On the source of unresolved Nyquist ghosting in EPI at high and ultrahigh field

Motivation: Residual Nyquist ghosting commonly confounds echo-planar images at ultrahigh field. The effect is not yet fully understood. Goal(s): To elucidate why standard ghost correction methods may fail at 7T. Approach: We analyzed the shift between odd and even echoes based on a linear, time-invariant (LTI) gradient model for different gradient axes and field strengths. In EPI experiments, we quantified the artifacts for corrections based on three navigators and the LTI-model. Results: Mechanical resonances of the gradient coils cause oscillating shifts between odd and even echoes. Through appropriate parameter choices or trajectory correction, they can be avoided or corrected for. Impact: Residual Nyquist ghosting in EPI at 7T can be reduced by avoiding or correcting for mechanical oscillations of the gradient coils. A deepened understanding of this effect allows for more reliable correction strategies and less confounded functional or diffusion-weighted MRI.