[ITK Community] [Insight-users] inverting a deformation field - problem

[Johnson, Hans J]

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<mailto:tim.bhatnagar at gmail.com>>
Date: Monday, December 9, 2013 at 8:06 AM
To: Bradley Lowekamp <brad at lowekamp.net<mailto:brad at lowekamp.net>>
Cc: insight-users <insight-users at itk.org<mailto: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<mailto: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<mailto: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<tel:%28604%29%20675-8845>
Fax: (604) 675-8820<tel:%28604%29%20675-8820>
Web: oibg.mech.ubc.ca<http://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<http://oibg.mech.ubc.ca>


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