You should change the document type to positive film as follows:
  Try
    VSTwain1.OpenDataSource()
    VSTwain1.Capability = DeviceCapability.ILightPath
    VSTwain1.CapType = CapType.OneValue
    VSTwain1.CapValue = 1     ' TWLP_TRANSMISSIVE
    VSTwain1.SetCap()         ' Set cap value
    VSTwain1.Acquire()
  Catch ex As TwainException
    MsgBox(ex.Message)
  End Try
This turns on the top light for transparencies or XRays.