Using $_SESSION and $this->session->userdata simultaneously

[eluser]Computerzworld[/eluser]
Hello. I am having a captcha script which generates captcha and the captcha code is stored in the session using $_SESSION. Now I want to retrieve this value in controller using $this->session->userdata but it doesn't retrieves that value in my controller. I tried to retrieve that value using $_SESSION also in the controller. But it also doesn't retrieves that value. Please help me how can I retrieve it? Thanks in advance.