Magento : How to check if admin is logged in within a module controller?

I'm creating a Magento Module. Within the controller, I want to check if an admin is logged in or not. So the controller only will be accessible if there is a logged in admin.

I'm trying to use this code on my controller.

Mage::getSingleton('core/session', array('name' => 'adminhtml')); 
$session = Mage::getSingleton('admin/session');

// Use the 'admin/session' object to check loggedIn status
if ( $session->isLoggedIn() ) {
   echo "logged in";
} else {
   echo "not logged in";
}

but I always get "not logged in", even if I'm already logged in to the magento admin.

Can anybody help me to resolve this issue?? any help will be much appreciated. Thanks