Neurosurgery 1992-98 March 1992, Volume 30, Number 3 402 A Comparison between Magnetic Resonance Imaging and Computed Tomography for Stereotactic Coordinate Determination Experimental and Clinical Study AUTHOR(S): Kondziolka, Douglas, M.D., M.S., F.R.C.S.C.; Dempsey, Peter K., M.D.; Lunsford, L. Dade, M.D.; Kestle, John R. W., M.D., F.R.C.S.C.; Dolan, Eugen J., M.D., F.R.C.S.C.; Kanal, Emanuel, M.D.; Tasker, Ronald R., M.D., F.R.C.S.C. Departments of Neurological Surgery (DK, PKD, LDL) and Radiology (LDL, EK), Presbyterian University Hospital, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, U.S.A.; and Division of Neurosurgery (DK, JRWK, EJD, RRT), The Toronto Hospital, University of Toronto, Toronto, Ontario, Canada Neurosurgery 30; 402-407, 1992 ABSTRACT: The spatial accuracy of magnetic resonance imaging (MRI) has not been established for stereotactic surgery. Magnetic susceptibility artifacts may lead to anatomical distortion and inaccurate stereotactic MRI coordinates, especially when targets are in regions of the brain out of the center of the magnetic field. MRI-guided stereotactic localization, however, provides better multiplanar target resolution than is available with computed tomographic (CT) scanning. Therefore, we compared the accuracy of stereotactic coordinates determined by MRI and CT studies in 41 patients (53 targets). Coordinates were measured in each plane and as vector distances between the target and the center of the stereotactic frame on axial or coronal MRI studies. Absolute axial plane MRI and CT distances varied an average of 2.13 ± 1.59 mm. The mean difference in measurements in the X (left-right) dimension was 1.19 mm and 1.55 mm in the Y (anterior-posterior) dimension. Central targets (located less than 2 cm from the frame center) had a mean MRI-CT difference of 2.09 ± 1.79 mm; peripheral targets (greater than 2 cm from the frame center) differed by 2.17 ± 1.3 mm. The voxel volumes were calculated for all compared images. Although differences between the physical properties of data acquisition with each imaging modality could explain the observed CT-MRI discrepancies, a 1-pixel difference in target selection could account totally for all the variance observed. MRI field strength (0.5 vs. 1.5 T) did not correlate with coordinate determination accuracy. We conclude that MRI-guided stereotactic localization can be used with confidence for most diagnostic, functional, and therapeutic stereotactic procedures. KEY WORDS: Computed tomography; Magnetic field; Magnetic resonance imaging; Stereotactic surgery INTRODUCTION The advantages of using magnetic resonance imaging (MRI) rather than computed tomographic (CT) scans in stereotactic surgery include the increased imaging resolution of the lesion or target (using contrast enhancement or different pulse sequences), direct nonreformatted multiplanar imaging and target coordinate determination, and reduced imaging artifacts produced by the stereotactic frame (13) . The use of MRI is especially beneficial when performing stereotactic surgery in patients with brain lesions or normal anatomical targets that are poorly demonstrated by CT scanning or contrast ventriculography. Stereotactic biopsies (13,17) , functional stereotactic surgery (6,7,9,11,18) , and stereotactic radiosurgery (10) all require MRI guidance for such patients. Because inhomogeneities in magnetic field gradients can lead to geometric image distortion (magnetic susceptibility artifacts), the accuracy of stereotactic MRI guidance has been questioned (6,14) . Optimal imaging can be attained by the frequent calibration of the MRI unit to standard test phantoms, the use of nonferromagnetic frames and fiducial systems, and the immobilization of the patient (5,7,8) . To determine whether MRI provides a consistent and accurate method to obtain stereotactic coordinates, we compared MRI-determined stereotactic target measurements with those obtained with CT scans. We also examined the possible effects of different magnetic field strengths during stereotactic imaging and whether MRI was more reliable in imaging central brain targets than in imaging peripheral targets. PATIENTS AND METHODS Stereotactic target coordinates were obtained in 41 patients using both MRI studies and CT scans. We compared these images for a total of 53 targets: 16 in patients undergoing stereotactic biopsies, 27 in patients undergoing stereotactic radiosurgery, and 10 in patients undergoing functional stereotactic surgery (thalamotomies or capsulotomies). This study was completed in 2 years using resources at two university teaching hospitals (University of Pittsburgh and University of Toronto). The Leksell stereotactic frame (Elekta Instruments, Tucker, GA) and coordinate-determination system were used in all patients to obtain both the CT and MRI data. The MRI-compatible stereotactic frame was constructed from a nonferromagnetic aluminum alloy. All imaging was performed using General Electric scanners (General Electric Medical Systems, Milwaukee, WI): for nonionic contrast-enhanced CT imaging, a 9800 CT scanner (field of view, 250 mm) was used; for MRI studies, either a 1.5-T Signa MRI scanner (26 patients, 30 targets) or a 0.5-T MAX scanner (15 patients, 23 targets). MR images were obtained at 3- or 4-mm slice intervals with no interval between slices. All CT slices were 5-mm thick and were scanned at 3-mm intervals. Using similar slice intervals, we attempted to obtain comparison images with a similar Z (superior-inferior) coordinate. With Redistribution of this article permitted only in accordance with the publisher’s copyright provisions.