Rebuild the Fulltext catalog
I was wondering how SQL Server proceeds while rebuilding the fulltext catalog.


From BOL : 

When you REBUILD a fulltext catalog, SQL Server rebuilds the entire catalog. When a catalog is rebuilt, the existing catalog is deleted and a new catalog is created in its place. All the tables that have full-text indexing references are associated with the new catalog. Rebuilding resets the full-text metadata in the database system tables.

I would suggest you to look into REORGANIZE depending on the fragment counts as described in my answer for Reorganize full text catalog is offline or online?