Why does the data adapter work but the reader does not?

If this works

        Dim strRootNodeName As String
        Dim dt As New DataTable

        Using cn As New SqlConnection(GlobalVariables.strConn)
            Using cmd As New SqlCommand(m_strNameLookupSQL, cn)
                cmd.Parameters.Add("@ItemId", SqlDbType.Int).Value = m_RootNodeId
                Using da As New SqlDataAdapter(cmd)
                    da.Fill(dt)
                    strRootNodeName = dt.Rows(0)("ItemName")
                End Using
            End Using
        End Using

Why would this not work?

        Dim strRootNodeName As String

        Using cn As New SqlConnection(GlobalVariables.strConn)
            Using cmd As New SqlCommand(m_strNameLookupSQL, cn)
                cmd.Parameters.Add("@ItemId", SqlDbType.Int).Value = m_RootNodeId
                Using rdr = cmd.ExecuteReader
                    rdr.Read()
                    strRootNodeName = rdr("ItemName")
                End Using
            End Using
        End Using