Dear Stefan,

The fslstats command is used on the partial volume output from FAST.
Multiplying the mean by the volume is just another way of summing up the partial
volume contributions from all voxels.  Using -V instead just gives you the number
of non-zero voxels, but we want to take into account the various partial volume
values (e.g. 0.15 representing 15% of that voxel being a certain tissue-type).
Doing it this way, by taking into account the partial volume, gives more accurate
results.

I hope this answers all three of your questions.
All the best (from China),
	Mark


On 9 Jun 2012, at 03:54, Stefan Kreisel wrote:

> Hi all.
> Looking at the sienax script it says somewhere near the bottom - if I've chosen the -2 option (just for simplicity) - ubrain=`echo "2 k $xa $xb * 1 / p" | dc -`; $xa comes from the first output of the fslstats -m -v command just a couple of lines above, $xb. ubrain is what my "raw" brain volume is going to be. There's a couple of things I keep tripping on:
> 1.) Why would you multiply the mean intensity of all voxels by the mm^3 of your image! (probably some nasty mathematical trick - right); I would have though it be easier to fetch the volume from fslstats -V.
> 2.) Having said that - why is that multiplication slighly off in comparison to fslstats -V (say 650507.49 in the report.sienax to 650511.125 using fslstats -V)?
> 3.) @ 2.) I first though it was a rounding inaccuracies - but probably not. If I run sienax without the -2 option and get the full breadth of tissue types (and no longer binary images of each) fslstats -V fails in full futility and one falls back to having to multiply mean by volume of image...
> So where's the trick I'm missing?
> Cheers and hope all of you are in China!?
> Stefan
>