Here's a different visualization of the problem (I'm in the same lab) - a gif going back and forth between using 8 and 9 cores. The problem isn't as much with changes in signal intensity but with changes in the pattern of distortions across number of threads.

We've also confirmed that repeated executions using the same number of cores give identical results, which to me seems to argue against the possibility that this is due to different orders of operations across executions (but is consistent with there being a different order of operations across number of threads). We've also confirmed this is restricted to 3dQwarp - 3dNwarpApply executes identically across different numbers of cores.

Finally - if this is just a known issue with multi-threaded computations in 3dQwarp, is there a 'correct' # of threads? Is it safest to default to a single thread, hopefully avoiding some of these problems?

Thanks!