Sup' teach.
But seriously, if it's your head of department Dear, anyone else use Hello in the initial email, you'll find that as the conversation goes on it'll move to hi, then just to a message and a name at the end, depending on how nice your lecturer is, normally they just want you to be respectful in the initial email.
Saying that my course leader replies to emails from his blackberry, so tends to reply in all caps and sometimes in text speak, as long as you've spoken to him before. I was a bit thrown when he replied to my email and used the term "Brill..."