The 18 month extension applied to math, technology, engineering and science graduates. Since you are clearly one of them, it should not be a problem for you to get an 18 month extension. There is some sort of rule saying that you should be employed in the area you have studied- I don't know how they'll implement that.
There was another thread about this long back. Just do a search. It had a link to the official immigration document released by the gov. I had read that but don't quite remember it.