In Slicer, scalar volumes are represented as vtkMRMLScalarVolumeNode. This class wraps a vtkImageData, which can hold multiple arrays of scalars (or other), even though only one is displayed (the 'ActiveScalar').

When loading an mha, nrrd, or vtk (structured points) volume, only one scalar will be loaded in the vtkMRMLScalarVolumeNode, whose name will be set to 'ImageScalars'.

When opening one of those multiple-scalar volumes in Paraview, it is possible to select which array/attribute to make active. In 3D Slicer, it IS possible to create such files in python: Slicer core can handle a change of active scalars in the 2D Views, and display the different attributes for only one volume node, even though saving those volumes would result in only the active scalar attribute being included.

Those issues can also be found when using a CLI module that needs to receive or pass those kinds of multi-scalar volumes.

Changes needed would be:

• Updating the reader/writer to allow loading and saving of multi-scalar volumes
• Updating the Volumes Module to allow for a selection of the active scalar within a list of the existing attributes