Set Document in DocumentViewer Async

Hi Experts,

i have a small problem with the DocumentViewer control. I am Rendering my document in a different thread, so far everything works fine, after that i want to display the fixeddocumentsequence in the viewer control. Here is where my problem starts i tried nearly everything but i cant set the document to the viewer, i always get a threading exception, that says that the document belongs to a differnt thread. I know what this exceptoin means, but i do not know why it's raised. 

Here is the code where I render the document and then try to display it in the viewer:

 System.Threading.Thread t = new System.Threading.Thread(new System.Threading.ThreadStart(() =>
{
    Build();
    System.Windows.Documents.FixedDocumentSequence se = Document.FixedDocumentSequence;
    viewer.Dispatcher.BeginInvoke((Action)(() =>
    {
        viewer.Document = se;
    }));
}));
t.SetApartmentState(System.Threading.ApartmentState.STA);
t.IsBackground = true;
t.Start();

Can anyone help me with this "hopefully" easy Problem?

Kind Regards

Manu