatial accuracy of magnetic resonance imaging (MRI)-guided stereotactic localization for gamma knife functional radiosurgery. METHOD: The spatial accuracy of MRI was evaluated by comparing stereotactic coordinates of intracranial targets, external fiducials, and anatomic stru ctures defined by computed tomographic and MRI studies of the Radionics sku ll phantom (Radionics, Inc., Burlington, MA), the Rando head phantom, and I I patients who underwent gamma knife functional radiosurgery. The distortio n in MRI:was assessed from computed tomographic and MRI fusion studies for these patients, as well as from MRI studies acquired by swapping the direct ion of the magnetic field gradients for five patients who underwent gamma k nife radiosurgery and three patients who underwent MRI-guided frameless sur gery. A follow-up program to compare the location of the created lesion wit h the intended target complemented the analysis. RESULTS: The average difference between computed tomographic and MRI stereo tactic coordinates of external fiducials, intracranial targets, and anatomi c landmarks was of the order of 1 pixel size (0.9 x 0.9 x 1 mm(3)) along th e x, y, and z axes. The average linear scaling along these axes as determin ed by fusion studies was approximately 0.8% and consistent with a single pi xel. The follow-up studies, available for seven patients, revealed good agr eement between the location of the created lesion and the intended target. CONCLUSION: The spatial accuracy of an MRI-based localization system can be comparable to computed tomography-based localization with the added benefi t of MRI resolution. Both machine- and object-related MRI distortions can b e reduced to an acceptable level with contemporary scanners, optimized scan ning sequences, and distortion-resistant stereotactic instruments.