[vtkusers] how to get the point of intersection

[xujf]

Hi,all
   I want to get the point of intersection of a line and an isosurface.this is my part of program:(written in python)
        #get a line
        rake=vtkLineSource()
        rake.SetPoint1(center[0],center[1],center[2])
        rake.SetPoint2(pointx,pointy,pointz)

        #get an isosurface of a volume
        marchingcubes=vtkImageMarchingCubes()
        marchingcubes.SetInput(reader.GetOutput())

the line\'s start point(center[0],center[1],center[2]) inside of the isosurface and its 
end point(pointx,pointy,pointz)is outside of the isosurface.so the isosurface and the line have
a point of intersection.how can I calculate the point\'s coordinates?

Thanks in advance