[Insight-users] inverting a deformation field - problem

[Tim Bhatnagar]

Hi Hans,

Thanks for that. So within the WarpImageFilter, the filter itself then is
essentially determining the inverse of my field, and applying the
deformation, voxel-by-voxel, to each voxel of my movingImage?

That explanation would fit my observations.. but I don't think
WarpImageFilter has the tools to invert the input displacement field and
apply it (the filter usually takes ~ 1-2 seconds to finish!)

Additionally, is there any truth to the rumour that exporting my vtk files
into Paraview could alter the coordinate system I'm using? That possibility
is on a whole other layer, and if true, could also be obfuscating my
methods..


On Mon, Dec 9, 2013 at 6:12 AM, Johnson, Hans J <hans-johnson at uiowa.edu>wrote:

>  Tim,
>
>  The moving image is not “pushed into the space of the fixed image” it is
> “pulled into the space of the fixed image” Displacement fields are defined
> in the space of the “fixed” image, and at each location of the fixed image
> the displacement field describes the offset physical location in the moving
> image that is interpolated.
>
>  Hans
>
>
>   From: Tim Bhatnagar <tim.bhatnagar at gmail.com>
> Date: Monday, December 9, 2013 at 8:06 AM
> To: Bradley Lowekamp <brad at lowekamp.net>
> Cc: insight-users <insight-users at itk.org>
> Subject: Re: [Insight-users] inverting a deformation field - problem
>
>    I appreciate the response, Brad.
>
>  Can you tell me how the implementation of the 'WarpImageFilter' works,
> such that when I input my Demons-generated displacement field, and my
> 'movingImage' (from Demons) as input, I get what looks like my 'fixedImage'
> (from Demons) as output... making me think that the displacement field is
> actually mapping my movingImage into my fixedImage?
>
>  Also, I use Paraview to visualize my field, and when visualized, it
> illustrates what you are suggesting (the mapping that would take fixed ->
> moving). But how then, does the WarpFilter use the SAME FIELD to warp my
> movingImage into a very nice estimation of my fixedImage?
>
>  Sorry if any of that is convoluted..
>
>
> On Mon, Dec 9, 2013 at 6:00 AM, Bradley Lowekamp <brad at lowekamp.net>wrote:
>
>> Hello,
>>
>>  It not uncommon for resampling-type algorithms to have some points be
>> mapped out side the the domain. This is why a boundary condition and
>> extrapolators are used. If there are just a limited number of pixel being
>> mapped out side, then the simplest way be by to just check if the pixel is
>> within the image's region.
>>
>>  ITK's registration frameworks solve for a transform which maps from the
>> "Fixed" image to the "Moving" image. This is well described int Chapter
>> Eight of the old Software guide:
>>
>>  http://www.itk.org/ItkSoftwareGuide.pdf
>>
>>  Brad
>>
>>   On Dec 8, 2013, at 6:38 PM, Tim Bhatnagar <tim.bhatnagar at gmail.com>
>> wrote:
>>
>>     Hello,
>>
>>  I've created a displacement field via Demons non-rigid registration. I
>> am now trying to create the inverse (using
>> InverseDisplacementFieldImageFilter), but when I am using the indexes to
>> ascribe the inverted field, my program crashes mid-way through, because it
>> appears I am trying to call a *negative* index (specifically, the 3rd
>> index parameter.. the 'z'-parameter).
>>
>> I don't do anything to the displacement field except write it to a vtk
>> file before i invert it.. can anyone suggest a solution? I thought I could
>> try and specify the 'direction' of the field, but I'm not sure where in my
>> pipeline I can set the direction of the *inverted* displacement field...
>>
>>  sorry if that's messy.. any thoughts would be appreciated!
>>
>> Thanks,
>>
>> Tim
>>
>> --
>> Tim Bhatnagar
>> PhD Candidate
>> Orthopaedic Injury Biomechanics Group
>> Department of Mechanical Engineering
>> University of British Columbia
>>
>> Rm 5000 - 818 West 10th Ave.
>> Vancouver, BC
>> Canada
>> V5Z 1M9
>>
>> Ph: (604) 675-8845
>> Fax: (604) 675-8820
>> Web: oibg.mech.ubc.ca
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>
>
> --
> Tim Bhatnagar
> PhD Candidate
> Orthopaedic Injury Biomechanics Group
> Department of Mechanical Engineering
> University of British Columbia
>
> Rm 5000 - 818 West 10th Ave.
> Vancouver, BC
> Canada
> V5Z 1M9
>
> Ph: (604) 675-8845
> Fax: (604) 675-8820
> Web: oibg.mech.ubc.ca
>
>
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-- 
Tim Bhatnagar
PhD Candidate
Orthopaedic Injury Biomechanics Group
Department of Mechanical Engineering
University of British Columbia

Rm 5000 - 818 West 10th Ave.
Vancouver, BC
Canada
V5Z 1M9

Ph: (604) 675-8845
Fax: (604) 675-8820
Web: oibg.mech.ubc.ca
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