Silke Hey1, Jos Koonen1, Zhaolin Chen1, Johan van den Brink1
1Philips Healthcare, Best, Netherlands
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.