[Insight-users] Problem in BSpline Registration of really small region

[Serena Fabbri]

Hi All,

I'd like to register a small portion of the Fixed Image with BSplineT, Mattes MI and Regular Step 
Gradient Descent Optimizer.
In particular I want to set BSpline Grid only on this portion of the Fixed image.

  I have set 
fixedRegion with appropriate size and origin and I have passed them to

registration->SetFixedImageRegion( fixedRegion );

and the same time I have used fixedRegion to determine
  GridSpacing( spacing );
  GridOrigin( gridOrigin );
  GridDirection( gridDirection );

I have followed the examples  in the ITKSoftGuide and i have set 
bsplineRegion.SetSize( totalGridSize );

but I have a doubt about GridRegion( bsplineRegion ):

what is the difference between bsplineRegion and fixedRegion???

and....The fixedRegion is really small, the its pixel size is: [9,25,38]
and the Fixed Image and Moving Image are [80,80,102], spacing=[4,4,1] mm

I use all pixel in the region for MMI calculus and I tuned fine the parameter of the optimizer but I get 
the error message that 
Description: itk::ERROR: MattesMutualInformationImageToImageMetric(0x1006af4f0): Too many 
samples map outside moving image buffer: 0 / 8550

The registration works only if I enlarge the size of the fixedRegion to [30,30,38], but then it contains 
features that I don't want to register.

How can I register a really small region ???
any idea?

Thank you a lot.

Serena.