EFFECT OF GEOMETRICAL DISTORTION CORRECTION IN MR ON IMAGE REGISTRATION ACCURACY

In this article we investigate the effect of geometrical distortion co rrection in MR images on the accuracy of the registration of X-ray CT and MR head images for both a fiducial marker (extrinsic point) method and a surface-matching technique. We use CT and T2-weighted MR image volumes acquired from seven patients who underwent craniotomies in a s tereotactic neurosurgical clinical trial. Each patient had four extern al markers attached to transcutaneous posts screwed into the outer tab le of the skull. The MR images are corrected for static field inhomoge neity by using an image rectification technique and corrected for scal e distortion (gradient magnitude uncertainty) by using an attached ste reotactic frame as an object of known shape and size, We define target registration error (TRE) as the distance between corresponding marker positions after registration and transformation. The accuracy of the fiducial marker method is determined by using each combination of thre e markers to estimate the transformation and the remaining marker to c alculate registration error. Surface-based registration is accomplishe d by fitting MR contours corresponding to the CSF-dura interface to CT contours derived from the inner surface of the skull. The mean point- based TRE using three noncollinear fiducials improved 34%-from 1.15 to 0.76 mm-after correcting for both static field inhomogeneity and scal e distortion. The mean surface-based TRE improved 46%-from 2.20 to 1.1 9 mm. Correction of geometrical distortion in MR images can significan tly improve the accuracy of point-based and surface-based registration of CT and MR head images. Distortion correction can be important in c linical situations such as stereotactic and functional neurosurgery wh ere 1 to 2 mm accuracy is required.