Technical Documentation - created_by_id and affected_part output

I am using the Technical Documentation application to create reports for the Quality Management qs_NCR ItemType. One of the qs_NCR properties is the affected_part which is implemented as an Item ItemType. I have also implemented a foreign property that refers to the  affected_part's name.

The method I am using to get the NCR information is:

ItemDocumentElement targetItem = targetElement as ItemDocumentElement;
if (targetItem != null) {

TableDocumentElement tableElement1 = (TableDocumentElement) this.Factory.NewTable("Table", 1, 4);

tableElement1.GetCell(0,0).AddChild(this.Factory.NewText("Title", "Part Name:"));
tableElement1.GetCell(0,1).AddChild(this.Factory.NewText("Text", targetItem.GetItemProperty("_affectedpartnumber", " ")));

tableElement1.GetCell(0,2).AddChild(this.Factory.NewText("Title", "Initiator:"));
tableElement1.GetCell(0,3).AddChild(this.Factory.NewText("Title", targetItem.GetItemProperty("created_by_id", " ")));



The output is the created_by_id id, not the user's name.

What is the most efficient way of outputting the user's name?