[Insight-users] Bspline-based registration example 6

[Eve]

Hi users,

I would like some clarifications on using multi-resolution bspline
registration. In the 2D multi-res bspline-based example#6, the second level
uses an 8 x 8 node grid (from 5 X 5). This means that more nodes are used,
and thus, less spacing between control points in the second level. Is it
always true that the 2nd level uses more nodes (less spacing) in multi-level
schemes? If so, this seems to be contradicted by a claim in the following
work:

"ITK Implementation of Deformable Registration Methods for Time-varying (4D)
Imaging Data"

available at
http://spiedl.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PSISDG00614100000161412J000001&idtype=cvips&prog=normal

The claim says: "the spacing between the control points is increased by a
factor of 2 [at the second level]" (p. 5)

Some other questions:

1. Are there heuristics of determining the optimal choice of 1) # of nodes
and 2) # of levels for nD-image? 

2. I tried a 2-level scheme, where in the 2-level, I input finer-sampled
images. Despite the change in resolution and more parameters (nodes), there
is no change in the deformation (though the metric did changed, and that 1st
level computed deformation was suboptimal)? I also tried decreasing the
number of nodes (due to the scheme in aforementioned paper), with the same
result. Is this reasonable?

3. Is it correct that more nodes don't necesary result in more accurate
registration due to possible unstable behaviour?

Thanks in advance,
Eve
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