Function TDataModule1.ScannerCheck(Source:String):Boolean;
// 'Source' is stored in a database or ini or registry...
var aImageEnIO:TImageEnIO;
    NewSource:String;
    ADevice : TIEAcquireSource;
begin
     result := FALSE;
     NewSource := Source;
     ADevice := StrToAcquireSource(Source);
     aImageEnIO := TImageEnIO.Create(self);
     try
        result := aImageEnIO.SetAcquireSource(ADevice.Api, ADevice.Location);
        if result then result :=  aImageEnIO.TwainParams.GetFromScanner;
        if (Source = '') or (not result) then
        begin
             ShowMessage('Scanner ist nicht bereit'+#13#10+
             'Ist der Scanner korrekt angeschlossen und eingeschaltet?');
             // ... dann Scanner auswählen
             result := aImageEnIO.SelectAcquireSource([ieaTwain, ieaWIA]);
             if result then
                NewSource := AcquireSourceToStr(aImageEnIO.SelectedAcquireSource);
             if NewSource <> Source then
             begin
                  // wollen Sie wirklich diesen Scanner auswählen:
                  // NewAcquisitionSource
                  // wenn ja, dann
                  ADevice := StrToAcquireSource(NewSource);
                  if MessageDlg('Wollen Sie diesen Scanner auswählen:?'+
                     +#13#10+AcquireSourceToStr(ADevice), mtConfirmation,
                     [mbYes, mbNo], 0, mbYes) = mrYes then
                  begin
                       Source := NewSource;
                       // store 'Source' to a database or ini or registry...
                       result :=  aImageEnIO.TwainParams.GetFromScanner;
                  end else result := FALSE;
             end;
        end;
     finally
        aImageEnIO.Free;
     end;
end;

procedure TMainForm.TestScanButtonClick(Sender: TObject);
var oldCursor:TCursor;
begin
     if not DataModule1.ScannerCheck then exit;
     oldCursor := Screen.Cursor;
     Screen.Cursor := crHourGlass;
     try
        result := MainForm.ImageEnIO1.Acquire;
     finally
        Screen.Cursor := oldCursor;
        SelectedBitmap.Assign(ImageEnIO1.IEBitmap);
        // or ImageEnView.IEBitmap.Assign(ImageEnIO1.IEBitmap);
     end;
end;