$thread[preview] in topic meta-description

[mrkiwi]

Hello everybody!
I'm stuck
Wanted to edit my threads meta-description this way:

PHP Code:

<meta name="description" content="$foruminfo[title_clean] > $threadinfo[title] > $thread[preview]" /> 


but variable $thread[preview] does not work at all.
($thread[preview] is a variable from threadbits and is presented in showforum - it's the first words from the first post, shown when you hover topic name with a mouse)

As far as i understand, i need to add to showthread.php some calls, but i even can't imagine where i should look at...

I'm not a programmer at all, it's just hobby, so i would be extremely grateful if someone can help me

Thanks in advance!

P.s. Or maybe there can be another way to insert the text of the first post on the page to the description tag?