DelphiTwain：如何知道从送纸器扫描完成

I am using TDelphiTwain component for scanning. When i start scanning from feeder, all scanning performed assynchronously and event OnTwainAcquire triggered when new image available for processing. But i can not find how to check that scanning of all douments already finished. When feeder tray is empty, then scanning stops and if i put new papers to the feeder it do not process it. So i need to know that it is finished and i can process received images. I can check that feeder is empty, but i can easy miss (with my check) the moment when tray was empty, so app will think that new images are coming, but they don't. I supposed there should be kind of events for such task (StartSession/StopSession), but i can't find it.I am using TDelphiTwain component for scanning.