Eddy current correction in diffusion‐weighted imaging using pairs of images acquired with opposite diffusion gradient polarity

Abstract

In echo-planar-based diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI), the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusion-sensitizing gradients. Correction methods that coregister diffusion-weighted and non-diffusion-weighted images suffer from the different contrast properties inherent in these image types. Here, a postprocessing correction scheme is introduced that makes use of the inverse characteristics of distortions generated by gradients with reversed polarity. In this approach, only diffusion-weighted images with identical contrast are included for correction. That is, non-diffusion-weighted images are not needed as a reference for registration. Furthermore, the acquisition of an additional dataset with moderate diffusion-weighting as suggested In diffusion-weighted echo-planar imaging (EPI), eddycurrent-related distortions add to the susceptibility-induced distortions that are generally seen on images measured by EPI. The magnitude and shape of these particular artifacts change with the chosen direction and strength of the applied diffusion-sensitizing gradients. Both diffusionweighted and non-diffusion-weighted images are necessary for the subsequent calculation of diffusion parameters such as apparent diffusion coefficients (ADCs) or anisotropy indices. Thus, significant miscalculations of maps depicting such parameters would occur if the acquired underlying images are warped against each other. Large errors are especially prominent in regions with high local contrast. While computational methods based on MR diffusion imaging, such as fiber tracking algorithms (1,2) or the mathematical modeling of high angular resolved diffusion data (3,4), become more and more sophisticated, attention must be paid to the reliability of the input data.Hence, an optimal distortion correction scheme for diffusion-weighted images still remains a goal.The high impact of eddy currents on diffusion-weighted imaging (DWI) is caused by strong diffusion-sensitizing magnetic field gradients flanked by short ramp times. Because in EPI the bandwidth in the phase-encoding direction is inevitably low, this direction is primarily prone to substantial eddy-current-generated distortions. Until now, several methods have been introduced to mitigate the problems caused by eddy-current-related distortions. Preemphasis settings may facilitate the generation of stabilized and, therefore, eddy-current-neutralizing gradient waveforms (5). Unfortunately, the application of a preemphasis technique does not fully eliminate eddy-currentrelated distortions. For this reason, other methods have been proposed in recent years that either reduce the eddy currents by modifications of the pulse sequence design or correct for the distortions during data postprocessing. Procedures using phantom scans as a reference have been suggested as well (6). Eddy-current-minimizing modifications of the...

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