Correction of MR image distortions induced by metallic objects using a 3D cubic B‐spline basis set: Application to stereotactic surgical planning

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Abstract
Metallic implants in MRI cause spin‐echo (SE) images to be distorted in the slice and frequency‐encoding directions. Chang and Fitzpatrick (IEEE Trans Med Imaging 1992;11:319–329) proposed a distortion correction method (termed the CF method) based on the magnitude images from two SE acquisitions that differ only in the polarity of the frequency‐encoding and slice‐selection gradients. In the present study we solved some problems with the CF method, primarily by modeling the field inhomogeneities as a single 3D displacement field built by 3D cubic B‐splines. The 3D displacement field was applied in the actual distortion direction in the slice/frequency‐encoding plane. To account for patient head motion, a 3D rigid body motion correction was also incorporated in the model. Experiments on a phantom containing an aneurysm clip showed that the knot spacing between the B‐splines is a very important factor in both the final image quality and the processing speed. Depending on the knot spacing and the image volume size, the number of unknowns range from a few thousands to over 100,000, leading to processing times ranging from minutes to days. Optimal knot spacing, a means of increasing the processing speed, and other parameters are investigated and discussed. Magn Reson Med 54:169–181, 2005.