EPI image reconstruction with correction of distortion and signal losses.

PURPOSE:To derive and implement a method for correcting geometric distortions and recovering magnetic resonance imaging (MRI) signal losses caused by susceptibility-induced magnetic field gradients (SFGs) in regions with large static field inhomogeneities in echo-planar imaging (EPI). MATERIALS AND METHODS:Factors to account for field inhomogeneities and SFGs were added in a traditional EPI equation that was a simple Fourier transform (FT) for expressing the actual k-space data of an EPI scan. The inverse calculation of this "distorted EPI" equation was used as a kernel to correct geometric distortions and reductions in intensity during reconstruction. A step-by-step EPI reconstruction method was developed to prevent complicated phase unwrapping problems. Some EPI images of phantom and human brains were reconstructed from standard EPI k-spaces. RESULTS:All images were reconstructed using the proposed multistep method. Geometric distortions were corrected and SFG-induced MRI signal losses were recovered. CONCLUSION:Results suggest that applying our method for reconstructing EPI images to reduce distortions and MRI signal losses is feasible.