[Insight-users] inverting deformation fields

[Anja Ende]

Hi Matt,

Thanks a lot for this explanation :)

AtB

Anja



On 10 April 2013 20:28, Matt McCormick <matt.mccormick at kitware.com> wrote:

> Hi Anja,
>
> Yes, sampling and the grid play an important role.
>
> The sampling locations of the output must be chosen, and they are not
> necessarily the sampling locations of the input.
>
> This is what the ResampleImageFilter and its Transform may operator in
> the "opposite" direction what is expected -- what is always computed
> is the output pixel location.
>
> HTH,
> Matt
>
> On Wed, Apr 10, 2013 at 7:17 PM, Anja Ende <anja.ende at googlemail.com>
> wrote:
> > Oh yes, of course! That is a subtle point.
> >
> > May I ask a follow up question then? In inverting a deformation field
> then,
> > how do we decide the sampling of this grid of the inverse field?
> >
> > Say our original deformation field has a displacement at every voxel and
> > these displacement vectors can point to any real location. So, is the
> > inverse deformation field going to be calculated at these real locations?
> > However, we need to have a uniform grid and this is not guaranteed by
> this
> > scheme now. Can someone explain this to me on how this is done?
> >
> > Sorry for asking these naive questions. I could use the ITK filters out
> of
> > the box but I would like to know some of these basics.
> >
> > Thanks,
> > Anja
> >
> >
> > On 10 April 2013 20:06, Matt McCormick <matt.mccormick at kitware.com>
> wrote:
> >>
> >> Hi Anja,
> >>
> >> If displacement vector field gives the displacement at location x, but
> >> the negative of the displacement at location x is not the inverse;
> >> what is required is negative of the displacement vector at x + \delta
> >> x.
> >>
> >> HTH,
> >> Matt
> >>
> >> On Wed, Apr 10, 2013 at 5:47 PM, Anja Ende <anja.ende at googlemail.com>
> >> wrote:
> >> > Hello all,
> >> >
> >> > This is a very naive question and I would be very grateful if someone
> >> > can
> >> > clarify this doubt for me.
> >> >
> >> > I see that there are quite a few filters and some very sophisticated
> >> > methods
> >> > for inverting deformation fields.
> >> >
> >> > If I have a dense deformation field, why is it not enough to simply
> >> > negate
> >> > the displacement vector to compute the inverse? I am sure there is a
> >> > simple,
> >> > logical explanation for this but I am having trouble figuring it out.
> >> >
> >> > Thanks a lot for your help!
> >> >
> >> > Anja
> >> >
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>
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