hi all, so i'm trying to use topup to correct the distortion in my FMRI EPI images...problem is, when i apply the warp i get this weird smearing effect at the back of the brain (see attachment) ... it isn't so clear on the raw image, but if you take the standard error of the BOLD time series, you can see it clearly. 

i'm using the function almost exactly as they do here:

https://cni.stanford.edu/wiki/Data_Processing

under gradient-reversal unwarping. i saw something kind of similar on another brain, but it wasn't nearly as extreme. 

am i doing something wrong, or is this to be expected? we're using multi-band 3 and 3.75mm isotropic with a TR of 0.69, if that makes any difference. 

thanks!

Russell