We have begun a set of pilot scans to see if some parameter combinations produce better subcortical and frontal signal in a reward task. So far all scans have been on a Siemens Prisma,
Of the two pilot people (so far), one has the crescent artifact and the other does not, and the appearance of the crescents in the different acquisitions is interesting. All of these images are voxelwise standard deviation, calculated over the entire run (no censoring, but extremely low motion), and on raw images (preprocessing is in progress).
First, here are sagittal views of a flat (left, scan 15) and tilted (right, scan 37) 2.4 mm isotropic run. The crescent artifact is visible in both; I marked the approximate ends with green arrows (click to enlarge). The multiband slice boundary is visible in both a fourth of the way up the image (red arrows).
Here are axial slices of the set of runs we have so far for this person. All are with the same color scaling (0 to 200); brighter is higher standard deviation. These are raw images, so the slice appearance varies quite a bit between the "flat" and "tilted" runs.
The 3p0 (3.0 mm isotropic voxels) images are generally more uniform and dark than the 2p4 runs, likely reflecting improved signal-to-noise, particularly in the middle of the brain. While the large vessels are brightest in all runs (as they should be), the runs with 2.4 mm voxels (2p4) generally have a "starburst" type effect (brighter in the center, darker towards the edges), which is worrying, particularly since we want good signal in reward areas.
I will share other observations on this blog as the piloting and analyses progress. Please contact me if you'd like to run your own analyses; we'd be happy to share and are very interested in others' thoughts.
UPDATE 18 April 2018: I've wondered before if head size was a factor in which people have the crescent artifact, using the total intracranial volume measurement produced by freesurfer as a proxy. I don't have those measurements yet, but they kindly allowed me to measure their heads as if fitting them for hats, and they were nearly identical: about 58 cm for the person without the crescent artifact, and about 57 for the person shown in this person (with the artifact). Both people have a normal healthy body size; the person without the artifact was a bit shorter (around 5'2") than the other (around 5'7"). So, at least for these two pilots, external head size doesn't seem to matter for the artifact.
more acquisition parameters below the jump