How the tensor is stored in NRRD

I use the 3DSlicer Diffusion module to load DWI, estimate its tensor and then store the tensor image into NRRD. I want to know how the tensor is stored in NRRD?
Is it stored as 6 upper triangle elements of the 3x3 tensor matrix or the lower triangle or others?
I also want to know which VTK class can be used to correctly load the tensor image?


Thank you very much!