BSplineTransform object has no attribute SetCoefficientImages

Algorithms
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1

I used SimpleITK v1.2 to perform a BSpline image registation. I used BSplineTransformInitializer to initialize the registration:

MeshSize = [NumControlPts] * MovIm.GetDimension()

InitialTx = sitk.BSplineTransformInitializer(FixIm, MeshSize)

I set the metric, optimizer, interpolator, etc. then get the final transform:

FinalTx = RegMethod.Execute(FixIm, MovIm)

The result looks fine. I want to store the transform parameters and deformation field in a DICOM registration object (DRO). The idea being that the DRO will be stored and when the same pair of FixIm and MovIm come about, I can avoid repeating the time-consuming deformable registration by parsing the DRO for the required parameters, create a BSpline transformation, apply the parsed parameters, and resample MovIm.

I have access to the grid size, spacing, origin and direction already but need the coefficient images. It seemed reasonable to get it from the final transform:

CoeffImX, CoeffImY, CoeffImZ = FinalTx.GetCoefficientImages()

but I got the error:

‘Transform’ object has no attribute ‘GetCoefficientImages’

I tried using the initial transform instead:

CoeffImX, CoeffImY, CoeffImZ = InitialTx.GetCoefficientImages()

which worked. I was puzzled by this result since both InitialTx and FinalTx are BSplineTransforms:

print(InitialTx)

itk::simple::BSplineTransform
BSplineTransform (00000267BA4D7330)
RTTI typeinfo: class itk::BSplineTransform<double,3,3>
Reference Count: 3
Modified Time: 19499183
Debug: Off
Object Name:
Observers:
none
CoefficientImage: [ 00000267B2E2FEA0, 00000267B2E2F3A0, 00000267B8BA8430 ]
TransformDomainOrigin: [-220.461, -137.269, -102.056]
TransformDomainPhysicalDimensions: [260.253, 260.253, 181.5]
TransformDomainDirection: 0.998543 -0.0277357 -0.046297
0.0277059 0.999615 -0.00128451
0.0463148 0 0.998927

GridSize: [11, 11, 11]
GridOrigin: [-250.992, -170.661, -126.226]
GridSpacing: [32.5316, 32.5316, 22.6875]
GridDirection: 0.998543 -0.0277357 -0.046297
0.0277059 0.999615 -0.00128451
0.0463148 0 0.998927

print(FinalTx)

itk::simple::Transform
BSplineTransform (00000267BA4D7330)
RTTI typeinfo: class itk::BSplineTransform<double,3,3>
Reference Count: 3
Modified Time: 19499183
Debug: Off
Object Name:
Observers:
none
CoefficientImage: [ 00000267B2E2FEA0, 00000267B2E2F3A0, 00000267B8BA8430 ]
TransformDomainOrigin: [-220.461, -137.269, -102.056]
TransformDomainPhysicalDimensions: [260.253, 260.253, 181.5]
TransformDomainDirection: 0.998543 -0.0277357 -0.046297
0.0277059 0.999615 -0.00128451
0.0463148 0 0.998927

GridSize: [11, 11, 11]
GridOrigin: [-250.992, -170.661, -126.226]
GridSpacing: [32.5316, 32.5316, 22.6875]
GridDirection: 0.998543 -0.0277357 -0.046297
0.0277059 0.999615 -0.00128451
0.0463148 0 0.998927

I acknowledge that the first lines are different, i.e. “itk::simple::BSplineTransform” for InitialTx and “itk::simple::Transform” for FinalTx - perhaps suggesting that they are of different classes. Yet “BSplineTransform” is on the next line for both. I’m not sure how to interpret these differences.

I’d like to know why I was unable to get the coefficient images from FinalTx, and whether it even makes sense to be getting it from the pre-registered transform InitialTx. In any case I thought I would proceed with what I have and see what it gets me.

I created a new BSpline transform, NewBsplineTx, using BSplineTransformInitializer as before. Then I tried to set the coefficient images:

NewBsplineTx.SetCoefficientImages((CoeffsImX_new, CoeffsImY_new, CoeffsImZ_new))

but got the error:

‘BSplineTransform’ object has no attribute ‘SetCoefficientImages’

After some searching I realised that SetCoefficientImages only became possible in SimpleITK v2.0 (although that release note contains “SetCoefficientImage”, i.e. missing “s”, which I assume to be a typo).

I created a new virtual environment and installed SimpleITK v2.0.2 yet to my surprise, I get the exact same error as before (I also tried SetCoefficientImage just to be sure but got the same error).

Any ideas on where I’m going wrong?

Thanks.

2

Check the results of type(FinalTx). The return type is sitk::Transform it needs to be converted to sitk::BSplineTransform:

FinalTx = sitk.BSplineTransform(FinalTx)
3

@blowekamp : Thanks for the tip on converting from sitk::Transform to sitk::BSplineTransform.

Interestingly, there was no difference between the arrays from the coefficient images obtained from InitialTx and FinalTx (after converting FinalTx as above).

Anyhow, what still remains is the question of why I get the error:

‘BSplineTransform’ object has no attribute ‘SetCoefficientImages’

when I try to set the coefficient images that I got from FinalTx to a new BSpline transform:

NewBsplineTx = sitk.BSplineTransformInitializer(FixIm, MeshSize)

that I can confirm is of type sitk::BSplineTransform.

My take on this and this is that it should be possible.

I tried both:

NewBsplineTx.SetCoefficientImages((CoeffImX, CoeffImY, CoeffImZ))

and

NewBsplineTx.m_pfSetCoefficientImages((CoeffImX, CoeffImY, CoeffImZ))

(not understanding what the “m_pf” is meant to mean).

Any clues on what I’m getting wrong?

4

This can be done with the constructor:

sitk.BSplineTransform([CoeffImX, CoeffImY, CoeffImZ])
5

@blowekamp: Thanks very much.

I was getting tripped up on trying SetCoefficientImages, meanwhile the answer was there all along - ironically at the same two places I linked above: BSplineTransform( const std::vector &coefficientImages, unsigned int order=3 ) :man_facepalming: