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Hi - with b=1k and b=2k, you should have significant kurtosis, and therefore you will get different-looking tensors (different MD but also FA), though these will be shifted and highly correlated. Fitting a single tensor to both data will give something in between and the data fitting won’t be that good in most of the brain tissue. Cheers, Saad. On 29 Sep 2016, at 16:17, Patrick <[log in to unmask]<mailto:[log in to unmask]>> wrote: Hi experts I've searched around the forum a bit but seem to be getting slightly differing answers to my question. I'm collecting multi b-value diffusion data (using a NODDI sequence). I have the matlab toolbox to reconstruct NODDI specific parameters (ODI, NDI etc) but I was wondering about the accuracy of using FSL to reconstruct my FA and MD maps. The sequence I use is a 2 shell sequence with 36 directions at b=1000 (+4 b0) and 72 directions at b=2000 (+4 b0). The two shells are collected interleaved and the data is processed in a single 4d data file. The resulting MD and FA maps look great but I want to ensure that FSL is reconstructing these maps accurately before proceeding with my quantitative data analysis. Thanks Patrick McCunn