No, indexes can be applied to any column or set of columns under 900 bytes wide in total.

http://www.sqlservercentral.com/articles/Indexing/68439/

http://www.sqlservercentral.com/articles/Indexing/68563/

http://www.sqlservercentral.com/articles/Indexing/68636/

Your index likely isn't covering, hence it's more efficient to just scan the table.

http://sqlinthewild.co.za/index.php/2009/01/09/seek-or-scan/