I'll be at OHBM 2025 next week (want to meet up? drop me a line), presenting a poster about the crescent artifacts (""Crescent" artifacts with multiband fMRI acquisitions: appearance, causes, and consequences"). The poster pdf is available via OHBM's system, or at https://osf.io/pzj39.
My blog post last December covers some of the same information, but preparing the poster refined my thoughts a bit. I still don't have as clear a sense as I'd like of exactly how much the artifact affects fMRI signal quality, but am confident that there is enough likelihood of a substantial negative impact that they shouldn't be ignored.
It seems to be a given in the MR physics literature that Nyquist ghosts appreciably degrade EPI signal to noise. Quantifying the impact in GLM results is difficult, however, especially after preprocessing, smoothing, in group analyses (artifact location and intensity varies across people), and when runs of different encoding directions are analyzed together. Qualitatively, I have seen crescents in single-subject statistical images (average activation, GLMs), but that is of course not the typical analysis. It could be informative to run a few comparison tests; for example, is the effect strength in frontal parcels (i.e., those in the path of the crescent artifact) worse in a group of participants with the artifact than without? Or in PA than AP encoding runs?
While the degree of impact is uncertain, I think there's enough evidence to recommend that crescent artifacts be one of the criteria for choosing acquisition protocols: select fMRI acquisition parameters so that crescent artifacts are minimized and/or appear in brain areas of low theoretical interest. Since the crescent artifact likely reduces BOLD signal quality somewhat, and is more often prominent in people with smaller brains, there's a risk of bias if an experimentally-important participant characteristic (e.g., age, gender) is associated with differences in head size; extra care should be taken in these cases.
