Geometric distortion correction of high‐resolution 3 T diffusion tensor brain images

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Abstract
Diffusion‐weighted images based on echo planar sequences suffer from distortions due to field inhomogeneities from susceptibility differences as well as from eddy currents arising from diffusion gradients. In this paper, a novel approach using nonlinear warping based on optic flow to correct distortions of baseline and diffusion weighted echo planar images (EPI) acquired at 3 T is presented. The distortion correction was estimated by warping the echo planar images to the anatomically correct T2‐weighted fast spin echo images (T2‐FSE). A global histogram intensity matching of the T2‐FSE precedes the base line EPI image distortion correction. A local intensity‐matching algorithm was used to transform labeled T2‐FSE regions to match intensities of diffusion‐weighted EPI images prior to distortion correction of these images. Evaluation was performed using three methods: (i) visual comparison of overlaid contours, (ii) a global mutual information index, and (iii) a local distance measure between homologous points. Visual assessment and the global index demonstrated a decrease in geometrical distortion and the distance measure showed that distortions are reduced to a subvoxel level. In conclusion, the warping algorithm is effective in reducing geometric distortions, enabling generation of anatomically correct diffusion tensor images at 3 T. Magn Reson Med, 2005.