On Oct 13th 2023, Nicolas Boulant presented an intriguing source of MRI image artifacts at the CMRR high field meeting in Minnesota. He suggested that the 3rd-order shim can result in amplified gradient trajectory imperfections. In low bandwidth FLASH, this can manifest as faint ghosts in the read direction shifted by a few pixels. In EPI, on the other hand, these trajectory errors can result in fuzzy ripples (low spatial frequency ghosts and shadings, not edge ghosts).
In a recent meta analysis of all openly available layer-fMRI datasets, I had found had that the fuzzy ripples are one of the main limits of high-quality layer-fMRI acquisition (see here) across vendors. So, I was curious whether the 3rd order shim might be partly related to this. In this blog post, I am describing my attempts to reproduce Nicola’s results and investigate the effect of the 3rd order shim on layer-fMRI protocols. I find that disconnecting the 3rd order shim can result in significantly better data quality. However, this finding is only visible for specific echo-spacings, which are either in the ‘forbidden frequencies’ or which have side bands in the forbidden frequencies.
This post does not imply that previous research was conducted sub-optimally. Since, it is common practice to optimize the EPI echo spacing in the piloting stage of each study, the frequencies with these artifacts are usually avoided anyway. Here, we confirm that this is a good practice.
Continue reading “3rd order shim for layer-fMRI: To get best data quality, avoid certain EPI frequencies.”