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

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Image Reconstruction
• Medical Physics

Background:

• Nyquist ghosts are artifacts in echo-planar imaging (EPI) that degrade image quality.
• Multishot EPI techniques are susceptible to these artifacts, especially on scanners with high-performance gradients or at ultra-high fields.
• Existing correction methods like linear phase correction (LPC) have limitations in addressing complex ghosting patterns.

Purpose of the Study:

• To develop a robust Nyquist ghost correction method specifically for multishot EPI.
• To address challenging ghosting artifacts encountered in advanced MRI systems.
• To enhance the fidelity and resolution of multishot EPI acquisitions.

Main Methods:

• Developed multishot dual-polarity GRAPPA (msDPG) by extending dual-polarity GRAPPA (DPG) to multishot readouts.
• Employed tailored DPG kernels to correct high-order phase differences not handled by LPC.
• Integrated advanced regularizers for physiologic inter-shot phase variation correction and proposed a calibration refinement method.

Main Results:

• msDPG demonstrated superior ghost correction compared to LPC, reducing the ghost-to-signal ratio (GSR) by over 50% in phantom and in vivo experiments.
• Achieved lower noise amplification than conventional DPG, particularly with large in-plane acceleration.
• Enabled high-fidelity, submillimeter diffusion imaging with regularized reconstruction.

Conclusions:

• The proposed msDPG method offers robust Nyquist ghost correction for multishot EPI.
• This technique facilitates high-fidelity submillimeter imaging, overcoming limitations of previous methods.
• msDPG is particularly effective on scanners with high-performance gradients and at ultra-high field strengths.