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

Area of Science:

• Magnetic Resonance Imaging
• Image Reconstruction
• Medical Physics

Background:

• Nyquist ghosts are artifacts in multishot echo-planar imaging (EPI).
• Existing correction methods like linear and non-linear phase correction struggle with complex phase variations, especially on high-performance scanners.
• Robust ghost correction is crucial for high-fidelity, submillimeter imaging.

Purpose of the Study:

• To develop a robust Nyquist ghost correction method for multishot EPI.
• To address challenging ghost artifacts on scanners with high-performance gradients and ultra-high fields.
• To enhance image quality and fidelity in advanced MRI applications.

Main Methods:

• Developed multishot dual-polarity GRAPPA (msDPG) by extending the dual-polarity GRAPPA concept to multishot readouts.
• Employed tailored DPG kernels to correct high-order phase differences not handled by linear phase correction (LPC) or non-linear phase correction (nLPC).
• 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 and nLPC, 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 using msDPG 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 beneficial for advanced MRI systems with high-performance gradients and ultra-high fields.