Dear Roberto,

> that's good news! (I didn't know what had happened that all my brain
> volumes were so similar :-) Thank you for you answers.
> So, I'll compute the determinant of the affine.mat matrix and multiply
> the volumes I computed from the jacobian by that value. What does the
> nitpicky solution involve?

you basically combine the affine part with the relevant partial  
derivative of the field for each of the components of the Jacobian  
matrix. This reduces to the multiplication of the determinants in some  
special cases where the off-diagonal elements of the affine and/or non- 
linear Jacobian are zero. In practice the differences are very small  

	jac(i,j,k) = (a11+(1.0/dx.Vxs_x())*dxdx->element(indx)) * (a22+(1.0/ 
dy.Vxs_y())*dydy->element(indx)) * (a33+(1.0/dz.Vxs_z())*dzdz- 
         jac(i,j,k) += (a12+(1.0/dx.Vxs_y())*dxdy->element(indx)) *  
(a23+(1.0/dy.Vxs_z())*dydz->element(indx)) * (a31+(1.0/ 
	jac(i,j,k) += (a13+(1.0/dx.Vxs_z())*dxdz->element(indx)) * (a21+(1.0/ 
dy.Vxs_x())*dydx->element(indx)) * (a32+(1.0/dz.Vxs_y())*dzdy- 
         jac(i,j,k) -= (a31+(1.0/dz.Vxs_x())*dzdx->element(indx)) *  
(a22+(1.0/dy.Vxs_y())*dydy->element(indx)) * (a13+(1.0/ 
         jac(i,j,k) -= (a32+(1.0/dz.Vxs_y())*dzdy->element(indx)) *  
(a23+(1.0/dy.Vxs_z())*dydz->element(indx)) * (a11+(1.0/ 
         jac(i,j,k) -= (a33+(1.0/dz.Vxs_z())*dzdz->element(indx)) *  
(a21+(1.0/dy.Vxs_x())*dydx->element(indx)) * (a12+(1.0/ 

> (Jasper, it would be great to have the full
> jacobian right from fnirt!)

I'll put it on the to-do list for the next release.

Good luck  J