Ok, so I did some experimentation. Results (for four slices) are shown below:
http://i.imgur.com/0c2bGyJ.png
So it appears topup with 15ms over estimates the negative regions, but the positive regions match better. For 60 ms it under estimates the positive regions, but matches the negative regions better. 29 ms seems somewhat inbetween (overestimates negative regions, and under estimates positive regions).
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