Multishot Dual Polarity GRAPPA: Robust Nyquist Ghost Correction for multishot EPI

TL;DR

This paper introduces msDPG, a novel ghost correction method for multishot EPI that significantly improves image quality and reduces artifacts, especially on advanced MRI scanners, enabling high-resolution imaging.

Contribution

The paper develops msDPG, extending DPG to multishot EPI with tailored kernels and calibration refinement, offering superior ghost correction over existing methods.

Findings

msDPG reduces ghost-to-signal ratio by over 50%

Achieves lower noise amplification compared to conventional DPG

Enables high-fidelity submillimeter diffusion imaging

Abstract

Purpose: This work aims to develop a robust Nyquist ghost correction method for multishot echo-planar imaging (EPI). The method helps correct challenging Nyquist ghosts, particularly on scanners with high-performance gradients or ultra-high fields. Methods: A method for multishot EPI ghost correction, called multishot dual-polarity GRAPPA (msDPG), is developed by extending the DPG concept to multishot readouts. msDPG employs tailored DPG kernels to address high-order phase differences between two EPI readout polarities, which cannot be fully addressed using linear phase correction (LPC). Advanced regularizers can be readily employed with the proposed msDPG for physiologic inter-shot phase variation correction during reconstruction. Additionally, a calibration refinement method is proposed to improve the quality of the DPG calibration data and enhance reconstruction performance. Results: Phantom and in vivo experiments on scanners with high-performance gradients and ultra-high fields demonstrated that msDPG achieved superior ghost correction performance than LPC, reducing the ghost-to-signal ratio (GSR) by over 50%. Compared to conventional DPG, msDPG provided images with lower noise amplification, particularly for acquisitions with large in-plane acceleration. Consequently, high-fidelity, submillimeter diffusion images were obtained using msDPG with regularized reconstruction. Conclusion: The proposed msDPG provides a robust Nyquist ghost correction method for multishot EPI, enabling submillimeter imaging with improved fidelity.