[vtkusers] HOW I CAN GET DATAEXTENT FROM VOLUME IMAGE?

[Adexe]

hi, I'm working with a volume image and 'll get  his extent.

vtkImageReader reader
    reader SetFilePrefix $datafile
   # puts "extension1: [reader GetDataExtent]"
    set extent [reader GetDataExtent]
    reader SetDataExtent [lindex $extent 0] [lindex $extent 1] [lindex 
$extent 2] [lindex $extent 3] [lindex $extent 4] [lindex $extent 5]

If I use GetDataExtent Method before set the extent,this no returns the 
extent , how I can do it?