How to Stream in 1080i

I'm trying to figure out how to setup my stream out to be 1080i. I see where you can resize the scene in the stream window, but it only lets you change the dimesnions and frame rate, and doesn't give you an option to choose between progressive and interlaced. It's defaulting to 1080p when I send through Mercury transport to a DeckLink card, but I need it to be 1080i. Is this a setting buried somewhere? or does the stream funciton only output in progressive?