Retrieving Child Matrix ItemID During Mass Update

Hello,

I am trying to create a mass update that copies the itemID into another field on the Item record.  I am doing this so that I can set that field as my Image Key in our SCA site. 


When I preview the Mass Update, in the case of Matrix items, child matrix items are prepending the itemID of their parent to the calculated value.  How can I retrieve just the itemID of the child matrix item and not have the parent itemID included?  Presently I am just using the {itemid} field reference.

Thanks,
Mike