How to determine which GridView row contains the cell that invoked a Modal Popup

I have a GridView whose rows contain an ItemTemplate with an ImageButton among other template definitions. The ImageButton causes a Modal Popup to be invoked. (See: http://forums.asp.net/t/2005757.aspx?How+to+retrieve+value+from+a+GridView+or+List+in+a+Modal+Popup+when+user+closes+popup+with+OK for how this is accomplished.) When the Modal Popup is closed, I need to determine somehow which row of the main GridView contains the ImageButton that invoked the Modal Popup.