[Insight-users] Warping w.o. overall intensity intergral change

[Torsten Rohlfing]

Hi Felix.

The intensity integral is conceptually a product of intensity times 
volume (area in 2D, length in 1D). So if you increase the volume of 
high-intensity regions due to warping, the overall intensity integral 
increases as a result.

The following are your options:

1. globally normalize the intensity integral, i.e., multiply the 
intensity of each pixel in the warped image with IntOriginal/IntWarped, 
where "IntOriginal" is the original integral and "IntWarped" is the 
integral after warping (but before normalization). This will guarantee a 
globally invariant intensity integral, but may be inappropriate for your 
application.

2. locally correct for volume changes by multiplying warped intensities 
with the Jacobian of the deformation at that pixel. This is physically 
the correct way to do things in some applications where signal piles up 
due to geometrical distortion, for example in echo-planar MR imaging.

Hope this helps.

Torsten

> Hello ITK Users,
>
> I am warping a functional measurment 3D volume using a spline grid for 
> pixel displacement, as discribed in the Softwareguide.
>
> The quality of the warped volume is fine by visual inspection is fine, 
> however,
> the overall intensity integral increase by x1.25. 
>
>
> For analysis in need the intensity integral invariant of course, so:
>
>         (0. why does the warping increase overall intensity)
>
>         1. Is there a way to maintain overall intensity values in warping 
> - i.e. gaussian interp. kernel etc. ?
>  
>
>   

-- 
Torsten Rohlfing, PhD          SRI International, Neuroscience Program
 Senior Research Scientist      333 Ravenswood Ave, Menlo Park, CA 94025
  Phone: ++1 (650) 859-3379      Fax: ++1 (650) 859-2743
   torsten at synapse.sri.com        http://www.stanford.edu/~rohlfing/

     "Though this be madness, yet there is a method in't"

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