[Insight-users] gaussian filtering in itkSymmetricForcesDemonsRegistrationFilter

[Corinne Mattmann]

Hi Luis,

Thanks for you fast reply.
Hmm... I've never used this threshold to stop the registration. I guess
you mean the metric which has lately been implemented in
itkDemonsRegistrationFunction? Can I set the threshold with
"SetMaximumRMSError"?
Why I was thinking about this extra smoothing: I expected this algorithm
to be "calculate deformable field and smooth it" - "calculate deformable
field and smooth it" - ... and I thought it was strange to output a
different deformable field than you use for the next iteration.
Another thing I was thinking about is, when you have a poorly registered
region and you have one step at the end with no filtering (always
assumed that you have no threshold to stop the algorithm), it might be
possible that this region gets completely wrong.
For me, the logical step to use a metric would be "calculate deformable
field, smooth it, calculate metric". However, I guess this
implementation would rise the computation time quite strongly.
I can't say, which implementation is better. The one with not smoothing
the field at the end has the advantage, that small regions with big
changes (which get always too small due to the smoothing) are registered
better.

Regards,
Corinne


-----Original Message-----
From: Luis Ibanez [mailto:luis.ibanez at kitware.com] 
Sent: Monday, March 08, 2004 6:14 PM
To: Corinne Mattmann
Cc: insight-users at itk.org
Subject: Re: [Insight-users] gaussian filtering in
itkSymmetricForcesDemonsRegistrationFilter




Hi Corinne,

The purpose of the smoothing in the implementation of
the Demons algorithm is to regularize the deformation
field.

In principle, what would be expected from this filter
is to return the deformation field that minimizes the
sum of squared differences between the fixed image and
the deformed moving image.  Since the algorithm is
stopped when the changed in this difference is under
a threshold, it is reasonable to return the latest
deformation field that produced this minimum value.

If the filter is changed to smooth the last deformation
field, a user will find the inconsistent situation in
which by using this (smoothed) deformation fields for
resampling the moving image, and recomputing the sum
of squared differences she/he will not be able to find
the same last metric value returned by the filter.

In addition, it is expected that if the algorithm really
converged, no further modification of the deformation
field should be necessary.


Do you find any particular reason for which the
deformation field should have this extra smoothing
after convergence ?


Please let us know,


    Thanks


      Luis



-------------------------
Corinne Mattmann wrote:

> Hi,
> 
> I've just noticed, that in itkSymmetricForcesDemonsRegistrationFilter
> the deformable field is always smoothed before performing an iteration
> (line 70 in itkSymmetricForcesDemonsRegistrationFilter.txx). Like
this,
> the output field (the one after the last iteration) is never filtered.
> Is there a reason not doing this?
> 
> Thanks,
> Corinne
> 
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