A comparison between magnetic resonance imaging and computed tomography for stereotactic coordinate determination.

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.