Dual-Polarity GRAPPA for simultaneous reconstruction and ghost correction of EPI data

Purpose: We seek improved image quality from accelerated EPI data, particularly at ultra-high field. Certain artifacts in EPI reconstructions can be attributed to nonlinear phase differences between data acquired using frequency encoding gradients of alternating polarity. These errors appear near regions of local susceptibility gradients, and typically cannot be corrected with conventional Nyquist ghost correction (NGC) methods. Methods: We propose a new reconstruction method that integrates ghost correction into the parallel imaging data recovery process. This is achieved through a pair of GRAPPA kernels that operate directly on the measured EPI data. The proposed Dual-Polarity GRAPPA (DPG) method estimates missing k-space data while simultaneously correcting inherent EPI phase errors. Results: Simulation results demonstrate that standard NGC is incapable of correcting higher-order phase errors, whereas the Dual-Polarity GRAPPA kernel approach successfully removes these errors. The presence of higherorder phase errors near regions of local susceptibility gradients is demonstrated with in vivo data. DPG reconstructions of in vivo 3T and 7T EPI data acquired near these regions show a marked improvement over conventional methods. Conclusion: This new parallel imaging method for reconstructing accelerated EPI data shows better resilience to inherent EPI phase errors, resulting in higher image quality in regions where higher-order EPI phase errors commonly occur.