Holy crescents, Batman!

Quite a few of the posts over the last year or so have arisen from things that catch my eye as I review the SMS/MB4 images we're collecting in our ongoing project, and this is another. For quick comparison, I make (with knitr; we may give mriqc a try) files showing slices from mean, standard deviation, and tSNR images for participants, runs, and sessions.

Some participants have obvious bright crescent-shaped artifacts in their standard deviation images (the examples above are from two people; both calculated from non-censored frames, after completing the HCP Minimal Preprocessing pipeline). Looking over people and runs (some participants have completed 6 imaging sessions, over months), people have the crescents or not - their presence doesn't vary much with session (scanning day), task, or movement level (apparent or real).

They do, however, vary with encoding direction: appearing in PA phase encoding runs only. Plus, they seem to vary with subject head size, more likely in small-headed people (large-headed people seem more likely to have "ripples", but that's an artifact for another day).

All that (and thanks to conversations with practiCal fMRI and @DataLoreNeuro) gave a hint: these crescents appear to be N/2 ghost artifacts.

Playing with the contrast and looking outside the brain has convinced me that the crescents do align with the edges of ghost artifacts, which I tried to show above. These are from a raw image (the HCP Minimal Preprocessing pipelines mask the brain), so it's hard to see; I can share example NIfTIs if anyone is interested.

So, why do we have the bright ghosts, what should we do about it, and what does that mean for analysis of images we've already collected? Suggestions are welcome! For analysis of existing images, I suspect that these will hurt our signal quality a little: we want the task runs to be comparable, but they're not in people with the crescent: voxels within the crescent areas have quite different tSNR in the PA and AP runs.

Holy crescents, Batman! (We've been watching the 1966 Batman TV series.)

afni: 3dTstat with not-censored timepoints only

In a few recent posts I've shown images of the mean and standard deviation (calculated across time for each voxel), for QC tests. These are easy to calculate in afni (example here), but the 3dTstat command I used includes all timepoints (TRs), unless you specify otherwise. As described previously, we've been using a threshold of FD > 0.9 for censoring high-motion frames before doing GLMs. Thus, I wanted to calculate the mean and standard deviation images only including frames that were not marked for censoring (i.e., restrict the frames used by 3dTstat). This was a bit of a headache to code up, so R and afni code are after the jump, in the hopes it will be useful for others.