Potential Impact of Time-Varying Gradient Delays in the EPI Train on Nyquist Ghost Correction

Silke Hey¹, Jos Koonen¹, Zhaolin Chen¹, Johan van den Brink¹

Reliable phase correction for long fMRI scans is crucial to ensure low ghost levels. Phase correction strategies are based on the measurement of gradient delays, usually using a phase navigator. In this work, we show that gradient delays vary across the echo train depending on the used echo-spacing frequency. This variation may change in the course of long fMRI acquisitions (several minutes). That is why, accurate and robust correction of the Nyquist ghost in EPI requires determination of the gradient delays at the echo time of the EPI sequence.