[ITK-users] BSpline Registration Parameters

[Stephen Parsons]

In the ITK Software Guide there is some general advice on how to approach
adjusting the parameters of registration. The following is said regarding
the Affine Transform:

5)  The MOST critical values of the registration process are the
      scaling parameters that define the proportions between
       the parameters of the transform. In your case, for an Affine
       Transform in 2D, you have 6 parameters. The first four are
        the ones of the Matrix, and the last two are the translation.

        The rotation matrix value must be in the ranges of radians
         which is typically [ -1 to 1 ], while the translation values are
         in the ranges of millimeters (your image size units).

        You want to start by setting the scaling of the matrix
        parameters to 1.0, and the scaling of the Translation
        parameters to the holy esoteric values:

               1.0   /  (  10.0 * pixelspacing[0]  *  imagesize[0]  )
               1.0   /  (  10.0 * pixelspacing[1]  *  imagesize[1]  )

         This is telling the optimizer that you consider that rotating
         the image by 57 degrees is as "significant" as translating
         the image by half its physical extent.

        Note that esoteric value has included the arbitrary number
        10.0 in the denominator, for no other reason that we have
         been lucky when using that factor. This of course is just a
         supersticion, so you should feel free to experiment with
         different values of this number.

        Just keep in mind that what the optimizer will do is to
        "jump" in a paramteric space of 6 dimension, and that the
       component of the jump on every dimension will be proporitional
       to 1/scaling factor * OptimizerStepLenght.     Since you put
       the optimizer Step Length to 0.1, then the optimizer will start
       by exploring the rotations at jumps of about 5degrees, which
       is a conservative rotation for most medical applications.

       If you have reasons to think that your rotations are larger or
        smaller, then you should modify the scaling factor of the  matrix
        parameters accordingly.

        In the same way, if you thinkl that 1/10 of the image size is too
        large as the first step for exploring the translations, then you
        should modify the scaling of  translation parameters accordingly.

I would like to know if anyone has similar advice regarding the
BSplineTransform. Any help is welcome!

Stephen Parsons
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