[Insight-users] Similarity Metrics - Clarification - Regions of Consideration

[Karthik Krishnan]

Pairs that satisfy the following criteria are evaluated using the metric.

     Iterate over all pixels in the FixedImageRegion (see SetFixedImageRegion)
       {
       If IsInside() Fixed image mask spatial object
          {
          map pixel index from fixed image to a point in the moving image through the CurrentTransform. 

          if MovingPixel IsInside() Moving Image Mask spatial object
            {
            // Compute metric
            }
         }
      }



M. Wirtzfeld wrote:
>
>
> Good evening,
>
> I am a novice ITK user and I am current working on an 
> image-registration project.  I would like to clarify the regions over 
> which similarity metrics are calculated.
>
>
> Based on very general descriptions of both the 
> itk::MeanSquaresImageToImageMetric and the 
> itk::NormalizedCorrelationImageToImageMetric classes ("Detailed 
> Description" section of the Doxygen documentation) and the more 
> detailed description of the 
> itk::MattesMutualInformationImageToImageMetric, it appears that these 
> metrics are calculated using a region of the fixed-image and a 
> corresponding moving-image region from an applied transform and 
> interpolator.
>
> The ITK Software Guide, page 415, Section 8.10, "Metrics", states: 
>  "Typically, the metric samples points within a defined region of the 
> fixed image. For each point, the corresponding moving image position 
> is computed using the transform with the specified parameters, then 
> the interpolator is used to compute the moving image intensity at the 
> mapped position."
>
>
> With this in mind, and considering the entire fixed-image area/region 
> unless noted otherwise, three cases come to mind:
>
> 1.)  No initial overlap (even with an applied initial tranformation) 
> of the fixed-image and moving-image.  In this obvious case, no metric 
> can be determined.
>
> 2.)  Partial overlap of the fixed-image and moving-image.  Considering 
> an initial identity-transform with no rotation/translation/etc., only 
> the common area/region of the fixed and moving images are used to 
> determine the metric.  If an initial rotation/translation/etc. is 
> applied along with the identity-transform, the common area/region 
> could be smaller or larger depending on the resulting movement.
>
> 3.)  Fixed-image bounded by moving-image.  As with 2., consider an 
> initial identity-transform.  Either because the fixed-image 
> area/region is smaller than the moving-image area/region or a 
> region-of-interest has been applied to the fixed-image, two 
> possibilities exist.  a.)  No initial rotation/translation/etc. - all 
> fixed-image area/region is used for metric determination.  b.)  Large 
> rotation/translation/etc. partial fixed-image area/region is used 
> depending on amount of movement.  Of course, a non-identity transform 
> could also change number of pixels/voxels considered.
>
>
> The ITK Software Guide (Section 8.10) indicates the 
> SetFixedImageMask() and SetMovingImageMask() methods of the 
> itk::ImageToImageMetric class can also be used to refine metric 
> calculations.   With respect to the SetFixedImageMask() method, an 
> itk::RegionOfInterestImageFilter could also be applied to define a 
> smaller, fixed-image working area/region?  How is the 
> SetMovingImageMask() typically applied?
>
>
> Thank you, 
>
> Michael.
>
>
> M. Wirtzfeld
> Robarts Imaging
> London, ON
>
> "There are three kinds of lies: lies, damned lies, and statistics." 
> - Benjamin Disrael
>
>
>
>
>
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>
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-- 
--
Karthik Krishnan
R & D Engineer,
Kitware Inc,
Ph: +1 518 3713971 x119
Fax: +1 518 3714573