[Insight-users] Similarity Metrics - Clarification - Regions of Consideration
M. Wirtzfeld
wirtzfeld at rogers.com
Tue Jul 15 01:24:58 EDT 2008
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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