[vtkusers] get the extent of the segmented image?

[xujf]

Hello,everyone:
  I have used itkfastmarchingimagefilter to get a segmented image from the medical image data.then I convert the 
segmented image data into vtkimagedata.next I want to get the extent of the segmented image(if the segmented image
is brain,I only want to get the extent of the brain,not the extent of the origin medical image data).how can I get
it in vtk?

Thanks in advance!
xujf