ls *.nii | python -c 'import sys, irtk; [sys.stdout.write( str(irtk.imread(line.rstrip(), dtype="float32").max())+"\n") for line in sys.stdin]'

import itk

pixelType = itk.UC
imageType = itk.Image[pixelType, 3]
readerType = itk.ImageFileReader[imageType]
writerType = itk.ImageFileWriter[imageType]
reader = readerType.New()
reader.SetFileName( "input.nii" )
reader.Update()

itk2np = itk.PyBuffer[imageType]
data = itk2np.GetArrayFromImage( reader.GetOutput() )

...

import SimpleITK as sitk

img = sitk.ReadImage( "input.nii" )
data = sitk.GetArrayFromImage( img )

...

output = sitk.GetImageFromArray( data )
output.SetSpacing( img.GetSpacing() )
output.SetOrigin( img.GetOrigin() )
output.SetDirection( img.GetDirection() )
sitk.WriteImage( output, "output.nii" )

import irtk

img = irtk.imread( "input.nii" )

...

irtk.imwrite( "output.nii", img )