Error With Converting RAS to IJK coordinates

Support
1

Hello Slicer Community,

I was using this older post as a guide to convert RAS to IJK coordinates: http://slicer-devel-archive.65872.n3.nabble.com/Get-coordinates-from-Fiducials-td4034132.html

But when I use “volumeNode.GetRASToIJKMatrix(rasToIJK)” I get an “AttributeError: ‘NoneType’ object has no attribute ‘GetRASToIJKMatrix’” Error call.

I’m using the Python interactor in 4.7.0, if that helps.

2

It means that volumeNode is not pointing to a node – probably the call to getNode(name) or a similar call failed. You can check this in the python interactor:

>>> n = getNode("foo")
>>> print(n)
None
3

Good call- I added print statements for everything up to that and I notice two things.

After print(volumeNode), the interactor just returns an empty line. And after print(volumeNode), the interactor returns “None”. Any pointers for how to fix that?

4

volumeNode is still not set to a MRML node… Whatever call you are making to assign volumeNode is failing (e.g.: volumeNode = ...), so you need to start there and figure out why.

There’s not much more we can say without some code demonstrating the problem. Please post a short example of what you are doing using data from the Sample Data.

5

I just want to get the transformation for RAS to IJK- I’m working with an older library that needs the voxel coordinates, not the mm coordinates.

I downloaded the sample data “MR-Head”, and placed a fiducial at (0,0,0) and used the same code- I gave me the same error though.

6 
volumeNode = slicer.util.getNode("FOO")
print volumeNode

None

import SampleData
sampleDataLogic = SampleData.SampleDataLogic()
sampleDataLogic.downloadMRHead()

volumeNode = slicer.util.getNode("MRHead")
print volumeNode

vtkMRMLScalarVolumeNode (0x7fe62896cff0)
ID: vtkMRMLScalarVolumeNode2
Class: vtkMRMLScalarVolumeNode
…

mat = vtk.vtkMatrix4x4()
volumeNode.GetRASToIJKMatrix(mat)
print mat

vtkMatrix4x4 (0x7fe628ecd4b0)
Debug: Off
Modified Time: 9287983
Reference Count: 1
Registered Events: (none)
Elements:
0 -1 0 133.929
-0 0 -1 116.786
0.769233 0 0 66.6502
-0 0 -0 1

7

Wonderful, thanks so much!

1 Like