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In RARE sequence, when multiple echoes with different phases are acquired within the same acquisition window (ADC) , it will lead to some bands across the image (and this situation is avoided using CPMG conditions!).
I was wondering, What exactly causes this artifact , specially from a K-space point of view?

Thank you in advance

A great, but complicated question, to hard to answer in the brief space herein.

As a start, I suggest looking at the following free online paper by Carmicheal et al from the following link:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2728202/