Thanks for pointing this out.  In this case, I was doubly accounting for
the relative volumes of voxels, so my units were out.  I have changed
the code (around line 89 of spm8/toolbox/DARTEL/spm_dartel_norm.m)
locally from:

    if jactransf,
        scal = abs(det(NI.mat(1:3,1:3))/det(NO.mat(1:3,1:3))); % Account
for vox sizes
        NO.dat(:,:,:,j,k,l)=f.*dt*scal;
    else

to:

    if jactransf,
        NO.dat(:,:,:,j,k,l)=f.*dt;
    else

This fixes the units.  Note that the effect is a constant scaling effect
if all original images have the same voxel sizes.  The results of any
statistical analysis will be identical, either with or without the
scaling (apart from the masking).

Fixes will be in the next set of updates.

Best regards,
-John

On Tue, 2009-10-06 at 01:27 -0700, cliff macnab wrote:
> John,
> 
> I think the module "create warped" has the same problem. Is there any easy way to fix this problem?
> 
> Thanks,
> 
> C.M.
> 
> 
> 
> ----- Original Message ----
> From: John Ashburner <[log in to unmask]>
> To: [log in to unmask]
> Sent: Tuesday, October 6, 2009 12:50:58 AM
> Subject: Re: [SPM] problems about dartel creat warped with Pres. Amount("modulation")
> 
> Which version do you have?  If you are using the original release of
> SPM8, then there was no correction for the change in voxel sizes between
> the native and spatially normalised images.  This was fixed in the
> updates, with the following change from:
>                     f    = dartel3('push',f,y,dim)*scal;
> to:
>                     scal =
> abs(det(NI.mat(1:3,1:3))/det(NO.mat(1:3,1:3))); % Account for vox sizes
>                     f    = dartel3('push',f,y,dim)*scal;
> 
> When you compute the volume by summing the voxels and multiplying by the
> volume of a voxel, the original version will require the volume of the
> native-space voxels, whereas the corrected version should give the
> correct answer by using the volume of the spatially normalised voxels.
> 
> I would suggest installing the updates:
> http://www.fil.ion.ucl.ac.uk/spm/software/spm8/#Updates
> 
> Best regards,
> -John
> 
> On Mon, 2009-10-05 at 09:06 -0700, cliff macnab wrote:
> > Dear SPMers,
> > 
> > I am using DATEL tools to creat warped images with modulation. My understanding about this procedure is that the volumes before and after warping are approximately same. However, my results indicated that they are quite different, up to 2 times when I warped a mask image. Is my understanding about this procedure wrong?
> > 
> > Thanks,
> > 
> > C.M.
> > 
> > 
> > 
> >      
> > 
-- 
John Ashburner <[log in to unmask]>